Photoinduction of Flower Identity in Vegetatively Biased Primordia

Correspondence to: Frederick D. Hempel, evoke{at}nature.berkeley.edu (E-mail), 510-642-4995 (fax).

	ABSTRACT

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

Far-red light and long photoperiods promote flowering in Arabidopsis. We report here that when 30-day-old vegetative plants were induced with a continuous light treatment enriched in far-red light, flowers developed directly from previously initiated primordia. Specifically, plants induced with our continuous incandescent-enriched (CI) treatment produced an average of two primary-axis nodes with a leaf/flower phenotype, indicating that approximately two leaf/paraclade primordia per plant produced an individual flower from tissue that typically would differentiate into a paraclade (secondary inflorescence). Assays for APETALA1::ß-glucuronidase activity during the CI photoinduction treatment indicated that the floral meristem identity gene APETALA1 was transcriptionally activated in primordia with a leaf/paraclade bias and in primordia committed to leaf/paraclade development. APETALA1::ß-glucuronidase activity levels were initially highest in young primordia but were not correlated strictly with primordium fate. These results indicate that primordium fate can be modified after primordium initiation and that developing primordia respond quantitatively to floral induction signals.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

After the transition to flowering in Arabidopsis, flowers are initiated from the apical meristem instead of leaves or paraclades (secondary shoots). In early-flowering ecotypes such as Landsberg erecta (Ler) and Nossen (No), strong photoperiodic treatments can induce mature vegetative plants to initiate flowers from the primary meristem within one plastochron (Hempel and Feldman 1994 , Hempel and Feldman 1995 ). These flowers typically are not associated with leaves, although Arabidopsis flower primordia evidently have a minor leaf component that is expressed when flowers are genetically ablated (Nilsson et al. 1998 ). The last leaves initiated on a photoinduced shoot apex develop as cauline leaves, largely due to their position and their developmental state at the time that the flower-inducing signals reach the shoot apex. In Arabidopsis, both cauline and rosette leaves develop in conjunction with secondary flowering shoots (paraclades), that is, leaves and paraclades develop from a common primordium—the leaf/paraclade primordium. The flowering of the paraclades associated with the cauline leaves typically occurs shortly after flowering begins on the primary shoot and after the initiation of one or more leaves from the paraclade meristem (Hempel and Feldman 1994 ; Grbic and Bleecker 1996 ).

Although the switch from leaf/paraclade to flower initiation on the primary shoot represents a relatively abrupt morphological change, a number of more gradual changes also occur during plant maturation. For example, the surface characteristics and shapes of the leaves change gradually in response to gibberellins and other developmental signals (Chien and Sussex 1996 ; Telfer et al. 1997 ). The flowering-response potential within the shoot apex generally increases with maturity, because older plants are more responsive to photoinduction treatments (Corbesier et al. 1996 ). These changes likely are due to a progressive increase in the sensitivity of primordia to photoinductive signals (Blazquez et al. 1997 ) and/or to changes in the balance of flower-promoting versus flower-repressing signals in the plant (Bernier et al. 1993 ; Schultz and Haughn 1993 ).

Within the shoot apex, floral meristem identity genes regulate flowering. One of these genes, LEAFY (LFY), is a critical regulator of the flowering response within the shoot apex, because above-threshold levels of LFY are required for establishing floral meristem identity (Weigel and Nilsson 1995 ; Blazquez et al. 1997 ) and for suppressing indeterminate meristem growth (Okamuro et al. 1996 ). A second floral meristem identity gene, APETALA1 (AP1), is also important for establishing floral meristems (Irish and Sussex 1990 ; Bowman et al. 1993 ). It is expressed predominately in young flower meristems (Mandel et al. 1992 ) and triggers flowering when ectopically expressed in transgenic plants (Mandel and Yanofsky 1995 ). AP1 promoter activity is useful as a marker for floral determination in Arabidopsis, because the AP1 promoter is active only in florally determined plants (Hempel et al. 1997 ).

The expression of above-threshold levels of floral meristem identity genes within primordia at the shoot apex requires above-threshold levels of flower-promoting signals from outside of the shoot apex (Bernier et al. 1981 ). It is the leaves that perceive the light signals that induce flowering and promote the transmission of flower-promoting signals to the shoot apex (Knott 1934 ; Corbesier et al. 1996 ). Genes likely to be involved in the production, transmission, and perception of flower-promoting signals have been identified in genetic screens for late-flowering mutants (Koornneef et al. 1991 ; Martinez-Zapater et al. 1994 ).

One of the mutants identified—constans (co)—has a semidominant phenotype, which might be expected of a gene involved in quantitative (threshold-based) signaling. CO is a putative transcription factor, and its expression is regulated by photoperiod (Putterill et al. 1995 ). Furthermore, constitutive expression of CO induces early flowering and the expression of floral meristem identity genes (Simon et al. 1996 ); recent work indicates that CO is positively regulated by blue light acting through a cryptochrome (CRY2) and negatively regulated by red light acting through a phytochrome (PHYB) (Guo et al. 1998 ).

The phyB mutant, which flowers early under both long-day and short-day conditions (Halliday et al. 1994 ; Reed et al. 1994 ), belongs to another broad class of mutants, the early-flowering mutants, that identify genes acting directly or indirectly to suppress flowering (Weigel 1995 ). Because phytochromes directly perceive light signals in the leaves, PHYB is likely involved in leaf-to-apex signaling during the induction of flowering. Interestingly, CO is expressed both in young leaves and in the shoot apex (Putterill et al. 1995 ); thus, CO also may be involved in leaf-to-apex signaling, although the precise location of CO activity still is not known.

It also is not known whether the flower-inducing signals from outside the shoot apex instruct meristems to make flower primordia or whether they directly instruct developing primordia to become flowers. In this report, we present results suggesting that primordia can respond directly to flower-inducing signals after they are initiated. Specifically, we describe the formation of flowers from young vegetatively biased leaf/paraclade primordia during a continuous incandescent-enriched (CI) photoinduction treatment. Analyses of AP1::ß-glucuronidase (GUS) activity within the shoot apices of photoinduced plants indicate that preexisting leaf/paraclade primordia express the floral meristem identity gene AP1 in response to the CI photoinduction treatment. The expression of AP1 in primordia initiated before the start of the photoinduction treatment and the formation of individual flowers from the youngest of the previously initiated primordia indicate that vegetatively biased primordia are not committed strictly to a specific fate at the time they are initiated.

	RESULTS

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

Photoinduction of a Leaf/Flower Phenotype
Previously, it was hypothesized that when vegetative Arabidopsis plants are induced to flower by strong photoinduction treatments, the chimeric flower/paraclade shoots that commonly appear on the primary shoot (below the lowest complete flower) represent vegetative primordia that were converted partially into flowers (Hempel and Feldman 1995 ). To test this "conversion" hypothesis, we induced a population of vegetative Ler plants with a CI photoinduction treatment to determine whether this very strong photoinduction treatment would induce the complete conversion of paraclade primordia into flower primordia. Table 1 provides a comparison of the CI treatment used in this study with the strong photoinductive treatment of Hempel and Feldman 1994 that was used to induce flowering in Ler plants. An average of six additional flower primordia was evident after 72 hr for the plants photoinduced with the CI treatment (Table 1).

Figure 1 shows a "leaf/flower" nodal phenotype that was common on the primary inflorescences of the plants subjected to the CI treatment. Leaf/flower nodes were formed directly above the uppermost leaf/paraclade on the primary shoot (Figure 1A and Figure 1B). Arabidopsis plants do not usually have leaves (bracts) subtending individual flowers, and in the previous study mentioned above, none of the flowers on the primary shoot was subtended by leaves (Hempel and Feldman 1994 ). Figure 2, however, indicates that most of the plants in the CI-induced population (83%) had at least one node with a macroscopic leaf subtending a flower (Figure 2C to E) and that ~14% had two nodes with macroscopic leaves subtending flowers (Figure 2E). Chimeric flower/paraclade shoots also were present on a subset of the plants. When present, they were located directly below the lowest complete flower on the primary shoot (Figure 2B and Figure 2D).

View larger version (68K): [in this window] [in a new window]   Figure 1. Leaf/Flower Nodes Produced on Plants Exposed to CI Photoinduction Conditions after 30 Short Days. (A) Primary inflorescence with one leaf/flower node (arrow). (B) Primary inflorescence with two leaf/flower nodes (arrows). The sepals and petals of the flower to the right have fallen away.

View larger version (19K): [in this window] [in a new window]   Figure 2. Primary Inflorescence Phenotypes Found within a Population of Ler Plants Induced with CI Photoinduction Conditions. (A) "Normal" inflorescence. The lowest flower, however, was always subtended by vestigial leaf tissue. (B) Inflorescence with a leaf/flower/paraclade node below a vestigial leaf/flower node. (C) Inflorescence with a single macroscopic leaf/flower node. On ~75% of these plants, the second lowest flower on the primary inflorescence was subtended by vestigial leaf tissue. (D) Inflorescence with a leaf/flower/paraclade node and a macroscopic leaf/flower node. An additional vestigial leaf/flower node also was evident on ~50% of these plants. (E) Inflorescence with two macroscopic leaf/flower nodes. Vestigial leaf/flower nodes were rare in this class of plants. In control populations of plants grown in long days, none of the plants had flowers on the primary inflorescence that were subtended by macroscopic or vestigial leaves.

We also found that fingerlike protrusions of tissue often subtended flowers low on the primary inflorescence (Figure 2A to D) and that all of the plants with no leaf/flower nodes had multicellular fingerlike protrusions subtending the lowest flower on the primary shoot. These structures were found below a flower on approximately three-fourths of the plants with one leaf/flower node but basically were absent from the plants with two leaf/flower nodes. In a previous study, similar structures were commonly evident under the uppermost paraclade on the primary shoot and were considered to be vestigial cauline leaves (Hempel and Feldman 1994 ). In that study, however, vestigial leaves were not present below individual flowers. In both cases, the fingerlike protrusions are indicative of the highly suppressed leaf component of a node; in this study, a leaf/flower node. Thus, the modal inflorescence phenotype within the CI-induced population was one with two leaf/flower nodes per plant (one with a macroscopic cauline leaf and one with a vestigial leaf).

Shoot Apex Morphogenesis and Flower Primordium Initiation
Figure 3 indicates that flower primordia were initiated rapidly after the start of the CI photoinduction treatment. After 72 hr of induction, ~14 flower primordia were present. The solid line drawn through the data points in Figure 3 indicates the number of flower primordia present between 72 and 120 hr after the start of the CI treatment. Before the 72-hr time point, the definitive morphological markers for leaf versus flower primordia (see Figure 4) were not evident. However, a curve roughly estimating the number of flower primordia present before the 72-hr time point is indicated by a dashed line (Figure 3). An acceleration of the primordium initiation rate is indicated. This acceleration is consistent with results showing that the rate of primordium initiation increases dramatically after the start of photoinduction (Hempel and Feldman 1995 ) and with data indicating that a photoinduced increase in mitotic activity in the closely related Sinapis alba (white mustard) occurs after an initial 26- to 30-hr lag period (Gonthier et al. 1987 ). The curve intercepts the y-axis at a value of 2. This value is equal to the total number of leaf/flower nodes found on plants with the modal phenotype (Figure 2C; see Discussion for details and rationale).

View larger version (16K): [in this window] [in a new window]   Figure 3. Initiation of Flower Primordia during a CI Photoinduction Treatment. A population of Ler plants was photoinduced under CI photoinduction conditions (Table 1) after 30 short days. Initiated flower primordia were evident clearly at 72, 96, and 120 hr after the start of the photoinduction treatment. A solid line is drawn through the data points; the dashed line represents an estimation of the number of flower primordia present at earlier time points in the photoinduction treatment (see Discussion). Error bars indicate standard deviation.

View larger version (115K): [in this window] [in a new window]   Figure 4. Shoot Apex Morphology of Plants Treated with the CI Photoinduction Treatment. (A) Shoot apex after 72 hr of CI photoinduction. The first four flower primordia are numbered in the order in which they were initiated (F1 to F4). The identity of these primordia is indicated by the sepal primordia that encircle their periphery. The first flower (F1) and the leaf primordium subtending it (arrow) developed from the same original leaf/paraclade primordium. The identity of a shoot primordium (question mark) initiated at the node below F1 is not yet evident. Although it is most likely a paraclade, it may be a flower or a flower/paraclade (see Figure 2). (B) Shoot apex after 96 hr of CI photoinduction. The first four flower primordia are labeled (F1 to F4). F1 is subtended by a leaf primordium (arrow) that will develop into a macroscopic leaf. (C) A second shoot apex after 96 hr of CI photoinduction. The first four flowers are labeled (F1 to F4). A chimeric flower/paraclade shoot (CS) is evident at the node below F1. A leaf primordium subtends this shoot (arrow), and the identity of the chimeric shoot is indicated by the abaxial sepal primordia (asterisks) and the initiation of a secondary flower primordium (arrowhead) on the abaxial side of the shoot. (D) Side view of the shoot apex shown in (B). The first two flowers (F1 and F2) and a paraclade (P) are labeled. F2 is subtended by a multicellular "finger" of leaf tissue (arrow). (E) Close-up of the leaf tissue subtending F2 in (D). Arrows indicate the leaf tissue and its close association with the developing flower primordium above it.

Figure 4 illustrates the patterns of primordium initiation and primordium differentiation evident at the shoot apices of the induced plants. The oldest flower primordia were clearly distinguishable from leaf and paraclade primordia at the 72-hr time point. At this time, leaf/flower nodes were evident, and their morphology indicated a common developmental origin for the leaf and flower (Figure 4A). The total number of leaf/flower nodes, however, was not always evident after 72 hr. For example, a second leaf/flower node may be present in Figure 4A, at the node below the evident leaf/flower node, but the identity of the shoot at this node (denoted by a question mark in Figure 4A) is not yet evident. In addition, the second flower (F2 in Figure 4A and Figure 4B) often is subtended by vestigial leaf tissue (Figure 2, Figure 4D, and Figure 4E).

After 96 hr of photoinduction, floral organ primordia were evident on the oldest flowers (Figure 4B and Figure 4C), and chimeric shoots were evident on a subset of the plants (Figure 4C). A consistent pattern evident at this time point was that the plants with fewer leaf/flower nodes typically were those that were less responsive to the induction treatment (as measured by total flower number). For example, Figure 4B shows the shoot apex of a plant that has 20 initiated flower primordia and a clearly evident leaf/flower node (with its flower component labeled F1), whereas Figure 4C shows the shoot apex of a plant with 16 initiated flower primordia and no clearly evident leaf/flower node. The shoot apex in Figure 4C, however, does have a chimeric flower/paraclade shoot subtended by a leaf. This shoot evidently represents the partial conversion of a paraclade primordium into a flower primordium.

Flower primordia that developed in conjunction with a macroscopic leaf primordium consistently developed and matured after the flower primordia immediately above them on the primary shoot (Figure 4A and Figure 4B). These differences typically persisted throughout the development of the inflorescence, because anthesis and fruit maturation also were delayed in the flowers subtended by leaves (Figure 1B).

Photoinduced Expression of AP1::GUS and Floral Meristem Identity
To characterize further the flowering response within the shoot apices of photoinduced plants, we measured the transcriptional regulation of the floral meristem identity gene AP1 by assaying for the activity of an AP1::GUS reporter. Previous work suggested that AP1::GUS activity is limited to plants that are committed to flowering, although activity was not limited strictly to flower primordia (Hempel et al. 1997 ). The purpose of analyzing AP1::GUS activity was to correlate the induction of flower meristem identity gene expression with the induction of flowers from previously vegetative primordia. The activation of the AP1::GUS transgene was characterized in the Ler and No ecotypes, both of which carried the transgene at the same locus (see Methods). Populations of Ler and No plants harboring the AP1::GUS gene were induced after 30 short days under CI photoinduction conditions in a single experiment. Table 2 shows that leaf/flower nodes commonly occurred on the plants within both the Ler and No populations, although at a slightly lower frequency than in the first experiment (Figure 2).

Figure 5 indicates that the two ecotypes responded similarly to the photoinduction treatment. No AP1::GUS activity was visible before the start of the photoinduction treatment (Figure 5A and Figure 5E). After 24 hr, however, AP1::GUS activity was evident in both the Landsberg erecta and Nossen apices, and expression was strongest in the youngest leaf/paraclade primordia. After 48 hr, AP1::GUS activity was evident in primordia throughout the shoot apex in both ecotypes, and flower primordia were evident on the flanks of the apical meristem (Figure 5C and Figure 5G). To identify the regions within the shoot apex with the highest levels of AP1::GUS activity, we used a diluted 5-bromo-4-chloro-3-indolyl ß-D-glucuronic acid (X-gluc) solution (one-quarter strength). Assays using the dilute solution indicated that the highest levels of activity occurred in the flower primordia (Figure 5D and Figure 5H) and that the patterns of expression were similar to reported patterns of mRNA expression (Mandel et al. 1992 ).

View larger version (112K): [in this window] [in a new window]   Figure 5. AP1::GUS Activity in CI-Photoinduced Ler and No Plants. In (A) to (C) and (E) to (G), incubation was with full-strength (2 mM) X-gluc solution. In (D) and (H) to (J), incubation was with quarter-strength (0.5 mM) X-gluc solution. GUS activity is indicated by the pink color. (A) and (E) No AP1::GUS activity is apparent at hour 0 of the CI photoinduction treatment. (B) and (F) At hour 24, the highest levels of AP1::GUS activity are evident in the youngest leaf primordia (arrows), which were initiated before the start of the CI treatment. (C) and (G) At hour 48, AP1::GUS activity is evident throughout the shoot apex. Arrows indicate cauline leaf primordia, and asterisks indicate flower primordia. (D) and (H) AP1::GUS activity is strongest within flower primordia (asterisks) at hour 48, whereas young leaf primordia (arrow) have lower levels of activity. Incubation with a dilute X-gluc solution was required to detect relative differences of activity at this time point. (I) Young paraclade primordia (p) had lower levels of AP1::GUS activity than did flower primordia (asterisks) at hour 72. The arrow indicates the leaf primordium subtending the paraclade primordium. (J) Flower primordia subtended by leaves (arrow) had similar levels of AP1::GUS activity when compared with other flower primordia at approximately the same stage of development. Asterisks indicate four flowers in the plane of the section.

AP1::GUS activity after 72 hr of induction is shown in Figure 5I and Figure 5J. Activity was relatively low in the paraclade primordia (Figure 5I) but high in the flower primordia, including the flower primordia, which developed in conjunction with a leaf primordium (Figure 5J). Relatively low levels of AP1::GUS activity were detected also in the developing leaf primordia at this time point (Figure 5I and Figure 5J).

AP1::GUS activity also was examined during the transition to flowering in plants grown only in long days. These results are shown in Figure 6. We expected that AP1::GUS activity would be lower in the young leaf primordia of these plants, because leaf/flower nodes were not found in long-day-grown populations (Table 2). Initially, no AP1::GUS activity was present in the long-day-grown plants (Figure 6A, Figure 6E, and Figure 6F). However, AP1::GUS activity was evident in the leaf primordia of the plants undergoing the transition to flowering in long days; contrary to initial expectations, levels of activity (Figure 6B and Figure 6G) were similar to those found in plants that were photoinduced with the CI photoinduction treatment (Figure 5B and Figure 5F). AP1::GUS activity was evident also throughout the shoot apices of the long-day-grown plants, and there was significant activity in nonmeristematic tissues, particularly after flower primordia had been initiated (Figure 6C, Figure 6D, and Figure 6H). AP1::GUS activity was first evident ~2 days earlier in the Ler ecotype. The overall patterns of AP1::GUS activity, however, were similar in the two lines (Figure 6).

View larger version (82K): [in this window] [in a new window]   Figure 6. AP1::GUS Activity in Long-Day-Grown Ler and No Plants. Plants were grown under long-day conditions (16 hr of light/8 hr of dark). Arrows indicate leaf primordia, and asterisks indicate flower primordia. All apices were incubated with full-strength (2 mM) X-gluc. (A) to (D) AP1::GUS activity in Ler. Activity was first evident on day 8, and flower primordia were first clearly evident on day 10. (E) to (H) AP1::GUS activity in No. Activity was first evident on day 10, and flower primordia were first clearly evident on day 12.

	DISCUSSION

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

Photoinduced Conversion of Vegetative Primordia into Flowers
Primordia initiated within vegetative Arabidopsis shoot apices typically produce a node comprised of both a leaf and a paraclade and thus are referred to as leaf/paraclade primordia (Hempel and Feldman 1995 ). The data reported here, however, indicate that very strong photoinduction treatments can induce the paraclade (shoot) region of a young leaf/paraclade primordium to become a flower, even when the leaf region of the primordium is committed to developing as a leaf. The result is an atypical "leaf/flower" phenotype. This phenotype appeared on approximately two nodes per plant in populations exposed to CI photoinduction conditions after 30 short days. One of the two nodes typically had a macroscopic cauline leaf associated with a flower, whereas the second node typically had only a vestigial leaf at the base of the flower (Figure 2 and Figure 4). We have concluded that these two leaf/flower nodes are derived from leaf/paraclade primordia initiated before the start of the CI photoinduction treatment and that the flower of a leaf/flower node develops from the converted paraclade region of a leaf/ paraclade primordium. This conclusion is denoted graphically in Figure 3 by a curve that intercepts the Y-axis at a value of 2.

Two lines of reasoning are consistent with this interpretation. First, in a previous study, no leaves (either macroscopic or vestigial) were found under individual flowers, and the production of leaves ceased within the first day of a less potent photoinduction treatment (Hempel and Feldman 1994 ). The presence of two leaf/flower nodes coupled with the fact that six additional flowers were initiated during the first 72 hr of the CI photoinduction treatment (Table 1) suggest that the flowers with subtending leaves developed from primordia initiated before the start of the CI treatment; that is, it is improbable that a stronger photoinduction treatment (the CI treatment) would induce the de novo formation of leaves under flowers when a weaker treatment did not (Table 1; Hempel and Feldman 1994 ).

Second, photoinduction experiments with white mustard indicate that increases in mitotic activity within the shoot apex do not occur until after 1 day of induction (Gonthier et al. 1987 ). The relevance of the white mustard study to Arabidopsis flowering is indicated by experiments showing that the responses of the two mustards to photoinduction treatments are qualitatively similar (Corbesier et al. 1996 ). Thus, the curve indicating the time of initiation of the first flower primordia (Figure 3, dashed line) is consistent with the assumption that the rapid acceleration of mitotic activity and the primordium initiation rate is preceded by an initial lag period during which the activity of the shoot apex is similar to that of the vegetative shoot apex (Hempel and Feldman 1995 ). The hypothesized slope of the curve during the first 24 hr of induction is estimated at one primordium per day, which is slightly more than the rate of primordium initiation calculated for vegetative plants (0.8 primordia per day; Hempel and Feldman 1994 ).

Alternatively, if it is assumed that the initiation of flowers did not begin until after the start of the induction treatment (i.e., after hr 0), it is implied that the rate of primordium initiation at the shoot apex would have to increase almost instantaneously from less than one primordium per day to >4.5 primordia per day. Such a rapid increase in shoot apex activity is not consistent with what is known about the activation of the shoot apex during floral induction in mustards (Gonthier et al. 1987 ; Corbesier et al. 1996 ).

Not all of the leaf/paraclade–to–leaf/flower conversions induced by the CI treatment were complete, because chimeric flower/paraclade shoots (Hempel and Feldman 1995 ) were common within the populations of plants induced with CI light. The chimeric flower/paraclade shoots formed on the primary inflorescence always were located below the lowest complete flower (Figure 2 and Figure 4), which is consistent with our conclusion that they represent partially converted primordia that also were initiated before the start of the CI photoinduction treatment.

Primordium Plasticity and the Progressive Specification of Primordium Identity
For simplicity, we have described the formation of flowers from leaf/paraclade primordia initiated on vegetative plants as "conversions," implying that the primordia initially had leaf/paraclade identity. Although it is known that the converted primordia would have differentiated into leaves and paraclades if the plants were not induced after 30 short days (Hempel and Feldman 1994 ), they clearly were not strictly fated to become leaves and paraclades early in their development. The converted primordia, however, presumably had partial leaf/paraclade identity early in their development, although that identity was overridden by the influx of above-threshold levels of flower-promoting signals into the shoot apex. This partial leaf/paraclade identity, which increases as a primordium develops on a vegetative plant, may be referred to as leaf/paraclade bias.

The term bias denotes a developmental tendency but not commitment; that is, it denotes a presumptive developmental fate, based on the inductive signaling that has already occurred (Grainger 1992 ). Primordia with a developmental bias, however, can take on alternative fates in response to signaling that overrides their bias. Although the concept of developmental bias has been used primarily to characterize animal primordia (Grainger 1992 ), it seems particularly useful for describing plant development, because the fates of plant cells and primordia often can be altered during their development by experimentally controlled changes in signaling (Battey and Lyndon 1990 ; Irish and Nelson 1991 ; van den Berg et al. 1995 ; Zachgo et al. 1995 ).

Figure 7 illustrates how the fates of existing primordia with leaf/paraclade bias are affected by photoinduction treatments and indicates the potential fates of the various vegetative primordia at the shoot apex of a 30-day-old plant (Figure 7A). If no induction treatment is given to a 30-day-old vegetative plant, all of the primordia present at the shoot apex develop into leaves and paraclades (Figure 7B; Hempel and Feldman 1994 ). However, the application of a strong photoinduction treatment induces anlagen to develop as flowers and initiated leaf/paraclade primordia to develop as cauline leaves and paraclades (Figure 7C; Hempel and Feldman 1994 , Hempel and Feldman 1995 ). The application of a CI photoinduction treatment (this study), however, induced both the anlagen and approximately two initiated primordia to produce individual flowers. It is also possible that an additional flower(s) may have been formed from an initiated primordium that had its leaf/paraclade bias overridden to the degree that no leaf identity was expressed morphologically (see Figure 7D).

View larger version (20K): [in this window] [in a new window]   Figure 7. Primordium Bias and Primordium Fates in 30-Day-Old Arabidopsis Shoot Apices. For simplicity, the shoot apices indicate all primordia present on a shoot apex (as though they were visible in a single plane). Primordia are numbered from youngest to oldest (1 to 6). Anlagen number is estimated at 2 (primordia 1 and 2) based on the domains of KNAT1 and LFY expression in the apical dome (Weigel et al. 1992 ; Lincoln et al. 1994 ). (A) Before photoinduction, all primordia at a 30-day-old apex are vegetative leaf/paraclade primordia. Hypothetical increases in primordium bias toward leaf and paraclade fate are indicated by increasing color density. (B) If no photoinduction treatment is given, all of the primordia present in (A) will develop as leaves and paraclades. (C) If a strong photoinduction treatment is given (Hempel and Feldman 1994 ), the anlagen (primordia 1 and 2) develop as flowers, whereas all initiated primordia (3 to 6) develop as leaves and paraclades. (D) If a CI photoinduction treatment is given (this study), the anlagen (primordia 1 and 2) develop as flowers. Initiated leaf/paraclade primordia committed to the expression of leaf fate (primordia 4 and 5) also produce individual flowers. Additional initiated primordia (e.g., primordium 3) also may develop as flowers without subtending leaves, if there is no commitment to leaf or paraclade fate within the primordium at the start of the inductive treatment.

The development of flowers from pre-existing vegetative primordia also occurs at the apex of photoinduced Lolium temulentum (Evans and Blundell 1996 ); in other cases, natural changes in temperature, hydration, and photoperiod affect the fates of primordia after they have developed bias and/or partial commitment toward a certain fate (Wycherley 1954 ; Deschamp and Cooke 1985 ; Battey and Lyndon 1990 ; Goliber and Feldman 1990 ). The fact that bias can be partially or fully nullified indicates that the specification of plant primordia is progressive and not instantaneous (Christianson and Warnick 1985 ; Kerstetter and Poethig 1998 ). It also is consistent with previous conclusions that primordia are not specified in accordance with strict cell division patterns or cell lineage (Poethig 1987 ; Kaplan and Hagemann 1991 ; van den Berg et al. 1995 ; Smith et al. 1996 ). One reason that plants can easily tolerate flexible specification processes and the teratologies that are an occasional by-product (Battey and Lyndon 1990 ; Hempel and Feldman 1995 ) is that they are iterative organisms. Because most plants produce numerous primordia, the precise specification of each individual primordium presumably is not of great importance.

Leaf/Flower Node Formation in Arabidopsis
The leaf/flower phenotypes discussed here differentiated in response to a potent CI photoinduction treatment, but the same general phenotype can occur on terminal flower1 (tfl1) mutants under standard growth conditions. tfl1 mutants produce leaf/flower nodes under long days (Shannon and Meeks-Wagner 1991 ; Alvarez et al. 1992 ), and their primary shoot meristems also terminate early, usually in a cluster of partial flowers (Alvarez et al. 1992 ). Furthermore, the severity of the tfl1 phenotype is moderated by photoperiod (Shannon and Meeks-Wagner 1991 ; Schultz and Haughn 1993 ). Thus, one general interpretation of the tfl1 mutant phenotype is that its meristems are highly sensitive to floral induction signals and therefore that TFL1 acts to prevent the activity of floral signals within indeterminate meristems (Alvarez et al. 1992 ; Bowman et al. 1993 ). The extension of prefloral development in plants that ectopically express TFL1 (Ratcliffe et al. 1998 ) is consistent with this interpretation. This suggests the possibility that TFL1, which is expressed at the base of the inflorescence meristem (Bradley et al. 1997 ), has a role in shielding indeterminate meristems from the signals that promote flower development (Liljegren and Yanofsky 1996 ; Ratcliffe et al. 1998 ).

Transgenic plants ectopically expressing the flower meristem identity genes LFY and AP1 also produce leaf/flower pheno-types throughout the primary shoot (Mandel and Yanofsky 1995 ; Weigel and Nilsson 1995 ), indicating that LFY and AP1 are critical integrators of the flower-promoting signals produced in response to photoinduction treatments. The LFY and AP1 overexpression phenotypes, however, do not indicate whether LFY or AP1 responds directly to signals produced outside the shoot apex. However, the rapid induction of these genes in the developing primordia of photoinduced plants (Blazquez et al. 1997 ; Hempel et al. 1997 ) indicates that this is a possibility (see also Huijser et al. 1992 ). The rapid response of LFY to CO expression (Simon et al. 1996 ) and to gibberellins (Blazquez et al. 1998 ) also suggests a relatively direct interaction between flower-inducing signals and the LFY promoter.

Although we have concluded that the leaf/flower nodes that were produced in response to the CI treatment represent conversions of primordia with a leaf/paraclade bias, the same phenotype may also arise by other mechanisms in wild-type plants. For example, in populations of No and Ler plants grown only in short days, leaves commonly subtend the lowest flower on the primary inflorescence (F.D. Hempel, unpublished data). One likely explanation for the appearance of leaf/flower nodes under short-day conditions is that mixed signals simultaneously promote both vegetative (leaf) and reproductive (flower) development in young primordia during the transition to flowering in short days (Hempel 1996 ). The appearance of a leaf under a flower in short days also may indicate the incomplete suppression of a normally cryptic leaf component that subtends the flower (see Nilsson et al. 1998 ).

Responses to Floral Induction Signals within the Shoot Apex
AP1::GUS activity was induced in vegetative primordia throughout the shoot apex after the start of the CI photoinduction treatment. The response of numerous primordia to signals that promote flowering is consistent with the hypothesis that primordia respond directly to developmental signals (Hempel 1996 ; Blazquez et al. 1997 ; Kerstetter and Poethig 1998 ). The last primordia initiated before the start of the photoinduction treatment were generally the most sensitive; they had the highest levels of AP1::GUS activity 24 hr after the induction treatment started (Figure 5F). This relatively strong response in these particular primordia is consistent with our conclusion that individual flowers developed from the last primordia initiated before the start of the CI photoinduction treatment (Figure 3 and Figure 7).

Interestingly, no clear differences in the levels of AP1::GUS activity were detected between CI-photoinduced plants and long-day-grown plants at approximately similar stages of development (cf. Figure 5 and Figure 6); under both growth conditions, AP1::GUS activity also was present in nodes that were committed to a leaf/paraclade fate. These results indicate that AP1 expression is not correlated strictly with a primordium's competence to develop into a flower and that the young primordia on older plants are more sensitive to the expression of floral meristem identity genes such as LFY and AP1 (Mandel and Yanofsky 1995 ; Weigel and Nilsson 1995 ).

Although the expression of floral meristem identity genes occurs throughout the apex during the transition to flowering (Figure 5 and Figure 6; Blazquez et al. 1997 ; Hempel et al. 1997 ), the developmental effects on leaf primordia are evidently minimal under normal circumstances. This, however, does not rule out the possibility that floral meristem identity genes can function within leaf primordia under certain circumstances. Interestingly, the ectopic expression of AP3 and PISTILLATA (PI) can induce petal tissue at the margins of cauline leaves (Krizek and Meyerowitz 1996 ), and it is at the margins of the cauline leaves that AP1::GUS activity is highest (F.D. Hempel, unpublished data). This suggests that the level of AP1 expression at the cauline leaf margin is sufficient for the specification of petal identity, that is, if the other organ identity genes required for petal identity, namely, AP3 and PI (Bowman et al. 1991 ), are also expressed.

It is clear from other studies that above-threshold levels of LFY and AP1 expression are critical for the assignment of floral identity to meristems that are competent to respond and that the amount of LFY and/or AP1 expression required for flower specification decreases as a plant matures (Mandel and Yanofsky 1995 ; Weigel and Nilsson 1995 ; Blazquez et al. 1997 ). Similar conclusions have also been reached with regard to flowering in Antirrhinum, in which above-threshold levels of the LFY ortholog FLORICAULA and the AP1 ortholog SQUAMOSA are required for flower specification (Coen et al. 1990 ; Huijser et al. 1992 ; Meijer et al. 1995 ; Bradley et al. 1996 ).

A more dramatic example of threshold-based flower induction, however, is that of Impatiens balsamina. In I. balsamina, continuous flower-promoting signals are required for complete flower specification (Krishnamoorthy and Nanda 1968 ; Debraux and Simon 1969 ; Battey and Lyndon 1988 ; Pouteau et al. 1997 ), and the dampening of leaf-to-apex induction signals results in the reversion of flowers to vegetative growth. This indicates that flower specification in I. balsamina is controlled by the leaves, even during floral organ differentiation (Pouteau et al. 1997 ). Whether there is a similar need for continuous leaf-to-apex signaling in Arabidopsis is not known. Although this may seem unlikely, because Arabidopsis inflorescence shoots do not undergo a reversion to vegetative growth when moved from florally inductive to noninductive conditions (Hempel et al. 1997 ), we cannot rule out the possibility that the seemingly irreversible commitment to flowering in Arabidopsis inflorescences is due to irreversible changes that occur outside the shoot apex.

	METHODS

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

Plant Materials and Growth Conditions
Wild-type Arabidopsis thaliana Landsberg erecta (Ler) plants were used in the first photoinduction experiment (Table 1 and Figure 1 Figure 2 Figure 3 Figure 4). The generation of a standard APETALA1 and ß-glucuronidase (AP1::GUS) line (AM154.5c/Nossen [No]) was described previously (Hempel et al. 1997 ). A second line (AM154.5c/Ler), with a reporter gene at the same genetic locus, was constructed by crossing AM154.5c/No into the Ler background four times. These lines were used in the second pair of experiments (Table 2 and Figure 5 and Figure 6).

See Hempel and Feldman 1995 for general growth conditions. Short-day (8-hr light/16-hr dark) preinduction light conditions and long-day (16-hr light/8-hr dark) light conditions were 100 µE m^-2 sec^-1, with 90% from cool-white bulbs and 10% from incandescent bulbs. Continuous incandescent-enriched (CI) photoinduction conditions are described in Table 1. The addition of incandescent light bulbs, which are high in far-red light (and decrease the red/far-red ratio), enhances the flowering response of Arabidopsis (Martinez-Zapater and Somerville 1990 ; Lee and Amasino 1995 ). Although we chose to promote flowering by adding far-red light and extending the light period, flowering also can be promoted by the addition of blue light (Guo et al. 1998 ).

The CI photoinduction treatment started after 30 days of vegetative growth in short days. Hr 0 of the photoinduction treatment occurred 8 hr into the continuous light period, when the plants were first exposed to a light extension. After ~1 week of CI photoinduction, the plants showed signs of stress, including anthocyanin production and slight bleaching. This may indicate that stress is partially responsible for the very strong flowering response in the induced plants.

Scanning Electron Microscopy, Morphological Analyses, and GUS Assays
Scanning electron microscopy and morphological analyses were as described previously (Hempel and Feldman 1994 , Hempel and Feldman 1995 ). All GUS assays, tissue sectioning, and microscopy were as described previously (Hempel et al. 1997 ). In some assays, the 2 mM 5-bromo-4-chloro-3-indolyl ß-D-glucuronic acid (X-gluc) solution was diluted with PBS buffer (three parts PBS to one part X-gluc solution) to indicate regions of relatively high AP1::GUS activity within shoot apices (Figure 5D and Figure 5H to J). Under dark-field optics, the X-gluc product appears pink.

	ACKNOWLEDGMENTS

We thank Detlef Weigel, Vava Grbíc, Andreas Gisel, Marty Yanofsky, Paula McSteen, and Jennifer Nemhauser for discussions and comments on the manuscript; Ale Mandel and Marty Yanofsky for the original AP1::GUS lines; Steve Ruzin and the Center for Biological Imaging for technical support and advice; and Paula Sicurello and the University of California, Berkeley, Electron Microscopy Laboratory for technical support and advice. This work was supported by U.S. Department of Agriculture Grant (No. 93-37304-9141) to L.J.F and a U.S. Department of Energy grant (No. 88ER13882) to P.C.Z. F.D.H. is a National Science Foundation postdoctoral fellow.

Received May 22, 1998; accepted August 17, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION RESULTS DISCUSSION METHODS REFERENCES

Alvarez, J., Guli, C.L., Yu, X.-H., and Smyth, D.R. (1992) terminal flower : A gene affecting inflorescence development in Arabidopsis thaliana.. Plant J. 2:103-116.

Battey, N.H., and Lyndon, R.F. (1988) Determination and differentiation of leaf and petal primordia in Impatiens balsamina.. Ann. Bot. 61:9-16[Abstract/Free Full Text].

Battey, N.H., and Lyndon, R.F. (1990) Reversion of flowering. Bot. Rev. 56:162-189.

Bernier, G., Kinet, J.-M., and Sachs, R.M. (1981). The Physiology of Flowering. I. The Initiation of Flowers. (Boca Raton, FL: CRC Press).

Bernier, G., Havelange, A., Houssa, C., Petitjean, A., and Lejeune, P. (1993) Physiological signals that induce flowering. Plant Cell 5:1147-1155[Free Full Text].

Blázquez, M.A., Soowal, L.N., Lee, I., and Weigel, D. (1997) LEAFY expression and flower initiation in Arabidopsis.. Development 124:3835-3844[Abstract].

Blázquez, M.A., Green, R., Nilsson, O., Sussman, M.R., and Weigel, D. (1998) Gibberellins promote flowering of Arabidopsis by activating the LEAFY promoter. Plant Cell 10:791-800[Abstract/Free Full Text].

Bowman, J.L., Smyth, D.R., and Meyerowitz, E.M. (1991) Genetic interactions among floral homeotic genes of Arabidopsis.. Development 112:1-20[Abstract].

Bowman, J.L., Alvarez, J., Weigel, D., Meyerowitz, E.M., and Smyth, D.R. (1993) Control of flower development in Arabidopsis thaliana by APETALA1 and interacting genes. Development 119:721-743[Abstract].

Bradley, D., Vincent, C., Carpenter, R., and Coen, E. (1996) Pathways for inflorescence and floral induction in Antirrhinum.. Development 122:1535-1544[Abstract].

Bradley, D., Ratcliffe, O., Vincent, C., Carpenter, R., and Coen, E. (1997) Inflorescence commitment and architecture in Arabidopsis.. Nature 275:80-83.

Chien, J.C., and Sussex, I.M. (1996) Differential regulation of trichome formation on the adaxial and abaxial leaf surfaces by gibberellins and photoperiod in Arabidopsis thaliana (L.) Heynh. Plant Physiol. 111:1321-1328[Abstract].

Christianson, M.L., and Warnick, D.A. (1985) Temporal requirement for phytohormone balance in the control of organogenesis in vitro.. Dev. Biol. 112:494-497[CrossRef].

Coen, E.S., Romero, J.M., Doyle, S., Elliott, R., Murphy, G., and Carpenter, R. (1990) floricaula: A homeotic gene required for flower development in Antirrhinum. Cell 63:1311-1322[CrossRef][ISI][Medline].

Corbesier, L., Gadisseur, I., Silvestre, G., Jacqmard, A., and Bernier, G. (1996) Design in Arabidopsis thaliana of a synchronous system of floral induction by one long day. Plant J. 9:947-952[CrossRef][ISI][Medline].

Debraux, G., and Simon, L. (1969) Induction expérimentale de fleurs prolifères chez l'Impatiens balsamina variété Buisson fleuri par perturbation de la photopériode. Can. J. Bot. 47:1791-1802.

Deschamp, P.A., and Cooke, T.J. (1985) Leaf dimorphism in the aquatic angiosperm Callitriche heterophylla.. Am. J. Bot. 72:1377-1387.

Evans, L.T., and Blundell, C. (1996) The acceleration of primordium initiation as a component of floral evocation in Lolium temulentum L. Aust. J. Plant Physiol. 23:569-576.

Goliber, T.E., and Feldman, L.J. (1990) Developmental analysis of leaf plasticity in the heterophyllous aquatic plant Hippuris vulgaris.. Am. J. Bot. 77:399-412[CrossRef].

Gonthier, R., Jacqmard, A., and Bernier, G. (1987) Changes in cell-cycle duration and growth fraction in the shoot meristem of Sinapis during floral transition. Planta 170:55-59[CrossRef].

Grainger, R.M. (1992) Embryonic lens induction shedding light on vertebrate tissue determination. Trends Genet. 8:349-355[ISI][Medline].

Grbíc, V., and Bleecker, A.B. (1996) An altered body plan is conferred on Arabidopsis plants carrying dominant alleles of two genes. Development 122:2395-2403[Abstract].

Guo, H., Yang, H., Mockler, T.C., and Lin, C. (1998) Regulation of flowering time by Arabidopsis photoreceptors. Science 279:1360-1363[Abstract/Free Full Text].

Halliday, K.J., Koornneef, M., and Whitelam, G.C. (1994) Phytochrome B and at least one other phytochrome mediate the accelerated flowering response of Arabidopsis thaliana L. to low red/far-red ratio. Plant Physiol. 104:1311-1315[Abstract].

Hempel, F.D. (1996) The morphology of the transition to flowering in mustards. Semin. Cell Dev. Biol. 7:391-400[CrossRef].

Hempel, F.D., and Feldman, L.J. (1994) Bi-directional inflorescence development in Arabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades. Planta 192:276-286[CrossRef][ISI].

Hempel, F.D., and Feldman, L.J. (1995) Specification of chimeric flowering shoots in wild-type Arabidopsis.. Plant J. 8:725-731[CrossRef][ISI][Medline].

Hempel, F.D., Weigel, D., Mandel, M.A., Ditta, G., Zambryski, P.C., Feldman, L.J., and Yanofsky, M.F. (1997) Floral determination and expression of floral regulatory genes in Arabidopsis.. Development 124:3845-3853[Abstract].

Huijser, P., Klein, J., Lönnig, W.-E., Meijer, H., Saedler, H., and Sommer, H. (1992) Bracteomania, an inflorescence anomaly, is caused by the loss of function of the MADS-box gene squamosa in Antirrhinum majus.. EMBO J. 11:1239-1249[ISI][Medline].

Irish, E.E., and Nelson, T.M. (1991) Identification of multiple stages in the conversion of maize meristems from vegetative to floral development. Development 112:891-898[Abstract].

Irish, V.F., and Sussex, I.M. (1990) Function of the apetala-1 gene during Arabidopsis floral development. Plant Cell 2:741-753[Abstract/Free Full Text].

Kaplan, D.R., and Hagemann, W. (1991) The relationship of cell and organism in vascular plants: Are cells the building blocks of plant form? Bioscience 41:693-703[CrossRef][ISI].

Kerstetter, R.A., and Poethig, R.S. (1998) The specification of leaf identity during shoot development. Annu. Rev. Cell Dev. Biol. 14:373-398[CrossRef][ISI][Medline].

Knott, J.E. (1934) Effect of a localized photoperiod on spinach. Proc. Am. Soc. Hort. Sci. 31:152-154.

Koornneef, M., Hanhart, C.J., and van der Veen, J.H. (1991) A genetic and physiological analysis of late flowering mutants in Arabidopsis thaliana.. Mol. Gen. Genet. 229:57-66[ISI][Medline].

Krishnamoorthy, H.N., and Nanda, K.K. (1968) Floral bud reversion in Impatiens balsamina under non-inductive photoperiods. Planta 80:43-51[CrossRef].

Krizek, B.A., and Meyerowitz, E.M. (1996) The Arabidopsis homeotic genes APETALA3 and PISTILLATA are sufficient to provide the B class organ identity function. Development 122:11-22[Abstract].

Lee, I., and Amasino, R.M. (1995) Effect of vernalization, photoperiod, and light quality on the flowering phenotype of Arabidopsis plants containing the FRIGIDA gene. Plant Physiol. 108:157-162[Abstract].

Liljegren, S.J., and Yanofsky, M.F. (1996) Genetic control of shoot and flower meristem behavior. Curr. Opin. Cell Biol. 8:865-869[CrossRef][ISI][Medline].

Lincoln, C., Long, J., Yamaguchi, J., Serikawa, K., and Hake, S. (1994) A knotted1-like homeobox gene in Arabidopsis is expressed in the vegetative meristem and dramatically alters leaf morphology when overexpressed in transgenic plants. Plant Cell 6:1859-1876[Abstract/Free Full Text].

Mandel, M.A., and Yanofsky, M.F. (1995) A gene triggering flower formation in Arabidopsis.. Nature 377:522-524[CrossRef][Medline].

Mandel, M.A., Gustafson-Brown, C., Savidge, B., and Yanofsky, M.F. (1992) Molecular characterization of the Arabidopsis floral homeotic gene APETALA 1.. Nature 360:273-277[CrossRef][Medline].

Martínez-Zapater, J.M., and Somerville, C.R. (1990) Effect of light quality and vernalization on late-flowering mutants of Arabidopsis thaliana.. Plant Physiol. 92:770-776[Abstract/Free Full Text].

Martínez-Zapater, J.M., Coupland, G., Dean, C., and Koornneef, M. (1994). The transition to flowering in Arabidopsis. In Arabidopsis, E.M. Meyerowitz and C.R. Somerville, eds (Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press), pp. 403–433.

Meijer, H., Saedler, H., and Huijser, P. (1995) Phase transition and inflorescence development in Antirrhinum majus.. Flowering Newslett. 20:37-43.

Nilsson, O., Wu, E., Wolfe, D.S., and Weigel, D. (1998) Genetic ablation of flowers in transgenic Arabidopsis. Plant J. in press.

Okamuro, J.K., den Boer, B.G.W., Lotys-Prass, C., Szeto, W., and Jofuko, D. (1996) Flowers into shoots: Photo and hormonal control of a meristem identity switch in Arabidopsis.. Proc. Natl. Acad. Sci. USA 93:13831-13836[Abstract/Free Full Text].

Poethig, R.S. (1987) Clonal analysis of cell lineage patterns in plant development. Am. J. Bot. 74:581-594[CrossRef][ISI].

Pouteau, S., Nicholls, D., Tooke, F., Coen, E., and Battey, N. (1997) The induction and maintenance of flowering in Impatiens.. Development 124:3343-3351[Abstract].

Putterill, J., Robson, F., Lee, K., Simon, R., and Coupland, G. (1995) The CONSTANS gene of Arabidopsis promotes flowering and encodes a protein showing similarities to zinc finger transcription factors. Cell 80:847-857[CrossRef][ISI][Medline].

Ratcliffe, O.J., Amaya, I., Vincent, C.A., Rothstein, S., Carpenter, R., Coen, E.S., and Bradley, D.J. (1998) A common mechanism controls the life cycle and architecture of plants. Development 125:1609-1615[Abstract].

Reed, J.W., Nagatani, A., Elich, T.D., Fagan, M., and Chory, J. (1994) Phytochrome A and phytochrome B have overlapping but distinct functions in Arabidopsis development. Plant Physiol. 104:1139-1149[Abstract].

Schultz, E.A., and Haughn, G.W. (1993) Genetic analysis of the floral initiation process (FLIP) in Arabidopsis.. Development 119:745-765[Abstract].

Shannon, S., and Meeks-Wagner, D.R. (1991) A mutation in the Arabidopsis TFL1 gene affects inflorescence meristem development. Plant Cell 3:877-892[Abstract/Free Full Text].

Simon, R., Igeño, M.I., and Coupland, G. (1996) Activation of floral meristem identity genes in Arabidopsis.. Nature 384:59-62[CrossRef][Medline].

Smith, L.G., Hake, S., and Sylvester, A.W. (1996) The tangled-1 mutation alters cell division orientations throughout maize leaf development without altering leaf shape. Development 122:481-489[Abstract].

Telfer, A., Bollman, K.M., and Poethig, R.S. (1997) Phase change and the regulation of trichome distribution in Arabidopsis thaliana.. Development 124:645-654[Abstract].

van den Berg, C., Willemsen, V., Hage, W., Weisbeek, P., and Scheres, B. (1995) Cell fate in the Arabidopsis root meristem determined by directional signaling. Nature 378:62-65[CrossRef][Medline].

Weigel, D. (1995) The genetics of flower development: From floral induction to ovule morphogenesis. Annu. Rev. Genet. 29:19-39[CrossRef][ISI][Medline].

Weigel, D., and Nilsson, O. (1995) A developmental switch sufficient for flower initiation in diverse plants. Nature 377:495-500[CrossRef][Medline].

Weigel, D., Alvarez, J., Smyth, D.R., Yanofsky, M.F., and Meyerowitz, E.M. (1992) LEAFY controls floral meristem identity in Arabidopsis. Cell 69:843-859[CrossRef][ISI][Medline].

Wycherley, P.R. (1954) Vegetative proliferation of floral spikelets in British grasses. Ann. Bot. 18:119-127