Plant Cell, Vol. 11, 1093-1104, June 1999, Copyright © 1999, American Society of Plant Physiologists
GEG Participates in the Regulation of Cell and Organ Shape during Corolla and Carpel Development in Gerbera hybrida
Mika Kotilainena,
Yrjö Helariuttaa,
Merja Mehtoa,
Eija Pöllänena,
Victor A. Albertb,
Paula Elomaaa, and
Teemu H. Teeria
a Institute of Biotechnology, Viikki Biocenter, P.O. Box 56, 00014–University of Helsinki, Finland
b Lewis B. and Dorothy Cullman Program for Molecular Systematics Studies, New York Botanical Garden, Bronx, New York 10458-5126
Correspondence to:
Mika Kotilainen, mika.kotilainen{at}helsinki.fi (E-mail), 358-9-70859366 (fax)
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ABSTRACT |
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The molecular mechanisms that control organ shape during flower development are largely unknown. By using differential hybridization techniques, a cDNA designated GEG (for Gerbera hybrida homolog of the gibberellin [GA]–stimulated transcript 1 [GAST1] from tomato) was isolated from a library representing late stages of corolla development in Gerbera. GEG expression was detected in corollas and carpels, with expression spatiotemporally coinciding with flower opening. In corollas and styles, GEG expression is temporally correlated with the cessation of longitudinal cell expansion. In plants constitutively expressing GEG, reduced corolla lengths and carpels with shortened and radially expanded stylar parts were found, with concomitant reduction of longitudinal cell expansion in these organs. In addition, in styles, an increase in radial cell expansion was detected. Taken together, these observations indicate a regulatory role for the GEG gene product in determining the shape of the corolla and carpel. The deduced amino acid sequence of the GEG gene product shares high similarity with previously characterized putative cell wall proteins encoded by GA-inducible genes, namely, GAST1, GIP (for GA-induced gene of petunia), and the GASA (for GA-stimulated in Arabidopsis) gene family. Our studies suggest that GEG, the expression of which can also be induced by application of GA3, plays a role in phytohormone-mediated cell expansion.
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INTRODUCTION |
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Plant cells do not migrate during development as do animal cells, and organ shape is deteminated by the organized and regulated control of cell expansion together with cell division (Meyerowitz 1997 
). Despite its importance, the molecular and genetic regulation of cell expansion is not well understood. Emerging data support a view that the direction and magnitude of the enlargement of the primary cell wall largely determine the expansion pattern and thereby the final shapes of the cells. Water uptake causes constant turgor pressure in plant cells, which is the driving force of cell expansion. Turgor pressure is counterbalanced by cell wall properties, such as the orientation of microfibrils. This in turn is thought to be regulated by cortical microtubules, one of the major components of cytoskeletal elements. It is believed that phytohormones, for example, gibberellin (GA) and auxin, in turn regulate the orientation of the cortical microtubules (Giddings and Staehelin 1991 ; Shibaoka 1991 ).
Compared with those of vegetative organs, the shape and size of floral organs are highly invariable. This suggests that their shape is under strict developmental control. During flower development, floral organs typically assume their final shape after mitotic activity has basically ceased, indicating that cell expansion plays an important role in determining organ shape (Pyke et al. 1991 ; Tsuge et al. 1996 ). For example, during corolla development in petunia and Antirrhinum, the induction of anthocyanin synthesis correlates with the end of cell division (Bianchi et al. 1978 ; Doodeman et al. 1985 ; Coen et al. 1986 ). In petunia, the unfolded flower is only 40% of its final length when cells cease to divide (Martin and Gerats 1993 ).
From an experimental point of view, the high degree of regularity of floral organ shape is advantageous for studying the basis of organogenesis in plants by using molecular and genetic approaches. In Gerbera (Gerbera hybrida: Asteraceae), the most prominent part of the corolla is the bladelike ligule, which results from a fusion of three petal lobes (Helariutta et al. 1993 ; Bremer 1994 ). The styles of carpels are fine and elongated nonphotosynthetic structures. By using differential hybridization of a corolla cDNA library, we isolated a cDNA, GEG (for Gerbera homolog of GAST1 gene). We propose that GEG plays a role in the regulation of cell shape during corolla and carpel development in Gerbera. GEG expression both spatially and temporally correlates with the opening of the corolla and with cessation of corolla elongation. In the carpel, induction of GEG expression coincides with the cessation of style elongation. In transgenic plants constitutively expressing GEG, corollas are shorter compared with those of nontransformed lines. Similar to that in the corolla, constitutive GEG expression causes shortening of the carpel but a concomitant radial expansion of the style. We found that epidermal cells of both the ligular part of the corolla and the style are reduced in length along organ axes. Radial expansion of the epidermal cells in styles was also observed.
A database search showed that GEG belongs to a gene family encoding putative small cell wall proteins with a cysteine-rich domain and a putative signal peptide sequence. This family includes GAST1 (for GA-stimulated transcript 1) of tomato, GASA-1 (for GA stimulated in Arabidopsis), GIP (for GA-induced gene of petunia), and RSI-1 (for root system inducible 1) of tomato (Shi et al. 1992 ; Taylor and Scheuring 1994 ; Herzog et al. 1995 ; Ben-Nissan and Weiss 1996 ). GEG expression was experimentally induced by treatment with GA3, which is similar to previous reports indicating that these genes are GA or auxin inducible. We hypothesize that GEG is part of a phytohormone-mediated cell expansion mechanism that functions during corolla and carpel development.
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RESULTS |
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Isolation of a GEG cDNA by Differential Screening
Both spatially restricted anthocyanin pigmentation patterns (Helariutta et al. 1993 ) and gene product accumulation patterns (Y. Helariutta, unpublished data) in various regions of the Gerbera ray floret corolla indicate region-specific control of gene expression along the longitudinal axis of the corolla. We performed several differential screening schemes along this axis during the late stages of corolla morphogenesis to isolate genes with differential expression within the corolla. In this context, we isolated a cDNA as a clone whose expression is stronger in the proximal part of the corolla than in the distal part. Based on high similarity to GAST1-like genes, we have named this cDNA GEG.
Sequence comparisons show that GEG belongs to a family of genes that are transcriptionally regulated by phytohormones in different plants. The predicted GEG protein has high sequence similarity with proteins encoded by genes whose expression is induced by GA (GAST1 of tomato, GASA1-4 of Arabidopsis, and GIP of petunia) or by auxin (RSI-1 of tomato) (Shi et al. 1992 ; Taylor and Scheuring 1994 ; Herzog et al. 1995 ; Ben-Nissan and Weiss 1996 ). All of the derived polypeptides have a putative signal sequence at their N termini, with cleavage sites predicted according to von Heijne 1986 . Because other targeting signals have not been identified, it has been proposed that these gene products are targeted into the extracellular space or into the cell wall. Among 60 C-terminal amino acids, there are 22 identical residues of which 12 are cysteines.
During DNA gel blot analysis, the probe (a 259-bp 3' fragment of the GEG cDNA, 90% noncoding) recognized one or two bands at the stringency used for RNA gel blotting. This most probably indicates that the expression analysis results presented below correspond to transcription of a single locus and that the two bands found in some digests were due to restriction length polymorphism in the heterozygous cultivar. During low-stringency DNA gel blot analysis, the full-length GEG cDNA probe recognized more bands, suggesting that there is a small gene family of GEG-like genes in the Gerbera genome (data not shown).
GEG mRNA Is Abundant in Corollas and Carpels
The developmental expression pattern of GEG was studied by using RNA gel blot analyses. The expression of GEG is highest in floral organs; in addition, a faint signal was detected in RNA from leaf blades. Strong GEG expression was observed in corolla tissue (both tube and ligule regions) and carpels, with more moderate signals in the scape (floral stem) and the receptacle (terminal enlargement of floral stem) (Figure 1). To understand the role of GEG in plant development, we focused on GEG expression in corollas and carpels.
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Figure 1.
RNA Gel Blot Hybridization Analysis Showing the Tissue Specificity of GEG Expression.
Autoradiography of an RNA gel blot probed with a 259-bp 3' fragment of the GEG cDNA (90% noncoding). Fifteen micrograms of total RNA was loaded per lane, and equal loading was confirmed by ethidium bromide staining. Organs covering several developmental stages were examined. Receptacle, terminal enlargement of floral stem; scape, floral stem.
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GEG expression was studied temporally at different stages of ray floret corolla development (Figure 2A to 2F; Helariutta et al. 1993 ). The expression correlates temporally with opening of both individual florets and the whole inflorescence, being induced at stage 7 (Figure 2G). Because of the large size of ray floret corollas in Gerbera, we were able to isolate RNA from various parts of the corolla over time and study the developmental induction pattern of GEG expression by using RNA gel blot analysis. The pattern is intriguing. Just before the opening of the flower—and unfolding of the corolla—the onset of GEG expression occurs almost simultaneously from both ends of the corolla. The very first signal can be seen in the proximal part of the corolla, more precisely, in the joint region of tube and ligule (Figure 2H, stage 7). During opening, the proximal expression proceeds in both directions: basipetally into the tube and acropetally into the ligule. Almost simultaneous to the onset of proximal expression, GEG expression also starts from the distal end of the corolla. This expression proceeds basipetally toward the middle of the ligule, where both proximal and distal expression domains meet just as the corolla has opened (Figure 2H, stage 8). GEG expression continues at a high level until senescence takes place (data not shown). In situ hybridization analysis of the GEG expression pattern revealed that the transcript can be detected both in the mesophyll and in the epidermis during opening of the corolla (Figure 3B).
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Figure 2.
Analysis of GEG Expression during Corolla and Carpel Development.
(A) to (F) Different developmental stages of Gerbera inflorescence, according to Helariutta et al. 1993 . (A) shows developmental stage 1; (B), stage 3; (C), stage 5; (D), stage 7; (E), stage 7.5; and (F), stage 8.
(G) The expression of GEG in carpel and corolla correlates with opening of both individual ray florets and the whole inflorescence. Numbers refer to developmental stage.
(H) Spatial partition of ray floret corolla. Regions are indicated above the gel. The onset of expression occurs from both ends of the corolla (stage 7, regions 2 and 7) and just as the corolla has opened. Both expression domains meet at the middle of the ligule (stage 8, region 5). Region 1 is the tubular part of ray floret corolla (tube); regions 3 (proximal region) to 7 (distal region) represent the ligular part of the corolla. GDFR1 is used as a loading control. The developmental stages (st.) are the same as presented in (A) to (F).
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Figure 3.
Analysis of GEG Expression in Carpel and Corolla by Using in Situ Hybridization.
In both organs (stage 7.5), GEG expression is seen in epidermal and parenchymatic cells as white silver grains. In situ analyses were conducted using the 35S-CTP–labeled antisense and sense (control, data not shown) RNA probes. The probes were transcribed from the same 3' fragment of GEG cDNA as was used in RNA gel blot analysis.
(A) Cross-section of carpel style.
(B) Cross-section of marginal region of proximal part of corolla ligule.
Bar in A = 100 µm for (A) and (B).
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In carpels, GEG expression was studied by gel blotting using RNA from samples taken before and after the opening of ray florets at developmental stages 6 and 8, respectively (Figure 2C to 2F; Helariutta et al. 1993 ). As shown in Figure 2G, the onset of GEG expression coincides with the opening of the florets. In the style, GEG mRNA was detected in the outer epidermis and in the parenchyma (cortex) but not in transmitting tissues (Figure 3A).
Temporal Correlation of GEG Expression with Cessation of Organ and Cell Elongation in Corollas and Carpels
Biometric analyses (Figure 4) of corolla growth show that before its opening, the corolla expands both longitudinally and laterally. Soon after opening, growth ceases in both directions (Figure 4A and Figure 4B). Temporally, GEG expression follows tightly the cessation of corolla growth, and it can be detected everywhere in corolla tissues just after opening (Figure 2D to 2F and Figure 2H).
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Figure 4.
Biometric Analysis of Corolla and Carpel Length and Width Development.
(A) and (B) Corolla length and width, respectively. Temporal GEG expression tightly follows the cessation of corolla expansion in both longitudinal and lateral directions (see also Figure 2) .
(C) Carpel length. In the carpel, cessation of elongation temporally correlates with GEG expression.
(D) Carpel width. Carpel styles do not expand in radial direction during or after the elongation period.
(E) Carpel cell length. In the carpel, the epidermal cells of the style elongate before opening of the floret but not later; thus, the retardation of cell elongation correlates with GEG expression.
Timing of different developmental stages (described by Helariutta et al. 1993 ) was measured by following the development of >50 inflorescences under our standard greenhouse conditions. The lengths and widths of both corolla and carpel, shown in (A) to (D), were measured from 15 to 30 outermost ray florets at each developmen-tal stage. Samples were collected from at least two different inflorescences. Carpel cell length (E) of 72 epidermal cells at each time point was measured. Cell lengths of 18 epidermal cells 200 to 400 µm below the stigma of each carpel were determined, and the average cell length of four carpels was measured. Numbers below the curves correspond to the development stage of the inflorescence (see Figure 2A to 2F). Error bars indicate the standard deviation.
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The temporal pattern of GEG expression along the apical–basal axis of the corolla made it important to analyze whether GEG expression correlates with cessation of cell elongation. Cell length was measured in the distal and central regions of corolla (Figure 2H, regions 7 and 5, respectively) just after stage 7 (7+) and at stage 8. At these stages, GEG mRNA is present in the distal region but reaches the central region just before stage 8 (Figure 2H). Cell length measurements revealed that cells in the distal region do not elongate, whereas in the central region, axial cell elongation takes place (Figure 5A). The cell length differences between stage 8 middle cells and other groups are statistically significant (rank sum tests; P < 0.001). Cell width growth was detected both in distal and middle parts of the corolla between stages 7+ and 8 (Figure 5B). Thus, GEG expression strictly correlates with the cessation of cell expansion along the apical–basal axis.
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Figure 5.
Cell Length and Width in Distal and Central Regions of Ray Floret Corolla Just before and after Opening (Stages 7+ and 8, Respectively).
GEG is expressed in the proximal region at both stages, whereas GEG expression reaches the central region just before stage 8 (see Figure 2H).
(A) Corolla cell length in microns. The cells in the distal region do not elongate, whereas the cells in the central region do. Thus, GEG expression strictly correlates with cessation of cell expansion along the apical–basal axis.
(B) Cell width (in microns) growth was detected both in distal and central regions of the corolla.
Cell length and width were measured with a vernier caliper using scanning electron micrographs. The length of 40 cells each in three distinct ray floret corollas was measured at each point. At the distal region of the corolla, measurements were done 1 mm from the tip. Cell widths were measured by counting cell numbers on 570-µm-long transverse lines and counting the average cell widths for each line (36 replicas). Error bars indicate the standard deviation.
tip, distal region; mid, central region.
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Similar to that in corolla, carpel organogenesis was characterized in more detail using biometric analyses (Figure 4). Carpel length and width in the outermost ray florets were measured at various stages of inflorescence development. The opening of ray floret corollas and the whole inflorescence coincides with a change in the longitudinal expansion of carpels. Elongation of the carpels takes place before the opening of the floret, being most rapid just before opening. After opening, the elongation of carpels ceased (Figure 2F and Figure 4C). In the radial direction, the styles do not expand a statistically significant amount during the elongation period or later (Figure 4D). Thus, in carpels, the cessation of elongation is also temporally correlated with GEG expression.
Cell elongation coincides with the patterns described in organs: the epidermal cells of the style elongate before the opening of the floret but not later (Figure 4E). Therefore, cell elongation, at least to a large degree, is responsible for the observed carpel growth described above. In both corollas and carpels, GEG expression correlates temporally with cessation of cell expansion along the apical–basal axis.
Transgenic Plants That Overexpress GEG Have Shorter Corollas
Detailed temporal analysis of the developmental regulation of GEG shows that its transcription correlates with cessation of cell elongation both in the corolla and in the carpel. Based on this observation, we hypothesized that the functional role of the GEG polypeptide is to suspend cell elongation. To test this hypothesis, we generated transgenic plants in which GEG expression was under the control of a constitutively active promoter. In these plants, constitutive GEG expression should lead to premature inhibition of cell elongation and to shorter organs with shorter cells.
The GEG cDNA was introduced into Gerbera plants under regulation of the cauliflower mosaic virus 35S promoter via Agrobacterium-mediated transformation. Four constitutively GEG-expressing lines were generated, and analyses of both the length and the width of 20 outermost ray floret corollas in four transgenic plants and control plants were conducted at developmental stage 9, when corolla growth has ceased. All four lines constitutively expressing GEG (m1, m2, m3, and m5) have shorter corollas compared with the nontransformed line and the two control lines transformed with GEG in an antisense orientation, resulting in no or a modest decline in GEG expression (Figure 6 and Figure 7A; antisense lines with significantly reduced GEG expression levels were not obtained). The differences are statistically significant (Student's t test/rank sum test; P < 0.001). In contrast, the corolla width in all plants of lines constitutively expressing GEG remained unchanged compared with control lines (Figure 7B).
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Figure 6.
Comparison of the Ray Floret Corollas of Wild-Type and Transgenic Plants.
Ray floret corollas of a nontransformed line (wt) are compared with those of an m3 transgenic plant constitutively expressing GEG before (stage 7), during (stage 7.5), and after (stage 9) the opening of ray florets.
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Figure 7.
Analysis of Corolla Length and Width for Four Lines Constitutively Expressing GEG and for the Control Lines.
(A) Corolla length (in millimeters). Length of the outermost ray floret corollas of wild-type plants (wt), constitutively GEG-expressing lines (m1, m3, m2, and m5) and two GEG antisense lines with no (t4) or a modest (t9; 80% remaining) decline of GEG expression. All four lines constitutively expressing GEG have statistically shorter corollas compared with the control lines.
(B) The corolla width (in millimeters) of all lines constitutively expressing GEG remained unchanged when compared with control lines. Forty outermost ray floret corollas of two inflorescences of each line at stage 9 were collected. Corolla width and length of four lines constitutively expressing GEG (m1, m3, m2, and m5), a nontransformed line (wt), and two antisense lines (with no or modest effect in GEG expression) were measured.
Error bars indicate the standard deviations.
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Constitutive Expression of GEG Decreases Cell Length in Corollas
More detailed analyses of corolla and carpel phenotypes were performed with two transgenic lines, m1 and m3, together with a nontransformed control line, as presented below. Constitutive expression of GEG in these transformants was verified by in situ hybridization of corolla cross-sections at developmental stage 6, when endogenous GEG expression is not yet present, and by RNA gel blot analysis of leaf tissues in which the endogenous expression is very low. In situ analyses show that all cell types overexpress GEG (data not shown).
In the corolla of both m1 and m3 lines and the control line, the length and the width of epidermal cells were measured at the central part of the proximal end of the ray floret corolla ligules on their adaxial sides at developmental stage 8 (Figure 8A). In plants constitutively expressing GEG, cell length was reduced in a statistically significant manner (Student's t test; P < 0.001), but no difference in cell width could be measured (Figure 8B, Figure 8C, Figure 9A, and Figure 9B). In conclusion, the major impact of constitutive GEG expression is the cessation of the axial cell expansion of epidermal cells in the ligule.
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Figure 8.
Analysis of the Effects of Constitutive GEG Expression on Corolla Epidermal Cells.
(A) The epidermal cells are organized into longitudinal files running along the apical–basal axis of the corolla. Measurements of cell dimensions took place in the area marked with a white box.
(B) Scanning electron microscopy of the adaxial (upper) side of the proximal part of a ray floret corolla of a nontransformed control line (stage 8). One of the epidermal cells is highlighted in yellow.
(C) Scanning electron microscopy of the corresponding region of an m1 line constitutively expressing GEG.
Bar beneath (C) = 75 µm for (B) and (C).
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Figure 9.
Cell Lengths and Widths in the Same Regions as Described in Figure 8 of m1 and m3 Lines Constitutively Expressing GEG and a Control Line.
In plants constitutively expressing GEG, cell length was reduced, but no difference in cell width could be measured. wt, control line.
(A) Corolla cell length in microns. The cell lengths in the two lines constitutively expressing GEG are shorter than those of the control, and the differences are statistically significant.
(B) Corolla cell width in microns. Cell widths of the lines constitutively expressing GEG did not differ from those of a control line.
Three ray floret corollas of transgenic lines m1 and m3 together with a nontransformed control line were collected (stage 8), and cell length and width were measured at the region marked with a white box in Figure 8A. In corollas, transverse lines were drawn on micrographs, and cells were chosen at intervals of 60 µm for length measurements. Cell length of ~200 cells was measured in m1, m3, and a control line. Cell width was measured by counting cell numbers on 570-µm-long transverse lines and counting the average cell width for each line (30 to 36 replicas; ~40 cells per line).
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Epidermal Cells of the Style are Shorter and Wider in Lines Constitutively Expressing GEG
Compared with the control line, transgenic lines constitutively expressing GEG had a decrease in carpel length and an increase in carpel radius (Figure 10A, Figure 10B, Figure 11A, and Figure 11B). A comparison of cell length and the width of style epidermal cells between m1 and m3 lines constitutively expressing GEG and the control line revealed a change in elongation pattern. Even before endogenous expression, at stage 6, statistically significant changes of cell length and width could be detected (Student's t test; P < 0.001). In m1 and m3 lines constitutively expressing GEG, cell length was reduced and the width was increased compared with the control line (Figure 10A, Figure 10B, Figure 11C, and Figure 11D). The constitutive expression phenotypes support the view that the GEG gene product regulates cell expansion in the axial dimension during carpel development as well as during corolla development. However, in the carpel, unlike in the corolla, we observed a concomitant opposite effect in the radial dimension.
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Figure 10.
Scanning Electron Microscopy of the Epidermis of the Stylar Part of the Carpel 300 µm below the Stigma.
For an m1 line constitutively expressing GEG, cell length was reduced, and the width was increased compared with the control line.
(A) A nontransformed control line.
(B) An m1 line constitutively expressing GEG at stage 6.
Bar in (B) = 100 µm for (A) and (B).
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Figure 11.
Analysis of the Effects of Constitutive GEG Expression on Epidermal Cells of the Carpel Style.
(A) Carpel length in millimeters. Carpel length of lines constitutively expressing GEG is shorter compared with that of a control line.
(B) Carpel width in microns. Compared with that of a control line, the carpel radius has increased in lines constitutively expressing GEG.
(C) Carpel cell length in microns. Comparison of cell length of m1 and m3 with a nontransformed control line reveals that epidermal cells of the style of lines constitutively expressing GEG are shorter than those of a control line.
(D) Carpel cell width in microns. The epidermal cells of constitutively GEG-expressing lines are wider than those of a control line.
Twelve to 20 carpels of two lines constitutively expressing GEG (m1 and m3) and a nontransformed control line (wt) were measured. Carpel length was measured at developmental stage 9; carpel width, epidermal cell length, and width were measured at stage 6. Cell length of 18 individual epidermal cells 200 to 400 µm below the stigma of each carpel and the average cell length of 12 to 20 carpels of each line were measured. The differences presented in (A), (B), (C), and (D) are statistically significant. Error bars indicate the standard deviations.
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GA and Regulation of GEG Expression in the Corolla
Because all of the homologous genes (see Introduction) are induced by phytohormones and because expression is spatially and temporally regulated, we studied whether GEG expression reacts to GA3. Application of GA3 upregulated GEG expression in ray floret corollas of detached inflorescences (Figure 12). A short pulse of GA3 was able to induce GEG in 2 hr. However, because maximal levels of GEG mRNA were not seen earlier than 24 hr after GA3 application, it is possible that GA stimulation of GEG expression is indirect or that a decline (rather than an increase) in GA concentration induces GEG expression.
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Figure 12.
GA Application Upregulates GEG Expression in Ray Floret Corollas.
RNA gel blot showing GEG expression after the addition of GA3 just before the opening of the inflorescence (stage 7+). The scape (floral stem) was cut 5 cm below the inflorescence.
(A) The control inflorescences were grown on 50 mM sucrose.
(B) Induction of GEG expression was detected when inflorescences were grown on 50 mM sucrose with 5 µm GA3.
(C) Induction of GEG expression was also detected when inflorescences were first incubated in 50 mM sucrose with 50 µm GA3 for 5 min and then transferred to 50 mM sucrose medium.
Approximately 10 ray floret corollas were collected for RNA isolation at each time point (indicated above gels in hours) after GA3 addition.
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We have also isolated the genomic 5' flanking sequence of GEG. It contains two sequence motifs that are found in the flanking regions of rice and barley 
-amylase genes whose expression is regulated by GA (Huang et al. 1990 ; Skriver et al. 1991 ). This further supports the idea that GEG expression is developmentally regulated by GA.
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DISCUSSION |
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GEG Participates in the Regulation of Cell Shape
The expression pattern of GEG correlates with completion of organ and cell elongation in both corollas and carpels. Furthermore, in transgenic plants, constitutive GEG expression demonstrates that excessive GEG production is able to cause alterations in organ and cell shape during corolla and carpel development. This suggests that GEG plays a role in determining cell shape during carpel and corolla morphogenesis, thus providing functional information for the role of GEG-like genes in plants.
In carpels, constitutive GEG expression reveals a negative interrelationship between longitudinal and radial growth. As described above, this is also evident in epidermal cells. However, in corollas, no radial expansion of epidermal cells due to constitutive GEG expression was observed. Furthermore, in carpels, no increase in style width was observed during the endogenous GEG expression stage (Figure 4D). This suggests that the primary role of GEG is to inhibit cell elongation. According to this hypothesis, constitutive GEG expresssion prematurely inhibits cell expansion in the longitudinal direction. This could allow the growth potential of the cell to be directed passively in the radial direction, as seen in the epidermal cells of the style. The alternative hypothesis that GEG would promote radial and inhibit longitudinal expansion simultaneously is less likely. To distinguish between these hypotheses, we need to study GEG function at a cellular level.
The angustifolia and rotundifolia mutations for leaf development (Tsuge et al. 1996 ) and several cell expansion mutations for root development in Arabidopsis (Aeschbacher et al. 1995 ; Hauser et al. 1995 ) point to an interrelationship between the longitudinal and radial dimensions of the cell. For example, the palisade cells of leaves in the angustifolia mutant exhibit restricted expansion in the leaf-width dimension and enhanced expansion in leaf thickness. In rotundifolia3, leaf morphology is affected in one direction only: cell elongation is reduced in the leaf-length (axial) dimension (Tsuge et al. 1996 ).
In our analyses, we have measured the dimensions of epidermal cells by using scanning electron microscopy. Because GEG expression was also observed in the underlying parenchymatic cells, it is probable that analogous changes in cell shape are occurring there. The parallels between cell expansion and organ expansion are evident, although it is possible that in addition to cell expansion, cell division events could also contribute to the determination of the final shape at the stages analyzed. However, during the stages that we have investigated, anthocyanin accumulation and flavonoid biosynthetic gene expression, which have been considered as markers for postmitotic cell differentiation, occurred (Martin and Gerats 1993 ).
GEG Homologs in Plants
The genes orthologous to GEG have been described earlier in various plants, for example, GAST1 (tomato), GASA1-4 (Arabidopsis), GIP (petunia), and RSI-1 (tomato) (Shi et al. 1992 ; Taylor and Scheuring 1994 ; Herzog et al. 1995 ; Ben-Nissan and Weiss 1996 ). The GEG-like gene/protein family shares several features that may suggest a role for GEG in regulating cell expansion. Based on our studies of GEG expression and the fact that several members (GEG, GAST1, and RSI-1) have been isolated by using differential screening methods, we conclude that the mRNA is relatively abundant, characteristic of a structural role for the gene product. Furthermore, the putative signal sequence and the absence of other targeting signals suggest that the gene products are secreted, possibly to the cell wall (Shi et al. 1992 ). Another characteristic feature is regulation of gene expression with phytohormones. The variability in the effective hormone indicates that the role of the genes may be downstream of various signal transduction pathways after their convergence. Taken together with the data from transgenic plants suggesting that the primary role of GEG may be inhibiting axial cell expansion, it is possible that the GEG-like function may be generally related to establishing cell wall properties during organogenesis in plants.
GEG-like gene products are characterized by a highly conserved C-terminal domain, with 12 invariable cysteine residues and an N-terminal domain variable in length and hydrophobicity. Because the genes are expressed in different developmental contexts, this might reflect involvement in similar but not necessarily identical processes. However, phylogenetic analysis using the nucleotide sequences corresponding to 60 amino acids of the conserved C terminus does not reveal subgroups that might reflect split functional relationships (data not shown).
No information about the function of these proteins has been reported until this study, but we can compare the other members of the gene family to GEG concerning their gene expression. Similar to GEG, two other members (GIP and GASA4) are expressed during corolla or carpel organogenesis. GIP expression in petunia corollas is reported to be the highest just before anthesis (stage 5). When the corolla reached maximum size (stage 7), no transcript was detected (Ben-Nissan and Weiss 1996 ). Thus, in contrast to GEG, GIP expression is transient and temporally correlates with the cell elongation period of corolla growth instead of its completion. If the role of GEG as a negative regulator of cell elongation is representative for GIP, the latter could be involved in preventing overelongation during cell expansion. However, there is also a requirement for competence to respond to GEG, because constitutive expression of GEG does not lead to a phenotype in all organs. In Gerbera, the competence may rise before GEG expression, but in petunia, GIP expression may be present before the competence develops, leading to a different timing of gene expression. Alternatively, GEG and GIP may be involved in the same process but act in an opposite manner. In this scheme, for example, the N-terminal variable domain of putative proteins could be responsible for allowing GEG to inhibit and GIP to promote cell elongation.
During style development, the expression pattern of Arabidopsis GASA4 has temporal and spatial similarities with GEG. GASA4 expression was also detected in the short stylar region of gynoecium at the stage during which the growth rate of overall gynoecium length ceases (Bowman 1994 ; Aubert et al. 1998 ). Therefore, GASA4 could have a role similar to GEG in regulating cell elongation in style development.
However, Aubert et al. 1998 reported that GASA4 in Arabidopsis is expressed in regions undergoing rapid cell division in various organs. Recently, Sablowski and Meyerowitz 1998 reported on a gene (NO APICAL MERISTEM, NAP) whose constitutive expression, like that of GEG, results in the inhibition of cell elongation. However, it was concluded that NAP probably functions in the transition between cell division and cell expansion during stamen and petal development, because in the absence of NAP this transition does not occur, and constitutive NAP expression delays the switch. The transient expression pattern of NAP between stages of frequent cell division and cell expansion further supports a transition function (Sablowski and Meyerowitz 1998 ). In contrast to NAP expression, GEG expression during corolla and carpel development is clearly induced after the active cell division phase and occurs simultaneously to the cessation of the elongation phase (as evidenced by the anthocyanin accumulation and Gerbera dihydroflavonol-4-reductase gene 1 [GDFR1] expression at this stage; Figure 2 and Figure 4), indicating that GEG is an inhibitor of cell elongation, as discussed above. If the inhibitory role of GEG in cell expansion is representative of GASA4 in this developmental context, then it is possible that the elongation of dividing cells is controlled by similar molecules as it is after the elongation stage.
GEG Belongs to a Class of Genes Abundantly Expressed in Styles and Corollas of Gerbera
We have previously isolated several genes from Gerbera that are functional during corolla morphogenesis. Interestingly, two of them have the same organ specificity as does GEG, involving epidermal and/or parenchymatic cells of carpel and corolla. In addition to GEG, this group of genes consists of GDFR1 and the nonspecific lipid transfer protein gene 1 (GLTP1) (Helariutta et al. 1993 ; Kotilainen et al. 1994 ). The style in Gerbera is nonphotosynthetic, differing from other well-studied species, and in addition has anthocyanin pigmentation. Although the spatial and temporal expression patterns of these genes are different, the data suggest that similar genetic programs are occurring during the differentiation of the epidermal and parenchymatic cell types of the corolla and style in Gerbera. It is likely that these differentiation-related molecules are regulated by programs/factors that are independent from or converge after the determination of floral organ identity, because the ABC model predicts that corolla and carpel development are triggered by a nonoverlapping set of regulatory factors in the floral meristem (Coen and Meyerowitz 1991 ). One such candidate is GA, which has been previously shown to regulate flavonoid biosynthetic genes and GAST1-like genes, including GEG, as has been described here.
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METHODS |
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Plant Material
Gerbera hybrida var Terra Regina used in this research was obtained from Terra Nigra BV (De Kwakel, Holland). The control and transgenic plants were grown under identical conditions (side by side) at the same time, and the age of plants was the same. Developmental stages of the inflorescence are described by Helariutta et al. 1993 . For all analyses, samples were collected from outermost ray florets (flowers) of the inflorescence, and each transgenic and control plant sample was harvested and treated at the same time.
Plant Transformation
Gerbera transformation was performed using Agrobacterium tumefaciens–mediated gene transfer, as described previously (Elomaa et al. 1993 , Elomaa et al. 1998 ). Transformation was verified by RNA gel blot analysis showing GEG (for Gerbera homolog of the gibberellin [GA]–stimulated transcript 1 [GAST1]) expression in leaves and by DNA gel blot analysis. The analyses were performed on clones of the original transgenic plants (T0).
Isolation of Plant DNA and RNA
Plant DNA was isolated using the method of Dellaporta et al. 1983 . Total RNA was isolated as described by Jones et al. 1985 or by using the RNeasy plant total RNA kit (Qiagen, Chatsworth, CA). Poly(A)+ RNA was isolated using oligo(dT) cellulose affinity chromatography (Sambrook et al. 1989 ).
Construction and Differential Screening of a Corolla cDNA Library
Polyadenylated RNA (5 µg) extracted from proximal parts of ray floret corollas at developmental stages 5 to 9 (Helariutta et al. 1993 ) was used to construct a cDNA library in the 
ZAPII vector (ZAP-cDNA synthesis kit; Stratagene, La Jolla, CA). From the nonamplified cDNA library, ~50,000 plaques were plated and transferred onto replica nylon membranes and then screened differentially with radiolabeled first-strand cDNA pools from the ray floret tube region of the proximal and distal parts of the ligule (first-strand cDNA synthesis kit; Amersham).
GEG cDNA was isolated as a clone that is expressed more strongly in the proximal part than in the distal part of the ligule. Two independent but similar cDNA clones were isolated, subcloned into a pUC18 derivative, and sequenced using the AutoRead kit (Pharmacia, Uppsala, Sweden). The 813-bp genomic fragment containing part of the GEG promoter was obtained by applying a 5' rapid amplification of cDNA ends–like polymerase chain reaction amplification on genomic DNA. The GEG cDNA sequence and sequence of the 5' flanking region of the GEG gene have been submitted to the EMBL database; the accession numbers are AJ005206 and AJ006273, respectively.
RNA Gel Blot Analyses and in Situ Hybridization
Fifteen micrograms of total RNA was loaded per lane. The amount of RNA to be loaded was measured spectrophotometrically, and equal loading was confirmed by ethidium bromide staining of rRNA bands. Electrophoresis and hybridizations were as described by Sambrook et al. 1989 . The 259-bp 3' fragment (of which 234 bp is from the noncoding region) served as the probe. Washing conditions of 0.2 x SSC (1 x SSC is 0.15 M NaCl and 0.015 M sodium citrate) and 0.1% SDS at 58°C were applied for all RNA blots. In situ hybridization was conducted as described previously by Kotilainen et al. 1994 using 35S-CTP–labeled antisense and sense (control) RNA probes. The probes were transcribed from the same fragment as was used in the gel blot studies under the T7 promoter in vector pSP72/73 (SP6/T7 transcription kit; Roche Diagnostics, Mannheim, Germany).
Scanning Electron Microscopic Analysis
Corolla and carpel samples of control and transgenic plants were collected and further treated side by side at the same time. They were fixed in FAA buffer (50% ethanol, 5% acetic acid, and 2% formaldehyde) overnight and then transferred through an ethanol series to 100% ethanol, critical point dried (Balzers CPD 030 critical point dryer; Bal-Tec, Balzers, Liechtenstein), and coated with platinum/palladium (agar sputter coater; Agar Scientific Ltd., Stansel, UK). Specimens were mounted on aluminium stubs using graphite adhesive or tape and examined with a scanning electron microscope (digital scanning microscope, model DSM 962; Karl Zeiss, Oberkochen, Germany) in the Electron Microscopy Laboratory of the Institute of Biotechnology, University of Helsinki.
Organ and Cell Measurements and Statistical Analysis
Organ length and width measurements were done with a vernier caliper in vivo, except for carpel width, which was measured from scanning electron micrographs 200 µm below the stigma. Cell length and width were measured by using scanning electron micrographs.
To study whether the differences in organ and cell expansion caused by constitutive GEG expression are statistically significant, we performed Student's t tests and/or rank sum tests. A parametric t test was used if the normality and equal variances of samples were confirmed (P values to reject < 0.050). A nonparametric rank sum test was used if either was not confirmed. The level of confidence is P < 0.001 in all statistically significant differences mentioned in this study.
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ACKNOWLEDGMENTS |
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We thank Philip Benfey, Xuemei Chen, Pekka Lappalainen, and three anonymous reviewers for valuable comments on the manuscript. We thank Eija Takala, Marja Huovila, and Anu Immonen for excellent technical assistance. We also thank Eija Saarikko and Anne Aaltonen for greenhouse care of plant material, Jyrki Juhanoja for his help in electron microscopy, Tapio Linkosalo for advice on statistical analyses, and Jari Penttinen for editing the figures. This work was partially funded by the Academy of Finland.
Received January 4, 1999; accepted March 29, 1999.
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ABSTRACT
INTRODUCTION
