IN THIS ISSUE

More Than Just a Surface Thing: Rice Infection by Magnaporthe grisea

The common names for plant diseases are as provocative as they are venerable: the blotches, blights, and bunts; the scabs, smuts, and rusts; mildews, powdery or downy. Many of the names are centuries old, appearing in the Bible and utilized by the ancient Romans to describe agricultural diseases that have plagued man—however directly or indirectly—over the epochs (see Hewitt 1998 ). The monikers have persisted into present-day parlance, however, not out of respect for any historical significance, but because of the corresponding pathologies that are as relevant to modern problems of agronomy as they were in past eras.

As the major human staple crop, rice is expected to be crucial to the sustenance of the growing global population well into the next century (see Lampe 1994 ). The International Rice Research Institute maintains, in fact, that in order to feed the growing population in Asia, the world will have to increase its rice output between now and 2020 by one-third. At the same time, however, ongoing industrialization is rapidly destroying vast tracts of cropland and depleting water supplies. In this context, diseases of rice take on titanic proportions. (Anyone who attended Lester Brown's lecture at the Annual Meeting of the ASPP in Baltimore this summer—or who has read his book [1995], Who Will Feed China?—has been warned of the dire consequences to be expected if increasing world demands on grains are not addressed.)

Rice blast disease, which is caused by the fungal agent, Magnaporthe grisea, represents one of the greatest pathological threats to rice crops. Recent, first-time, outbreaks in California suggest that this threat is by no means confined to developing countries. Beyond its obvious agronomic relevance, however, M. grisea has become a fascinating experimental system for assessing many aspects of the plant–microbe interactions that in themselves comprise an essential subdiscipline of modern plant science. Mechanisms of host recognition, host range limitation and gene-for-gene complementarity, and host penetration have all been addressed in studies of the fungal phytopathogen (Talbot et al. 1996 ; Adachi and Hamer 1998 ; Lauge and De Wit 1998 ; see also Knogge 1996 ).

Like many fungal pathogens that infect intact plants, M. grisea demonstrates a certain biological elegance, not completely understood, in initiating a developmental regimen that culminates in the penetration of the plant body. This elegance includes recognition steps that preclude the initiation of fungal differentiation on inappropriate surfaces, such as a wet mineral surface, for example, and yet promote growth on the living plant surface. Subsequent to recognition and fungal germination, the intracellular milieu of the undifferentiated "germling" must be signaled to initiate a developmental regimen of genetic activity that results in infection. Underlying this biological elegance are signal transduction pathways, which are characteristic of all life forms; the ways that any set of signal transduction events is mediated and concluded, of course, are specific for each particular life form.

To focus first on the extent to which the signal transduction machinery of M. grisea overlaps with that of other eukaryotes, several well-characterized second-messenger systems have been elucidated in fungal growth. Specifically, germination of conidia (i.e., asexual spores) upon contact with the plant surface normally results in the growth of a germ tube. Invasion of the host then requires that the end of the germ tube elaborate an appressorium, a dome-shaped, melanized cell that eventually generates sufficient internal turgor pressure so as to penetrate the plant cuticular surface. When conidia are allowed to germinate in a nutrient solution or upon a hard surface that otherwise fails to mimic the hydrophobic nature of the plant surface—a glass slide, for example—the germ tube fails to elaborate an appressorium and instead continues to grow vegetatively.

Significantly, the "default" pathway of vegetative germ tube growth can be overridden in the presence of exogenous cyclic AMP (cAMP), suggesting that inductive surfaces (i.e., those that are hydrophobic) act through the mediation of a biological cAMP signal (Lee and Dean 1993 ). The gene that encodes adenylate cyclase (MAC1) has been cloned from M. grisea and shown to be essential to appressorium development (Choi and Dean 1997 ). Moreover, the cAMP-mediated promotion of appressorium development appears to involve the CPKA gene product, that is, the catalytic subunit of protein kinase A (Adachi and Hamer 1998 ). Upstream of cAMP production, MAGB, a gene that encodes a G protein alpha subunit, is also essential for appressorium development, and magB mutants can in fact be rescued by an exogenous supply of cAMP (Liu and Dean 1997 ). The upstream importance of an M. grisea G protein might also be reflected in the implicated role of diacylglycerol in appressorium formation (Thines et al. 1997 ).

Similarly, signaling cascades that involve mitogen-activated protein kinases (MAPKs) appear to be involved in appressorium development and function. Indeed, two distinct genes in M. grisea that encode MAPKs have previously been cloned and established as essential in these regards (i.e., PMK1 [ Xu and Hamer 1996 ] and MPS1 [Xu et al., 1998]). In this issue of THE PLANT CELL, a third MAPK-encoding gene is characterized. On pages 2045–2058, Dixon et al. introduce the OSM1 gene, so named in recognition of its osmosensory role. The authors' intent in isolating the gene was to follow up on previous work in which glycerol, at molar concentrations, had been established as the primary solute within the appressorium responsible for maintaining the considerable turgor pressure necessary for penetrating the plant cuticle (de Jong et al. 1997 ). In this way, the signaling cascade underlying a specific appressorial response (i.e., glycerol accumulation) was the ultimate target of investigation.

The investigators' approach, quite reasonably, was based upon consideration of the MAPK-dependent high-osmolarity glycerol (HOG) pathway that had been elucidated for a fungal relative of M. grisea, namely, the budding yeast, Saccharomyces cerevisiae. The HOG response culminates, through the hyperosmotic stress–induced activation of a specific MAPK kinase (MAPKK) and upstream MAPKK kinases (MAPKKKs), in the phosphorylative activation of the MAPK encoded by the HOG1 gene (Hog1p; see Gustin et al. 1998 ). The activated Hog1p, in turn, promotes the synthesis of enzymes directly responsible for producing glycerol, the solute that accumulates so as to maintain yeast turgor in the face of hyperosmotic stress. In light of this cascade of events, leading to glycerol production in yeast, the authors employed primers, synthesized according to the yeast HOG1 gene sequence, for use in a polymerase chain reaction (PCR) that successfully amplified the OSM1 gene from M. grisea.

Despite the authors' clear success in exploiting yeast/M. grisea analogies, both in terms of the HOG1/OSM1 gene similarity (~90%) and the accumulation of glycerol in the yeast cell/appressorium, the two organisms prove to have evolved in surprisingly distinct ways. One distinction arises in that the OSM1 gene turns out to have little to do with glycerol accumulation in the appressorium: null osm1 mutants grown under standard conditions produce appressoria that function normally by virtue of molar levels of glycerol. Nevertheless, OSM1, like the yeast HOG1 gene, is essential to mycelial resistance to hyperosmotic stress—but not by inducing glycerol-synthesizing enzymes. Rather, OSM1 function promotes the accumulation of arabitol, the mycelial solute that the authors now show to be responsible for turgor maintenance in M. grisea.

The reformulation of signaling pathways for purposes specific to a given phytopathogen is an intriguing theme that continues to emerge in studies of plant–microbe interactions. Evolutionary processes in which messenger systems are maintained and yet endowed with novel "messages" appear to have supported the establishment of the distinct and narrow niches that generally characterize phytopathogens. The apparent transformation of the high-osmolarity "glycerol" (HOG) signaling system from yeast into a high-osmolarity "arabitol" pathway in M. grisea may have arisen through the evolutionary modulation of a signal cascade. Against such a background of evolutionary usurpation and rearrangement of signal transduction cascades, Dixon et al. conclude their discussion by offering a model as to how various MAPK genes may differentially negotiate processes of signal "inputs" and "outputs" in M. grisea.

A second paper in this issue approaches the exigencies of appressorium development in M. grisea from another direction. On pages 2013–2030, DeZwaan et al. turn our attention not to the downstream, effector end of the signal cascades that determine appressorium function, but instead present a novel mutant of M. grisea that is deficient in the upstream events of appressorium differentiation. Using a method of insertional mutagenesis, the authors isolate a gene, designated PTH11, the mutational inactivation of which results in compromised pathogenesis. Careful analysis of pth11 mutants shows that they are unaffected with regard to vegetative growth both in vitro and upon inoculation into wounded plant tissue, fertile and competent to produce conidia, and, indeed, produce functional appressoria on inductive surfaces at a rate that is ~15% that of the isogenic wild-type strain.

Most significantly, the sequence of PTH11 is suggestive of a transmembrane protein characterized by nine putative membrane insertion domains and a hydrophilic C-terminal domain. The authors therefore surmise that Pth11p may play a role in appressorium development as a response to surface cues, and a series of experiments supports such a conclusion. First, translational fusion of Pth11p with the green fluorescent protein results in detectable fluorescence of the cell membrane. Secondly, although mutants form appressoria on noninductive (hydrophilic) surfaces at a rate comparable to the wild type, they are greatly deficient, relative to wild-type rates of appressoria formation, in responding to the inductive cues of the leaf surface. Functionally, the protein appears to feed into the signal transduction pathways that are cAMP- and diacylglycerol-dependent; an exogenous supply of either metabolite significantly raises rates of appressorium development on the inductive leaf surface.

Intriguingly, the work presented in this issue by DeZwaan et al. on the putative transmembrane Pth11 protein echoes many of the signal transduction themes outlined in the contribution from Dixon et al. that describes a novel MAPK. Above all, it becomes apparent that appressorium functionality in M. grisea emanates from the complex interplay of signal transduction motifs that are integrated, often by surprising means, into a highly specific whole. One example of this unexpected complexity is offered in the observation that exogenous cAMP, but not diacylglycerol, restores pathogenicity to pth11 mutants, whereas either metabolite can restore appressorium formation. In addition, the Pth11 protein appears to promote appressorium differentiation not only in response to a surface providing a hydrophobic environment, but also as cued by cutin monomers, the only known alternative surface-inductive cue. Previously, the two cues had been presumed to operate by unrelated pathways.

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Related articles in Plant Cell:

Magnaporthe grisea Pth11p Is a Novel Plasma Membrane Protein That Mediates Appressorium Differentiation in Response to Inductive Substrate Cues

Todd M. DeZwaan, Anne M. Carroll, Barbara Valent, and James A. Sweigard
Plant Cell 1999 11: 2013-2030. [Abstract] [Full Text]  

Independent Signaling Pathways Regulate Cellular Turgor during Hyperosmotic Stress and Appressorium-Mediated Plant Infection by Magnaporthe grisea

Katherine P. Dixon, Jin-Rong Xu, Nicholas Smirnoff, and Nicholas J. Talbot
Plant Cell 1999 11: 2045-2058. [Abstract] [Full Text]  

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