A Rho3 Homolog Is Essential for Appressorium Development and Pathogenicity of Magnaporthe grisea

Abstract

The small GTPase Rho3 is conserved in fungi and plays a key role in the control of cell polarity and exocytosis in yeast. In this report, we show that a Rho3 homolog, MgRho3, is dispensable for polarized hyphal growth in the rice blast fungus Magnaporthe grisea. However, MgRho3 is required for plant infection. Appressoria formed by the Mgrho3 deletion mutants are morphologically abnormal and defective in plant penetration. Conidia of the Mgrho3 deletion mutants are narrower than those of the wild-type strain and delayed in germination. Transformants expressing a dominant negative Mgrho3 allele exhibit similar phenotypes as the Mgrho3 deletion mutant, while transformants expressing a constitutively active allele of MgRho3 can produce normal conidia but remain defective in appressorium formation and plant infection. In contrast, overexpression of wild-type MgRho3 increases the infectivity of M. grisea. Our results reveal a new role for the conserved Rho3 as a critical regulator of developmental processes and pathogenicity of M. grisea.Magnaporthe grisea is an ascomycete pathogen of important cereal crops, such as rice, barley, and wheat. It causes rice blast, one of the most severe fungal diseases of rice throughout the world (33, 35). The fungus infects rice plants in a sophisticated manner, like many other foliar pathogens. Germ tubes produced from conidia differentiate into specialized infection structures called appressoria and then use the enormous turgor pressure generated in appressoria for plant penetration (8). Mature appressoria develop thin penetration pegs to physically pierce the host surface and enter plant epidermal cells. At early stages, the penetration peg contains high concentrations of actin filaments (3) and recruits the rice cell membrane to form the extrainvasive hyphal membrane, while infection hyphal growth and its co-opt plasmodesmata for cell-to-cell movement (18). It has been hypothesized that actin and cytoskeleton elements may be involved in the reestablishment of polarized growth by determining the penetration site and in stabilizing the tip of the penetration peg (15). After penetration, the peg differentiates into infectious hyphae that grow inter-and intracellularly and result in development of blast lesions.Signal transduction pathways that regulate infection-related morphogenesis have been extensively studied in M. grisea during the past few years (34, 41). In general, cyclic AMP (cAMP) signaling is involved in surface recognition and initiation of appressorium formation (1,20,22,44). However, appressorium formation is regulated by the Pmk1 mitogen-activated protein kinase pathway (25,42,46). The pmk1 deletion mutant fails to form appressoria and is nonpathogenic. One putative transcription factor regulated by PMK1 is MST12, which is homologous to Saccharomyces cerevisiae Ste12 and essential for pathogenesis (27). The mst12 deletion mutant forms melanized appressoria that have normal appressorium turgor but fail to develop penetration pegs, probably due to cytoskeleton defects i...

Feedback Regulation of SIN by Etd1 and Rho1 in Fission Yeast

“…p190 GAP is inhibited by phosphorylation and activated by PP2A-mediated dephosphorylation (Mori et al 2009). Similar results have been recently shown for Rho GAP activation in a number of different eukaryotic cells (Sopko et al 2007;Zheng et al 2007;Toure et al 2008;McAvoy et al 2009;Mori et al 2009;Wolfe et al 2009). Since rga5D and rga1D null mutants suppress lethality of Etd1-depleted cells, inhibition of Rho1 by PP2A-Pab1 could be exerted through dephosphorylation and activation of these Rho1 GAPs (see model in Figure 8).…”

MoTea4-Mediated Polarized Growth Is Essential for Proper Asexual Development and Pathogenesis in Magnaporthe oryzae

“…The clakel2 mutant formed abnormal appressoria on glass slides; however, mutant hyphal growth on potato dextrose agar medium and conidiation, though with a reduced amount of spores, was similar to that of the wild type (18). In M. oryzae, Zheng et al (28) have shown that Magnaporthe grisea Rho3 (MgRho3) is dispensable for vegetative hyphal growth but not for plant infection, as the appressoria formed by the ⌬Mgrho3 strain are morphologically abnormal and defective in plant penetration. However, it is important to note that unlike in Magnaporthe, the deletion of TEAC in A. nidulans leads to significantly reduced vegetative growth (10) and a likely decrease in conidiation.…”

“…Members of the Rho family of small GTP-binding proteins act as pivotal signaling switches and play a key role in morphogenesis during pathogenic development in M. oryzae (28,29). In Aspergillus nidulans, cell end marker proteins TeaA (Tea1), TeaR (Mod5), KipA (Tea2), and TeaC (Tea4) have been shown to be important for polarized hyphal growth (10,24).…”

Function of small GTPase Rho3 in regulating growth, conidiation and virulence of Botrytis cinerea

“…Dünkler and Wendland (2007) found that the Rho3 was required for the polarized growth and cell separation in Candida albicans. Zheng et al (2007) reported the Rho3 was dispensable for polarized hyphae growth, but required for the appressorium development and virulence in Magnaporthe oryzae.…”