Differences between appressoria formed by germ tubes and appressorium-like structures developed by hyphal tips in Magnaporthe oryzae

MoTup1 is required for growth, conidiogenesis and pathogenicity of Magnaporthe oryzae

The general transcriptional repressor Tup1 proteins play important regulatory roles in the growth and development of fungi. In this report, we characterized MoTup1, a protein homologous to Tup1 of Saccharomyces cerevisiae, from M. oryzae. Disruption of MoTUP1 resulted in severe mycelial growth reduction and a defect in conidiogenesis. We found that MoTup1 is required for the maintenance of cell wall integrity by regulating the expression of the genes involved in cell wall biosynthesis. Pathogenicity assays indicated that the ΔMotup1 mutants lost the ability to invade both rice and barley hosts. Moreover, observation of rice epidermis penetration showed that the hyphal tips of the mutants could still form appressorium-like structures, but were unable to invade host cells. Taken together, our results demonstrate that M. oryzae MoTup1 is an important regulatory factor in fungal growth, development and pathogenesis on hosts.

Role of the MoYAK1 protein kinase gene in Magnaporthe oryzae development and pathogenicity

Conidiation and appressorium differentiation are key processes for polycyclic dissemination and infection in many pathogens. Our previous study using DNA microarray led to the discovery of the MoYAK1 gene in Magnaporthe oryzae that is orthologous to YAK1 in Saccharomyces cerevisiae. Although the mechanistic roles of YAK1 in S. cerevisiae have been described, roles of MoYAK1 in M. oryzae, a phytopathogenic fungus responsible for rice blast, remain uncharacterized. Targeted disruption of MoYAK1 results in pleiotropic defects in M. oryzae development and pathogenicity. The ΔMoyak1 mutant exhibits a severe reduction in aerial hyphal formation and conidiation. Conidia in the ΔMoyak1 are delayed in germination and demonstrate decreased glycogen content in a conidial age-dependent manner. The expression of hydrophobin-coding genes is dramatically changed in the ΔMoyak1 mutant, leading to a loss of surface hydrophobicity. Unlike the complete inability of the ΔMoyak1 mutant to develop appressoria on an inductive surface, the mutant forms appressoria of abnormal morphology in response to exogenous cyclic adenosine-5'-monophosphate and host-driven signals, which are all defective in penetrating host tissues due to abnormalities in glycogen and lipid metabolism, turgor generation and cell wall integrity. These data indicate that MoYAK1 is a protein kinase important for the development and pathogenicity of M. oryzae.

Activation of Mst11 and Feedback Inhibition of Germ Tube Growth in Magnaporthe oryzae

Appressorium formation and invasive growth are two important steps in the infection cycle of Magnaporthe oryzae that are regulated by the Mst11-Mst7-Pmk1 mitogen-activated protein kinase (MAPK) pathway. However, the molecular mechanism involved in the activation of Mst11 MAPK kinase kinase is not clear in the rice blast fungus. In this study, we functionally characterized the regulatory region of Mst11 and its self-inhibitory binding. Deletion of the middle region of Mst11, which contains the Ras-association (RA) domain and two conserved phosphorylation sites (S453 and S458), blocked Pmk1 activation and appressorium formation. However, the MST11(ΔRA) transformant MRD-2 still formed appressoria, although it was reduced in virulence. Interestingly, over 50% of its germ tubes branched and formed two appressoria by 48 h, which was suppressed by treatments with exogenous cAMP. The G18V dominant active mutation enhanced the interaction of Ras2 with Mst11, suggesting that Mst11 has stronger interactions with the activated Ras2. Furthermore, deletion and site-directed mutagenesis analyses indicated that phosphorylation at S453 and S458 of Mst11 is important for appressorium formation and required for the activation of Pmk1. We also showed that the N-terminal region of Mst11 directly interacted with its kinase domain, and the S789G mutation reduced their interactions. Expression of the MST11(S789G) allele rescued the defect of the mst11 mutant in plant infection and resulted in the formation of appressoria on hydrophilic surfaces, suggesting the gain-of-function effect of the S789G mutation. Overall, our results indicate that the interaction of Mst11 with activated Ras2 and phosphorylation of S453 and S458 play regulatory roles in Mst11 activation and infection-related morphogenesis, possibly by relieving its self-inhibitory interaction between its N-terminal region and the C-terminal kinase domain. In addition, binding of Mst11 to Ras2 may be involved in the feedback inhibition of cAMP signaling and further differentiation of germ tubes after appressorium formation.

Screening of a synthetic peptide combinatorial library to identify inhibitors of the appressorium formation in Magnaporthe oryzae

The rice blast disease caused by Magnaporthe oryzae is one of the most devastating diseases of cultivated rice. One of the most important stages in the infective cycle of M. oryzae is the formation of the dome-shaped structure called appressorium. The purpose of the present study was to identify novel peptides to control the rice blast disease by blocking the appressorium formation through screening of a synthetic peptide combinatorial library. As result of the screening, a set of 29 putative bioactive peptides were identified, synthesized and assayed in comparison with the previously identified peptide PAF104. The peptides MgAPI24, MgAPI40 and MgAPI47 showed improved inhibitory activity on the M. oryzae appressorium formation. Our data show that these peptides have a differential effect on two developmental structures: appressoria and appressorium-like structures. Antimicrobial assays against M. oryzae and other non-target microorganisms showed a weak or no toxicity of these peptides, demonstrating their specific activity blocking the appressorium formation. Therefore, the outcome of this research would be useful in the development of novel target-oriented peptides to use in plant protection.

Verticillium transcription activator of adhesion Vta2 suppresses microsclerotia formation and is required for systemic infection of plant roots

Six transcription regulatory genes of the Verticillium plant pathogen, which reprogrammed nonadherent budding yeasts for adhesion, were isolated by a genetic screen to identify control elements for early plant infection. Verticillium transcription activator of adhesion Vta2 is highly conserved in filamentous fungi but not present in yeasts. The Magnaporthe grisea ortholog conidiation regulator Con7 controls the formation of appressoria which are absent in Verticillium species. Vta2 was analyzed by using genetics, cell biology, transcriptomics, secretome proteomics and plant pathogenicity assays. Nuclear Vta2 activates the expression of the adhesin-encoding yeast flocculin genes FLO1 and FLO11. Vta2 is required for fungal growth of Verticillium where it is a positive regulator of conidiation. Vta2 is mandatory for accurate timing and suppression of microsclerotia as resting structures. Vta2 controls expression of 270 transcripts, including 10 putative genes for adhesins and 57 for secreted proteins. Vta2 controls the level of 125 secreted proteins, including putative adhesins or effector molecules and a secreted catalase-peroxidase. Vta2 is a major regulator of fungal pathogenesis, and controls host-plant root infection and H2 O2 detoxification. Verticillium impaired in Vta2 is unable to colonize plants and induce disease symptoms. Vta2 represents an interesting target for controlling the growth and development of these vascular pathogens.

PAF104, a synthetic peptide to control rice blast disease by blocking appressorium formation in Magnaporthe oryzae

Magnaporthe oryzae is the most devastating pathogen of rice and the main cause of crop losses worldwide. The successful management of blast disease caused by this fungus is a clear necessity. The synthetic peptide PAF104 has been characterized by its inhibition of M. oryzae appressorium formation on hydrophobic surfaces. Growth and the ability of conidia to germinate was not affected by PAF104, indicating the lack of toxicity on fungal conidia. The addition of the cutin monomer 1,16-hexadecanediol does not interfere with the inhibitory effect of PAF104 on in vitro hydrophobic surfaces. On the other hand, inhibition of appressorium formation by PAF104 was nullified by the exogenous addition of cAMP. Our results suggest that PAF104 affects the Pmk1 pathway by repression of the gene expression of MoMSB2, which encodes a sensing surface protein, and the mitogen-activated protein/extracellular signal-regulated kinase kinase kinase MST11. The pathogenicity of M. oryzae was reduced after PAF104 treatment specifically blocking appressorium formation. Our results support PAF104 as a promising compound to control rice blast disease by blocking a specific target related to appressorium formation, a process essential for infection of rice leaves. Moreover, PAF104 is proposed as a lead compound to develop novel specific fungicides with improved properties.