The bZIP transcription factor BIP1 of the rice blast fungus is essential for infection and regulates a specific set of appressorium genes.

The rice blast fungus Magnaporthe oryzae differentiates specialized cells called appressoria that are required for fungal penetration into host leaves. In this study, we identified the novel basic leucine zipper (bZIP) transcription factor BIP1 (B-ZIP Involved in Pathogenesis-1) that is essential for pathogenicity. BIP1 is required for the infection of plant leaves, even if they are wounded, but not for appressorium-mediated penetration of artificial cellophane membranes. This phenotype suggests that BIP1 is not implicated in the differentiation of the penetration peg but is necessary for the initial establishment of the fungus within plant cells. BIP1 expression was restricted to the appressorium by both transcriptional and post-transcriptional control. Genome-wide transcriptome analysis showed that 40 genes were down regulated in a BIP1 deletion mutant. Most of these genes were specifically expressed in the appressorium. They encode proteins with pathogenesis-related functions such as enzymes involved in secondary metabolism including those encoded by the ACE1 gene cluster, small secreted proteins such as SLP2, BAS2, BAS3, and AVR-Pi9 effectors, as well as plant cuticle and cell wall degrading enzymes. Interestingly, this BIP1 network is different from other known infection-related regulatory networks, highlighting the complexity of gene expression control during plant-fungal interactions. Promoters of BIP1-regulated genes shared a GCN4/bZIP-binding DNA motif (TGACTC) binding in vitro to BIP1. Mutation of this motif in the promoter of MGG_08381.7 from the ACE1 gene cluster abolished its appressorium-specific expression, showing that BIP1 behaves as a transcriptional activator. In summary, our findings demonstrate that BIP1 is critical for the expression of early invasion-related genes in appressoria. These genes are likely needed for biotrophic invasion of the first infected host cell, but not for the penetration process itself. Through these mechanisms, the blast fungus strategically anticipates the host plant environment and responses during appressorium-mediated penetration.

A machine learning-based chemoproteomic approach to identify drug targets and binding sites in complex proteomes.

Chemoproteomics is a key technology to characterize the mode of action of drugs, as it directly identifies the protein targets of bioactive compounds and aids in the development of optimized small-molecule compounds. Current approaches cannot identify the protein targets of a compound and also detect the interaction surfaces between ligands and protein targets without prior labeling or modification. To address this limitation, we here develop LiP-Quant, a drug target deconvolution pipeline based on limited proteolysis coupled with mass spectrometry that works across species, including in human cells. We use machine learning to discern features indicative of drug binding and integrate them into a single score to identify protein targets of small molecules and approximate their binding sites. We demonstrate drug target identification across compound classes, including drugs targeting kinases, phosphatases and membrane proteins. LiP-Quant estimates the half maximal effective concentration of compound binding sites in whole cell lysates, correctly discriminating drug binding to homologous proteins and identifying the so far unknown targets of a fungicide research compound.

Methionine biosynthesis is essential for infection in the rice blast fungus Magnaporthe oryzae.

Methionine is a sulfur amino acid standing at the crossroads of several biosynthetic pathways. In fungi, the last step of methionine biosynthesis is catalyzed by a cobalamine-independent methionine synthase (Met6, EC 2.1.1.14). In the present work, we studied the role of Met6 in the infection process of the rice blast fungus, Magnaporthe oryzae. To this end MET6 null mutants were obtained by targeted gene replacement. On minimum medium, MET6 null mutants were auxotrophic for methionine. Even when grown in presence of excess methionine, these mutants displayed developmental defects, such as reduced mycelium pigmentation, aerial hypha formation and sporulation. They also displayed characteristic metabolic signatures such as increased levels of cysteine, cystathionine, homocysteine, S-adenosylmethionine, S-adenosylhomocysteine while methionine and glutathione levels remained unchanged. These metabolic perturbations were associated with the over-expression of MgCBS1 involved in the reversed transsulfuration pathway that metabolizes homocysteine into cysteine and MgSAM1 and MgSAHH1 involved in the methyl cycle. This suggests a physiological adaptation of M. oryzae to metabolic defects induced by the loss of Met6, in particular an increase in homocysteine levels. Pathogenicity assays showed that MET6 null mutants were non-pathogenic on both barley and rice leaves. These mutants were defective in appressorium-mediated penetration and invasive infectious growth. These pathogenicity defects were rescued by addition of exogenous methionine and S-methylmethionine. These results show that M. oryzae cannot assimilate sufficient methionine from plant tissues and must synthesize this amino acid de novo to fulfill its sulfur amino acid requirement during infection.

The crucial role of the Pls1 tetraspanin during ascospore germination in Podospora anserina provides an example of the convergent evolution of morphogenetic processes in fungal plant pathogens and saprobes.

Pls1 tetraspanins were shown for some pathogenic fungi to be essential for appressorium-mediated penetration into their host plants. We show here that Podospora anserina, a saprobic fungus lacking appressorium, contains PaPls1, a gene orthologous to known PLS1 genes. Inactivation of PaPls1 demonstrates that this gene is specifically required for the germination of ascospores in P. anserina. These ascospores are heavily melanized cells that germinate under inducing conditions through a specific pore. On the contrary, MgPLS1, which fully complements a DeltaPaPls1 ascospore germination defect, has no role in the germination of Magnaporthe grisea nonmelanized ascospores but is required for the formation of the penetration peg at the pore of its melanized appressorium. P. anserina mutants with mutation of PaNox2, which encodes the NADPH oxidase of the NOX2 family, display the same ascospore-specific germination defect as the DeltaPaPls1 mutant. Both mutant phenotypes are suppressed by the inhibition of melanin biosynthesis, suggesting that they are involved in the same cellular process required for the germination of P. anserina melanized ascospores. The analysis of the distribution of PLS1 and NOX2 genes in fungal genomes shows that they are either both present or both absent. These results indicate that the germination of P. anserina ascospores and the formation of the M. grisea appressorium penetration peg use the same molecular machinery that includes Pls1 and Nox2. This machinery is specifically required for the emergence of polarized hyphae from reinforced structures such as appressoria and ascospores. Its recurrent recruitment during fungal evolution may account for some of the morphogenetic convergence observed in fungi.

Magnaporthe grisea avirulence gene ACE1 belongs to an infection-specific gene cluster involved in secondary metabolism.

The avirulence gene ACE1 from the rice blast fungus Magnaporthe grisea encodes a polyketide synthase (PKS) fused to a nonribosomal peptide synthetase (NRPS) probably involved in the biosynthesis of a secondary metabolite recognized by Pi33 resistant rice (Oryza sativa) cultivars. Analysis of the M. grisea genome revealed that ACE1 is located in a cluster of 15 genes, of which 14 are potentially involved in secondary metabolism as they encode enzymes such as a second PKS-NRPS (SYN2), two enoyl reductases (RAP1 and RAP2) and a putative Zn(II)(2)Cys(6) transcription factor (BC2). These 15 genes are specifically expressed during penetration into the host plant, defining an infection-specific gene cluster. A pORF3-GFP transcriptional fusion showed that the highly expressed ORF3 gene from the ACE1 cluster is only expressed in appressoria, as is ACE1. Phenotypic analysis of deletion or disruption mutants of SYN2 and RAP2 showed that they are not required for avirulence in Pi33 rice cultivars, unlike ACE1. Inactivation of other genes was unsuccessful because targeted gene replacement and disruption were inefficient at this locus. Overall, the ACE1 gene cluster displays an infection-specific expression pattern restricted to the penetration stage which is probably controlled at the transcriptional level and reflects regulatory networks specific to early stages of infection.

Fungi have three tetraspanin families with distinct functions.

BACKGROUND: Tetraspanins are small membrane proteins that belong to a superfamily encompassing 33 members in human and mouse. These proteins act as organizers of membrane-signalling complexes. So far only two tetraspanin families have been identified in fungi. These are Pls1, which is required for pathogenicity of the plant pathogenic ascomycetes, Magnaporthe grisea, Botrytis cinerea and Colletotrichum lindemuthianum, and Tsp2, whose function is unknown. In this report, we describe a third family of tetraspanins (Tsp3) and a new family of tetraspanin-like proteins (Tpl1) in fungi. We also describe expression of some of these genes in M. grisea and a basidiomycete, Laccaria bicolor, and also their functional analysis in M. grisea. RESULTS: The exhaustive search for tetraspanins in fungal genomes reveals that higher fungi (basidiomycetes and ascomycetes) contain three families of tetraspanins (Pls1, Tsp2 and Tsp3) with different distribution amongst phyla. Pls1 is found in ascomycetes and basidiomycetes, whereas Tsp2 is restricted to basidiomycetes and Tsp3 to ascomycetes. A unique copy of each of PLS1 and TSP3 was found in ascomycetes in contrast to TSP2, which has several paralogs in the basidiomycetes, Coprinus cinereus and Laccaria bicolor. A tetraspanin-like family (Tpl1) was also identified in ascomycetes. Transcriptional analyses in various tissues of L. bicolor and M. grisea showed that PLS1 and TSP2 are expressed in all tissues in L. bicolor and that TSP3 and TPL1 are overexpressed in the sexual fruiting bodies (perithecia) and mycelia of M. grisea, suggesting that these genes are not pseudogenes. Phenotypic analysis of gene replacementmutants Deltatsp3 and Deltatpl1 of M. grisea revealed a reduction of the pathogenicity only on rice, in contrast to Deltapls1 mutants, which are completely non-pathogenic on barley and rice. CONCLUSION: A new tetraspanin family (Tsp3) and a tetraspanin-like protein family (Tpl1) have been identified in fungi. Functional analysis by gene replacement showed that these proteins, as well as Pls1, are involved in the infection process of the plant pathogenic fungus M. grisea. The next challenge will be to decipher the role(s) of tetraspanins in a range of symbiotic, saprophytic and human pathogenic fungi.

Tripartite chimeric pseudogene from the genome of rice blast fungus Magnaporthe grisea suggests double template jumps during long interspersed nuclear element (LINE) reverse transcription.

BACKGROUND: A systematic survey of loci carrying retrotransposons in the genome of the rice blast fungus Magnaporthe grisea allowed the identification of novel non-canonical retropseudogenes. These elements are chimeric retrogenes composed of DNA copies from different cellular transcripts directly fused to each other. Their components are copies of a non protein-coding highly expressed RNA of unknown function termed WEIRD and of two fungal retrotransposons: MGL and Mg-SINE. Many of these chimeras are transcribed in various M. grisea tissues and during plant infection. Chimeric retroelements with a similar structure were recently found in three mammalian genomes. All these chimeras are likely formed by RNA template switches during the reverse transcription of diverse LINE elements. RESULTS: We have shown that in M. grisea template switching occurs at specific sites within the initial template RNA which contains a characteristic consensus sequence. We also provide evidence that both single and double template switches may occur during LINE retrotransposition, resulting in the fusion of three different transcript copies. In addition to the 33 bipartite elements, one tripartite chimera corresponding to the fusion of three retrotranscripts (WEIRD, Mg-SINE, MGL-LINE) was identified in the M. grisea genome. Unlike the previously reported two human tripartite elements, this fungal retroelement is flanked by identical 14 bp-long direct repeats. The presence of these short terminal direct repeats demonstrates that the LINE enzymatic machinery was involved in the formation of this chimera and its integration in the M. grisea genome. CONCLUSION: A survey of mammalian genomic databases also revealed two novel tripartite chimeric retroelements, suggesting that double template switches occur during reverse transcription of LINE retrotransposons in different eukaryotic organisms.