MOLECULAR MECHANISMS UNDERLYING FUNGAL PATHOGENESIS IN THE RICE BLAST FUNGUS, MAGNAPORTHE ORYZAE
Global food security is seriously affected by fungal pathogens of the Magnaporthaceae family. Two of the most damaging fungal pathogens affecting the staple foods rice and wheat belong to this family: the rice blast fungus, Magnaporthe oryzae, which also causes a very serious disease in wheat, and the causal agent of take-all disease of wheat, Gaeumannomyces graminis.
The process of infection of leaves by M. oryzae has been extensively studied. We have shown that it can undergo a different set of developmental events that are typical of root-infecting pathogens such as G. graminis. Extraordinary genomic resources are now available for the rice blast fungus. Currently, the genetic dissection and comparative analysis of these two different pathways of plant attack by M. oryzae and their regulation constitute an exciting challenge.
M. grisea penetrates the stele. Confocal imaging of radial sections of a two-week-old rice and barley seedlings infected with GFP-tagged M. grisea.
Organ-specific infection mechanisms
The recognition of plant cell surfaces (leaves/roots) leads to the development of different infection structures (appressoria or hyphopodia) before fungal penetration. It is not known if the hyphopodium is an intermediate step that takes place before the formation of a mature appressorium or is an independent developmental process. Several approaches including lectin cytochemistry, gene expression analysis of infection-related genes and well characterised mutants were used for a detailed analysis of this morphogenetic development. Our results reveal genetic commonalities between appressorium and hyphopodium differentiation suggesting that hyphopodia could represent primitive appressoria. Thus, hyphopodium formation is likely to be an intermediate step before mature appressorium development (Tucker et al, 2010).
Post-transcriptional gene regulation mechanisms
RNA-binding proteins play a central role in post-transcriptional mechanisms that control gene expression. Identification of fungal-specific RNA-binding proteins is helping to unravel post-transcriptional networks and cellular processes that confer identity to the fungal kingdom.
RNA metabolism is controlled by RNA-binding proteins which play a crucial role in a number of steps between de novo transcription and protein synthesis (Sesma, 2016).
We have characterised two proteins involved in RNA metabolism, the karyopherin exportin-5 and the RNA-binding protein Rbp35. These two proteins are required for fungal virulence (Franceschetti et al., 2011; Rodríguez-Romero et al., 2015; Tucker et al., 2010). Rbp35 is not essential for fungal viability but regulates the length of 3’UTRs of transcripts with developmental and virulence-associated functions. The lack of clear Rbp35 orthologues in yeast, plants and animals indicates that Rbp35 is a novel auxiliary protein of the polyadenylation machinery of filamentous fungi.
We also found that different classes of sRNAs are altered in Exp5 and Rbp35, which suggests that in addition to other functions, these two proteins participate in different sRNA biogenesis pathways. M. oryzae is predominantly a clonally propagating organism, reproducing by conidial production from disease lesions. Increasing our knowledge of sRNA pathways can help us to understand mechanisms of transposon silencing and can reveal effective alternative control methods against M. oryzae.
- Regulación post-transcripcional en respuesta a estrés ambiental en el hongo de la piriculariosis del arroz MINECO, 2015- 2018. PI: Ane Sesma.
- Regulación traduccional y abordaje de su potencial biotecnológico para mejorar la adaptación de las plantas a estreses bióticos y abióticos 2014- 2018 (Comunidad de Madrid / EU). PIs: Ane Sesma (CBGP/UPM), Carmen Castresana (CNB/CSIC), y Mar Castellano (INIA).
- Post-transcriptional networks regulating organ-specific and general infection mechanisms in the rice blast fungus. Marie Curie Reintegration grant 2012-2016 (FP7-2011-CIG). PI: Ane Sesma.
- Infection mechanisms in the rice blast fungus Magnaporthe oryzae. MICINN, 2012-2015. PI: Ane Sesma.
- Dissection of distinct pathogenesis-related developmental processes in the rice blast fungus Magnaporthe grisea. David Philips BBSRC fellowship, 2005-2010. PI: Ane Sesma.
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