An array of signal-specific MoYpd1 isoforms determines full virulence in the pathogenic fungus Magnaporthe oryzae

Magnaporthe oryzae is placed first on a list of the worlds top ten plant pathogens with the highest scientific and economic importance. The pathogen is highly evolved to sense environmental changes quickly to invade different tissues of its host plant successfully. Here, we found alternative splicing (AS) as a previously unknown mechanism of how the pathogen may handle rapidly changing situations during in planta growth. The AS facilitates the production of multiple mRNAs and, consequently, protein isoforms with distinct biological functions from a single gene. The locus MGG_07173 occurs only once in the genome of M. oryzae and encodes the phosphotransfer protein MoYpd1p, which plays an important role in the high osmolarity glycerol (HOG) signaling pathway for osmoregulation. Originating from this locus, at least three MoYPD1 isoforms are produced in a signal-specific manner. The transcript levels of these MoYPD1-isoforms were individually affected by external stress, which is equally present in host plant tissues. Potassium salt (KCI) stress raised MoYPD1_T0 and MoYPD1_T2 abundance, whereas osmotic stress by sorbitol elevates MoYPD1_T1 levels. In line with this, signal-specific nuclear translocation of green fluorescent protein-fused MoYpd1p isoforms in response to stress was observed. The protein isoforms MoYpd1p_T0 and MoYpd1p_T2 were detected in the nucleus following KCl treatment, whereas MoYpd1p_T1 did likewise upon sorbitol stress. Mutant strains that produce only one of the MoYpd1p isoforms are less virulent, suggesting a combination thereof is required to invade the host successfully. Using in silico protein-protein interaction studies, we were able to assign different roles to respective isoforms in the HOG signaling pathway and predicted interactions with other signaling pathways. In summary, we demonstrate the signal-specific production of MoYpd1p isoforms that individually increase signal diversity and orchestrate virulence in the rice blast fungus M. oryzae. Authors summaryThe worlds population rises rapidly and a major problem is the global food security. Magnaporthe oryzae is placed first on a list of the worlds top ten plant pathogens with the highest scientific and economic importance since it causes blast, the most devastating disease of cultivated rice, which is the major food source for more than half of the world population. In this study, we demonstrate signal-specific production of a signaling protein, MoYpd1p, and show for the first time that the isoforms individually increase signal diversity and orchestrate virulence in the rice blast fungus M. oryzae. These results will help to better understand the molecular basis of pathogenicity of the rice blast fungus and consequently open the door to develop new strategies for plant protection and food security. In addition, our study is a valuable contribution to the basic research area of signaling mechanisms in in filamentous pathogenic fungi, and will thus be of high interest for a broad readership of the scientific community.