Identification and Characterization of Auxin/IAA Biosynthesis Pathway in the Rice Blast Fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae has been known to produce the phytohormone auxin/IAA from its hyphae and conidia, but the detailed biological function and biosynthesis pathway is largely unknown. By sequence homology, we identified a complete indole-3-pyruvic acid (IPA)-based IAA biosynthesis pathway in M. oryzae, consisting of the tryptophan aminotransferase (MoTam1) and the indole-3-pyruvate decarboxylase (MoIpd1). In comparison to the wild type, IAA production was significantly reduced in the motam1 Delta mutant, and further reduced in the moipd1 Delta mutant. Correspondingly, mycelial growth, conidiation, and pathogenicity were defective in the motam1 Delta and the moipd1 Delta mutants to various degrees. Targeted metabolomics analysis further confirmed the presence of a functional IPA pathway, catalyzed by MoIpd1, which contributes to IAA/auxin production in M. oryzae. Furthermore, the well-established IAA biosynthesis inhibitor, yucasin, suppressed mycelial growth, conidiation, and pathogenicity in M. oryzae. Overall, this study identified an IPA-dependent IAA synthesis pathway crucial for M. oryzae mycelial growth and pathogenic development.

Automated summary

The rice blast fungus Magnaporthe oryzae produces the phytohormone auxin/IAA, which is crucial for its mycelial growth and pathogenic development. This study identified an indole-3-pyruvic acid (IPA)-dependent IAA synthesis pathway in M. oryzae, consisting of the enzymes tryptophan aminotransferase (MoTam1) and indole-3-pyruvate decarboxylase (MoIpd1).