Function of hydroxycinnamoyl transferases for the biosynthesis of phenolamides in rice resistance to Magnaporthe oryzae

Phenolamide (PA) metabolites play important roles in the interaction between plants and pathogens. The putrescine hydroxycinnamoyl transferase genes OsPHT3and OsPHT4positively regulate rice cell death and resistance to Magnaporthe oryzae. The bZIP transcription factor APIP5, a negative regulator of cell death and rice immunity, directly binds to the OsPHT4promoter to regulate putrescine-derived PAs. Whether other hydroxycinnamoyl transferase (HT) genes also participate in APIP5-mediated immunity remains unclear. Surprisingly, we find that genes encoding agmatine hydroxycinnamoyl transferases OsAHT1 and OsAHT2, tryptamine hydroxycinnamoyl transferases OsTBT1 and OsTBT2, and tyramine hydroxycinnamoyl transferases OsTHT1 and OsTHT2, responsible for the biosynthesis of polyamine-derived PAs are all up-regulated in APIP5-RNAi transgenic plants compared with segregated wild-type rice. Furthermore, both OsAHT1/2and OsTBT1/2are induced during M. oryzaeinfection, showing expression patterns similar to those previously reported for OsTHT1/2and OsPHT3/4. Transgenic plants overexpressing either OsAHT2-GFPor OsTBT1-GFPshow enhanced resistance against M. oryzaeand accumulated more PA metabolites and lignin compared with wild-type plants. Interestingly, as demonstrated for OsPHT4, APIP5 directly binds to the promoters of OsAHT1/2, OsTBT1/2, and OsTHT1/2, repressing their transcription. Together, these results indicate that the HT genes are common targets of APIP5 and that PAs play critical roles in rice immunity.