OXYLIPINS OTHER THAN JASMONIC ACID ARE XYLEM-RESIDENT SIGNALS REGULATING SYSTEMIC RESISTANCE INDUCED BY TRICHODERMA VIRENS IN MAIZE

While multiple long-distance signals were identified for pathogen-induced systemic acquired resistance (SAR), mobile signals for symbiont-induced systemic resistance (ISR) are less understood. We used ISR-positive and -negative mutants of both maize (Zea mays) and the beneficial fungus Trichoderma virens and identified 12-oxo-phytodienoic acid (12-OPDA) and α-ketol of octadecadienoic acid (KODA) as important ISR signals. We show that a 13-lipoxygenase mutant, lox10, colonized by wild type T. virens (TvWT) lacked ISR response against Colletotrichum graminicola, but instead displayed induced systemic susceptibility (ISS). Oxylipin profiling of xylem sap derived from combinations of T. virens-treated plants revealed that 12-OPDA and KODA levels correlated with ISR. Transfusing sap supplemented with 12-OPDA or KODA increased receiver plant resistance in a dose-dependent manner, with 12-OPDA being capable of restoring ISR of both TvWT-treated lox10 or T. virens Δsm1 strain, a mutant unable to induce ISR. Unexpectedly, JA was not involved, as the JA-deficient opr7opr8 mutant retained the capacity for T. virens-induced ISR. Transcriptome analysis of TvWT-treated B73 revealed upregulation of 12-OPDA biosynthesis and OPDA-responsive genes, but downregulation of subsequent JA biosynthesis and JA response genes. We propose a model that differential regulation of 12-OPDA and JA in response to T. virens colonization results in ISR induction.-