Disruption ofZea mays isochorismate synthase1decreases PHENYLALANINE AMMONIA LYASE activity and suppresses hypersensitive response-induced metabolism

Abstract

AbstractISOCHORISMATE SYNTHASE (ICS) catalyzes the isomerization of chorismate to isochorismate, an essential precursor in the biosynthesis of the Photosystem I electron carrier phylloquinone and of one of two pathways for the biosynthesis of the defense response hormone salicylic acid (SA). We characterized aZea mays ics1mutant for impacts on metabolism, photosynthesis, and immune signaling. Phylloquinone was reduced in the mutant resulting in low electron transfer rates and high electron backflow rates. SA accumulation induced by autoactive alleles of the nucleotide-binding leucine-rich repeat (NLR) geneResistance to Puccinia sorgi1(Rp1)requiredics1. Induced accumulation of SA was not required for lesion formation by the autoactiveRp1-D21#4allele. Metabolomic analyses and SA supplementation ofRp1-D21#4mutants,ics1-1mutants andRp1-D21#4; ics1-1double mutants demonstrated that most hypersensitive response-induced metabolism requiredics1but this was independent of SA accumulation. Both the PAL and ICS pathways contributed to SA biosynthesis in maize as labeled phenylalanine was incorporated into SA glucoside. Maizeics1-1mutants had low PHENYLALANINE AMMONIA LYASE activity, accumulated phenylalanine, and decreased abundance of phenylalanine derived metabolites. This demonstrates that the ICS and PAL pathways interact by a yet unknown mechanism complicating the interpretation of SA biosynthesis in plants from genetics alone.