OXI1 induces immunity by coordinating N-hydroxypipecolic acid, salicylic acid and camalexin synthesis

Expression of OXI1 is induced by a number of stress conditions and regulates the interaction of plants with pathogenic and beneficial microbes. In this work, we generated Arabidopsis OXI1 knock out and genomic OXI1 overexpression lines and show by transcriptome, proteome and metabolome analysis that OXI1 triggers ALD1, SARD4 and FMO1 expression to promote the biosynthesis of pipecolic acid (Pip) and N-hydroxy pipecolic acid (NHP). OXI1 contributes to enhanced immunity by induced SA biosynthesis via CBP60g-induced expression of SID2 and camalexin accumulation via WRKY33-targeted transcription of PAD3. OXI1 regulates genes involved in ROS generation such as RbohD and RbohF. OXI1 knock out plants show enhanced expression of nuclear and chloroplast genes of photosynthesis and enhanced growth under ambient conditions, while OXI1 overexpressing plants accumulate NHP, SA, camalexin and ROS and show a gain-of-function cell death phenotype and enhanced pathogen resistance. The OXI1 gain-of-function phenotypes are completely suppressed when compromising NHP synthesis in the fmo1 or ald1 background, showing that OXI1 regulation of immunity is mediated via the NHP pathway. Overall, these results show that OXI1 plays a key role in basal and effector-triggered plant immunity by regulating defense and programmed cell death via biosynthesis of SA, NHP and camalexin.