Unraveling the Role of 12- cis -Oxo-Phytodienoic Acid in the Wound Response of Arabidopsis thalian a: Insights from Transcriptomic and Complementation Studies

In addition to jasmonoyl-isoleucine (JA-Ile), a well-established signaling molecule for plant growth and defense, its precursor, cis -12-oxo-phytodienoic acid (OPDA), is thought to possess independent signaling functions. Its perception in vascular plants is still uncharacterized. Several OPDA functions in Arabidopsis were inferred from a mutant that is affected in the function of the OPDA REDUCTASE3 (OPR3), catalyzing the conversion of OPDA within peroxisomes. Recently, opr3 plants were found to accumulate JA-Ile via a cytosolic OPR2-mediated bypass. Given the uncoupling of OPDA and JA biosynthesis in the JA-deficient mutant opr2opr3 , potential OPDA signaling was investigated by a transcriptome approach comparing wild type, opr2opr3 and the JA- and OPDA-deficient mutant allene oxide synthase . Dissecting the wound response of seedlings revealed that OPDA lacked a transcriptional signature, and that previously characterized OPDA-response genes were wound-induced independently of OPDA. Exogenous application of OPDA to opr2opr3 seedlings led to JA-Ile formation and signaling even in absence of OPR2 and OPR3 and resulted in activation of sulfur assimilation. These divergent responses to endogenously synthesized and applied OPDA suggest a compartmentalization of endogenous OPDA which was investigated by a trans-organellar complementation approach. OPR3 complemented the opr2opr3 mutant in terms of fertility and wound-induced JA-Ile production irrespective of its subcellular localization. In vitro enzymatic activity of OPR3, however, showed conversion of OPDA and 4,5-didehydro-JA (4,5-ddh-JA), therefore not allowing to conclude which compound is translocated. Dissecting the conversion of either OPDA or 4,5-ddh-JA by OPR2 and OPR1 organelle variants pointed to a strong OPDA compartmentalization supporting its lacking signaling capacity.