LDL2 and PAO5 genes are essential for systemic acquired resistance in Arabidopsis thaliana

A partly infected plant becomes more resistant to subsequent infections by developing systemic acquired resistance (SAR). Primary infected tissues produce signals that travel to systemic tissues for SAR-associated priming of defense-related genes. The mechanism through which mobile signals contribute to long-lasting infection memory is mostly unknown. RSI1/FLD, a putative histone demethylase, is required for developing SAR. Here, we report that two other FLD homologs, LSD1-LIKE2 (LDL2) and POLYAMINE OXIDASE 5 (PAO5), are required for SAR development. The mutants of LDL2 and PAO5 are not defective in local resistance but are specifically impaired for SAR. The mutants are defective in salicylic acid accumulation and priming of defence-related genes such as PR1, FMO1, and SnRK2.8. LDL2 and PAO5 are expressed in systemic tissues upon SAR induction by pathogens or SAR mobile signal azelaic acid. The ldl2 and pao5 mutants generate SAR mobile signals like wild-type (WT) plants but fail to respond to the signal at the systemic leaves. Both LDL2 and PAO5 proteins contain polyamine oxidase (PAO) domains, suggesting their involvement in polyamine metabolism. Exogenous applications of polyamines such as spermine and spermidine activate SAR in WT and rescue SAR defects of ldl2 and pao5 plants. Inhibition of polyamine biosynthetic gene arginine decarboxylase blocks SAR development. Results altogether demonstrate specific non-redundant roles of LDL2 and PAO5 in SAR development with their possible involvement in polyamine metabolism.