Systemic Acquired Resistance signaling molecule N-hydroxypipecolic acid is involved in Age-Related Resistance in Arabidopsis thaliana

Little is known about how mature Arabidopsis thaliana (Arabidopsis) plants become competent to initiate Age-Related Resistance (ARR) to normally virulent Pseudomonas syringae pv tomato (Pst). RNA-Sequencing was used to identify genes that contribute to ARR by comparing the response to Pst in young susceptible 3.5 weeks post germination (wpg) to mature 6 wpg ARR-competent plants. Comparative transcriptomics revealed that ARR shares many genes with Pathogen Associated Molecular Pattern (PAMP) Triggered Immunity, Effector Triggered Immunity and Systemic Acquired Resistance, providing additional evidence for the emerging idea that plant immune pathways are intertwined and interconnected. ARR-responding leaves expressed genes for the biosynthesis of N-hydroxypipecolic acid (NHP), a signaling molecule involved in Systemic Acquired Resistance (SAR) during the establishment of a primed state in systemic leaves. NHP biosynthesis mutants (ald1-1, fmo1-1) were ARR-defective and accumulated less intercellular salicylic acid (SA) compared to wild-type Col-0, indicating a requirement for functional NHP biosynthesis genes for intercellular SA accumulation during ARR. SA perception by NPR1/4 proteins was required for ARR as demonstrated by the ARR-defective phenotype of the SA-signaling mutant, npr1-1 npr4-4D. NHP biosynthesis genes were still expressed in npr1-1 npr4-4D, suggesting that similar to SAR, NHP functions upstream of NPR1-mediated signaling during ARR. Unlike ARR-incompetent plants (3, 4, 5 wpg untreated), ARR-competent plants (6 wpg untreated), expressed NHP biosynthesis genes, accumulated NHP and expressed immunity-associated cell-surface receptors RLP23 and RLP28, supporting the idea that NHP contributes to initiating a developmentally controlled ARR competent primed state in mature Arabidopsis that is similar to the primed state established in distant leaves during SAR.