Investigating the Role of N-Hydroxypipecolic Acid and Salicylic Acid During Age-Related Resistance in Arabidopsis thaliana

Abstract

Little is still known about what allows mature Arabidopsis thaliana (Arabidopsis) plants to respond with an Age-Related Resistance response (ARR). To better understand how mature plants initiate and establish ARR, gene expression in the leaves of young and mature plants responding to Pseudomonas syringae pv. tomato (Pst) was investigated using RNA-sequencing analysis. Genes involved in the biosynthesis of N-hydroxypipecolic acid (NHP) were upregulated in ARR-responding leaves leading to the idea that NHP, a signaling molecule in Systemic AcquiredResistance (SAR) may also be required for ARR. The ARR response was examined in NHP biosynthesis mutants and revealed that NHP biosynthesis is required for ARR. During ARR, NHP biosynthesis mutants were also shown to accumulate less salicylic acid (SA) compared to wild-type leaves in response to Pst. Healthy untreated leaves had modest accumulation of NHP and modest expression of several cell-surface receptors was observed compared to the healthy untreated leaves of young plants, suggesting that ARR competence in mature untreated plants involves a primed/immune ready state similar to what is observed in systemic leaves of plants induced for SAR in a local leaf. The ARR response also requires the accumulation of intercellular SA which is involved in inhibiting biofilm-like aggregate formation of Pst in mature plant leaves. To understand how SA is transported from the cytosol to the intercellular space during ARR, the ARR response of the PDR-type transporter mutants pdr8-4 and pdr12-3 was examined. The pdr8-4 pdr12-3 double mutant was partially ARR-defective and SA accumulation in leaf intercellular spaces was reduced by ~50% compared to wild-type mature plants during ARR, demonstrating that PDR8 and PDR12 are required for the intercellular localisation of SA during ARR. To obtain evidence that PDR8 and PDR12 act as transporters of SA, SA transporter assays were performed with yeast expressing PDR8 and PDR12. PDR8- and PDR12-expressing yeast accumulated less intracellular SA than empty vector containing yeast cells, suggesting that PDR8 and PDR12 act as SA transporters. Together, this work found ARR shared signaling components with SAR and found additional support for SA as an antimicrobial and signaling molecule during ARR.