Polyamines are small amines that accumulate during stress and contribute to disease resistance through as yet unknown signaling pathways. Using a comprehensive RNAsequencing analysis, we show that early transcriptional responses triggered by each of the most abundant polyamines (putrescine, spermidine, spermine, thermospermine and cadaverine) exhibit specific quantitative differences, suggesting that polyamines (rather than downstream metabolites) elicit defense responses. Signaling by putrescine, which accumulates in response to bacteria that trigger effector triggered immunity (ETI) and systemic acquired resistance (SAR), is largely dependent on the accumulation of hydrogen peroxide, and is partly dependent on salicylic acid (SA), the expression of ENHANCED DISEASE SUSCEPTIBILITY (EDS1) and NONEXPRESSOR of PR GENES1 (NPR1). Putrescine elicits local SA accumulation as well as local and systemic transcriptional reprogramming that overlaps with SAR. Loss-of-function mutations in arginine decarboxylase 2 (ADC2), which is required for putrescine synthesis and copper amine oxidase (CuAO), which is involved in putrescine oxidation, compromise basal defenses, as well as putrescine and pathogen-triggered systemic resistance. These findings confirm that putrescine elicits ROS-dependent SA pathways in the activation of plant defenses.

Genetic divergence between populations can lead to reproductive isolation. Hybrid incompatibilities (HI) represent intermediate points along a continuum toward speciation. In plants, genetic variation in disease resistance () genes underlies several cases of HI. The progeny of a cross between Arabidopsis () accessions Landsberg (L, Poland) and Kashmir2 (Kas2, central Asia) exhibits immune-related HI. This incompatibility is due to a genetic interaction between a cluster of eight ) (- genes (-) from L and central Asian alleles of a -family receptor-like kinase () from Kas2. In characterizing mutants altered in L/Kas2 HI, we mapped multiple mutations to the -like L locus. Analysis of these () mutants reveals complex, additive and epistatic interactions underlying - L locus activity. The effects of these mutations were measured on basal defense, global gene expression, primary metabolism, and disease resistance to a local isolate ( Gw) collected from Gorzów (Gw), where the Landsberg accession originated. Gene expression sectors and metabolic hallmarks identified for HI are both dependent and independent of - L members. We establish that mutations suppressing immune-related L/Kas2 HI do not compromise resistance to Gw. QTL mapping analysis of Gw resistance point to as the causal locus. This work provides insight into the complex genetic architecture of the- L locus and immune-related HI in Arabidopsis and into the contributions of - genes to HI and defense.

Polyamines are involved in defense against pathogenic microorganisms in plants. However, the role of the polyamine putrescine (Put) during plant defense has remained elusive. In this work, we studied the implication of polyamines during pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) in the model species Arabidopsis thaliana . Our data indicate that polyamines, particularly Put, accumulate in response to non-pathogenic Pseudomonas syringae pv. tomato DC3000 hrcC and in response to the purified PAMP flagellin22. Exogenously supplied Put to Arabidopsis seedlings induces defense responses compatible with PTI activation, such as callose deposition and transcriptional up-regulation of several PTI marker genes. Consistent with this, we show that Put primes for resistance against pathogenic bacteria. Through chemical and genetic approaches, we find that PTI-related transcriptional responses induced by Put are hydrogen peroxide and NADPH oxidase ( RBOHD and RBOHF ) dependent, thus suggesting that apoplastic ROS mediates Put signaling. Overall, our data indicate that Put amplifies PTI responses through ROS production, leading to enhanced disease resistance against bacterial pathogens.