POWDERY MILDEW RESISTENT4-dependent cell wall deposition is a consequence but not the cause of temperature-induced autoimmunity

Abstract

Plants possess a well-balanced immune system that is required for defense against pathogen infections. In autoimmune mutants or necrotic crosses, an intrinsic temperature-dependent imbalance leads to constitutive immune activation, resulting in severe damage or even death of plants. Recently, cell wall depositions were described as one of the symptoms following induction of the autoimmune phenotype in Arabidopsis saul1-1 mutants. However, the regulation and function of these depositions remained unclear. Here, we show that cell wall depositions, containing lignin and callose, were a common autoimmune feature and were deposited in proportion to the severity of the autoimmune phenotype at reduced ambient temperatures. When plants were exposed to reduced temperature for periods insufficient to induce an autoimmune phenotype, the cell wall depositions were not present. After low temperature intervals, sufficient to induce autoimmune responses, cell wall depositions correlated with a point of no return in saul1-1 autoimmunity. Although cell wall depositions were largely abolished in saul1-1 pmr4-1 double mutants lacking SAUL1 and the callose synthase gene GSL5/PMR4, their phenotype remained unchanged compared to that of the saul1-1 single mutant. Our data showed that cell wall depositions generally occur in autoimmunity, but appear not to be the cause of autoimmune phenotypes.

OUP accepted manuscript

“…In autoimmune mutants, temperature may lead to constitutive immune responses that can severely damage the plant or even have lethal consequences. Hessler et al (2021) show that, although not causal, the formation of cell wall depositions generally occurs in Arabidopsis autoimmunity and depends on reduced temperature. In saul1-1 autoimmunity mutants, low temperature stimuli are elegantly used as a switch to induce first callose deposition and then immune response phenotypes.…”