The Polycomb protein LHP1 regulates Arabidopsis thaliana stress responses through the repression of the MYC2-dependent branch of immunity.

Polycomb repressive complexes (PRCs) have been traditionally associated with the regulation of developmental processes in various organisms, including higher plants. However, similar to other epigenetic regulators, there is accumulating evidence for their role in the regulation of stress and immune-related pathways. In the current study we show that the PRC1 protein LHP1 is required for the repression of the MYC2 branch of jasmonic acid (JA)/ethylene (ET) pathway of immunity. Loss of LHP1 induces the reduction in H3K27me3 levels in the gene bodies of ANAC019 and ANAC055, as well as some of their targets, leading to their transcriptional upregulation. Consistently, increased expression of these two transcription factors leads to the misregulation of several of their genomic targets. The lhp1 mutant mimics the MYC2, ANAC019, and ANAC055 overexpressers in several of their phenotypes, including increased aphid resistance, abscisic acid (ABA) sensitivity and drought tolerance. In addition, like the MYC2 and ANAC overexpressers, lhp1 displays reduced salicylic acid (SA) content caused by a deregulation of ICS1 and BSMT1, as well as increased susceptibility to the hemibiotrophic pathogen Pseudomonas syringae pv. tomato DC3000. Together, our results indicate that LHP1 regulates the expression of stress-responsive genes as well as the homeostasis and responses to the stress hormones SA and ABA. This protein emerges as a key chromatin player fine tuning the complex balance between developmental and stress-responsive processes.