Conferencia Plenaria

ALVAREZ ME

Salta

Congreso; Conf. Ranwel Caputto-Ciclo 2019- LV Reunión Anual de SAIB y Congreso PABMB; 2019

SAIB

Plants use sophisticated strategies to cope with microbial infections. This includes physical and chemical barriers that restrict a wide range ofinvaders, and a complex surveillance system that detects adapted pathogens that overcome the mentioned defenses. The plant innate immunesystem is based on a large family of pattern recognition receptors (PRR) and nucleotide-binding leucine-rich repeat proteins (NLR) that recognizemicrobial components at the cell surface or the intracellular level, respectively, to then activate defense networks. Over the past years, we havestudied early defense responses induced by stimulation of PRR/NLR in the Arabidopsis?Pseudomonas syringae pv. tomato (Pst) pathosystem.Among them, we were recently interested in the epigenetic control of plant immunity. Our finding that Arabidopsis tissues infected with Pst losebasal pericentromeric heterochromatin compaction led us to evaluate the causes and consequences of such a response. Normally, these regions arekept condensed by the accumulation of repressive epigenetic marks, such as 5-methyl cytosine (5-mC) and dimethylated H3 histone lysine 9(H3K9me2). We found that after infection, 5-mC is reduced at CG, CHG, and CHH sites of centromeric repeats and pericentromeric transposableelements (pTE). This occurs without replication suggesting that active demethylation causes chromatin relaxation. Furthermore, several studiesreported that infection triggers de-repression of TEs with consequent activation of proximal PRR/NLR genes, and genome-wide methylationanalysis described the multiple alterations of DNA methylation that occur in infected cells. However, the impact of DNA methylation on plantimmunity is complex and requires further study. A clear example is limited information that exists on the behavior of pTE, which are the majorityin the genome, under biotic stress. We found that Pst-infection triggers early expression of pTE and their subsequent re-silencing by small RNAs(sRNAs) through RNA-directed DNA methylation (RdDM). Interestingly, these sRNAs also map to PRR/NLR genes that are distal from pTEs.Moreover, Arabidopsis mutants that lose repression of pTEs and accumulate homologous sRNAs constitutively express distal PRR/NLR genes.Our findings indicate that the epigenetic state pTEs affects plant immune responses, and the de-repression of these elements leads to the activationof immune receptor genes by trans-acting mechanisms. This type of regulation has been recently suggested for other pathosystems, and could stillbe replicated in other stress conditions, suggesting new roles of pTEs in adaptation