Long-term somatic memory of salinity unveiled from physiological, biochemical and epigenetic responses in two contrasting rice genotypes

Plants are constantly exposed to environmental fluctuations, that may occur in a single day or over longer periods. In many cases, abiotic stresses are transient and recurrent, impacting how plants respond in subsequent adverse conditions. Adaptation mechanisms may occur at the physiological, biochemical and molecular level, modifying transcriptional response, regulatory proteins, epigenetic marks or metabolites. Here, we aimed to uncover the different strategies that rice uses to respond to recurrent stress. We tested varieties with contrasting behavior towards salinity (tolerance or sensitivity) and imposed salt stress (150 mM NaCl) during 48 h at vegetative and/or reproductive stages. After 48 h of stress in reproductive stage, leaves and roots were harvested separately or otherwise the plants were submitted to a 24 h recovery, prior to sample harvesting. Plants submitted to a recurrent stress responded differently from those suffering a single stress event. In the case of the sensitive genotype, recurrent stress led to lower Na/K ratio in roots and lower hydrogen peroxide accumulation and lipid peroxidation in leaves, but maintenance of global DNA methylation levels. In the tolerant genotype, recurrent stress did neither affect the Na/K ratio nor the stomatal conductance, although the levels of superoxide anion and hydrogen peroxide accumulation were lower, as also observed for global levels of DNA methylation. Our work shows that a short pre-exposure to salt stress may improve rice tolerance to subsequent stress, trough biochemical, physiological and epigenetic processes, with more significant changes visible in the tolerant genotype.