Salt acclimation and ionic homeostasis are associated with priming and acquired memory in rice plants

Abstract

This thesis aimed to test the hypotheses that the pre-exposure of rice plants to eustressors in the early stages of life can generate stress memory. This memory would be responsible for promoting more efficient metabolic responses under salt conditions and to achieve an acclimation state faster. Thus, the first part of the thesis consists of a contextualization on the importance of studies on soil salinity and rice production for food security. Then, a brief review of concepts of great relevance in the context of research, such as acclimation and stress of plants, as well as the concepts of eustress and priming and their relevance in the scientific environment. Subsequently, we started the experimental part, seeking to identify salinity dose capable of promoting eustressor effects in rice plants. For this, we conducted an experiment with different salinity doses (low - 16 mM NaCl; moderate - 25 mM; and high - 50 mM) applied in exposure-recovery cycles to identify possible differences in the responses due to stress repetition, as well as the spatial effect of ionic accumulation on tissues. The results indicate that saline doses from low to moderate, when applied in the vegetative phases, present potential for stress memory formation. So, the exposed plants have morphological alterations in relation to the control plants, yet without severely compromising crop productivity. From the results of this first experiment, a second experiment was carried out using moderate salinity (25 mM NaCl for 7 days) as priming agent at different stages of plant development to identify the effect of application time on metabolic and morphological responses. The tested treatments were priming during germination and during traditional transplanting moment (V6) followed by saline stress (50 mM NaCl), imitating the actual condition of exposure of seedlings after transplanting. As a result, we obtained that the seed priming (SPS) presented high beneficial potential for the crop. Despite reduction in leaf length, there was an increase in dry biomass production and tillering compared to the control treatment. On the other hand, the priming treatment applied at the time of transplanting (VPS) showed a strong distressor effect, with lower parameters than those obtained by the salt stress treatment alone (S). In addition, it was possible to visually identify the premature necrosis of the flag leaf tissues, although they did not present toxic ions' accumulation. Together, the results of these studies show the great phenotypic plasticity of rice, presenting differential responses in each tissue and adapting according to the moment of exposure to the stressful factor, stress intensity, acclimation state. More studies are needed related to the epigenetic part to unravel the mechanisms behind this plasticity, considering the space-time scale of these organisms with such complex and dynamic behavior.