Bases bioquímicas e moleculares para tolerância ao estresse salino em cultivares de arroz

Abstract

Rice (Oryza sativa) is a grass that stands out for being one of the most consumed cereals in the world. Due to the world population growth and the food demand, any scenario that causes loss of productivity in this crop generates serious economic impacts. Thus, salinity of soils and irrigation water is a limiting factor for rice cultivation, as it impairs many of its physiological processes and negatively affects plant growth and development. However, the extent of this sensitivity is genotype-dependent and some cultivars have mechanisms that allow them to acclimate to salt stress and thus avoid major losses in their production. Thus, the objective of this work was to investigate how salt stress influences the metabolism of different rice cultivars, establishing connections between changes in growth and photosynthetic efficiency and ionic, osmotic and oxidative mechanisms involved in salt stress tolerance and susceptibility. For this, two experimental steps were established. In the first one, growth, gas exchange and ion accumulation were evaluated in five rice cultivars (BRS Pepita, BRS Sertanejo, BRS Esmeralda, BRS São Francisco and BRS Primavera) in order to determine which are more salt stress sensitive or tolerant. The results showed that salinity promoted severe reductions in plant growth, however, a better biomass production was observed in BRS São Francisco, which coincided with higher CO2 assimilation rates and lower Na+ accumulation in this cultivar. Conversely, the cultivar BRS Esmeralda was the one that suffered the most from the damage caused by salinity. After that, the second stage aimed to compare and explain the physiological, biochemical, morphological and molecular responses of the cultivars BRS Esmeralda (more sensitive to salinity) and BRS São Francisco (more tolerant to salinity) when subjected to saline stress. Confirming the findings of step 1, the cultivar BRS São Francisco was efficient in reducing the adverse effects of salinity on growth and gas exchange. In addition, under saline stress, the leaves of cultivar BRS São Francisco presented higher effective quantum efficiency of PSII, higher photochemical extinction coefficient and higher electron transport rate, while the relative excess energy in PSII and the non-photochemical quenching were reduced compared to BRS Esmeralda cultivar. Although salinity promoted a significant increase in Na+ and Cl- levels, mainly in BRS Esmeralda cultivar, the harmful effects were less aggressive in the more tolerant cultivar. Positive regulation of SOS and NHX gene expressions revealed that this cultivar has mechanisms to control Na+ accumulation in cytosol. Reduced photosynthetic capacity and cytotoxic effects of Na+ caused damage to chloroplasts ultrastructure in the sensitive cultivar. The excess of salts also promoted the increase in the levels of H2O2 and thiobarbituric acid reactive substances (TBARS), which were partially attenuated in the cultivar

BRS São Francisco. Nevertheless, the antioxidant enzymes activity and the content of ascorbate and glutathione were dependent on the time of stress exposure, organ and cultivar analyzed, and only the increase in ascorbate peroxidase activity (in roots) seems to have been a mechanism of acclimatation to excess salts used by the cultivar BRS São Francisco. In addition, the synthesis of compatible solutes was also inconsistent, with no correlation between the accumulation of organic solutes (which could contribute to osmotic adjustment), and the tolerance or susceptibility of the cultivar to saline stress. Based on these results, it can be concluded that salt stress tolerance involves multifactorial alterations and that the higher survivability of cultivar BRS São Francisco seems to be more related to Na+ exclusion and compartmentalization and to increased photosynthetic capacity and less to osmotic and oxidative homeostasis.