Análise fisiológica e proteômica de plantas de sorgo sob estresse salino e nutridas com diferentes fontes de nitrogênio

Abstract

Salinity is an abiotic stress that causes significant damages to several crops yield around the world, especially in arid and semiarid regions, like the Brazilian Northeast. In this context, a search for cultivation strategies and the elucidation of the mechanisms of tolerance to that stress have become more and more important for agricultural production in saline soils. Lately, it has been shown that ammonium (NH4+) provided as nitrogen source activates pivotal mechanisms to the acclimation of sorghum plants to salt stress. In this work a detailed investigation in the proteome of forage sorghum (Sorghum bicolor L. Moench) genotype CSF20 under salt stress and fed with either nitrate (NO3-) or NH4+ was performed, aiming to identify salt-responsive proteins and correlate them with tolerance mechanisms to the stress. The presentation of this work was divided in two chapters. The first one, chapter I, exhibits a review with the state-of-the-art of the main subjects addressed throughout the study. In chapter II, in the form of article, growth data, physiological responses and the proteomic study of leaves of sorghum plants are presented. The results clearly demonstrated that NH4+-fed sorghum plants displayed better performance under salt stress than those ones supplied with NO3-, given higher biomass values, such as leaf area and root and shoot dry mass. The greater sensitivity of nitrate-grown plants was associated with massive decreases in roots and shoots dry masses, excessive Na+ accumulation and low K+/Na+ ratio in tissues. The proteomic study revealed that a large number of proteins are differentially modulated by salt stress, and this regulation is highly influenced by the nitrogen source. In general, 115 proteins underwent some sort of alteration in reponse to the stress, like increase or decrease in the abundance, repression or de novo synthesis. Under salt stress, NO3--fed plants showed regulation in 67 proteins being 28 positively regulated, 23 negatively regulated, 6 repressed and 10 de novo synthesized. The identification of these proteins by LC-ESI-MS-MS revealed that they are involved mainly in carbon/photosynthetic metabolism (52%), energetic metabolism (21%), response to stress (9%) antioxidant defense system (5%) and other cellular processes (13%). In contrast, NH4+-grown plants displayed alteration in the expression of 53 proteins, being 25 positively salt-regulated, 10 negatively regulated, 4 suppressed and 14 de novo synthesized. Out of this total of proteins, 42% are associated with carbon/photosynthetic metabolism, 28% with energetic metabolism, 13% with response to stress, 2% with the antioxidant defense system and 15% other cellular processes. The comparison of the protein profile of plants under salt stress (NO3- salt × NH4+ salt) showed that 35 proteins are differentially expressed in leaves, being 13 with greater abundance in tissues of NH4+-fed plants, 11 in those grown with NO3-, 8 NH4+-growth exclusive and 3 NO3- -treated specific. Data suggest that, under salinity, the better performance of NH4+ is related not only to the expression of stress-responsive proteins, but also to the activation of a more efficient energetic metabolism, which provides energy to defense mechanisms of those plants. Such results allow the maintenance of the photosynthetic apparatus efficiency and are crucial for the greater tolerance to salt stress. These findings give signs of how proteome modulation can result in a better acclimation to salt stress. The results offer new perspectives to the development of biotechnological strategies in order to acquire more salt-tolerant genotypes.