Estudo de mecanismos de regulação do sódio citoplasmático em cultivares de Vigna unguiculata (L.) WALP. submetidas à salinidade

Abstract

In the Northeast of Brazil, there are some salinized areas and others in the process of salinization, what drastically reduces plant productivity. Many studies have been done regarding the effect of saline stress on plant growth and development. However, there is little knowledge about the mechanisms of acclimatization to salinity in the different plant organs, particularly in the maintenance mechanisms of sodium ion homeostasis in Vigna unguiculata, involving: enzymes, channels and transporters. For this, two cultivars with different degrees of susceptibility to saline stress were used: Pitiúba (tolerant) and Setentão (sensitive). The objective of the present work was to understand the interrelationships between the H+-ATPases and the SOS1 transporter, as well as the interrelationships between the vacuolar NHX transporter and the vacuolar proton pumps and their roles on regulating the Na+ cytoplasmic concentration in Vigna unguiculata, in response to saline stress. For this, morphological parameters were evaluated: roots, stems and leaves length, root, stems and leaves dry mass, ratio Root/Aerial, and biochemical parameters: proline, malondialdehyde, Na+ and K+ ions and the ratio Na+/K+, at 5 different concentrations (0, 25, 50, 75 and 100 mM) of NaCl. Molecular parameters in the two cultivars Pitiúba and Setentão were also analyzed in plants with 10 days after sowing, submitted to NaCl (100 mM) for 120 hours. SOS1, H+-ATPase type P and type V, pyrophosphatases and NHX encoding genes were identified through searches in Vigna unguiculata databases. The gene expression was evaluated in plants with 10 days after sowing, submitted to 100 mM NaCl, at the times: 0, 6, 12, 24 and 48 hours after stress application, using PCR technique in real-time (qPCR). The entirely evaluation of the data allowed to establish the differences in the form of acclimatization to the saline stress between the two cultivars. The parameters such as visual aspects, dry mass, leaf area and Na+/K+ ratio gave support for Pitiúba to be classified as a salt tolerant cultivar in relation to the Setentão. The parameters, growth and percentage of water were not conclusive. The Pitiúba cultivar, presented a constitutive proline concentration in the leaves greater than the Setentão cultivar and being, therefore, better pre-acclimatized to tolerate saline stress. The highest lipid peroxidation in the leaves was observed in Pitiúba. The database searches revealed that V. unguiculata presents at least 1 SOS1 gene, 13 H+-ATPase type P genes, 7 NHX genes, 2 PPase genes and 1 V-ATPase subA gene. Of these genes, SOS1, H+-ATPase 06 and 48, NHX1, NHX2, PPase1 and V-ATPase subA were induced in response to saline stress in a differentiated way in relation to organ and cultivar analyzed. From these data it was possible to infer that the exclusion of Na+ in Pitiúba occurs mainly in the roots, while in the Setentão it occurs more intensely in the leaves. Vacuolar compartmentalization of Na+ is higher in the roots of both cultivars. However, while Pitiúba increased expression of NHX1, Setentão increased expression of NHX2. NHX induction was accompanied by the mRNA increase of PPase1 in both cultivars and V-ATPase in Pitiúba. In leaves, the SOS1 gene was most expressed in the Setentão, revealing a co-expression with two H+-ATPase genes (H+-ATPase 06 and 48), whereas NHX1 expression increased only in Pitiúba. In general, the data suggest that the cultivars of Vigna unguiculata (Pitiúba and Setentão) used different strategies to minimize the deleterious effects of saline stress.