Efeito do óxido nítrico no fluxo de auxina e no estresse oxidativo em raízes de Triticum aestivum em resposta ao alumínio

Abstract

Aluminum is an element widely distributed in soils, in which the species Al3+ is considered one of the most detrimental to plant growth and development. The effect of Al3+ on indole-3-acetic acid (IAA) flow and the role of nitric oxide (NO) as a signaling molecule in plant responses to abiotic stresses are documented. This study has focused on the investigation of a possible relationship among NO production, IAA flow and the efficiency of the enzymatic antioxidant system during the response of Triticum aestivum cvs. BH1146 (Al3+-tolerant) and Anahuac (Al3+-sensitive). The NO basal levels were higher in Al3+-tolerant plants than those of Al3+-sensitive ones, being found mainly in cells of vascular cylinder. The treatment with 75 µmol L-1 Al3+ for 48 h stimulated NO production in root cells of both cultivars with Al3+-tolerant plants accumulating much higher amounts of this free radical, detected mainly in epidermal and endodermal cells. Exposure of roots for 24 h to 300 µmol L-1 Snitrosoglutathione (GSNO; an NO donor), NO3- or L-arginine (L-arg) (both substrates for NO synthesis) decreased Al3+ accumulation particularly in the cytosol, nuclei and cell membranes in both cultivars. Regardless of the stress, Al3+-tolerant plants did not exhibit a significant number of dead cells. The stress, however, intensified cell death in roots of Al3+-tolerant plantlets, while the pre-treatments with GSNO, NO3- or L-arg attenuated this event in such cultivar. These same pre-treatments decreased superoxide anion levels in roots of both cultivars in response to Al3+. The Al3+ stress diminished lipid peroxide levels more intensively in roots of Al3+-tolerant plantlets. Such levels further decreased in roots of GSNO-, NO3-- or L-arg-treated plantlets upon Al3+ stress. The superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities, but not the levels of lignin or oxidized protein, were stimulated in roots of Al3+-tolerant plantlets upon stress. Additionally, GSNO, NO3- or Larg affected SOD, CAT and APX activities in both cultivars upon Al3+ stress, at different extents. Under physiological conditions, IAA flow in roots was more intense in Al3+-tolerant plantlets. The Al3+ stress reduced IAA flow in roots of both cultivars while the pre-treatment with GSNO, NO3- or L-arg restored IAA flow to the normal levels, regardless the cultivar. In addition, GSNO was found to suppress the effect of a known inhibitor of IAA eflux by stimulating the distribution of such auxin on cells of protoderm, promeristem, procambium and cortex in roots of boot cultivars. Overall, substrates for NO synthesis and an NO donor were shown to contribute for the response of Triticum aestivum roots to Al3+ by reestablishing the IAA flow and controlling the oxidative burst in such organs.