Preliminary studies on the involvement of biosynthesis of cysteine and glutathione concentration in the resistance to B toxicity in sunflower plants

Boron (B) is an essential plant micronutrient which can be phytotoxic to plants when present in soils in high concentrations. However, despite its agronomic importance, B toxicity is poorly understood. In the present work, we study the response of the biosynthesis of cysteine and glutathione concentration under conditions B toxicity (500 mM H3BO3), given that this biosynthetic pathway is essential in the resistance and adaptability of plants to different types of stress. Our results show that B toxicity does not affect cysteine synthesis, with values of the enzymatic activity of serine acetyl transferase and of O -acetylserine(thiol)lyase being similar to that of control plants grown without B toxicity. On the contrary, the enzymatic activities of g-glutamyl-cysteine synthetase and glutathione synthetase were significantly inhibited by B toxicity, with the low foliar concentrations of glutathione causing phytotoxicity of this micronutrient in the plants. The data presented in the present work are novel in being the first to demonstrate that B toxicity impides the conversion of cysteine to glutathione. Here, in addition, we show that external the application of cysteine, and especially of glutathione, significantly reduces B toxicity, these plants reflecting a foliar biomass similar to that of control. # 2003 Elsevier Ireland Ltd. All rights reserved.