Arsenite and arsenate stress differently affect auxin distribution in rice roots and brassinosteroids restore it sustaining root system plasticity

Rice is a worldwide cultivated crop that serves as an important source of food for the human population, but it is also the simplest route for arsenic (As) contamination of the food chain. The As inorganic forms, arsenate [As(V)] and arsenite [As(III)], are the highly toxic As species found in the soil and the most easily absorbed by the roots. The absorption of As(V) prevails in aerobic soils while that of As(III) is favored in anaerobic soils. As(V) is converted to As(III) in the roots, although small amounts of As(V) also remain in the plant organs. The root system is the first target of the action of both As forms. The mechanisms of action of As(V) and As(III) are still widely unknown. Understanding them is essential for selecting rice genotypes with a lower capacity of As uptake and transport to the caryopses, thus improving food safety. Auxin is the phytohormone necessary for the development and plasticity of the root system, and its action is modulated by endogenous/exogenous brassinosteroids (BRs), mainly under stress conditions. The research aim was to deepen the knowledge of the mechanisms triggered by As(III) or As(V) in rice roots with particular attention to the role played by the interaction between auxin transport and BRs. We show that As(III) is the main As species present in rice roots regardless of the As(III) or As(V) forms supplied to the growing medium. Arsenic alters auxin distribution in both adventitious and lateral roots, but strongly in the latter ones. The application of an exogenous BR, the 24-epibrassinolide (eBL), combined with As(III) or As(V) strongly increases the expression of the OsPIN2 and OsAUX1 genes involved in auxin transport, thus contributing to restore the correct auxin distribution altered by As, and mainly by As(III), with higher effects on the LRs. Moreover, eBL increases the antioxidant activity in the roots in the presence of As, but only when combined with As(V).