Brassinosteroids interact with nitric oxide in the response of rice root systems to arsenic stress

Brassinosteroids (BRs), an emerging class of phytohormones, affect numerous plant physiological and metabolic processes and can improve plant defense systems to counteract metalloid phytotoxicity. Nitric oxide (NO), a reactive nitrogen species (RNS), behaves as a signalling molecule activating plant cellular responses to various environmental conditions. Brassinosteroids induce NO synthesis through nitrate reductase (NR) and NO synthase (NOS) activities. Arsenite and arsenate, inorganic forms of the metalloid arsenic (As), cause both soil pollution and many disorders in numerous plants, including important crops like rice, due to the oxidative stress generated by the imbalance between RNS and reactive oxygen species (ROS). Rice is very susceptible to As toxicity because both As availability and solubility are high in flooded paddy fields in many cultivated areas. The research aims to investigate the effects of BRs on the rice root systems exposed to 10-4 M Na2HAsO40.7 H2O [As(V)] or 2.5 × 10-5 M NaAsO2 [As(III)], highlighting the induced cyto-histological events and dissecting the NO role in the root response. A specific concentration (10-7 M) of 24-epibrassinolide (24-eBL), an exogenously applied BR, increases lateral root (LR) formation of more than 50% in the presence of As(III) or As(V). In addition, eBL attenuates the thickening of the cell walls induced by As in the outermost root cortical layers of LRs and in the adventitious roots (ARs) by reducing of ⁓ 50% the lignin deposition, while it restores the As(v)-altered NO levels by increasing OsNOS1 expression and the cellular NO distribution.