ROOT SYSTEM PLASTICITY IS ESSENTIAL TO OVERCOME AS TOXICITY IN RICE: THE ROLE OF BRASSINOSTEROIDS

Rice (Oryza sativa L.) is a staple food for approximately half of the global population but is frequently cultivated in polluted environments, posing a risk to human health due to potential toxic substance accumulation. Arsenic (As) is a prevalent contaminant in rice paddies, with inorganic forms, particularly trivalent arsenic [As(III)] and pentavalent arsenic [As(V)], being easily absorbed by the roots and transported through the xylem to aerial parts. The root system of rice, consisting of embryonic and post-embryonic adventitious roots and their laterals, is the first organ affected by arsenic in the environment. Brassinosteroids (BRs), steroidal phytohormones, regulate numerous physiological and metabolic processes, including root organogenesis. Recently, BRs gained scientific interest for their involvement in plant stress responses. However, their mechanisms in increasing plant tolerance to stress, particularly arsenic stress in rice, remain elusive. This study explored the impact of the most biologically active form of BRs, 24-epibrassinolide (eBL), on the root system architecture of rice seedlings grown in vitro under As(V) or As(III) toxicity. Effects were analyzed at morphological and cyto-histological levels. The study also investigated the impact of eBL on the content and biosynthesis of nitric oxide (NO), a crucial reactive nitrogen species involved in plant stress response transduction. Results revealed that applying exogenous eBL at a specific concentration stimulated lateral root (LR) formation in the presence of As(III)/As(V) and reduced lignin thickening in the cell walls induced by As in the outermost cortical layers of LRs and adventitious roots (ARs). Moreover, eBL restored disrupted NO levels caused by As(V) by increasing the expression of OsNOS1 and facilitating cellular NO distribution. These findings shed light on how BRs can protect plants from stress induced by toxic elements like arsenic, offering potential applications of this phytohormone in cultivating important crops like rice, which are often affected by arsenic contamination.