Morphological responses of two rice genotypes to inorganic arsenic and the protective role of the synthetic strigolactone analogue GR24

The toxic semimetal arsenic (As) can be present in soils at high concentrations due to both anthropogenic activities and natural events. In polluted paddy fields, it is easily absorbed by rice (Oryza sativa L.) roots and accumulates in these organs, altering their development and hormonal homeostasis (1). Compared with other cereals, rice is more efficient in As uptake by roots and translocation to the shoot, thus becoming the main food source of As exposure for humans. Plants have endogenous defence mechanisms that include physical barriers, such as cell wall thickening, and chemical strategies, including the production of secondary metabolites and phytohormones, to cope with environmental stress. Among the various phytohormones, an emerging group is represented by strigolactones (SLs), terpenoid lactones derived from carotenoids. These active biomolecules are synthesized mainly by plant root systems in very low amounts. Strigolactones play a key role in various plant biological functions through cross-talk with other phytohormones. They are involved not only in developmental processes but also in responses to abiotic stresses (2). However, the mechanisms by which they mitigate these types of stresses in plants, especially when supplied exogenously, remain unclear. The aim of this research is to investigate whether SLs alleviate the harmful effects of inorganic As (iAs) forms in rice plants. To achieve this goal, seeds of two Oryza sativa genotypes, Carnaroli and Zhonghua 11, were sown in vitro in the presence or absence of iAs (25 μM NaAsO2 or 100 μM Na2HAsO4.7H2O), with or without four concentrations of GR24 (1.25, 2.5, 5, and 10 μM), a synthetic analogue of SLs widely used in experimental research. At the end of the culture period, rice seedlings were collected and analysed at the morphological level. The results show that Carnaroli is less sensitive to iAs than the Zhonghua 11 genotype. In both genotypes, GR24 did not negatively affect seedling development when supplied alone; instead, it showed a stimulatory effect on shoot, primary root (PR), and adventitious root (AR) elongation in Carnaroli. Additionally, when GR24 was combined with iAs at specific concentrations, the negative effects of the metalloid were recovered in both genotypes, as evidenced by fresh weight, shoot growth, and lateral root density. These results represent a starting point, and further analyses are required to gain deeper insight into the actual function of exogenous SLs in counteracting the negative effects of iAs in Oryza sativa, potentially leading to the application of these molecules in sustainable agriculture. 1) D. Piacentini, F. Della Rovere, F. Lanni, M. Cittadini, M. Palombi, L. Fattorini, V. Cecchetti, M.M. Altamura, G. Falasca (2023) Environ Exp Bot, 209, 105287 2) A. Bhoi, B. Yadu, J. Chandra, S. Keshavkant (2021) Planta, 254(2), 28