Evaluation of the sensitivity to inorganic arsenic of Carnaroli rice plant (Oryza sativa L.)

Italian rice contributes to more than one-half of the European production, with the provinces of Vercelli, Pavia, Novara, and Milan having the largest number of paddy fields for its cultivation. Carnaroli (Oryza sativa subsp. japonica) is among the most cultivated and marketed Italian rice varieties, exported worldwide thanks to the high nutritional and organoleptic qualities and cooking seal of its grains, as well as good yield. It originated in Lombardy 80 years ago from a breeding between Vialone and Lencino varieties (1). However, it is still an understudied variety in relation to tolerance to environmental stresses, particularly arsenic (As). Arsenic contamination has become a serious environmental issue worldwide and rice is one of the main routes through which As reaches humans. Indeed, rice plants are more efficient at absorbing As than other cereal crops, particularly when As is present in its inorganic forms, arsenite (AsIII) and arsenate (AsV), with AsIII being more toxic than AsV. Inorganic As (iAs) has been classified as Group 1 carcinogens by IARC, and the European Commission has recently set new limits (Commission Regulation EU 2023/465) to reduce its presence in rice-based foods, as a measure to fight cancer and other adverse effects in humans. The flooded cultivation practice promotes the accumulation of iAs in rice roots, altering their development and hormonal homeostasis (2), often leading to phytotoxicity. The As presence in paddy fields points up a global urgent need to select rice varieties able to exclude As from the grains. This research aims to deepen knowledge on AsIII and AsV toxicity on the development of different rice genotypes with special attention to Carnaroli. Rice plantlets were grown in vitro to reduce and control the effect of abiotic factors, and analyzed at the morphological, cytohistological, and chemical levels. The responses of Carnaroli to treatments with iAs, added at toxic concentrations to the culture media, were compared with those of the As-sensitive Zhonghua 11 (3). Our first results show that Carnaroli exhibited low sensitivity to iAs, although its root uptake of As was higher than in Zhonghua 11. Only the treatment with AsIII caused a significant reduction in lateral root (LR) production compared to the Control in Carnaroli, whereas this decrease was significantly more pronounced, and with both the iAs forms, in Zhonghua 11. These results were confirmed by histological analysis of the adventitious roots, which are the main component of rice root system, where anomalies in development and protrusion of their LRs were detected. Further analyses are underway to better understand the different effects of AsIII and AsV on rice plant development, and the possible mechanisms which allow different rice genotypes to tolerate or not As toxicity.