Response of different rice (Oryza sativa L.) genotypes to inorganic arsenic exposure: Carnaroli vs Zhonghua 11

Rice (Oryza sativa L.) is a staple food and one of the most important grain crops worldwide. Italy is the leading rice producer in Europe, accounting for more than fifty percent of the total production and more than two-hundred varieties. Italian rice production is gathered in the regions of Piedmont and Lombardy. In particular, the province of Vercelli is the largest producer of the Carnaroli variety (O. sativa subsp. japonica) which originated in Lombardy eighty years ago from a breeding between the Vialone and Lencino varieties. The Carnaroli cultivar is still the most cultivated today and is exported all over the world thanks to nutritional and organoleptic properties of its grains. However, this variety has not yet been sufficiently studied in terms of tolerance to arsenic (As) of the plant. Rice is one of the main As exposure food sources for humans. Inorganic arsenic (iAs) is steadily present in paddy fields wherein it is easily absorbed by plant roots and, gathering in these organs, alters their development as well as hormonal homeostasis (1). Paddy rice is more efficient in iAs uptake than other cereals and, although most of this toxic element is sequestered in the root, it also moves up the plant to the edible fruits. The content of iAs in Italian rice varies greatly due to complex interactions between physico-chemical soil properties, agronomic practices, and rice cultivars. (2) Given its toxicity, authoritative organizations such as the European Commission have established strict guidelines and new limits (EU 2023/465) to reduce the iAs presence in rice-based foods, as a measure to counteract its harmful effects in humans. Nowadays, it would be essential to select rice varieties capable of excluding or limiting As accumulation in the grains. The aim of this research is to deepen the knowledge on the tolerance to iAs of different rice genotypes, particularly Carnaroli variety. Seeds of Carnaroli were sown in vitro in presence, or not, of arsenite (AsIII) or arsenate (AsV), added at sublethal concentrations to the culture media. After ten days, rice seedlings were analyzed at the morphological, cytohistological and chemical levels. The responses of this variety were compared with those of the As-sensitive Zhonghua 11 (O. sativa subsp. japonica) (3). The results indicate that the Carnaroli seedlings showed a greater tolerance to iAs, despite their higher root uptake compared to Zhonghua 11. Lateral root (LR) production was significantly reduced in Carnaroli only when treated with AsIII compared to the Control, while this reduction was significantly higher in Zhonghua 11 with both iAs forms. Histological analysis of adventitious roots, the main components of the O. sativa root system, showed abnormalities in LR development, confirming the results of morphological analysis. Further analyses are currently underway to understand in depth the mechanisms that allow the different rice genotypes to cope with the presence of iAs in the substrate and to evaluate the involvement of the shoot in the sequestration and detoxification of this dangerous metalloid.