Nitric oxide and phytohormones interaction in the response of the rice root system to Cadmium or Arsenic

Soil pollution by cadmium (Cd) and arsenic (As) is a worldwide concern due to their toxicity chemical stability. Oryza sativa L. (rice) is a model organism for monocot plants and is also one of the most relevant sources of toxic metals for humans. Changes in the root intracellular content and distribution of phytohormones, such as auxin or brassinosteroids represent a tool through which plants change their architecture in response to the stress. In addition to phytohormones, plants can rely on a plethora of small signal molecules such as nitrogen monoxide (nitric oxide – NO) able to promptly sense and transduce the stress signals, thanks also to their interaction with plant hormones. However, the complex relationship between NO and auxin or brassinosteroid, during the plant response to toxic metals is still poorly understood. The aim of the study was to evaluate if exogenous supplementations with a specific NO-donor (sodium nitroprusside, SNP) compound mitigate Cd or As stress in the root system of rice seedlings grown in vitro, as well as to understand if the molecule interacts with auxin or brassinosteroids during the root stress response. Our results show that exogenous treatments with SNP mitigate the inhibition of the rice root system induced by Cd, but not that induced by As, through an increase of the intracellular NO levels. Moreover, by the use of the OsDR5::GUS system, we show that SNP treatments restore the IAA distribution in the adventitious root apices altered by both the pollutants. In addition, exogenous brassinosteroid treatments strongly increase the root endogenous NO levels, reduced by As, and the transcripts of OsNOS1, a gene involved in NO biosynthesis. However, the brassinosteroid-mediated increase of NO does not mitigate the morpho/histological anomalies induced by the pollutants in the root system.