Nitric oxide and phytohormones interaction in the response of the rice root to toxic metals

To deal with environmental challenges, plants must rely on a sophisticated and tightly regulated defense system to protect cells/tissues/organs in close contact with the stressor. Under stress conditions, changes in the intracellular content and distribution of phytohormones such as auxins and brassinosteroids, are critical in modulating morphogenic responses that eventually lead to adaptation. In addition, increasing evidence indicates that hormones often interact with other endogenous molecules, originating complex signaling cascades which ensure the fast and efficient perception of the stress signal [1]. Among these signaling molecules, the gaseous nitrogen monoxide (nitric oxide – NO) is ubiquitous in plant systems and is involved in a plethora of mechanisms ranging from development to resistance and defense to biotic and abiotic stresses. Depending on its cellular levels, NO protects plants from the stress-induced oxidative damage either by directly reacting with reactive oxygen species (ROS) or by increasing the enzymatic or non-enzymatic antioxidant systems. In addition, by chaining the structure of target proteins through S-nitrosylation, NO can influence their activities and cellular functions, ultimately leading to the plant response [2]. Through the employment of pharmacological and genetic approaches in different plant species, it has been demonstrated that NO and auxins or brassinosteroids strongly interact at multiple, diversified levels during the regulation of plant developmental processes, such as during root organogenesis [1]. However, the complex mechanisms underlying NO action in interacting with auxins or brassinosteroids, during pollutant stress responses are still poorly understood and need to be better investigated, especially in the root system, which is the first plant organ system to be exposed to soil pollution. Cadmium (Cd) and Arsenic (As) are toxic pollutants often found in contaminated soils with an adverse impact on food security by reducing crop yields and by making food unsafe for consumption. Oryza sativa L. (rice), being one of the most consumed food in the world, constitutes an important source of potential Cd or As contamination, since it is often grown in contaminated paddy fields [3]. In this sense, increasing the knowledge of the mechanisms through which plants cope with stress conditions will help to create correct agricultural practices aimed at increasing food security to meet the world’s growing demand for food. The aim of the study was to evaluate if exogenous supplementations with a specific NO-donor (sodium nitroprusside, SNP) mitigate Cadmium (Cd) or Arsenic (As) in the root system of rice seedlings grown in vitro as well as to understand if the molecule interacts with auxins 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. Also, SNP treatments restore the root IAA distribution monitored by the OsDR5::GUS system altered by both the pollutants, highlighting an interaction between NO and auxin. In addition, exogenous brassinosteroid treatments strongly increase the root endogenous NO levels, reduced by As, and the transcript of OsNOS1, a gene involved in NO biosynthesis suggesting that NO acts downstream of the hormone signal. However, the brassinosteroid-mediated increase of NO does not result in a better morpho/histological response of the root system to the pollutant. In animal systems, NO has been shown to influence key aspects of epigenetic regulation that include histone posttranslational modifications, DNA methylation, and microRNA levels, up to being considered the new architect of epigenetic landscapes. There is evidence that NO may be a similar key regulator of epigenetic modifications also in plants [4]. In this sense, further analyses are needed to better evaluate the nature of the NO role in the adaptative response of the root system to Cd and As. The research is supported by Progetti Ateneo Sapienza University of Rome, grant number: RM1221815D29A543