Ricerc@Sapienza

Oryza sativa L. is a worldwide food-crop frequently growing in cadmium (Cd) polluted soils. High Cd concentrations alter plant development and, in particular, the root-system, both by affecting auxin metabolism and by triggering reactive oxygen/nitrogen species (ROS/RNS), thereby affecting rice yield. In addition, Cd2+ easily enters in the rice root cells through passive transport, reaching the grains after xylem-tophloem transfer, thus becoming a threat to food security. Nitrogen monoxide (nitric oxide – NO) is a ubiquitous gaseous molecule involved in numerous animal and plant physiological processes, and it is also a mediator of plant development and of abiotic/biotic stresses response. Various reports highlight that NO has an important role in alleviating heavy metal toxicity and reducing the oxidative damages in plant organs either by enhancing the activity of antioxidant enzymes or by directly scavenging ROS. On the other hand, heavy metal-induced accumulation of NO was reported to be responsible for heavy metal toxicity. Indeed, NO can act either as a stress-inducing agent or as a protective molecule depending on its concentration, the plant tissue or age, and the type/severity of stress.
At optimal levels, NO interacts with auxins [both indole-3 butyric acid (IBA) and indole-3 acetic acid (IAA)] during root growth and development. An auxin-induced NO production during many plant root responses has been suggested trough the modulation of the activity of enzymes involved in NO biosynthesis, while studies carried out with exogenous application of NO-specific donor compounds (i.e. sodium-nitroprusside, SNP) have demonstrated the involvement of the signal molecule in auxin metabolism, transport and signalling. However, the complex mechanisms underlying the interaction between NO and auxin during the metal stress is still poorly understood and need to be better investigated, together with further elucidations
about the multifaceted role of NO (i.e. as a mitigating or a stressor agent) during Cd toxicity. To this aim, the effects of Cd toxicity on rice root anatomy/morphology and on H2O2 and O2●ˉ production, and the possible recovery by NO, was evaluated after 100μM Cd exposure, combined or not with SNP at 50μM. Moreover, endogenous IAA/IBA contents, transcription-levels of OsYUCCA1 and OsASA2 IAA-biosynthetic-genes, and expression of the IAA-responsive DR5::GUS construct were analysed, and the NO-epifluorescence levels measured.
Our results show that exogenous treatments with the NO-donor SNP increase intracellular root NO levels in in vitro grown rice seedlings not exposed to Cd and restore the NO-levels reduced by the heavy metal. In addition, SNP treatments mitigate both the increase in the HPLC-measured root IAA levels and the alteration of its distribution monitored by the DR5::GUS system due to the toxic metal exposure. Notably, treatments with Cd alone or combined with SNP reduced YUCCA1 expression compared to the Control, while no effects were detected on ASA2, suggesting no involvement of the two IAA biosynthetic genes in the Cd-related increase of the IAA levels detected.
Finally, the enhanced cellular NO-content alleviates the Cd-induced root morphological and histological damages and the root H2O2 and O2●ˉ overproduction. Moreover, exogenous NO decreases the heavy-metal uptake. All together our data highlight the beneficial effects of the NO in alleviating Cd toxicity in rice.