Title : Salt tolerance in rice: plant strategies and genetic improvement.
Abstract:
Salinity tolerance is a complex trait and, despite many efforts to obtain rice plants resistant to salt, few results have been achieved and a deeper understanding of the tolerance mechanisms is needed. By studying two Italian rice varieties with contrasting salt response, we demonstrated the involvement of both cell and whole-plant mechanisms in salt tolerance. In cultured cells, the tolerant variety showed higher activity levels of antioxidant enzymes in control conditions and a more consistent increase in these activities after salt treatment. Several genes regulated by salt stress involved in ROS signalling and detoxification showed rapid upregulation in tolerant cells. The tolerant variety also exhibited rapid upregulation of K+ transporter genes and ion homeostasis recovery during the stress. In plants, the tolerant variety responded more effectively to osmotic and ionic stress. In roots, an increase in levels of H2O2 was observed as early as 5 minutes after treatment. Consequently, the expression of genes involved in perception, signal transduction and response to salt were induced at earlier times when compared to susceptible plant roots. Transcriptomic analyses supported the set-up of an adaptive program consisting of allocating sodium preferentially to roots, restricting it to the oldest leaves and activating regulatory mechanisms of photosynthesis in new leaves. As a consequence, plants resumed growth even under prolonged salt stress. Overall, our findings show that a tight control on ROS is fundamental in triggering a coordinated response resulting in adaptation instead of senescence in salt treated rice plants.
