Transmembrane transporters and salt tolerance in temperate japonica rice

Several investigations aimed at identifying molecular tools useful for the selection and/or the constitution of high-yield salt tolerant rice have been successfully carried out, concerning in particular indica and/ or tropical rice genotypes. Te global warming process is nowadays determining the intrusion of saline wedge into coastal fresh-water streams, and the soil salt concentration of many European rice areas, where temperate rice cultivars are mainly grown, is more and more increasing. In order to identify molecular markers and/or new loci related to salt tolerance, a Genome Wide Association Study (GWAS) has been carried out using a panel of 277 japonica rice accessions. Te panel has been subjected to Genotyping By Sequencing and phenotyping concerning tolerance to a mild-salt stress soil condition (5 dS m-1) expressed at the 4th-5th leaf developmental stage using the Standard Evaluation Score (SES) proposed by IRRI. On the basis of GWAS, a QTL including a few genes that in the indica rice genome are localized within the major salinity tolerance-related QTL ‘SalTol’ have been identified. Among them, the Os01g0337500 gene encoding the vacuolar H+-pyrophosphatase 6 (OsOVP6) is present. Since the role of the OsOVP6 activity is considered central in regulating the cellular Na+ homeostasis in both roots and leaves, investigations comparing some elements of the complex mechanisms involved in this process have been carried out. A physiological approach evaluating this possibility has been conducted in two japonica rice varieties (Galileo and Virgo) that resulted salt-tolerant, in one japonica rice variety (PL12) known to be quite salt-susceptible, and in the salt-tolerant indica inbred genotype FL478 (containing the ‘SalTol’ QTL) as reference genotype. Te root and shoot Na+/K+ ratio, Na+ influx and K+ efflux, H+ extrusion activity, cytosolic and vacuolar pH by in vivo 31P-NMR techniques were evaluated in roots of the four rice genotypes. Te results obtained, together with the electrophysiological evaluation of the whole root Na+ conductance, allow to define a picture that may explain the different salt tolerance observed among the rice genotypes analyzed. As a whole, these results confirm the interest towards a deep allele mining analysis, concerning OsOVP6, within the most significant members of the japonica rice accession panel under investigation.