The Role of HKT Transporters in Salinity Tolerance of Tomato

Salinity stress is the most widespread and the most severe abiotic stress that plants face. More than 800 million hectares worldwide, representing more than 6% of world's land area, are affected by salinity. Tomato is one of the most important horticultural crops. However, due to the progressive salinization of irrigated land, areas for optimal growth of tomato are being reduced all over the world. To overcome this problem, many attempts have been made to increase the salinity tolerance of tomato using wild tomato species, as these species are a useful source of genes involved in salinity tolerance that can be transferred to cultivated tomato lines. In order to identify tomato accessions to be used in breeding programmes to develop salinity tolerant tomato lines, in Chapter 2, we treated 93 tomato accessions with NaCl and we measured their Na + and K + tissue concentrations. Our results showed a great variation in ion accumulation between all accessions tested. From the data collected in this Chapter, we selected a group of 24 accessions showing either a high or a low accumulation of Na + in the leaves. This group of accessions was used in a more in depth analysis of salinity tolerance performed in Chapter 3. The experiments reported in Chapter 3 aimed to find a trait or gene(s) that breeders could use to select for in new breeding programs. In this analysis, we hoped to couple gene expression to variation in levels of ions and organic molecules. In this Chapter, we analysed the expression of several genes directly or indirectly involved in Na + homeostasis in planta, such as HKT, SOS, NHX, LHA and AVP. We also analysed the expression of P5CS, a key enzyme in proline biosynthesis. Our results showed that Na + , K + , Cl-, proline and sucrose concentrations did not correlate with salt sensitivity or tolerance. Nevertheless, several significant correlations between the expression of genes and Na + accumulation were observed. For instance, both Na + concentrations in the leaves and stems were positively correlated with SlHKT1;2 expression in the roots, and Na + concentration measured in the roots correlated positively with SlHKT1;1 expression also in the roots. These results suggest that Na + exclusion or inclusion and tissue tolerance evolved independently in tomato plants. As a