Can the transcriptional regulation of NHX1, SOS1 and HKT1 genes handle the response of two pomegranate cultivars to moderate salt stress? Salt-tolerance of two pomegranate cultivars

Molecular mechanisms underlying plant functioning under salt conditions have not been completely elucidated, especially in a recalcitrant and less studied fruit trees such as pomegranate (Punica granatum L.). Here, we identified and characterized the expression of NHX1, HKT1 and SOS1 to understand their role in mediating Na+ and K+ transport, translocation and intracellular compartmentation in two pomegranate cultivars (Wonderful and Parfianka) during the first hours of a moderate salt stress (100 mM). In Wonderful, salt treatment significantly increased the Na+ content only in mature leaves (ML) at 3 h after the beginning of the irrigation (2-fold higher than controls), however a concomitant decrease of K+ content was observed (-33%). A significant decrease of NHX1 and SOS1 levels was observed in ML of Wonderful starting from 10 h. Salt irrigation significantly increased expression levels of these genes at all time points in young leaves of Wonderful (YL; with the exception of NHX1 at 24 h) and led to a 7-fold induction of HKT1 in roots. In Parfianka, salt treatment did not affect the Na+ content, irrespective of leaf age. A significant increase of K+ content was observed only in ML at 3 h (+46%). However, NHX1 gene expression was downregulated at the same time in ML of Parfianka, while it was upregulated in YL. An opposite trend was observed in relation to SOS1 expression. Our finding reinforces the idea that difference between cultivars in ion homeostasis and salt tolerance is associated with transcriptional regulation of NHX1, HKT1 and SOS1 genes, these being members of three major Na+ transporters gene families.