Salinity tolerance in wheat: rethinking the targets.

Wheat is a major staple food in the human diet, but its production under current climate scenarios is problematic given the predicted extent of land salinization and the fact that wheat is highly sensitive to soil salinity. This work aims to critically assess previous breeding efforts and the pros and cons of targeting Salt Overly Sensitive 1 (SOS1) and High-affinity K+  Transporter 1 (HKT1) genes to improve salinity stress tolerance in wheat. We argue that overexpressing SOS1 genes encoding Na+/H+ exchangers for Na+ removal from root to the rhizosphere may come with the caveat of increased loading of Na+ into the xylem and its delivery to the shoot, as well as numerous pleiotropic effects. Similarly, targeting HKT1 transporters for removing Na+ from the shoot comes with significant yield penalties due to the high carbon cost of osmotic adjustment; this strategy is also limited by the relatively small capacity of the root to store excessive Na+ without experiencing toxicity symptoms. We suggest that targeting tissue tolerance traits such as K+ retention in mesophyll and vacuolar Na+ sequestration in the shoot will be able to deliver better outcomes. We also call for a better understanding of the structure-function relationships of various isoforms of key proteins involved in maintenance of Na+ and K+ homeostasis and a need for more in-depth physiological studies of wheat species with the DD genome, a key contributor to tissue tolerance traits. Our arguments are supported by a bioinformatic analysis of the number of orthologs for some key genes between hexaploid (AABBDD) and tetraploid (AABB) wheats and their structural differences.