Xylem ion loading and its implications for plant abiotic stress tolerance

Plant adaptive potential is critically dependent on efficient communication and coordination of resource allocation and signalling between above and below-ground plant parts. Control of xylem ion loading plays an important role in this process. This review focuses on the molecular identity, tissue-specific expression patterns, and transcriptional and post-translational regulation of transporters mediating xylem loading of Na+, K+, and Cl- in plants grown under abiotic stress conditions such as drought, salinity, and soil flooding. The data are discussed in the context of breeding crops for stress resilience, which remains one of the highest priorities for dealing with the global food security challenge. This resilience was present in wild ancestors, but has been lost during domestication of crop species and exacerbated by the selection for higher yielding cultivars over the last 100 years. Thus, progress in the field requires a major rethink of current paradigms in crop breeding and the targeting of previously unexplored genes and traits. We argue that control of xylem ion loading is one of these traits and represents an unexplored opportunity for genetic improvement of plant germplasms.