Biological Roles of NHX vacuolar antiport: achievements and prospects of plant breeding

NHX antiporters are membrane proteins that mediate cation and proton exchanges across vacuolar membranes. In particular, they catalyze the Na+-H+ exchanges and, consequently, sequester Na+ into vacuoles. The latter mechanism protects essential enzymatic reactions in the cytoplasm from toxicity due to excess Na+ levels, while adjusting turgor by means of vacuolar Na+ as an osmoticum. The function of vacuolar NHX antiporters in plants has been studied primarily in the context of salinity tolerance. The expression of induction of NHX-type genes in response to NaCl treatment was observed for most NHX-type genes characterized, and their overexpression allowed to confer an improved tolerance to salinity, initially in transgenic model plants (Arabidopsis thaliana (L.) Heynh. and tobacco) and, subsequently, in cultivated plants (tomato, rice, wheat, etc.). As many genes encoding these antiporters have been cloned from both salt-sensitive and salt-tolerant species and because multiple isoforms can be identified in genomes that have been sequenced, it is likely that these antiporters assume functions at various levels beside providing tolerance to salinity. Indeed, in addition to playing a role in other known functions such as pH regulation, ion homeostasis, and osmotic and turgor regulation, they also have an important role in diverse physiological processes, including control of cell growth and proliferation, vesicle biogenesis and trafficking, protein targeting, and more specifically, leaf development, blue-flower coloration, and grape berry maturation. If we take into account the high probability that vacuolar NHX antiporters play a role in multiple physiological processes, it would be possible, by exploiting the available information, to use several molecular methods, such as overexpression or silencing of NHX-type genes, to achieve the selection and improvement of interesting plant characters, including flower coloration, fruit maturation, and tolerance to salinity and drought.