Identification and characterization of a salt-stress-induced plasma membrane H+-ATPase in tomato

Plant plasma membrane H+-ATPases (PM H+-ATPases) energize the secondary transport of Na+ from the cytosol across the plasma membrane and so may play a role in the plant response to salt stress. A PM HI-ATPase gene in rice is closely linked to a locus responsible for increased salt resistance. PM HI-ATPases are encoded by large gene families, including 10-12 in tomato (Lycopersicon esculentum). Salt stress stimulates the accumulation of PM ATPase transcripts in a variety of plants but the effect of salt stress on the expression of specific isoforms has not been investigated. We isolated a partial-length cDNA clone of a novel tomato PM H+-ATPase gene from salt-stressed expanded leaf tissue and characterized its expression in response to salt and osmotic stress. The gene, LHA8, is a member of the subfamily including AHA1, AHA2, AHA3, AHA4, AHA9, LHA4, PHA2 and PMA4, and is most closely related to the Nicotiana plumbaginifolia gene PMA6. LHA8 transcript accumulation is induced by NaCl exposure. LHA8 is not expressed at detectable levels in roots or expanded leaves and is present at very low levels in unexpanded leaves. LHA8 expression is induced in expanded leaves, unexpanded leaves and roots. Induction appears to be specific to the ionic, rather than the osmotic, effects of NaCl because iso-osmotic levels of polyethylene glycol do not induce message accumulation.