Salt-inducible isoform of plasma membrane H+ATPase gene in rice remains constitutively expressed in natural halophyte, Suaeda maritima

To look into a possible involvement of plasma membrane H+ATPase (PM-H+ATPase, EC 3.6.3.6) in mitigation of physiological disturbances imposed by salt stress, response of the enzyme was studied in two Oryza sativa Indica cultivars, salt-tolerant Lunishri and non-tolerant Badami, and a natural halophyte Suaeda maritima after challenge of the young plants with NaCl. Significant increase in activity of the enzyme was observed in response to NaCl in all the test plants with S. maritima showing maximum increase. Protein blot analysis, however, did not show any increase in the amount of the enzyme (protein). RNA blot analysis, on the other hand, revealed significant increase in transcript level of the enzyme upon NaCl treatment. In the rice cultivars, salt treatment also induced expression of a new isoform of PM-H+ATPase gene, not reported so far. The induced transcript showed maximum homology to OSA7 (O. sativa PM-H+ATPase isoform 7). Similar transcript message, however, remained constitutively present in S. maritima, along with the transcript of another isoform of PM-H+ATPase showing resemblance to OSA3 (O. sativa PM-H+ATPase isoform 3). The latter was the only PM-H+ATPase isoform expressed in both the rice cultivars not exposed to NaCl. In the salt-treated test plants, both rice and S. maritima, the salt-inducible PM-H+ATPase isoform resembling OSA7 was expressed in much greater amount than that resembling OSA3. Appearance of a new PM-H+ATPase transcript, besides increase in the enzyme activity, indicates the important role of the enzyme in maintaining ion-homeostasis in plants under salt stress, enabling them to survive under saline conditions.