Discussion

In the present study, the salt concentrations (0.5% under submerged conditions or 1.0% on paper towel) maintained inhibited growth of seedlings. It was evident that root growth was more inhibitory to the effects of salt.

When adenylate kinase activity was measured in the japonica variety, root tissues exhibited an enhanced activity while shoot tissues did not do so. These observations were further confirmed by Western blot. Increased activity of the enzyme was not confined to a specific region of roots. Results from tissue print analysis suggested that though adeny-late kinase is mainly localized to vascular tissues and other peripheral tissues under saline conditions, induced expression of the protein can occur even in cortical tissues.

Adenylate kinase is found in several subcellular localizations. In rice, localization of the enzyme at tissue and cellular levels has been done previously .9 It is distributed mainly to the cytosol fraction. Therefore, in the absence of chloroplastic fractions, the increased enzyme activity in root tissues, shown here in several experiments, can possibly be attributed to the increase in cytosol fraction. However, no remark can be addressed to the observed difference between roots and shoots in their adenylate kinase activity, though root growth was shown to be more sensitive to salinity. It remains to be known whether there is any correlation between high sensitivity of root growth and increased response of adeny-late kinase in roots to salinity stress.

In order to see whether there is any varietal difference for the relationship between salt stress and adenylate kinase activity, a salt tolerant variety and a salt sensitive variety were used. Induction of enzyme activity was seen in the salt sensitive variety. This may imply that in salt sensitive rice plants adenylate kinase may play some role in adenylate homeostasis in response to salt stress. The response of the adenylate kinase level to salinity stress is an interesting phenomenon.

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