Chloroplast fructose-1,6-bisphosphatase from Oryza differs in salt tolerance property from the Porteresia enzyme and is protected by osmolytes.

Salinity exerted a distinctly differential effect on fructose-1,6-bisphosphatase (EC. 3.1.3.11) isolated from salt-sensitive and salt-tolerant rice (Oryza sativa) varieties. Cytosolic and chloroplastic isoforms of the enzyme from salt-sensitive rice seedlings exhibited decreased catalytic activity during growth in the presence of salt. Furthermore, chloroplastic fructose 1,6-bisphosphatase purified from salt-sensitive (O. sativa cv. IR26) and from the wild halophytic rice Porteresia coarctata differed in their in vitro salt tolerance property although they exhibited otherwise identical biochemical and immunological properties. This decline in enzyme activity was not correlated with de novo synthesis of the chloroplastic fructose-1,6-bisphosphatase protein in the presence of salt. The inhibitory effect of increasing concentration of NaCl on in vitro enzymatic activity could be prevented by preincubation of the enzyme with a number of osmolytes with an effectiveness in the order polyol>sugars. Further, the intrinsic tryptophan fluorescence of the purified rice enzyme is altered in vitro with increasing NaCl concentration which could be prevented by preincubation with inositol. Purified chloroplastic fructose-1.6-bisphosphatase from P. coarctata however, exhibits no such inhibition of enzyme activity in vitro or alteration in tryptophan fluorescence with increasing NaCl concentration.