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-bisphosph atase (EC. 3.1.3.11) isolated from salt-sensitive and salt-tolerant rice (O ryza sativa) varieties. Cytosolic and chloroplastic isoforms of the enzyme from salt-sensitive rice seedlings exhibited decreased catalytic activity d uring growth in the presence of salt. Furthermore, chloroplastic fructose 1 ,6-bisphosphatase purified from salt-sensitive (O, sativa,a cv, IR26) and f rom the wild halophytic rice Porteresia coarctata differed in their in vitr o salt tolerance property although they exhibited otherwise identical bioch emical and immunological properties. This decline in enzyme activity was no t correlated with de novo synthesis of the chloroplastic fructose-1,6-bisph osphatase protein in the presence of salt. The inhibitory effect of increas ing concentration of NaCl on in vitro enzymatic activity could be prevented by preincubation of the enzyme with a number of osmolytes with an effectiv eness in the order polyol > sugars. Further, the intrinsic tryptophan fluor escence of the purified rice enzyme is altered in vitro with increasing NaC l concentration which could be prevented by preincubation with inositol. Pu rified chloroplastic fructose-1,6-bisphosphatase from P. coarctata however, exhibits no such inhibition of enzyme activity in vitro or alteration in t ryptophan fluorescence with increasing NaCl concentration. (C) 2001 Elsevie r Science Ireland Ltd. All lights reserved.