The role of the bifunctional enzyme, fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase, in carbon partitioning during salt stress and salt tolerance in Rice (Oryza sativa L.)

A fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase (F6P2K/F26BPase) gene was found to exhibit different transcript expression levels after 48 h of salt treatment between the salt-tolerant rice line, LPT123-TC171, and its original cultivar, LPT123, by differential RNA display and confirmed by RT-PCR. DNA sequence similarity analysis suggests this is the F6P2K/F26BPase gene located on chromosome 5. During 9 days of salt stress induced by inclusion of 85 mM NaCl in the nutrient solution, the growth of LPT123, but not LPT123-TC171, was inhibited with reduced dry and fresh weights, suggesting the higher salt tolerance of LPT123-TC171. Under these saline conditions, both rice cultivars significantly increased leaf sucrose levels and decreased the proportion of carbon assimilated to starch, but this was significantly more marked in the salt-tolerant LPT123-TC171 cultivar. Under normal culture conditions F6P2K/F26BPase transcripts could not be detected in LPT123 leaves, but were readily detected in LPT123-TC171. Salt stress for 72 h induced and up-regulated F6P2K/F26BPase expression levels in LPT123 and LPT123-TC171 cultivars, respectively, with higher levels of F6P2K/F26BPase transcripts being found in LPT123-TC171. Correspondingly, an increase in F6P2K and F26BPase enzymatic activities were observed in LPT123, but in LPT123-TC171, only a significant increase in F26BPase activity was seen, leading to a significant reduction in the F6P2K/F26BPase activity ratio after 9 days of salt treatment in LPT123-TC171. In correlation with the observed F6P2K/F26BPase activity ratios, the average leaf F26BP levels significantly decreased after 9 days salt-stress while the sucrose levels increased. These data support the role of a decrease in F26BP levels leading to an increased partition of carbon to sucrose, and suggest that this may contribute to salt-stress tolerance in rice (Oryza sativa L.).