Relationships between nitrite reductase activity and genotype-dependent callus growth in rice cell cultures

The nitrite ion content and activity of nitrate reductase and nitrite reductase were examined in scutellum-derived calluses of rice varieties using a modified R2 medium. (medium A) and a medium derived from the modified R2 medium (medium B). In medium A, marked differences were observed in callus growth between the varieties. The calluses of the poor-growth varieties accumulated significantly more nitrite ions during the culture period than did the good-growth varieties. Callus growth rate was negatively correlated with the nitrite ion content, indicating that the calluses of the poor-growth varieties were injured by toxic nitrite ions, which lead to browning and inhibited growth. The calluses of the poor-growth varieties had significantly lower levels of nitrite reductase activity than good-growth varieties, On the other hand, no between-group differences were observed in the nitrate reductase activity. These results indicate that the higher nitrite ion levels observed in the poor-growth varieties resulted from a lower ability to reduce nitrite and that nitrite reductase activity is one of the physiological factors that correlates with differences between varieties in rice cell cultures. In medium B, the calluses of the poor-growth varieties grew as well as the good-growth varieties, bur also had significantly lower levels of nitrite reductase. Nitrate reductase activity was repressed in the calluses of both varieties in medium B compared to culture in medium A. The results suggest that repressed nitrate reductase activity causes the calluses of poor-growth varieties to accumulate only trace amounts of nitrite ions despite lower nitrite reductase activity and as a result, callus growth improved in medium B.