Role of xylem sap nitrate in the regulation of nitrate reductase gene expression in leaves of barley (Hordeum vulgareL.) seedlings

The role of xylem sap nitrate in the regulation of nitrate reductase (NR) in leaves was investigated in barley (Hordeum vulgare L.) seedlings. Upon the exposure of barley roots to nitrate at concentrations ranging from 0.1 to 5 mm, rapid accumulation of NADH-specific NR mRNA was observed in both leaves and roots. In the same treatment, the concentration of xylem sap nitrate also rapidly increased during the early feeding period and reached a steady state at concentration ranging from 15 to 50 mm. Patterns of NR mRNA accumulation in leaves were closely linked with the increase in the nitrate concentration in the xylem sap, but not with the amount of nitrate stored in the leaf tissues. High levels of NR mRNA in intact leaves were observed only when the xylem sap nitrate reached mm levels. Supply of different concentrations of nitrate to detached leaves also indicated that mm levels of nitrate supplied through the xylem were required for adequate accumulation of NR mRNA in leaves. When nitrate-grown seedlings were transferred to a nitrate-free medium, a parallel decline in both the concentration of xylem sap nitrate and NR mRNA level was observed, while the nitrate concentrations in leaf tissues remained constant. These results indicate that continuous supply of mm levels of nitrate through the xylem is important for maintaining the high levels of NR mRNA in barley leaves.