Regulation of the expression of ferredoxin-glutamate synthase in barley

We have investigated the regulation of ferredoxin-glutamate synthase (Fd-GOGAT) in leaves of barley (Hordeum vulgare L. cv. Maris Mink) at the mRNA, protein and enzyme activity levels. Studies of the changes in Fd-GOGAT during plant development showed that the activity in shoots increases rapidly after germination to reach a maximum (on a fresh-weight basis) at day 10 and then declines markedly to less than 50% of the maximal activity by day 30, this decline being correlated with an equivalent loss of Fd-GOGAT protein. Growing the plants in darkness reduced the maximum activity attained in the shoots, but did not affect the overall pattern of the changes or their timing. The activity of Fd-GOGAT increased two- to three-fold within 48 h when etiolated leaves were exposed to light, and Northern blots indicated that the induction occurred at the mRNA level. However, whilst a carbon source could at least partially substitute for light in the induction of nitrate reductase activity, no induction of Fd-GOGAT activity was seen when etiolated leaves were treated with either sucrose or glucose. Interestingly, the levels of Fd-GOGAT mRNA and activity remained high up to a period of 16 h or 72 h darkness, respectively. Compared with plants grown in N-free medium, light-grown plants supplied with nitrate had almost two-fold higher Fd-GOGAT activities and increased Fd-GOGAT mRNA levels, but nitrate had no effect on the abundance of the enzyme or its mRNA in etiolated plants, indicating that light is required for nitrate induction of barley Fd-GOGAT.