REGULATION OF NITRATE REDUCTASE TRANSCRIPT LEVELS BY GLUTAMINE ACCUMULATING IN THE LEAVES OF A FERREDOXIN-DEPENDENT GLUTAMATE SYNTHASE-DEFICIENT GLUS MUTANT OF ARABIDOPSIS-THALIANA, AND BY GLUTAMINE PROVIDED VIA THE ROOTS

The regulation by glutamine of the leaf transcript level corresponding to the Arabidopsis thaliana (L.) Heynh. nitrate reductase gene nia2 w as examined using a novel approach: we took advantage of the ability o f a ferredoxin-dependent glutamate synthase-deficient gluS mutant of A . thaliana to accumulate glutamine in the leaves when illuminated unde r conditions that favour photorespiration. The accumulation of glutami ne in gluS mutant leaves and the concomitant decline in the leaf gluta mate pool were not correlated with a reduction in the foliar nia2 tran script level. This result indicates that glutamine may not exert a neg ative control of the leaf nia2 transcript pool. The pattern of diurnal nia2 mRNA oscillation did not change upon illumination of the gluS mu tant in air, although the leaf glutamine level remained high during th e diurnal cycle. The amplitude of the diurnal fluctuation in nia2 tran script abundance, therefore, does not seem to depend on the size of th e leaf glutamine pool (which normally fluctuates in opposite phase). T his result also appears to argue against a role of glutamine as an eff ective repressor of nia2 transcript accumulation. The application of a solution containing 100 mM glutamine to the roots of A. thaliana resu lted in an increase in the leaf glutamine level and in a decrease in t he leaf nia2 transcript level. Net CO2 uptake and chlorophyll fluoresc ence quenching by attached leaves of A. thaliana were determined as a control of the physiological status of the plants and remained unaffec ted by the glutamine treatment. However, there was a decrease in the f oliar nitrate level. The negative effect on the nia2 transcript pool e xerted by exogeneous glutamine may, therefore, be explained as a resul t of the down-regulation of nitrate-uptake permeases in the roots by g lutamine.