Tobacco plants that lack expression of functional nitrate reductase in roots show changes in growth rates and metabolite accumulation

When tobacco is provided with a high nitrate supply, only a small amount of the nitrate taken up by the roots is immediately assimilated inside the ro ots, while the majority is transported to the leaves where it is reduced to ammonium. To elucidate the importance of root nitrate assimilation, tobacc o plants have been engineered that showed no detectable nitrate reductase a ctivity in the roots. These plants expressed the nitrate reductase structur al gene nia2 under control of the leaf-specific potato promoter ST-LS1 in t he nitrate reductase-mutant Nia30 of Nicotiana tabacum, Homozygous T2-trans formants grown in sand or hydroponics with 5.1 mM nitrate had approximately 55-70% of wild-type nitrate reductase acivity in leaves, but lacked nitrat e reductase acivity in roots. These plants showed a retarded growth as comp ared with wild-type plants. The activation state of nitrate reductase was u nchanged; however, diurnal variation of nitrate reductase acivity was not a s pronounced as in wild-type plants. The transformants had higher levels of nitrate in the leaves and reduced amounts of glutamine both in leaves and roots, while roots showed higher levels of hexoses (3-fold) and sucrose (10 -fold). It may be concluded that the loss of nitrate reductase acivity in t he roots changes the allocation of reduced nitrogen compounds and sugars in the plant. These plants will be a useful tool for laboratories studying ni trate assimilation acid its interactions with carbon metabolism.