THE USE OF MUTANTS AND TRANSGENIC PLANTS TO STUDY NITRATE ASSIMILATION

The nitrate assimilatory pathway has been the matter of intensive gene tic and molecular analysis over the past decade. Mutants impaired in t he expression of nitrate reductase have been characterized in a number of plant species. Molecular analysis of the Nia gene coding for nitra te reductase has been the basis for a three-domain model of the struct ure of the enzyme, in agreement with biochemical and genetic data. Mut agenesis and antisense strategies have led to the description of nitri te reductase deficiencies. The molecular analysis of the corresponding Nii genes has provided invaluable information on the structure of nit rite reductase. Recently, a gene involved in nitrate uptake has also b een identified. The regulation of the nitrate assimilatory pathway has been investigated. Analysis of the regulation of the pathway at the m olecular level has shown evidence for the involvement of nitrate, Ligh t and/or sucrose, and reduced nitrogen in the regulation. Surprisingly , no bonafide regulatory mutant specific to this pathway has been iden tified so far in higher plants. This may reflect the redundancy of reg ulatory genes. The deregulated expression of one or the other step of the pathway obtained by ectopic expression of the corresponding genes is a new approach to study the physiological role of these regulations . Elements of the pathway have also been successfully used as transpos on traps, or negatively selectable markers for other purposes. Finally , the identification at the molecular level of regulatory genes and st ructural elements involved in transport and storage of nitrate, or in the biosynthesis of cofactors of nitrate and nitrite reductases, will be the goal of the next decade.