Genetic analysis of nitrate reductase-deficient tobacco plants regenerated from mutant cells

Abstract

Fifteen nitrate reductase (NR)-deficient mutants that had been selected from amphihaploid cell cultures of Nicotiana tabacum and regenerated to fertile amphidiploid plants were genetically analyzed through crosses. All the 15 mutants proved to be allelic. Segregation among F2 progeny from crosses between mutant and wild-type plants and among testcross progeny showed the regenerated plants to be homozygous double mutants. The NR deficiency is conferred by two unlinked recessive nuclear mutations, which thus define a pair of duplicate loci (nia1 nia2). These loci were identified as the structural genes for the apoprotein of NADH-NR. — Two mutants (Nia28, Nia30) were characterized further. Both Nia28 and Nia30 plants had less than 2% of wild-type NR activity, were resistant to chlorate and incapable of sustained growth on nitrate or in soil. However, they grew normally on media containing ammonium succinate (with or without nitrate). Growth test showed that Nia28 seedlings do not utilize nitrate, whereas Nia30 seedlings utilize nitrate at a very small rate. — The examination of single mutants (nia1-28/28 and nia2-28/28) revealed that either of the two loci is able to produce wild-type levels of NR activity. NR activity and chlorate sensitivity respond to nia + gene dosage only at the very early seedling stage. At later developmental stages, both the basal and the nitrate induced levels of NR activity were found to be independent of the number of nia + genes, indicating complete compensation of gene dosage effects by regulatory mechanism.