METHYLAMMONIUM RESISTANT MUTANTS OF NICOTIANA-PLUMBAGINIFOLIA ARE AFFECTED IN NITRATE TRANSPORT

This work reports the isolation and preliminary characterization of Ni cotiana plumbaginifolia mutants resistant to methylammonium. Nicotiana plumbaginifolia plants cannot grow on low levels of nitrate in the pr esence of methylammonium. Methylammonium is not used as a nitrogen sou rce, although it can be efficiently taken up by Nicotiana plumbaginifo lia cells and converted into methylglutamine, an analog of glutamine. Glutamine is known to repress the expression of the enzymes that media te the first two steps in the nitrate assimilatory pathway, nitrate re ductase (NR) and nitrite reductase (NiR). Methylammonium has therefore been used, in combination with low concentrations of nitrate, as a se lective agent in order to screen for mutants in which the nitrate path way is de-repressed. Eleven semi-dominant mutants, all belonging to th e same complementation group, were identified. The mutant showing the highest resistance to methylammonium was not affected either in the ut ilization of ammonium, accumulation of methylammonium or in glutamine synthase activity. A series of experiments showed that utilization of nitrite by the wild-type and the mutant was comparable, in the presenc e or the absence of methylammonium, thus suggesting that the mutation specifically affected nitrate transport or reduction. Although NR mRNA levels were less repressed by methylammonium treatment of the wild-ty pe than the mutant, NR activities of the mutant remained comparable wi th or without methylammonium, leading to the hypothesis that modified expression of NR is probably not responsible for resistance to methyla mmonium. Methylammonium inhibited nitrate uptake in the wild-type but had only a limited effect in the mutant. The implications of these res ults are discussed.