Functional analysis of an Arabidopsis T-DNA "Knockout" of the high-affinity NH4+ transporter AtAMT1;1

NH4+ acquisition by plant roots is thought to involve members of the NH4+ transporter family (AMT) found in plants, yeast, bacteria, and mammals. In Arabidopsis, there are six AMT genes of which AtAMT1;1 demonstrates the highest affinity for NH4+. Ammonium influx into roots and AtAMT1;1 mRNA expression levels are highly correlated diurnally and when plant nitrogen (N) status is varied. To further investigate the involvement of AtAMT1;1 in high-affinity NH4+ influx, we identified a homozygous T-DNA mutant with disrupted AtAMT1;1 activity. Contrary to expectation, high-affinity (NH4+)-N-13 influx in the amt1;1:T-DNA mutant was similar to the wild type when grown with adequate N. Removal of N to increase AtAMT1;1 expression decreased high-affinity (NH4+)-N-13 influx in the mutant by 30% compared with wild-type plants, whereas low-affinity (NH4+)-N-13 influx (250 muM-10 mm NH4+) exceeded that of wild-type plants. In these N-deprived plants, mRNA copy numbers of root AtAMT1;3 and AtAMT2;1 mRNA were significantly more increased in the mutant than in wild-type plants. Under most growth conditions, amt1;1:T-DNA plants were indistinguishable from the wild type, however, leaf morphology was altered. However, when grown with NH4+ and sucrose, the mutant grew poorly and died. Our results are the first in planta evidence that AtAMT1;1 is a root NH4+ transporter and that redundancies within the AMT family may allow compensation for the loss of AtAMT1;1.