Abstract
Nitrate uptake is the first step of the nitrate assimilation pathway. In Physcomitrella patens, transcript levels of the five NRT2 genes, encoding nitrate transporters, were low in ammonium-grown cells and drastically increased in several hours after transfer of the cells to nitrate-containing medium or nitrogen-free medium, indicating that all the NRT2 genes are inducible. Expression of the NRT2 genes was induced in a shorter time under high CO2 conditions than under low CO2 conditions and was not observed in the dark, suggesting close relation of NRT2 expression to carbon assimilation. In higher plants, ammonium assimilation products repress expression of the NRT2 genes, but the actual signaling molecule remains to be elucidated. In P. patens, the repressive effect of ammonium was abolished by an inhibiter of glutamine synthetase but not by an inhibiter of glutamate synthase, indicating that glutamine acts as the regulator of NRT2 expression.
