Transcription factor WRKY23 is involved in ammonium-induced repression of Arabidopsis primary root growth under ammonium toxicity.

Although WRKY transcription factors (TFs) are known to be involved in the regulation of plant root development, the mechanisms by which these TFs regulate plant tolerance to ammonium (NH4+) toxicity remain unclear. To identify the molecular mechanisms underlying NH4+-induced repression of primary root growth and NH4+ sensitivity in Arabidopsis, wild-type (Col-0) and mutant (wrky23) plants were treated with 10 mM KNO3 (control) or 5 mM (NH4)2SO4 (NH4+ toxicity) for 7 days. Under NH4+ toxicity, the fresh weight of wrky23 mutant was significantly lower than that of Col-0 plants, and the NH4+ concentration in wrky23 roots was significantly higher than that in Col-0 roots. However, we observed no significant differences between the two genotypes under the control treatment. Ammonium transporter AMT1;2 expression was induced in wrky23 roots but not in Col-0 roots. The transcript levels of cytosolic glutamine synthetase-encoding genes and activity of glutamine synthetase did not differ significantly between wrky23 and Col-0. Furthermore, the fluorescence and staining patterns of DR5::GFP and DR5::GUS, respectively, were more pronounced under NH4+ toxicity than under the control treatment. Collectively, our results indicate that AMT1;2 expression was induced in the wrky23 mutant in response to NH4+ toxicity, leading to NH4+ accumulation in the roots and primary root growth repression. Under NH4+ toxicity, both auxin transport and distribution were affected, and auxin accumulation in the root tips inhibited primary root growth in the wrky23 mutant. Our study provides important insights into the molecular mechanisms by which WRKY23 TF regulates plant responses to NH4+ toxicity.