Nitrate responses of Arabidopsis cho1 mutants: obvious only when excess nitrate is supplied.

We reported a loss-of-function of an Arabidopsis double AP2 transcription factor CHOTTO1 (CHO1) gene results in the altered responses to high concentrations of nitrate (approximately 50 mM). Nitrate up to 10 mM promotes growth of the wildtype seedling, but inhibits it under higher concentrations. The cho1 seedlings responded to nitrate up to 10 mM similarly to the wildtype, but the inhibitory effect of excess nitrate is less prominent in the mutants. This phenotype is restricted to the cotyledons, and growth of the hypocotyl and roots of the cho1 mutants is inhibited by excess nitrate. The cho1 mutations caused the upregulation of two nitrate transporter genes, AtNRT1.4 and At1g52190. Altered nitrate distribution and storage may explain the phenotypes of the cho1 mutants.