Ammonium Transporter (BcAMT1.2) Mediates the Interaction of Ammonium and Nitrate in Brassica campestris

The provision of ammonium (NH(4) (+)) and nitrate (NO(3) (−)) mixture increases the total nitrogen (N) than the supply of sole NH(4) (+) or NO(3) (–) with the same concentration of total N; thus, the mixture contributes to better growth in Brassica campestris. However, the underlying mechanisms remain unknown. In this study, we analyzed NH(4) (+) and NO(3) (–) fluxes using a scanning ion-selective electrode technique to detect under different N forms and levels in B. campestris roots. We observed that the total N influxes with NH(4) (+) and NO(3) (−) mixture were 1.25- and 3.53-fold higher than those with either sole NH(4) (+) or NO(3) (−). Furthermore, NH(4) (+) and NO(3) (–) might interact with each other under coexistence. NO(3) (–) had a positive effect on net NH(4) (+) influx, whereas NH(4) (+) had a negative influence on net NO(3) (–) influx. The ammonium transporter (AMT) played a key role in NH(4) (+) absorption and transport. Based on expression analysis, BcAMT1.2 differed from other BcAMT1s in being upregulated by NH(4) (+) or NO(3) (−). According to sequence analysis and functional complementation in yeast mutant 31019b, AMT1.2 from B. campestris may be a functional AMT. According to the expression pattern of BcAMT1.2, β-glucuronidase activity, and the cellular location of its promoter, BcAMT1.2 may be responsible for NH(4) (+) transport. Following the overexpression of BcAMT1.2 in Arabidopsis, BcAMT1.2-overexpressing lines grew better than wildtype lines at low NH(4) (+) concentration. In the mixture of NH(4) (+) and NO(3) (–), NH(4) (+) influxes and NO(3) (–) effluxes were induced in BcAMT1.2-overexpressing lines. Furthermore, transcripts of N assimilation genes (AtGLN1.2, AtGLN2, and AtGLT1) were significantly upregulated, in particular, AtGLN1.2 and AtGLT1 were increased by 2.85–8.88 times in roots, and AtGLN1.2 and AtGLN2 were increased by 2.67–4.61 times in leaves. Collectively, these results indicated that BcAMT1.2 may mediate in NH(4) (+) fluxes under the coexistence of NH(4) (+) and NO(3) (–) in B. campestris.