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

The provision of ammonium (NH4 (+)) and nitrate (NO3 (-)) mixture increases the total nitrogen (N) than the supply of sole NH4 (+) or NO3 (-) 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 NH4 (+) and NO3 (-) 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 NH4 (+) and NO3 (-) mixture were 1.25- and 3.53-fold higher than those with either sole NH4 (+) or NO3 (-). Furthermore, NH4 (+) and NO3 (-) might interact with each other under coexistence. NO3 (-) had a positive effect on net NH4 (+) influx, whereas NH4 (+) had a negative influence on net NO3 (-) influx. The ammonium transporter (AMT) played a key role in NH4 (+) absorption and transport. Based on expression analysis, BcAMT1.2 differed from other BcAMT1s in being upregulated by NH4 (+) or NO3 (-). 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, beta-glucuronidase activity, and the cellular location of its promoter, BcAMT1.2 may be responsible for NH4 (+) transport. Following the overexpression of BcAMT1.2 in Arabidopsis, BcAMT1.2-overexpressing lines grew better than wildtype lines at low NH4 (+) concentration. In the mixture of NH4 (+) and NO3 (-), NH4 (+) influxes and NO3 (-) 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 NH4 (+) fluxes under the coexistence of NH4 (+) and NO3 (-) in B. campestris.