Cloning and characterization of the ammonium transporter genes BaAMT1;1 and BaAMT1;3 from Chinese kale

Chinese kale (Brassica alboglabra L.) is a popular vegetable rich in important nutrients. Fertilization with appropriate ammonium:nitrate ratios enhances biomass production and quality. AMT-type ammonium transporters have been shown to mediate ammonium uptake across the plasma membrane. However, very little is known about the molecular regulation of growth and development by ammonium in Chinese kale, including how ammonium regulates the expression of AMT1 genes. In this study, we identified and characterized two AMT1 genes from B. alboglabra, BaAMT1;1 and BaAMT1;3. The full-length open reading frames of BaAMT1;1 and BaAMT1;3 were 1512 bp and 1515 bp, respectively. Transient expression of the fusion proteins pBE-EGFP-BaAMT1;1 and pBE-EGFP-BaAMT1;3 in onion epidermal cells indicated that these transporters are located on the plasma membrane. BaAMT1;1 and BaAMT1;3 were functional in yeast and complemented a mutant defective in ammonium transport. BaAMT1;1 was expressed in vegetative organs and at high levels in roots, while BaAMT1;3 expression was root specific. In addition, we observed opposite responses of BaAMT1;1 and BaAMT1;3 expression to nitrogen starvation and ammonium resupply in roots. These results provide new insights into the molecular mechanisms underlying ammonium absorption in Chinese kale.