VviAMT4;1 Is a High-Affinity Ammonium Transporter in Table Grape

Ammonium transporters (AMTs) are a class of membrane-associated proteins that play crucial roles in the uptake and transport of ammonium (NH₄⁺ or NH₃). In this study, an ammonium transporter-encoding gene VviAMT4;1 was isolated and identified from table grape 'Yanpu No.2'. Notably, the expression level of VviAMT4;1 varied significantly across different organs or tissues of 'Yanpu No.2', and the highest expression level was detected in the roots of both tissue-cultured seedlings and 5-year-old mature trees. Expression of VviAMT4;1 was significantly up-regulated under NH₄⁺ depletion throughout the whole of tissue-cultured seedlings. Yeast mutant functional complementation indicates that the recombinant strain pYES2-VviAMT4;1/31019b restored growth under different pH conditions. ¹⁵N isotope-labeled uptake kinetics analysis demonstrated that VviAMT4;1 is a typical high-affinity ammonium transporter, with a Kₘ value of 49.58 ± 4.66 μmol·L^-1 and a Vₘₐₓ value of 3.29 μmoles·min^-1·μg^-1 cells. Moreover, VviAMT4;1 can mediate the weak uptake and utilization of methyl amine (MeA⁺) in yeast cells. The VviAMT4;1-mediated NH₄⁺ uptake process may suffer from feedback inhibition by endogenous NH₄⁺ enrichment. This study provides insights into understanding the molecular mechanisms of N transport and utilization in fruit trees.