Overexpression of JrAMT2 in walnut (Juglans regia L.) rootstock enhances nitrogen level in grafted wild-type walnut scions

Nitrogen is one of the most important essential macronutrients, which acts on a physiological basis as a key limiting factor for plant productivity and crop yield. As the primary source of inorganic nitrogen, ammonium is the preferential source for root uptake, especially in nitrogen-deficient plants, because its assimilation requires less energy than nitrate. To date, several ammonium transporter (AMT) genes have been isolated and partially characterized from plants other than walnut. The growth and yield of walnut trees require abundant nitrogen because of its large size. For the purpose of decreasing production costs and reducing environmental pollution, higher nitrogen utilization of walnut rootstock is desirable. In this study, an ammonium transporter gene, encoding a 52.39 kDa poly-peptide of 461 amino acids, designated JrAMT2, was isolated from walnut. JrAMT2 was expressed in all tissues of tissue cultured walnut shoots, with the highest expression in roots. Overexpression of JrAMT2 in walnut shoots resulted in darker green foliage and higher nitrogen concentration. In vitro graft experiments showed overexpression JrAMT2 rootstock can significantly increase the nitrogen level in the WT scion. This result indicates a potential approach to development of better nitrogen utilization rootstocks for walnut and other tree crops.