Root Development in Poplar Overexpressing pxAlaAT3 Differs Depending on the Form of the Applied Exogenous Nitrogen

Alanine aminotransferase is essential for plant nitrogen assimilation. It catalyzes the reversible reaction that transfers an amino group from alanine to a-ketoglutarate and helps regulate primary metabolic homeostasis. Unfortunately, how this gene contributes to nitrogen metabolism is unclear. In this study, the regulatory effects of nitrogen on AlaAT3 expression were investigated via qRT-PCR analyses. Differences in root morphology were observed among AlaAT3-overexpressing plants exposed to diverse forms of nitrogen. Moreover, glutamine synthetase, nitrate reductase, and nitrite reductase activities were differentially affected by the nitrogen forms. The transcriptome sequencing data indicated that genes preferentially expressed in OL plants treated with ammonium were related to transcriptional-translational pathways, whereas genes preferentially expressed in OL plants treated with nitrate were mainly associated with carbohydrate metabolism and lipid metabolism. MapMan annotations revealed OL-ammonium genes were mainly related to hormone metabolism, transcription factors, and protein synthesis and catabolism, while OL-nitrate genes were primarily related to amino acid synthesis, major CHO metabolism, lipid synthesis and catabolism, and the cell wall. Notably, some genes associated with amino acid synthesis and catabolism, polysaccharide catabolism, flavonoids, minor CHO metabolism, and polysaccharide synthesis, including genes encoding MYB, AuxIAA, ARF, WRKY, C2H2, and TCP transcription factors, had the opposite expression trends in the OL and wild-type plants, possibly reflecting the importance of these genes in OL plants. These findings provide an important theoretical basis for elucidating the AlaAT3 function, with potential implications for the utility of the transgenic line.