Genome-Wide Analysis of the Aspartate Aminotransferase Family in Brassica rapa and the Role of BraASP1 in Response to Nitrogen Starvation

Nitrogen (N) is the most important fertilizer for increasing crop production, as it is absorbed by various N transporters and metabolized by a series of enzymes. Aspartate aminotransferase (ASP) facilitates the conversion of Glu to Asp for N storage. Chinese cabbage is a typical leafy vegetable that requires a large amount of N for growth. To investigate the functions of <i>BraASPs</i>, 10 members of the ASP gene family in <i>Brassica rapa</i> (<i>B. rapa</i>) were identified. Phylogenetic analysis and collinearity comparisons of ASP members among <i>B. rapa</i>, <i>Arabidopsis thaliana</i> (<i>A. thaliana</i>), <i>Oryza sativa</i> (<i>O. sativa</i>), <i>Brassica napus</i> (<i>B. napus</i>), and <i>Brassica oleracea</i> (<i>B. oleracea</i>) were conducted to examine evolutionary associations and genome duplication events across species. Multiple cis-acting elements associated with stress responses were identified in the promoters of <i>BraASPs</i>, suggesting their diverse roles. Members of <i>BraASPs</i> were expressed in roots, stems, flowers, siliques, and leaves, with the highest expression in leaves. Their expression levels increased rapidly at 3 h under low N conditions, peaked at 6 h, and returned to low levels at 24 h. Based on transcriptomic data, <i>BraASP1b</i> was identified as a candidate gene in <i>B. rapa</i> under low N stress, localized in the nucleus and cytoplasm. Overexpression of <i>BraASP1b</i> in <i>A. thaliana</i> resulted in a higher biomass than Col-0 under low N conditions.