Genome-wide identification and expression divergence of the NDPK gene family in Brassica juncea under abiotic stress.

Nucleoside diphosphate kinases (NDPKs) constitute a multifunctional enzyme family that is widely distributed in prokaryotes and eukaryotes and plays important roles in energy metabolism, stress perception, signal transduction, and reactive oxygen species homeostasis. However, this gene family has not been systematically characterized in Brassica juncea. Here, we performed a genome-wide identification and comprehensive analysis of the BjNDPK gene family by integrating bioinformatic analyses with expression and physiological assays. Eighteen non-redundant BjNDPK genes were identified and classified into five phylogenetic groups with distinct structural features. Cis-acting element analysis showed that the promoters of several members contain drought-responsive elements. Collinearity analysis indicated complex subgenomic rearrangement and selective retention during the allotetraploid evolution of B. juncea. Expression profiling showed that many BjNDPK genes differed among tissues and responded to PEG-induced osmotic stress and salt stress. Reference-gene stability analysis identified qBjActin as the most stable reference gene under the tested conditions. Physiological assays showed that PEG treatment increased the content of H2O2 in B. juncea leaves, whereas an Arabidopsis thaliana atndpk2 (corresponding to BjNDPK7 in B. juncea) mutant maintained greater root elongation than the wild type under mannitol treatment. These findings describe the structural, evolutionary, and expression characteristics of the BjNDPK family and provide candidate genes for future functional studies of abiotic-stress responses in polyploid B. juncea.