Genome-Wide Characterization of High-Affinity Nitrate Transporter 2 (NRT2) Gene Family in Brassica napus

Nitrate transporter 2 (NRT2) plays an essential role in Nitrogen (N) uptake, transport, utilization, and stress resistance. In this study, the NRT2 gene family in two sequenced Brassica napus ecotypes were identified, including 31 genes in 'Zhongshuang11' (BnaZSNRT2s) and 19 in 'Darmor-bzh' (BnaDarNRT2s). The candidate genes were divided into three groups (Group I-III) based on phylogenetic analyses, supported by a conserved intron-exon structure in each group. Collinearity analysis revealed that the large expansion of BnaZSNRT2s attributed to allopolyploidization of ancestors Brassica rapa and Brassica oleracea, and small-scale duplication events in B. napus. Transcription factor (TF) binding site prediction, cis-element analysis, and microRNA prediction suggested that the expressions of BnaZSNRT2s are regulated by multiple factors, and the regulatory pattern is relatively conserved in each group and is tightly connected between groups. Expression assay showed the diverse and differentiated spatial-temporal expression profiles of BnaZSNRT2s in Group I, but conserved patterns were observed in Group II/III; and the low nitrogen (LN) stress up-regulated expression profiles were presented in Group I-III, based on RNA-seq data. RT-qPCR analyses confirmed that BnaZSNRT2.5A-1 and BnaZSNRT2.5C-1 in Group II were highly up-regulated under LN stress in B. napus roots. Our results offer valid information and candidates for further functional BnaZSNRT2s studies.