A boron transporter, BnaC3.BOR1, is critical for boron regulation in roots, stem integrity, and floral organs in Brassica napus L.

Brassica napus L. is an allotetraploid plant that contains numerous boron (B) transporter genes, the functions of which are largely unknown. Here, we identified BnaC3.BOR1, a gene expressed in various tissues, including roots, stems, leaves, and floral organs. In yeast, BnaC3.BOR1 expression significantly reduces cellular B concentration. When grown in B-poor conditions, BnaC3.BOR1 mutation significantly lowered B concentrations in the xylem sap. BnaC3.BOR1 mutants exhibited stunted growth, including shorter roots, reduced shoot biomass, and leaf curling, compared to wild-type B. napus. In low-B soil, loss of BnaC3.BOR1 function resulted in shorter plant height and abnormal floral organ development with lower B levels, which ultimately reduced yield at the reproductive stage. Furthermore, BnaC3.BOR1 is expressed in multiple tissues, with specific expression in the regions below the nodes of stems. Mutants grown in B-poor soil exhibited stem cracking, and the B concentration in these cracks was lower than in wild-type plants. These findings indicate that BnaC3.BOR1 possesses the same conserved role in roots and flowers as previously discovered BOR1s, while having a distinct, specific role in stems. Our study provides a clear example of the functional diversification of B transporter homologous genes.