Genome-wide identification of BOR genes in poplar and their roles in response to various environmental stimuli

Abstract

Abstract Boron (B) is one of the microelements that is essential for plant growth and fitness. The uptake and distribution of B is facilitated by B transporters (BORs) that participate in many physiological and biochemical processes. In the current study, we identified 519 BOR proteins from 86 plant species utilizing their latest genomic database. Evolutionary analysis demonstrated that the BORs in plants can be clustered into three groups. Groups 1 and 2 represent the previously defined “BOR1-like” and “BOR4-like” proteins, respectively. Evidence from an evolutionary tree suggested that the BORs in flowering plants originated from two distinct ancestors: BOR1-like genes were from Bryophyta, whereas BOR4-like genes were produced after the segregation between gymnosperms and angiosperms. A total of ten PtrBOR genes were identified in the genome of Populus trichocarpa, with five members in Group 1 and Group 2, respectively. The identified PtrBOR genes were distributed across 9 different chromosomes and contained 10–12 introns. Each PtrBOR protein harbored three conserved HCO3− transporter domains and at least 10 transmembrane domains (TMDs). Gene duplication analysis indicated that segmental duplication played important roles in expanding this gene family, and these gene pairs have been undergoing purifying selection. In silico analysis of gene expression levels within tissues implicated BOR1-like genes and PtrBOR 6/9 in B uptake and reproductive organ formation, respectively. Coexpression analysis of PtrBOR genes uncovered their functions in nitrogen metabolism and response to drought, salt and heavy metal stresses, as well as pollen tube formation. In addition, the expression levels of the PtrBOR genes were verified by quantitative real-time PCR in roots and leaves from “NL895” (P. × euramericana) subjected to diverse abiotic stresses, including low B (LB), high B (HB), drought, cadmium (Cd), low nitrogen (LN) and salt conditions. Transcripts of BOR1-like genes were much more abundant than those of BOR4-like genes, whereas BOR4-like genes displayed much more sensitivity than BOR1-like genes in the roots of “NL895” in response to all tested abiotic stresses. In particular, PtrBOR6 from the BOR4-like group showed the largest variations among these PtrBOR genes and was induced under all the tested conditions in roots and leaves. These results will aid in the elucidation of the different roles played by the BOR1-like and BOR4-like genes in the growth, development, and stress response in poplar.