PbBPC4 involved in a xylem-deficient dwarf phenotype in pear by directly regulating the expression of PbXND1

Dwarfing is an important agronomic trait in fruit breeding. At present, dwarf cultivars or dwarfing rootstocks are used for high-density planting. Although some dwarf rootstocks have been used in the cultivation of pear (Pyrus bretschneideri Rehd), the breeding of dwarf pear rootstocks or cultivars is still sorely lacking. A previous study reported that PbXND1 results in a xylem-dwarf phenotype in pear trees. However, the regulatory mechanism upstream of PbXND1 is unclear. In this study, we identified PbBPC4 as an upstream regulatory factor of PbXND1 in yeast one-hybrid assays. In β-glucuronidase staining and dual-luciferase assays, PbBPC4 enhanced the activity of the PbXND1 promoter. Tobacco plants overexpressing PbBPC4 showed decreased plant height because of a reduced xylem size. Similar changes in the xylem was observed in transgenic pear roots; those overexpressing PbBPC4 showed reduced xylem size, and those with silencing PbBPC4 expression showed increased xylem size, greater density of xylem vessels, and a larger proportion of the xylem out of the total cross-section area. Expression analyses showed that PbBPC4 increases the transcription of PbXND1, leading to reduced transcript levels of genes involved in the positive regulation of xylem development, ultimately resulting in a xylem-deficient dwarf phenotype. Taken together, our results reveal the mechanism by which PbBPC4 participates in the regulation of xylem development via directly altering the expression of PbXND1, thus leading to the dwarf phenotype in pear. These findings have reference value for the breeding of dwarf pear trees.