BpGH3.5, an early auxin-response gene, regulates root elongation in Betula platyphylla x Betula pendula

The GH3 family is an important class of early auxin-response genes involved in the development of the hypocotyls and roots in Arabidopsis thaliana, but the role of this gene family in woody plants is poorly understood. In this study, we cloned a GH3-like gene from Betula platyphylla x Betula pendula (birch) named BpGH3.5 and produced transgenic birch lines that overexpressed either a sense or antisense version of the BpGH3.5 gene using Agrobacterium-mediated transformation. We found that both types of transgenic lines exhibited short primary and lateral roots in vitro, which was caused by a smaller sized root apical meristem with a fewer cells compared with the non-transgenic plant as observed in paraffin sections. The qRT-PCR results showed that the expression of genes associated with auxin and cytokinin metabolism and signaling changed in the transgenic lines. These results indicated that cytokinin and auxin crosstalk caused a small meristem, short-root phenotype in BpGH3.5 transgenic lines. In addition, transgenic sense and antisense BpGH3.5 lines showed reduced indole-3-acetic acid and N-1-napthylphthalamic-acid sensitivity. Taken as a whole, our results suggested that BpGH3.5 had a complex function and sophisticated mechanism of regulation in woody plants.