XTH genes impact growth habit determination of twining common bean vine revealed by brassinosteroid treatment

Brassinosteroids impact the development of G-fibers - specialized cells that generate tension in plants. To explore the functional and genetic relationships between G-fibers and twining stems of Phaseolus vulgaris, we applied an active brassinosteroid and a brassinosteroid inhibitor to perturb G-fiber development and probed these phenotypes through gene expression and anatomical analyses. Brassinosteroid treatment generated phenotypes that affected three key features of twining: elongation, circumnutation, and G-fiber development. We examined anatomical and biochemical changes in the G-fibers through cross-sections, macerations, and immunohistochemistry. RNA sequencing and differential gene expression analysis allowed us to identify unique gene expression patterns for each treatment. Brassinosteroid treatment led to significantly elongated internodes with disrupted circumnutation and long, thin-walled G-fibers. By contrast, inhibitor treatment produced short internodes with thick G-fibers. These phenotypes corresponded with significant differential expression of xyloglucan endotransglucosylase/hydrolase (XTH) genes, both at the onset of elongation and later, during G-layer deposition. Detection of xyloglucan in the G-layer, along with in situ hybridization, confirmed active xyloglucan remodeling after twining. Our results confirm the presence of xyloglucan in the G-layer of common bean, underscoring its importance in G-fiber function, and suggest a regulatory role for XTH genes in shaping the twining growth habit through modulation of cell wall properties.