Red light accelerates adventitious root primordium initiation and differentiation in Phoebe bournei cuttings by modulating auxin signaling and hormonal balance

Abstract

Adventitious root formation in stem cuttings of Phoebe bournei, an economically and ecologically significant woody species, is a major bottleneck in clonal propagation. To elucidate the impact of spectral quality on early adventitious root development, we characterized the morphological, hormonal, and transcriptomic dynamics of primordium initiation and differentiation under white, red, and blue light. Under red light, cuttings exhibited the earliest and most pronounced formation of white filamentous structures at lenticels, indicative of root primordium initiation, as early as 20 days after cutting, whereas white and blue light treatments showed delayed responses. Paraffin section analysis confirmed the induced origin of root primordia from cortical and vascular cambial tissues, with red light accelerating their differentiation. Hormonal profiling revealed that although red and blue light transiently increased indole-3-acetic acid levels in the basal bark, the persistently elevated abscisic acid in apical buds across all treatments likely suppressed polar auxin transport. Notably, red light downregulated brassinosteroid and gibberellin levels in the base, which could alleviate growth inhibition. Transcriptome analysis of phloem tissues identified significant differential expression of auxin-responsive genes, including members of the ARF (e.g., PbARF6, PbARF13), GH3, and SAUR gene families, which were specifically upregulated under red light. Collectively, our findings suggest that red light accelerates the early induction and differentiation of adventitious root primordia in P. bournei cuttings by modulating hormonal homeostasis and activating key auxin signaling pathways. This work provides a physiological and molecular basis for optimizing light conditions in the vegetative propagation of P. bournei.