Phosphorus-induced enhancement of adventitious root formation in K2 apple rootstocks: hormonal and gene expression insights

Adventitious root (AR) formation is a crucial process for the clonal propagation of horticultural crops. Phosphorus (P), a key macronutrient, plays an essential role in various physiological and molecular processes involved in AR formation, including cell division and development. However, its role in apple rootstock remains underexplored. This study investigates the effects of three P treatments—low P (LP), medium P (MP), and high P (HP)—along with a control group (CK, containing the basal P level of standard MS medium)—on AR formation in K2 apple rootstock stem cuttings. The results demonstrated that LP treatment significantly increased both rooting percentage and the number of ARs per cutting compared to CK, whereas HP treatment inhibited both. Anatomical analysis revealed that LP-treated cuttings exhibited enhanced AR primordia formation, accompanied by increased endogenous levels of indole-3-acetic acid (IAA), while levels of zeatin riboside (ZR), abscisic acid (ABA), and brassinosteroid (BR) were reduced. Transcriptome analysis identified 1,009 differentially expressed genes (DEGs), including 383 upregulated and 626 downregulated, enriched in metabolic and hormone signaling pathways. LP altered the expression of key hormone-related genes—ARF1 and GH3.1 (auxin signaling), B-ARR1 (cytokinin signaling), PYR/PYL1 (ABA signaling), and BAK1a (BR signaling)—which may synergistically promote AR initiation and development. These results reveal the physiological and molecular basis of LP-enhanced AR formation and provide insights into nutrient–hormone interactions in root development.