ARF6 integrates auxin and gibberellin signaling to promote stone cell lignification in pear via the HB49-MYB169 module.

Stone cells originate from secondary cell wall thickening and contain abundant lignin. Their excessive accumulation compromises pear fruit quality, yet the endogenous hormonal mechanisms governing stone cell formation remain unclear. Here, co-expression network analysis using transcriptome data - the flesh of 206 sand pear accessions sampled at the critical stage of stone cell formation and multitissues of 'Dangshansuli' - showed that auxin biosynthetic gene YUCCA6 and auxin signaling activator ARF6 were strongly co-expressed with lignin-related genes. A positive correlation was observed between endogenous indole-3-butyric acid (IBA) levels and the rate of stone cell accumulation. Moreover, both YUCCA6 overexpression and exogenous IBA treatment enhanced lignin deposition in pear, demonstrating that auxin promoted stone cell lignification. Additionally, ARF6 promoted stone cell lignification. Mechanistic analyses revealed that ARF6 activated HB49, which subsequently upregulated MYB169, a key transcriptional activator of lignin biosynthesis, thereby promoting stone cell lignification. Notably, ARF6 interacted with repressors IAA27 or RGL3, which suppressed HB49 activation; this repression was alleviated by auxin or gibberellin treatment, which restored ARF6-mediated activation of HB49. These results establish ARF6 as an integrator of auxin and gibberellin signaling pathways and provide new mechanistic insights into auxin-gibberellin crosstalk in regulating stone cell lignification.