Heavy fruit load inhibits axillary bud outgrowth initially through weakening its sink strength and auxin efflux in citrus

Excessive and random outgrowth of axillary buds (AxBs) produce a mass of summer shoots, resulting in overly tall and dense canopies that complicate disease and pest control. Heavy fruit load reduces the number of summer shoots. However, the underlying molecular mechanism for this suppression remains unclear. In this study, we found that transcript levels of two well-characterized dormancy marker genes (CsBRC1 and CsDRM1) were significantly higher in AxBs from fruit-bearing branchlet (FB) than from vegetative branchlet (VB). However, de-fruiting significantly decreased them within 9 days. Transcriptome analysis identified 5638 differentially expressed genes (DEGs) after de-fruiting, many of which are associated with sugar and phytohormone homeostasis. Interestingly, transcript levels of sink strength-related genes (CsCwINV6 and CsSUS6) and auxin-efflux carrier gene CsPIN1a were significantly increased within 2 h after de-fruiting. Furthermore, their expression was markedly lower in FB AxBs than in VB AxBs and was significantly induced by sucrose supplementation. Collectively, these results suggested that a heavy fruit load suppresses the expression of sink strength-associated genes and CsPIN1a, thereby reducing sugar availability and impairing auxin export from axillary buds, ultimately inhibiting the outgrowth of AxBs. These findings provide valuable insights into the molecular basis of poor vegetative growth under heavy fruit load and contribute to simplifying canopy management through rationalizing fruit load.