The light-harvesting complex protein LHCB4 fine-tunes temperature-regulated flowering via chloroplast retrograde signaling in pear

Flowering is crucial for plant reproductive success and is regulated by both endogenous and external factors. However, the mechanisms by which ambient temperature influences flowering in pear (Pyrus spp.) remain poorly understood. In this study, we observed that elevated temperatures induce early flowering and alter the expression levels of pear light-harvesting complex Lhcb4 (PpyLhcb4) and other flowering-related genes. Notably, the heterologous expression of PpyLhcb4 in Arabidopsis increased chlorophyll content and delayed flowering under normal and high-temperature conditions, suggesting that PpyLhcb4 inhibits flowering by promoting chlorophyll biosynthesis. We also identified a base substitution (A to C) within the PpyLhcb4 promoter region in 2 pear varieties exhibiting different flowering times. This base substitution affected the CArG-box, which influences PpyLhcb4 promoter activity. Furthermore, the specific binding of the transcription factors PpyAP1 and PpyMADS1 to the PpyLhcb4 promoter was demonstrated. At 22 °C, PpyLhcb4 expression was not regulated by AP1 but was inhibited by MADS1. However, at 30 °C, both AP1 and MADS1 were upregulated by PpyLhcb4 expression. Moreover, we showed that PpyLHCB4 interacts with PpyPIF3 to sense environmental changes. In summary, this study elucidates the molecular mechanisms by which PpyLhcb4 regulates flowering, which involves an intracellular signaling pathway originating from the chloroplast that responds to environmental changes to balance plant growth and development.