Comprehensive Histological, Endogenous Hormone, and Transcriptome Analysis to Reveal the Mechanism of Hormone Regulation Mediating Pepper (Capsicum annum L.) Fruit Size

As the only harvest organ of pepper, fruit size is an important yield determinant. To elucidate the molecular mechanisms underlying pepper fruit size, we performed histological, physiological, and transcriptomic analyses on the pepper varieties QB6 (large fruit) and CXJ82 (small fruit). High contents of auxin and cytokinin in the early stage of fruit development promoted the rapid division of fruit cells in both varieties, which provided sufficient cells for subsequent fruit enlargement. High gibberellin accumulation induced the elongation and expansion of QB6 pericarp cells. Transcriptome analysis showed that genes related to cell division, cell wall polysaccharide degradation, and photosynthesis were highly expressed in QB6 fruit, likely contributing to its larger size. In the hormone–signal transduction factor–gene interaction network, GID6, GID1, IAA12, MYC30, and SAUR36 exhibited high correlations with numerous genes related to cell division, the cell wall, and photosynthesis, emerging as key signal transduction factors for the hormone-mediated regulation of pepper fruit size. Weighted gene co-expression network analysis identified the transcription factors OFP20, HD-ZIP6, and HD-ZIP13 as fundamental for pepper fruit size regulation. Our results expand the understanding of hormone regulation of pepper fruit size, providing a foundation for the breeding and improvement of excellent pepper varieties.