Research progress in the regulatory effects of plant hormones on fruit growth and development

Plant hormones, or phytohormones, act as fundamental chemical orchestrators guiding the intricate process of fruit development—from fertilized flower to mature, seed-dispersing organ. This developmental journey comprises a sequence of precisely timed and coordinated stages: fruit set, involving ovary activation and abscission prevention; a phase of intensive cell division that establishes final cell number; a crucial period of cell expansion governing fruit volume and size; and finally, ripening, which remodels fruit texture, biochemistry, and aroma in preparation for seed dispersal. This review systematically synthesizes current knowledge on how five classical plant hormones—Auxin (AUX), Gibberellins (GAs), Cytokinins (CK), Abscisic Acid (ABA), and Ethylene (ET)—govern developmental transitions, with emphasis on their molecular machinations and the sophisticated crosstalk underlying their regulatory networks. Aux, primarily synthesized via the tryptophan-dependent TAA1/TARs-YUCs pathway, signals through the well-characterized TIR1/AFB-Aux/IAA-ARF ubiquitination- degradation cascade. This pathway is crucial during fruit set, where it promotes ovary development and can induce parthenocarpy independently of fertilization. During the early fruit growth, AUX regulates cell division by modulating genes such as the Small Auxin Up RNA (SAUR) family (e.g., AcoSAUR50 in pineapple influencing fruit shape, MdSAUR36, a negative regulator in apple) and various auxin response factors (ARFs) (e.g., SlARF9 controlling cell division layers and SlARF12 affecting cell expansion in tomato). GAs, diterpenoids biosynthesized through enzymes including ent-copalyl pyrophosphate synthase, entkaurene synthase, ent-kaurene oxidase, ent-kaurenoic acid oxidase, gibberellin 20 oxidase, and gibberellin 3 oxidase, are perceived by the gibberellin insensitive dwarf1 (GID1) receptor, leading to degradation of DELLA repressor proteins. GAs are widely recognized for inducing parthenocarpy in fruits such as citrus, apple, and pear, and serve as critical drivers of cell elongation during fruit expansion, as illustrated by the VvDELLA2-VvCEB1 module that activates expansion genes in grape. CTK, central regulators of cell proliferation, is activated through the IPT-LOG pathway, with homeostasis maintained by cytokinin oxidase/dehydrogenases (CKXs). The CK signal is transduced via a multi-step phosphorylay known as the two-component system (TCS), involving histidine kinase receptors (AHKs), phosphotransfer proteins (AHPs), and type-B response regulators (type-B ARRs). The importance of CTK in establishing the foundational cell population of fruits is underscored evidence that suppressing CKXs expression or applying synthetic CTK such as N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU) consistently results in larger fruits in species including apple, grape, and jujube, by extending the cell division period. The transition to the final stage of fruit development ripening is predominantly governed by ABA and ET, which often define climacteric and non- climacteric ripening physiologies. ABA, synthesized through the rate- limiting 9- cis epoxy carotenoid dioxygenase (NCED) enzyme, signals via the core PYR/PYL-PP2C-SnRK2-ABF/AREB pathway. In non-climacteric fruits such as strawberry and grape, ABA acts as the primary hormonal trigger, coordinating sugar accumulation, pigment synthesis (e.g., anthocyanins), and fruit softening. Recent pioneering work in strawberry has revealed novel, mechanistic insights, including the FvABF3-FvALKBH10B-FvSEP3 cascade involving mRNA methylation dynamics, and the FaRIPK1-FaTCP7-FaSTP13/FaSPT module connecting ABA signaling to sugar transport. ET, the quintessential ripening hormone in climacteric fruits (e.g., tomato, banana, apple, and persimmon), is synthesized from methionine through the SAMS-ACS-ACO pathway. Its signal perception follows a linear pathway: ET→ETR/ERS receptors→CTR1 kinase→EIN2→EIN3/EILs transcription factors→ERF transcription factors. This cascade directly activates genetic reprogramming responsible for cell wall disassembly (via polygalacturonase and pectate lyase), chlorophyll degradation and carotenoid synthesis, starch hydrolysis, and production of volatile aroma compounds. Functional specificity within this pathway is exemplified by different ethylene response factor (ERF) members (e.g., DkERF18 and DkERF8 in persimmon) selectively activating distinct sets of downstream genes involved softening and coloration. A paradigm shift in plant physiology has been the realization that these hormonal pathways do not operate in isolation but function as interconnected nodes within a dense, dynamic crosstalk network. Characterized by synergistic, antagonistic, and sequential interactions, this network ensures precise timing and coordination of developmental transitions. Key interactions include: the IAA-GA interplay during fruit set and early growth, where DELLA proteins physically interact with ARF and AUX proteins to integrate signals; extensive ET-ABA cross-communication, where ET transcriptionally upregulates ABA biosynthesis genes (e.g., NCEDs in persimmon and kiwifruit) to promote ripening, while ABA modulates ET production and sensitivity; and the complex regulatory loops in which high IAA and GA levels in young fruits suppress ABA accumulation, followed by ABA establishing a self- rein-forcing positive feedback loop to drive ripening in strawberries. Additionally, hormones such asbrassinosteroids, jasmonates, and methyl jasmonate are increasingly recognized as important modulators of these core pathways, adding further complexity and robustness to the regulatory system. The translation of this fundamental knowledge into agricultural practice has been extensive and impactful. The strategic application of exogenous plant growth regulators, their synthetic analogs, and specific inhibitors has become indispensable for achieving high yield, superior quality, and enhanced postharvest management. Common horticultural practices now include: using GA3 and CPPU to reliably induce parthenocarpy and enhance fruit size in plants such as kiwifruit, grape, and mango; applying ethephon (an ET-releasing compound) to synchronize ripening in climacteric fruits like tomato and mango for commercial harvest; employing ABA sprays to improve coloration and sugar content in grapes and strawberries; and using ET action inhibitors such as 1-MCP to significantly extend storage and shelf-life in apples, pears, and other fruits by blocking ET receptors and delaying ripening and senescence. The emerging role of melatonin in promoting fruit set and development further expands the toolkit for hormonal manipulation, often acting through the modulation of existing hormone pathways such as those of GAs. In conclusion, the precise and coordinated regulation by plant hormones, mediated through intricate crosstalk networks, constitutes the foundation of fruit development and maturation. Future research directions are set to leverage multi-omics integration (genomics, transcriptomics, proteomics and metabolomics) and advanced gene-editing technologies (e.g., CRISPR-Cas9) to elucidate the spatial and temporal dynamics of these networks at a single-cell resolution, particularly in economically important but genetically complex perennial crops. Ultimately, a deeper, systems- level mechanistic understanding will enable more precise breeding strategies and the development of next-generation, hormone-based management protocols, paving the way for sustainable, high-efficiency, and quality-oriented horticultural production systems capable of meeting evolving global demands.