Insights into the molecular regulation of premature fruit drop - what we have learned from mango (Mangifera indica) and other fruit crops

Fruit abscission is a complex physiological process influenced by the availability of carbohydrates, along with environmental, genetic, and hormonal cues. In perennial tree crops, such as mango (Mangifera indica L.), the abscission of fruitlets prior to maturity directly impacts yield and is a major hurdle for commercial production. Studies have demonstrated that five classes of phytohormones play a pivotal role in regulating the abscission process, including auxins, cytokinins, gibberellins (GA), ethylene and abscisic acid. Naturally occurring and synthetic hormones, known as plant growth regulators (PGRs), have been applied to manage fruit abscission in mango, with varying success. Here we performed a meta-analysis on available published PGR studies in mango, and found that individual applications of gibberellic acid (GA₃), or synthetic versions of auxin (1-naphthaleneacetic acid) or cytokinin (forchlorfenuron) improve fruit retention and yield. Applying combinations of these PGRs has been shown to further improve fruit retention in other species but requires further exploration in mango. Carbohydrate availability also plays a critical role in fruit abscission. Experimental manipulations resulting in carbon-limiting conditions, such as defoliation coupled with girdling, consistently increase fruitlet drop. This response is thought to be mediated by hormonal pathways with low carbohydrate levels reducing auxin signalling while enhancing ethylene biosynthesis and sensitivity, tipping the balance toward abscission. This review provides a comprehensive overview of the current understanding of the molecular mechanisms governing mango fruit abscission, highlighting the intricate interactions between hormones and their application for improvement of fruit retention. Understanding these interactions will enable future studies to develop targeted interventions for improved fruit retention.