Immature fruit abscission is triggered by hormonal changes in the seed coat of avocado

Avocado (Persea americana Mill.) is an economically important tree crop that exhibits a high rate of immature fruit abscission (IFA), reducing yield. As the seed coat derived from recently abscised fruitlets displays a senescence phenotype, we hypothesized that the seed coat plays a critical role in initiating IFA. Here, we show that fruitlets fated to abscise undergo growth arrest before shrinking and detaching from the tree. Comparative RNAseq analysis indicates that growth arrest is associated with a transcriptome reprogramming that is first initiated in the seed coat then transmitted to pericarp and embryo. Gene expression and hormone profiling results indicate that fruitlet growth arrest is associated with a decline in auxin activity and an increase in abscisic acid, the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid, and the bioactive jasmonate, jasmonoyl-isoleucine, in the seed coat. At a late stage of growth arrest, transcriptomic signatures further suggest that a dormancy-like program of development is induced in the seed and a senescence phenotype is activated in the seed coat. Together, our data indicate that avocado IFA is initiated by hormone-driven transcriptome reprogramming that functions to transition the seed coat to a senescence program of development, which induces growth arrest, seed dormancy and ultimately, fruitlet abscission.