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

Avocado (Persea americana) 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 this maternal organ 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 RNA-seq analysis indicated that growth arrest is associated with a transcriptome reprogramming that is first initiated in the seed coat and then transmitted to pericarp and embryo. Gene-expression and hormone-profiling results indicated 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 suggested 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.