2018 ASHS Annual Conference

Fruit drop is one of the many HLB symptoms and is the main cause of yield loss in citrus production. Results from the UF-CREC citrus breeding program show that Sugar Belle has tolerance to HLB, good yield, vigorous growth, and can retain its fruit for 30 days past optimum maturity. Compared to Hamlin, which is very susceptible to HLB and subjected to a high fruit drop rate. Our hypothesis is that Sugar Belle abscission zone (AZ) has a low expression of cell wall degradation, ethylene, auxins and abscisic acid pathway biosynthesis, thereby decreasing fruit abscission when compared to Hamlin AZ. Twenty Sugar Belle and Hamlin fruits were collected from 16 trees (8 per variety) in November 2016. Fruits were pulled from stems and the force to detach was measured. Fruit that detached with a force below 5 kgf were considered loose fruit, and equal or above 5 kgf were considered tight fruit. Fruit abscission zone RNA from homogenous samples of loose and tight Sugar Belle and Hamlin fruit were extracted and sequenced. Over 16,000 genes were differently expressed. A threshold of four-fold change was used to filter the full sequence data resulting in 699 different expressed genes. Genes related to pectin, carbohydrate transport, response to hormones, and abscisic acid signaling were downregulated in Hamlin. Lignin-related genes were overexpressed in Hamlin. Upregulation of ethylene and cell wall degradation related genes were found in Sugar Belle AZ, indicating initiation of the abscission pathway, however Sugar Belle fruit remain attached to the stems. Our RNAseq results suggests that low fruit drop in Sugar Belle is not associated with altered expression of abscission related genes. Further, anatomical and physio-chemical studies could explain why Sugar Belle fruit endure the abscission process.