2019 ASHS Annual Conference

In Florida, it has been documented that Huanglongbing [HLB (Candidatus Liberibacter asiaticus)] elevates preharvest fruit drop in affected citrus trees, especially for sweet orange (Citrus sinensis) cultivars. In contrast, the mandarin hybrid ‘Sugar Belle’ (previously known as ‘LB8-9’) [Citrus reticulata × (C. paradise ×C. reticulata)], which was released by the Citrus Research and Education Center, University of Florida/IFAS (Lake Alfred, FL) in 2009, is tolerant to HLB and has relatively low preharvest fruit drop. To increase our understanding of the mechanism that causes the various tendencies of preharvest fruit drop for different citrus in the presence of HLB, the objective of this study was to analyze the global transcriptome of abscission zones (AZ) from mature fruit of field-grown HLB-affected sweet orange and ‘Sugar Belle’ mandarin trees. For the citrus used in the experiment, a set of mature fruit were left on trees (unharvested) during commercial harvest for the evaluation of fruit drop. The fruit drop rate of ‘Midsweet’ sweet orange was 8% at 2 weeks after the commercial harvest day (WAH) and reached 25% at 4 WAH; ‘Hamlin’ sweet orange had 23% of fruit dropping at 2 WAH and more than 30% at 4 WAH. In contrast, for ‘Sugar Belle’ mandarin, the drop rate remained below 8% within 5 WAH. The results indicate that ‘Hamlin’ sweet orange had a higher tendency to drop than ‘Sugar Belle’ mandarin as early as 2 WAH. For the mature fruit that were likely to abscise [fruit detachment force (FDF) < 5.5 kgF], there were only 26 differentially expressed genes (DEGs) in the AZ of ‘Hamlin’ sweet orange compared to ‘Sugar Belle’ mandarin at 2 WAH. For the fruit inclined to retain on the tree (FDF > 5.5 kgF), there were 125 DEGs between the two citrus. The extremely low number of DEGs (less than 1% of total mapped reads) in the two contrasts suggest that the underlying biochemical and physiological mechanism of mature fruit abscission is highly conversed among citrus species with different tendencies of preharvest fruit drop and with different susceptibility to HLB.