New insight in Huanglongbing-associated mature fruit drop in citrus and its link to oxidative stress

Abstract

As Huanglongbing (HLB) becomes epidemic throughout Florida, there is a sharp rise in mature fruit drop prior to harvest in affected citrus, leading to a substantial reduction in yield. To investigate how HLB increases mature fruit abscission, this research evaluated the drop rate and compared the global gene expression using RNA sequencing in HLB-susceptible ‘Hamlin’ sweet orange (Citrus sinensis) versus HLB-tolerant ‘LB8-9’ mandarin {‘Clementine’ mandarin (C. reticulata) and ‘Minneola’ tangelo [hybrid of ‘Duncan’ grapefruit (C. paradisi) and ‘Dancy’ tangerine (C. reticulata)]}. Consistent with their susceptibility to HLB documented in the literature, the drop rate of mature fruit was significantly higher in ‘Hamlin’ sweet orange than ‘LB8-9’ mandarin. Between the two citrus, there were 368 differentially expressed genes (DEGs); 310 and 58 were upregulated and downregulated, respectively, in the abscission zone (AZ) of ‘Hamlin’ sweet orange compared to ‘LB8-9’ mandarin fruit. Although at the time of collection, fruits of both citrus were still attached to the tree branch (fruit detachment force > 6 kgf), upregulation of the DEGs related to cell wall loosening in ‘Hamlin’ sweet orange indicates that the process of abscission was already initiated in the AZ of this citrus. The pattern of DEGs related to the metabolism and signal transduction of hormones, specifically ethylene, auxin, cytokinin, abscisic acid, and gibberellic acid, also suggests that HLB likely brings about changes in endogenous hormone balance that promote mature fruit abscission. Interestingly, DEGs encoding antioxidants were upregulated, suggesting a demand for detoxification mechanisms against oxidative stress in the AZ of ‘Hamlin’ sweet orange fruit. Additionally, gene ontology (GO) terms for cell death and senescence, two of consequences of oxidative stress, were highly enriched. Together, the results of transcriptome analysis presented herein provide evidence that for HLB-susceptible ‘Hamlin’ sweet orange, oxidative stress caused by the pathogen infection likely results in cell wall modification, leading to cell separation and eventually fruit abscission. The results further suggest that ‘LB8-9’ mandarin may have an advanced antioxidant system to mitigate the pathogen-induced oxidative stress, thereby contributing to its tolerance to HLB.