Submitted to: American Chemical Society Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 4/13/2016
Publication Date: N/A
Technical Abstract: One symptom of citrus Huanglongbing (HLB) disease is excessive pre-harvest fruit drop, and as the severity of HLB has increased throughout Florida, fruit drop has become the most severe ever recorded. To understand the mechanisms underlying HLB-associated pre-harvest fruit drop, the transcriptome in the citrus calyx abscission zone (AZ-C) was analyzed and compared for asymptomatic ‘Hamlin’ fruit harvested from asymptomatic trees (A), fruit dropped from HLB-symptomatic trees upon shaking the trees (D), and fruit retained on HLB-symptomatic trees after shaking the trees (R). Each treatment had 3 replicates of 20 fruit from 10 trees, from which the dropped fruit were collected and the retained fruit harvested. The AZ-C tissue was excised and used for RNA isolation and subsequent RNA-sequencing analysis. Comparison of the transcriptomic profiles revealed 1209, 1739 and 927 significantly (p<0.05) differentially expressed genes between D and R, D and A, R and A, respectively. Gene ontology (GO) and KEGG pathway enrichment analysis of the significantly differentially expressed genes indicated that response to stimulus, plant-type cell wall modification, secondary metabolism were among the significant biological processes involved in HLB-related fruit abscission, and the most significant GO term was ‘response to chitin’ (FDR = 1.2e-07); while among KEGG pathways, ‘alpha-linolenic acid metabolism’ was the most significant (FDR< 0.005), of which the product is jasmonate. Consistently up-regulated genes in D versus R and A included genes related to biotic stress, lipid metabolism, secondary metabolism, whereas genes related to carbohydrate metabolism, cell wall (cellulose synthesis and cell wall modification) were consistently down-regulated. In the hormone category, genes related to synthesis and signaling of ethylene and jasmonate were consistently up-regulated, while abscisic acid (ABA), auxin, brassinosteroid, cytokinin, and gibberellin were down-regulated. The gene expressions for hormone related genes and chitinase genes were validated by qRT-PCR. The results indicate that different from water stress or carbon shortage induced abscission (which is characterized by increases in ABA synthesis), HLB-associated pre-harvest fruit abscission is related to elevated ethylene and jasmonate synthesis and signaling possibly due to the responses to external biotic stimulus.