2017 ASHS Annual Conference

Citrus greening or huanglongbing (HLB) disease is associated with an increase in pre-harvest fruit drop, for which the molecular mechanisms remain unknown. Recent studies have implicated a secondary infection by fungus Lasiodiplodia theobromae (Diplodia) in enhancing fruit drop from HLB-affected trees. In order to understand the molecular basis of the HLB-associated fruit abscission, transcriptomes in citrus calyx abscission zones were analyzed and compared among healthy fruit (H) from healthy (CLas negative) trees, and fruit dropped (D) or retained (R) from HLB-affected (CLas positive) trees upon shaking the trees, by means of RNA-Sequencing analysis (RNA-Seq). Results showed up-regulated biotic stress, lipid and secondary metabolism, whereas genes related to carbohydrate metabolism and cell wall were down-regulated. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of differentially expressed genes (DEGs) indicated that the most significant GO term was “response to chitin” (p= 9.70E-09), while “response to jasmonic acid” and “phenylpropanoid biosynthesis” were also significant. Among KEGG pathways, “alpha-linolenic acid metabolism” was the most significant (p < 0.005), which produces jasmonates (JA). In addition, “plant hormone signal transduction” and “phenylpropanoid biosynthesis” were also significant pathways (p < 0.05). Hierarchical clustering analysis indicated genes related to synthesis and signaling of ethylene (ET) and JA were consistently up-regulated, while abscisic acid, auxin, brassinosteroid, cytokinin, and gibberellin were generally down-regulated in D versus R and H fruit. In agreement with the hormone expression profiles, substantial numbers of downstream JA/ET-responsive defense (the hallmark of anti-fungal defense) related genes were up-regulated in D fruit as well. In agreement, significantly higher Diplodia level in D than in R and H fruit, and fruit ethylene production was positively correlated with Diplodia level. Quantitative RT-PCR confirmed RNA-Seq results for 30 representative genes covering each of the DEG categories. 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 likely in response to a secondary fungal infection.