Abstract
Citrus greening disease or huanglongbing (HLB) is associated with excessive pre-harvest fruit drop. To understand the mechanisms of the HLB-associated fruit abscission, transcriptomes were analyzed by RNA sequencing of calyx abscission zones (AZ-C) of dropped “Hamlin” oranges from HLB-diseased trees upon shaking the trees (Dd), retained oranges on diseased trees (Rd), dropped oranges from healthy shaken trees (Dh), and retained oranges on healthy trees (Rh). Cluster analysis of transcripts indicated that Dd had the largest distances from all other groups. Comparisons of transcriptomes revealed 1047, 1599, 813, and 764 differentially expressed genes (DEGs) between Dd/Rd, Dd/Dh, Dh/Rh, and Rd/Rh. The gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses indicated hormone signaling, defense response, and secondary metabolism were involved in HLB-associated fruit abscission. Ethylene (ET) and jasmonic acid (JA) synthesis/signaling-related genes were upregulated in Dd, while other phytohormone-related genes were generally downregulated. In addition, genes related to JA/ET-activated defense response were upregulated in Dd as well. Consistent with the phytohormone gene expression data, increased levels (p < 0.05) of ET and JA, and a decreased level (p < 0.05) of abscisic acid were found in Dd compared with Rd, Dh or Rh. Lasiodiploidia theobromae level in Dd AZ-C was higher than the other fruit types, confirmed by qPCR, indicating AZ-C secondary fungal infection of HLB fruit may exacerbate their abscission. This information will help formulate effective strategies to control HLB-related abscission.
Highlights
The shedding or abscission of citrus-reproductive organs is a highly regulated process that is influenced and activated in response to exogenous and endogenous stimuli
Before RNAsequencing analysis, CLas titer in abscission zones (AZ)-C tissue was measured and the results verified that the fruits from HLBdiseased trees (Dd and Rd) were infected with CLas, while the fruits from healthy trees (Dh and Rh) were free of CLas (Fig. S1)
C revealed the upregulation of genes involved in ET and jasmonic acid (JA) pathways, and JA/ET-activated defense response, as well as secondary metabolism in the HLB-affected fruit that were undergoing abscission (Dd) compared with those not undergoing abscission (Rd)
Summary
The shedding or abscission of citrus-reproductive organs is a highly regulated process that is influenced and activated in response to exogenous (changing environmental conditions, interactions with microorganisms) and endogenous (physiological modifications) stimuli. The process takes place at anatomically distinct cell layers termed abscission zones (AZ)[1]. Bud and flower abscission takes place at the abscission zone A (AZ-A) located between twig and peduncle, whereas fruitlet and fruit abscission occurs at the AZ located at the calyx between the pericarp and the floral disc (calyx abscission zone or AZ-C)[2]. AZ comprises distinct layers of small cytoplasmic dense cells that forms at an early stage of pedicel development and proliferates during fruit development[3]. Major events leading to abscission have been classified into three phases to occur: differentiation and development of the AZ tissue, acquisition of competence to respond to abscission signals, and execution (activation) of abscission following perception of abscission signals[3]. Molecular studies in model plants (tomato and Arabidopsis) have identified key molecular components involved in the development of AZ or regulation of
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