HLB-associated preharvest fruit abscission is mediated by jasmonate/ethylene signaling triggered by secondary fungal infection

Authors: Zhao, Wei; Baldwin, Elizabeth - Liz; Bai, Jinhe; Plotto, Anne; IREY, MIKE - Southern Gardens Citrus

Submitted to: Proceedings of Florida State Horticultural Society
Publication Type: Proceedings
Publication Acceptance Date: 10/31/2016
Publication Date: 8/2/2017
Citation: Zhao, W., Baldwin, E.A., Bai, J., Plotto, A., Irey, M. 2017. HLB-associated preharvest fruit abscission is mediated by jasmonate/ethylene signaling triggered by secondary fungal infection. Proceedings of Florida State Horticultural Society. 129:198-202.

Interpretive Summary: Citrus greening or huanglongbing (HLB) disease results in pre-harvest fruit drop representing up to 30% of the crop. An opportunistic fungus was found to be associated with this fruit drop that normally causes a postharvest decay of the fruit. To better understand this, orange trees were shaken and the dropped fruit collected and the fruit remaining on the tree harvested. Their abscission zones (where the stem meets the fruit) were collected and their gene expression compared to see what genes were being expressed to a more or less degree. It turned out that the fungus was more associated with the abscission zones of the fruit that fell off the tree, and the genes in that abscission zone that were expressed were for the plant hormone ethylene, an abscission promoting hormone, and not for abscisic acid, which would be synthesized in response to drought or carbohydrate stress due to HLB-induced root dieback or thinning tree canopy (less leaves). Therefore, the fruit fell off the tree due to the fungal pathogen, which induced the fruit to produce ethylene that led to fruit drop rather than due to HLB-effects on the root and leaf reduction.

Technical Abstract: One symptom of citrus huanglongbing (HLB) is excessive pre-harvest fruit drop. Recently, higher incidence of Lasiodiplodia theobromae (Diplodia) was found in HLB-symptomatic orange calyx abscission zones (AZ-C) than in non-symptomatic fruit, and the infection was positively correlated with the reduction in fruit detachment force (FDF), suggesting that Diplodia infection may be involved in the HLB-related pre-harvest fruit drop. To verify the direct correlation of HLB-related fruit drop with the fungal infection, HLB-affected sweet orange trees (Hamlin) were shaken manually during the harvest season. The fruit that dropped from trees upon shaking were collected (D), and the fruit retained on trees after shaking were harvested (R). Fruit ethylene production was measured, and the titers of Diplodia and CLas in AZ-C of D and R fruit were analyzed. Gene expression levels related to signaling and biosynthesis of phytohormone (jasmonate, ethylene, and abscission) were evaluated by RT-qPCR. The results indicate that significantly higher Diplodia titers in D than in R fruit; more than half of D fruit but none of R fruit exhibited ethylene production, which was positively correlated with Diplodia titer in D fruit. Genes related to synthesis and signaling of ethylene (ET) and jasmonates (JA) were up-regulated, while abscisic acid (ABA) were down-regulated. The results suggest that different from water stress or carbon shortage induced abscission (which is characterized by induced abscisic acid), HLB-associated pre-harvest fruit abscission is mediated by JA/ET signaling, which was triggered by secondary fungal infection at calyx abscission zone.