Coronatine and abscission in citrus

Coronatine is a polyketide phytotoxin produced by several plant pathogenic Pseudomonas spp. The effect of coronatine on abscission in Citrus sinensis L. Osbeek 'Hamlin' and 'Valencia' orange fruit, leaves, fruitlets, and flowers was determined. Coronatine at 200 mg.L-1 significantly reduced fruit detachment force of mature fruit, and did not cause fruitlet or flower loss in 'Valencia'. Cumulative leaf loss was 18% with coronatine treatment. Coronafacic acid or coronamic acid, precursors to coronatine in Pseudomonas syringae, did not cause mature fruit abscission. Ethylene production in mature fruit and leaves was stimulated by coronatine treatment, and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and 12-oxo-phytodienoate reductase (12-oxo-PDAR) gene expression was upregulated. A slight chlorosis developed in the canopy of whole trees sprayed with coronatine, and chlorophyll content was reduced relative to adjuvant-treated controls. Leaves formed after coronatine application were not chlorotic and had chlorophyll contents similar to controls. Comparison of coronatine to the abscission compounds methyl jasmonate, 5-chloro-3methyl-4-nitro-pyrazole and ethephon indicated differences in ethylene production and ACO and 12-oxo-PDAR gene expression between treatments. Leaf loss, chlorophyll reduction and low coronatine yield during fermentation must be overcome for coronatine to be seriously considered as an abscission material for citrus.