Regulation of ethylene biosynthesis in citrus leaves infected with Xanthomonas campestris pv. citri

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Abstract
Inoculation of Poncirus trifoliata (L.) Raf. x Citrus Paradisi Macf. (Swingle citrumelo) leaves with Xanthomonas campestris pv. citri (Hasse) Dye (Xc). a nursery strain of bacteria causing citrus canker, induced significant ethylene production. No ethylene biosynthesis was induced by Xanthomonas campestris pv. dieffenbachiae (McCulloh and Pirrone) Dye (Xd), a non pathogen to citrus. Leaves inoculated with Xc had a higher endogenous 1‐aminocyclopropane‐1‐carboxylic acid (ACC) content than the leaves treated with aminoethoxyvinylglycine (AVG) or streptomycin before inoculation. All the inhibitors of ethylene biosynthesis tested were effective in suppression of ethylene production from Xc inoculated leaves. The inhibitors also suppressed the rate of disease progression. Major points of regulation of pathogen induced ethylene appear to reside at the transcriptional and/or translational levels, as indicated by the inhibitor studies. It appears that the pathogen‐induced ethylene biosynthesis in this case is closely associated with disease progression and that denovo induction and activation of ACC synthase is the key step in the induction of ethylene production. The induction of ethylene production as a result of pathogen invasion may be a plant defense response as it leads to the abscission of leaves which helps retard the spread of the pathogen into the plant‐system.