The contribution of host and pathogen to ethylene biosynthesis in Penicillium digitatum-infected citrus fruit

The biosynthesis of ethylene by grapefruit (Citrus paradisi Macf) cv. “Marsh Seedless” 6 days after inoculation with either an ethylene producing or a non-ethylene-producing isolate of Penicillium digitatum, was studied. Relatively low levels of ethylene were produced by the apparently healthy region, at the edge of rot in fruits infected with either the ethylene producing or the nonethylene-producing isolate of the fungus. Higher levels of production were found in regions of the peel showing infection symptoms, but only in fruit infected with the ethylene-producing isolate. The production of symptoms and the ACC content of the peel of fruits inoculated with the ethylene-producing isolate was similar to that in fruits inoculated with the non-producing isolate.

The use of radiolabelled precursors showed that the ethylene produced by the healthy parts of the fruit originated from methionine, while the ethylene produced by the infected parts of the fruit originated mostly from glutamic acid. Furthermore, ethylene production by the healthy part of the fruit was markedly enhanced by 1-aminocyclopropane-1-carboxylic acid (ACC), and to a lesser extent by CuSO₄, but it was inhibited by aminoethoxyvinylglycine (AVG). In contrast the production of ethylene by infected peel and by P. digitatum in culture was not affected by ACC, but was markedly inhibited by CuSO₄, and, to a lesser extent, by AVG.

These results suggest that ethylene production in the healthy part of the fruit is of plant origin, whereas the markedly enhanced production of ethylene by the P. digitatum-infected regions, is mostly or entirely offungal origin.