Citrus fruits infected with the fungus Penicillium digitatum substantially increase the production of the
plant hormone ethylene. In this study, the regulation of
ethylene biosynthesis in Citrus sinensis-infected fruits and its putative involvement in an active defence response against P. digitatum
infection is examined.
Ethylene production is demonstrated as being the result of the co-ordinated and differential up-regulation of at least three
ethylene biosynthetic genes: ACS1, ACS2, and ACO. Blocking
ethylene perception by
1-MCP resulted in an increased
ethylene production and ACS2 expression during
infection and mechanical wounding, suggesting that this gene is negatively regulated by
ethylene. ACO expression was induced by
ethylene in the absence of wounding or
infection, although further results indicate that its induction during the course of
infection may not be primarily mediated by
ethylene. Treatment with
1-MCP also increased susceptibility to Penicillium decay, showing an involvement of
ethylene perception in promoting defence responses in citrus fruits. The changes in the expression of two defence-related genes up-regulated during
infection were also studied: the ones coding for
phenylalanine ammonia-lyase (PAL) and an acidic class II
chitinase (ACR311). The onset of PAL expression after mechanical wounding or inoculation was not changed in 1-MCP-pretreated fruits, while its later increase during the course of
infection was abolished.
Chitinase gene induction was more related to mechanical damage and was partially repressed by
ethylene. These studies indicate distinct possible regulatory mechanisms of plant fruit defence genes in the context of
fungal infection and
ethylene perception.