Study of relative expression of defence-related genes in orange fruit in response to antagonist-pathogen interaction

Pseudomonas spp. produce a wide spectrum of phytotoxic compounds, some of which correlated with post-harvest disease control. Additionally, biocontrol agents can induce a range of defence mechanisms of resistance in citrus tissue that result in a broad spectrum of metabolic modifications such systemic acquired resistance, induced systemic resistance and reactive oxygen species. In the present study we investigated the expression level of genes that encode chitinase (CHI1), gluthatione peroxidase (GPX), phenylalanine-ammonia lyase (PAL), and allene oxide synthase (AOS) in 'Tarocco' sweet orange [Citrus sinensis (L.) Osbeck] treated with P. syringae biocontrol strain and the green mold pathogen Penicillium digitatum. Quantitative reverse transcriptase PCR (qRT-PCR) showed that antagonistic P. syringae cells and P. digitatum applied singly increases the transcription of chitinase (CHI1) in the flavedo relative to the untreated control. Interestingly, the transcription of CHI1 in the flavedo was markedly induced by application of P. syringae challenged by P. digitatum, and it was pronounced 24 and 48 h after inoculation. P. syringae inoculation, alone or followed by challenge inoculation with P. digitatum, was weakly effective in enhancing AOS, GPX1, and PAL1 gene expression, whereas P. digitatum alone strongly enhanced the expression of AOS and GPX1 (24 h after inoculation) and PAL1 (48 h after inoculation) in orange tissues. Overall, we assume that CHI1 gene is most likely to be part of the molecular mechanisms involved in pathogen defence responses in citrus fruit.