An insight into the extracellular proteome in the Citrus limon/Phoma tracheiphila interaction

Mal secco of citrus is a vascular disease caused by the mitosporic Ascomycete Phoma tracheiphila which is particularly destructive on lemon (Citrus limon)1. A major physiological effect of mal secco on infected citrus trees is the impairment of water transport in the xylem. However, the mechanism of pathogenesis of this disease is only partly known1. Recently, it has been demonstrated that the oxidative stress may play a role in the interaction between P. tracheiphila and C. limon 1,2. Currently available and emerging proteomic techniques have made possible the investigation of very complex protein systems 3 and have contributed to a better understanding of the pathogen/host-plant interactions in several pathosystems. This study was aimed at comparing the extracellular proteome expressed in vitro by P. tracheiphila in a liquid medium containing leaf disks of ‘Monachello’ lemon (the most tolerant commercial cultivar of lemon) with the proteome expressed by the fungus in the same medium without leaf disks. Proteome analysis was carried out combining 1D SDS-PAGE, in-gel trypsin digestion, nHPLC/nESI-MSMS and database search. The proteins identified were classified into two groups: enzymes with oxido-reductive functions and therefore involved in the response to the oxidative stress and proteins responsible for the hydrolytic processes of cellular walls. The results obtained confirm that oxidative stress inducers and hydrolytic enzymes are involved in the interaction between P. tracheiphila and its hosts and suggest that some gene products belonging to these two groups of proteins play a key role in the mechanism of interaction between the causal agent of mal secco and the host.