Plants naturally express variable levels of resistance against different groups of their pathogens. This kind of primary defense response is known as basal, general, partial, polygenic or multigenic resistance that is controlled by several genes. Basal resistance partially protects plants against challenge infection by pathogens and decreases the progress and destructive effects of disease. It is obtained by cooperation of multiple molecular and cellular defense responses and involvement of various signaling pathways. In this study, the molecular and cellular changes of a partially resistant (Sunny 6066) and a susceptible (Rio Grande) tomato cultivar after infection with necrotrophic soil-borne fungus Rhizoctonial solani, causing seedling damping-off, were compared. The expression of defense related genes such as chitinase (LOC544149) and peroxidase (CEVI-1) in infected tomato cultivars was investigated using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). This method revealed considerably elevated levels of expression for both genes in the partially resistant cultivar compared to the susceptible cultivar. Cytological observations of infected tomato seedling samples revealed that lower level of disease symptoms in the partially resistant cultivar is associated with decreased plant colonization by the pathogen. One of the most prominent facets of basal plant defense responses is the formation of physical barriers at sites of attempted fungal penetration. These structures are produced around the sites of potential pathogen ingress to prevent pathogen progress in plant tissues. We investigated formation of lignin, as one of the most important structural barriers affecting plant resistance, using thioglycolic acid assay. A correlation was found between lignification and higher level of resistance in Sunny 6066 compared to Rio Grande cultivar. These findings suggest the involvement of chitinase, peroxidase, and lignin formation in defense responses of tomato plants against R. solani as a destructive phytopathogen.

Keywords