Rhizoctonia solani is one of the most destructive pathogens causing foot rot disease on tomato. 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 R. solani 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 elevated levels of expression for both genes in the partially resistant cultivar compared to the susceptible cultivar. 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 pathogen. 1. Introduction Tomato (Lycopersicon esculentum) is one of the most important vegetables in the world which suffers from various fungal diseases [1, 2]. Foot rot disease of tomato plants was found in various greenhouses in Iran. Symptoms were characterized by soft rot of the seedling near the soil line. Rhizoctonia solani was consistently isolated from the damaged plant tissues. Rhizoctonia solani is a species complex composed of several anastomosis groups (AGs). This pathogen can survive in soil within diseased plant material as mycelia or sclerotia during unfavorable environmental conditions for several years. The pathogen is transported in infested soil or through movement of diseased plant tissues. Potential for seed borne inoculum also exists. In nature, usually R. solani has asexual reproduction and exists primarily as vegetative mycelium and/or sclerotia. The teleomorph of R. solani, Thanatephorus cucumeris, is classified in the phylum Basidiomycota. Formation of basidiospores on diseased host plants in nature is rarely observed. In favorable environmental conditions, following infection of the host plant by R. solani, sexual spores are formed on specialized structures called basidia. Four spores
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