Bacterial canker of tomatoes - histological characterization of cultivar resistance and seed transmission

Abstract

Clavibacter michiganensis subsp. michiganensis (Cmm) causes bacterial canker in tomatoes representing a significant threat to worldwide tomato production. In order to demonstrate potential resistance mechanisms in tomato cultivars differing in susceptibility to the pathogen, inoculations were carried out in the greenhouse followed by histological examinations of infected transverse stem sections using histochemical staining methods. In the greenhouse trials, cultivars 'Moneymaker' and 'Lyconorma' showed significantly stronger disease symptoms than 'Irat L3' and 'Hawaii 7998' four weeks after inoculation with Cmm. More occlusions of the xylem vessels, but less tyloses were found in the two susceptible cultivars than in the less susceptible cultivars. Further plant responses to the pathogen like accumulation of phenolic compounds in the xylem parenchyma, xylem lignification or callose deposition were observed in stems of infected plants, but these reactions occurred to a similar extent in all four cultivars. Since enhanced accumulation of phenolic substances, lignin and callose was detected in infected plants of all four cultivars they appear to be components of basal resistance of tomato to Cmm. With immunofluorescence microscopy, Cmm was localized on the surface of the seed coat but not in the deeper seed layers or the embryo of a moderately susceptible tomato cultivar. After seed germination, bacteria were directly transmitted to the radicle, hypocotyl and cotyledons of seedlings emerging from infected seeds, suggesting an external rather than a systemic transmission of the bacteria from seed to the plant shoot.