Abstract:
Tumor formation by Agrobacterium tumefaciens (AT) was significantly increased when the grape stem explants were planted and inoculated basal end upwards. When stem pieces of Catawba and Chancellor cultivars of Vitis vinifera were inoculated basal end upwards, these exhibited a 40% increase in tumor occurrence and a 3-4 times increase in tumor size. The enhanced tumor production reflects basipetal auxin accumulation in the grape stem explants. This in vitro inoculation procedure provided an efficient method to study pathogenicity of AT strains on a large scale in grape cultivars.
Some necrosis accompanied tumor formation following in vitro inoculation in normal upwards direction. In some cases, tumors were produced in early stages but they became necrotic later. It might be possible that the higher auxin production in transformed cells lead to higher auxin oxidase activity which might be responsible for this necrosis.