Crown Gall on Castor Bean Leaves

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Abstract
Secondary tumours were formed on the cotyledonary leaf petiole, the hypocotyl, and first true leaf of castor bean seedlings after inoculating the blades of the cotyledonary leaves with Agrobacterium tumefaciens. Depending on the strain of bacteria employed, 0 to 80 per cent of the plants developed secondary tumours. The ability of different strains to initiate secondary tumours was not obviously correlated with their relative effectiveness in initiating primary tumours. Though all produced primary tumours, five out of ten auxotrophic strains failed to yield secondary tumours, whereas only one out of 14 prototrophic strains failed to do so. Both the number of plants developing secondary tumours and the frequency with which these tumours occurred on different parts of the plant were positively correlated with the concentration of the primary inoculum. Tumours also developed on the cotyledonary leaf petiole and on the hypocotyl after vacuum infiltration of A. tumefaciens into the blade of cotyledonary leaves. In most instances (9 out of 11 plants) no tumours were formed on the blade of the infiltrated leaf. Thus, tumour formation equivalent to secondary tumours can occur in the absence of a primary tumour or an overt primary wound. Excision of inoculated leaves showed that the stimulus for secondary tumour formation moves from the blade to the hypocotyl within 24 h. Attempts to demonstrate the presence of a sub-cellular tumour-initiating agent in homogenates of inoculated leaves were unsuccessful. A. tumefaciens, however, was found in the petiole of the cotyledonary leaf and in the hypocotyl within 24 h of inoculation. The migrating agent responsible for secondary tumour formation in castor beans is concluded to be intact bacteria.

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