Introduction of rolC into Marubakaidou [ Malus prunifolia Borkh. var. ringo Asami Mo 84-A] apple rootstock via Agrobacterium tumefaciens

We attempted to produce new dwarf rootstock cultivars with high rooting ability from cuttings by introducing the rolC gene derived from Agrobacterium rhizogenes. Southern blot analysis demonstrated that one to three copies of the transgene had been integrated in the plant genome of rolC transformants. UidA expression was stable throughout development in whole plants. The introduction of rolC resulted in short plants with shortened internodes, smaller leaves, and reduced apical dominance. Adventitious root formation from cutting stem explants was induced earlier in rolC transformants than in pIG121Hm-transformants. The cytokinin-related response was altered in rolC transformants. When the shoot apex was cut from rolC transformants and transferred to medium containing 1 mg/l benzyl adenine, it formed a callus with abnormal multi-shoot primordia that was unable to develop further and then ceased growth. However, these could develop into whole plants when cultured on plant growth regulator-free medium. Four phenotypic groups of rolC transformants were identified. One group displayed both reduced height and shortened internodes; a second group had reduced height with normal internode lengths; the third group had normal height with shortened internodes; and the fourth group was phenotypically similar to control plants. No correlation between phenotype and transcriptional activity of the rolC gene was observed. Some of the transformants are good candidates for dwarf rootstocks that maintain high rooting ability.