In Vitro Rooting of Fruit Trees by Agrobacterium rhizogenes

Several almond cultivars are strongly recalcitrant to rooting even when cultivated in vitro. So far, transformation of leaf tissue with Agrobacterium tumefaciens, carrying the plasmid pBinGUSint, has been reported. In analogy to Ti-plasmid, Ri-plasmid of A. rhizogenes can also be transferred to tissues of almond. In this case, basal part of microcuttings is infected and roots are expected to be formed from transformed tissue at site of the infection. Many factors influence the rooting response: the genetic background of the cultivar and its susceptibility to the bacterial strains, concentration of the bacterial suspension, the presence of auxin in the medium, etc. The cultivar Supernova shows a very strong sensitivity to the bacterial infection, and the rooting percentage on hormone free medium reached about 60% versus 0% of the control. When localized infection with Agrobacterium rhizogenes, combined or not with IAA, was applied to the base of the apple root- stock MM 106 microcuttings, which is an easy-to-root genotype, the response was rather inhibitory: without infection, IAA treatment induced most of the expiant (95%) to root, and 22.5% of expiants rooted even in the hormone free medium; after the infection, the rooting is strongly inhibited both with or without IAA treatment. Evidence of transgenic nature of roots obtained by infection is indicated by hybridization between total DNA and the fragment of pBinl9::Eco 15 containing rol genes.