Root system and rooting ability modification

Horticulturally valuable cultivars or rootstocks often show very poor rooting ability. Rooting can be improved by inoculating A. rhizogenes by wounding the basal part of in vitro microcuttings. These methods induce rooting in recalcitrant species such as almond (Rugini 1984; Strobel and Nachmias 1985, Damiano et al. 1995; Archilletti et al. 1995), walnut (Caboni et al. 1996)) and also in other woody species such as olive, grape, apple (Rugini 1986; Tepfer and Casse-Delbart 1987; Patena et al. 1988; Scorza 1991; Owens 1995; Gribaudo and Schubert 1990). According to our results in olive and cherry, few roots became transgenic, it seems that these results support the hypothesis that the partial integration of T-DNA has a possible inductive role on the non-transgenic neighbour cells or perhaps some unknown substances are present in the bacterial secretions (Rugini et al. 2000a). Rooting ability, number of roots and mass of roots increases when rol genes are overexpressed in plants, such as in kiwi fruit expressing rolABC genes (Rugini et al. 1991, 1997, 2000b), in apple ‘M.26’ rootstock and grape both expressing RiT-DNA (Lambert and Tepfer 1992; Nakano et al. 1994) or rolB (Zhu 2001; Welander et al. 1998).

In species which naturally make suckers from roots and are recalcitrant to regenerate shoots in vitro, a simple infection with A. rhizogenes induces root formation and makes transgenic shoots easily selectable to show different morphology, i.e. rootstock MRS/5 (Rugini and Gutierrez-Pesce 1999).