Phytohormones modification

Genes encoding enzymes for phytohormone production (ipt, iaaH, iaaM) or other related genes such as rol A, B, C,D have been isolated from strains of Agrobacterium rhizogenes and A. tumefaciens and Pseudomonas (Tepfer 1984; Slightom et al. 1986; Spena et al. 1987; Cardarelli et al. 1987a; Schmulling et al. 1988; Capone et al. 1989). The ipt gene codes are for the isopentyl transferase, the first enzyme in the cytokinin biosynthetic pathway, while both iaa/M and iaa/H genes are codifying for enzymes (tryptophan-2 monoxigenase and indoleacetamide hydrolase respectively) involved in the pathway of IAA synthesis. Regarding rol genes, their functions as transcription products are not completely clear. RolB and rolC are probably responsible for the betaglucosidase activity and are able to release active auxines (rolB) and active cytochinines (rolC) from conjugated glucoside (Estruch et al. 1991a, 1991b). In addition a tirosine-phosphatase activity associated to rolB has been demonstrated, explaining the strong morphogenic action in root organogenesis (Filippini et al. 1996). Transgenic plants with chimeric construct with gus+rolB, revealed that the proteins of rolB and rolC are localised in the plasmatic membrane and in the cytosol respectively.

The stable integration of these genes in plants which include fruit crops, under constitutive promoter control, showed altered phenotype in morphology (usually except for rolB) and could alter resistance to diseases, positively or negatively, according to the prevalence of expression of auxins or cytokinins respectively (see Section 3.7). Fruit crops, such as pear and trifoliate orange, transgenic for rolABC, showed a reduction in size, in internode length and in leaf area (Kaneyoshi and Kobayashi 1999), and in active gibberellin synthesis. The association of more rol genes modifies morphology and biotic and abiotic stress resistance. Kiwi fruit expressing rolABC (Rugini et al. 1997, 2000b), as well as cherry rootstock Colt (Gutierrez-Pesce et al. 1998; Rugini and Gutierrez- Pesce 1999), apple (Lambert and Tepfer 1992), papaya (Rugini et al. 1994), expressing the T-DNA of A. rhizogenes, showed ‘hairy root’ phenotype and morphological similarity to tobacco transgenic for the oat phyA, having in common the internode length reduction, reduced apical dominance, late vegetative period and increased chlorophyll content (Wanger et al. 1991; Cherry et al. 1991a; Whitelam and Harberd 1994). Furthermore, the rol genes can determine alterations in floral morphology, probably due to the polyamine content variation and modifications in the architecture of the root system, reduced pollen and seed production, abundant and partially geotropic root system, increased rooting ability and juvenility reduction (Cardarelli et al. 1987; Jouanin et al. 1987; Spena et al. 1987; Vilaine and Casse-Delbart 1987; Vilaine et al. 1987). In addition, when used as rootstock, rol transgenes seem to influence scion by reducing growth, indicating that some of the products of those genes (primary product transcripts or translated or secondary products, induced from their expression) can migrate from the transgenic tissues to nontransgenic ones. Regarding rolD, this reduces the growth and promotes early blossom in tobacco (Trovato et al. 1997; Mauro et al. 1996). This gene may be a candidate for fruit tree transformation.