Histological and ultrastructural analysis of A. rhizogenes-mediated root formation in walnut cuttings

The induction of adventitious roots is crucial for vegetative propagation in plant or micropropagation in vitro of many woody species, yet many woody species are totally recalcitrant to rooting This recalcitrance may be associated with various histologically detectable factors, such as the inability of the explant cells, after stimulation, to organize root meristemoids. In English walnut (J. regia L.), the propagation by grafting on seedling rootstocks is an expensive process, yet the alternative practice of grafting on black walnut seedling rootstocks makes trees vulnerable to the lethal blackline disease. In vitro, root formation may be a response to wounding per se or it may be associated with the presence of root inducers, such as auxin. However, for recalcitrant woody species, auxin may not induce root formation. As an alternative or complementary strategy, localized infection with Agrobacterium rhizogenes may be used and has recently been adopted for walnut microcuttings.