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

This chapter discusses the histological and ultrastructural changes occurring in the agroinfected cuttings of a recalcitrant walnut genotype cultured under conditions inductive for rooting to determine the way agrobacteria trigger root formation in the stem of a recalcitrant woody cutting, their migration in the explant, and the cytological events resulting from the combined presence of infection and exogenous auxin. 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.