Mechanisms of primordium formation during adventitious root development from walnut cotyledon explants

In walnut (Juglans regia L.), an otherwise difficult-to-root species, expla nts of cotyledons have been shown to generate complete roots in the absence of exogenous growth regulators. In the present study, this process of root formation was shown to follow a pattern of adventitious, rather than prima ry or lateral, ontogeny: (i) the arrangement of vascular bundles in the reg ion of root formation was of the petiole type; (ii) a typical root primordi um was formed at the side of the procambium within a meristematic ring of a ctively dividing cells located around each vascular bundle; (iii) the devel oping root apical meristem was connected in a lateral way with the vascular bundle of the petiole. This adventitious root formation occurred in three main stages of cell division, primordium formation and organization of apic al meristem. These stages were characterized by expression of LATERAL ROOT PRIMORDIUM-1 and CHALCONE SYNTHASE genes, which were found to be sequential ly expressed during the formation of the primordium. Activation of genes re lated, to root cell differentiation started at the early stage of primordiu m formation prior to organization of the root apical meristem. The systemat ic development of adventitious root primordia at a precise site gave indica tions on the positional and biochemical cues that are necessary for adventi tious root formation.