EXPERIMENTAL INDUCTION OF VASCULAR TISSUES IN CALLUS OF ANGIOSPERMS

Wetmore , Ralph H. (Harvard U., Cambridge, Mass.), and John P. Rier . Experimental induction of vascular tissues in callus of angiosperms . Amer. Jour. Bot. 50(5): 418–430. Illus. 1963.—Callus tissues in established maintenance culture lack morphological and physiological organization. Such callus consists of homogeneous parenchyma. Movement of auxin and sugar, therefore, must be along diffusion gradients. The only vascular tissues occurring in callus are induced. Experimental induction of vascular tissues has been successful in callus of 3 sp. of the Oleaceae: a tree, Fraxinus americana, and 2 shrubs, Syringa vulgaris and Ligustrum vulgare; another tree, Salix purpurea, var. lambertiana; a vine, Parthenocissus tricuspidata; and an herb, Helianthus tuberosus. In each of these species, an auxin (IAA or NAA in these studies) and a sugar (sucrose or glucose in these studies) prove necessary for the induction and complete differentiation of xylem and phloem in callus tissues. Varying concentrations of sugar alter the proportions of xylem to phloem: low concentrations, 1.5%–2.5%, favor xylem formation; high, 3%–4%, favor phloem. Middle concentrations, 2.5%–3.5%, favor the presence of xylem and phloem, usually with a cambium between. The almost universal association of xylem and phloem may have its explanation in this middle concentration of sugar. Grafting of apices into callus or direct application of appropriate concentrations of an auxin and a sugar in agar to the surface of callus causes nodules of vascular tissue to be formed, mostly in a circular pattern when seen in section transverse to the axis of orientation of the callus in the medium. The diameter of this circle varies directly with the auxin concentration at the place of application, 0.05 mg/liter giving a narrow, and 1 mg/liter, a wide pith. In individual nodules, xylem is characteristically oriented towards the center of the callus and the phloem towards the outside. Variable cross-sectional views of nodule distribution in calli under different treatments suggest experimental approaches to understanding stelar patterns. The induction and differentiation in callus of xylem and phloem tissues has no relation to conduction. Any use of vascular tissues can occur only after their induction.