We confirm the hypothesis that Agrobacterium tumefaciensinduced galls produce ethylene that controls vessel differentiation in the host stem of tomato (Lycopersicon esculentum Mill.). Using an ethylene-insensitive mutant, Never ripe (Nr), and its isogenic wild-type parent we show that infection by A. tumefaciens results in high rates of ethylene evolution from the developing crown galls. Ethylene evolution from isolated internodes carrying galls was up to 50-fold greater than from isolated internodes of control plants when measured 21 and 28 d after infection. Tumor-induced ethylene substantially decreased vessel diameter in the host tissues beside the tumor in wild-type stems but had a very limited effect in the Nr stems. Ethylene promoted the typical unorganized callus shape of the gall, which maximized the tumor surface in wild-type stems, whereas the galls on the Nr stems had a smooth surface. The combination of decreased vessel diameter in the host and increased tumor surface ensured water-supply priority to the growing gall over the host shoot. These results indicate that in addition to the well-defined roles of auxin and cytokinin, there is a critical role for ethylene in determining crown-gall morphogenesis.Infection of sensitive plants by Agrobacterium tumefaciens is known to induce crown galls. Tumor growth is initiated by the integration and expression of the T-DNA of the bacterial Ti plasmid within the plant nDNA. The T-DNA encodes enzymes catalyzing the synthesis of high levels of auxin, cytokinin, and opines (Weiler and Spanier, 1981;Zambryski et al., 1989;Schell et al., 1994). Aloni et al. (1995) found that an A. tumefaciens-induced crown gall caused the development of pathologic xylem in the centripetal direction within the host stem of castor bean. This pathologic xylem was characterized by narrow vessels, giant rays, and an absence of fibers. A similar anatomy was induced experimentally in stems of elm seedlings by the ethylene-releasing agent ethrel (Yamamoto et al., 1987), indicating the possible involvement of the hormone ethylene in crown-gall development. Therefore, Aloni et al. (1995) suggested that the high auxin levels induced by the T-DNA-encoded genes iaaM and iaaH (Thomashow et al., 1984(Thomashow et al., , 1986 stimulate ethylene synthesis at the base of the crown gall, where high auxin streams originating in the tumor merge. Therefore, tumor-induced ethylene might affect crown-gall morphogenesis and differentiation of host tissues adjacent to the tumor.A. tumefaciens-induced crown galls cause poor xylem development in grapevine, which impairs water flow to the young parts of the shoot above the gall (Agrios, 1988). Aloni et al. (1995) proposed the "gall-constriction hypothesis" to explain the mechanism that gives water-supply priority to the growing gall over the host shoot. The hypothesis proposes that the growing gall retards the development of its host shoot by decreasing vessel diameter in the host, substantially reducing the supply of water to the upper parts of the shoot. ...

The regulatory effect of cytokinin on the formation of secondary xylem fibers was studied in the hypocotyl of young Helianthus annuus L. plants. Positive correlation was found between the kinetin supplied (0.25-0.5 micrograms/gram) to the growth medium and the rate of fiber formation within and between the vascular bundles. Reducing the root originated cytokinin supply, either by root removal or by lowering the transpiration rate, diminished the number of newly formed secondary xylem fibers. This decrease was considerably reversed in the presence of 0.5 microgram/gram kinetin. Early pulse exposure of kinetin had a temporary promoting effect on fiber differentiation at low concentrations and a permanent inhibitory effect at high concentration.Fiber differentiation is dependent on signals originating in the leaves (1, 2, 13). The signals for fiber differentiation flow in a strictly polar fashion from the leaves downward along several internodes and induce fibers along their flow (1, 2). The role of the leaves in primary phloem fiber differentiation in Coleus can be replaced, both qualitatively and quantitatively, by exogenous application of combined IAA and GA3. When various combinations of both hormones are applied, high concentrations of IAA stimulate rapid differentiation of fibers with thick secondary walls, while high levels of GA3 result in long fibers with thin walls (3).Cytokinin is both a limiting and controlling factor in the early stages of secondary xylem fiber differentiation in cultured hypocotyl segments of Helianthus, at the time when many cell divisions occur in the explants. Later stages of fiber differentiation can occur in the absence of cytokinin. At high concentrations, zeatin is more effective than kinetin (4). Kinetin increases the length of secondary xylem fibers in cultured wood of Adhatoda (12). Aloni (4) proposed that the mechanism which controls and determines the early stages of fiber differentiation is based on interaction of three major hormonal signals: IAA plus GA3 from the leaves with zeatin from the root apices. He also suggested that the basis for correlation between the development of the plant body and the differentiation of its supportive tissues is based on their common dependence on the same shoot/root feedback control signals.However, there is no evidence frotn intact plants on the role of cytokinin in fiber differentiation. Torrey et al. (17) suggested that the difficulty in proving the cytokinin involvement in in vivo systems is because most of the tissues and organs of higher plants contain a considerable level of root-supplied cytokinin throughout most of their life cycle. The difficulty in studying cytokinin participation in morphogenetic, as well as other processes in the plant is further increased by the lack ofanticytokinins and by the fact that excised plant organs or tissues are capable of synthesizing cytokinin under certain experimental conditions (8).We were able to show cytokinin involvement in fiber differentiation in intact young plants of Helian...