THE 3-DIMENSIONAL STRUCTURE OF VASCULAR TISSUES IN AGROBACTERIUM TUMEFACIENS-INDUCED CROWN GALLS AND IN THE HOST STEMS OF RICINUS-COMMUNIS L

The three-dimensional pattern of phloem and xylem in 10-d- to two-mont h-old tumors induced by Agrobacterium tumefaciens (C58) and in adjacen t Ricinus communis L. stem tissues was studied in thick sections by cl earing with lactic acid and by staining with lacmoid. The crown galls contained two types of vascular strands: treelike branched bundles, wh ich developed towards the tumor surface in fast-growing regions, and g lobular bundles in the slowly developing parts. Both types of vascular bundles contained xylem and phloem and were continuous with the vascu lar system of the host plant. The tumor bundles were interconnected by a dense net of phloem anastomoses, consisting of sieve tubes but no v essels. These vascular patterns reflect the apparent synthesis sites, concentration gradients and flow pathways of the plant hormones additi onally produced in the tumors upon expression of the T-DNA-encoded gen es. The A. tumefaciens-induced crown gall affected vascular differenti ation in the host stem. In the basipetal direction, the tumor induced more xylem differentiation directly below it, where the crown-gall bun dles joined the vascular system of the host. In the centripetal direct ion, the crown gall caused the development of pathologic xylem charact erized by narrow vessels, giant rays and absence of fibers. On the oth er hand, most probably as a consequence of its gibberellic acid conten t, the host plant stimulated a local differentiation of regenerative p hloem and xylem fibers with unique ramifications, only at the base of the tumor. However, fibers were absent from the main body of the crown gall. The study shows that A. tumefaciens-induced crown galls are cha racterized by a sophisticated network of vascular tissues in the tumor and are accompanied by a perturbated vessel system in the host. The h ormonal mechanisms controlling vascular differentiation in the tumor a nd neighboring host tissues are discussed. In addition, the ''gall con striction hypothesis'' is proposed for explaining the mechanism which gives priority in water supply to the growing gall over the host shoot .