The distribution of the tissues in the scion and rootstock is essential in the correct establishment of the graft, in particular the vascular meristematic tissue (2,11). The stems of tomato have the typical structure of dicotyledonous, mesophytic plants, with a vascular cylinder that appears discontinuous in the early stages of life, but which later becomes continuous with the interfascicular regions (12,13). This distribution allows for grafting, unlike the dispersed vascular pattern of monocotyledonous plants (2,10,14).
The cells walls at the line of contact between the scion and rootstock commonly appeared thickened. These structures are thought to play a key role in graft establishment, taking part in the recognition and adhesion of the two sides of the graft (15–21). The increase in thickness is related to the deposition of wall polysaccharides by the protoplasts of nearby cells and the compacting of necrotic remains from the cut (20). The later thinning of these walls is accompanied by the restitution of the symplastic pathway owing to the formation of plasmodesmata (15,18,22) and the clearing of cell wastes (7,23).
The tissues close to the cut also undergo profound changes (2,10). In tomato, the callus is large, but this would not appear to have a bearing on the success of the graft; in other dicotyledonous species the callus can be very small (in some cases almost imperceptible), yet grafting is successful (23,24).
The callus is generated by the de-differentiation of cells close to the wound (10,25,26), but it is hard to say which types are most involved. In the present work, the location and appearance of the cells and tissues point to the callus is formed largely from live cells associated with the vascular tissue. This might be related to the preferential expression of the transcription factor WIND1 by the vascular meristematic tissue (26). WIND1 is involved in de-differentiation and callus formation via the activation of a signalling pathway in which cytokinins participate (26,27,28). Auxins also have the capacity to induce callus formation (25), although in graft establishment their induction of vascular differentiation is likely more important (29,30–34). These hormones, which travel in a basipetal direction (i.e., downward from the tip of the scion), move through the stem, transported by PIN-FORMED (PIN) transport proteins (35). These transporters can alter their position in the cell surface towards wounds (36,37). Their accumulation in the scion vasculature, provoked by the cut, would seem to induce nearby cells to become meristematic. The differentiation of new vasculature occurs in different directions, away from the pre-existing vascular tissues, and these new branches may connect with pockets of vasculature forming in the callus. As reported in other studies (15,22,38), vascular connections between the rootstock and scion are clearly visible at around 10 days after grafting. The connection of the scion and rootstock tissues leaves a permanent mark at the graft junction, especially in the vascular tissue.
The formation of adventitious roots by the scion is a response to the stress caused by the wound (39,40). The process is directly related to the blockage of the basipetal movement of auxins at the cut and their accumulation in that area too (41,42). The production of these roots is not helpful in graft establishment, indeed, rooting could prevent the graft being successful (42). However, it could be useful in nurse-rooting, a method of propagation that requires rooting occur (2,41). Sala et al. (40) identified two types of adventitious root, produced at different times after grafting: one with and one with no parenchyma envelope.