Histological Changes Associated with the Graft Union Development in Tomato

Abstract

Despite the importance of grafting in horticultural crops such as tomato (Solanum lycopersicum L.), the structural changes that occur during the graft establishment are little understood. Using histological techniques, the present work examines the time course of changes on the anatomical structure of the graft junction in functional tomato homografts and compares it to that of heterografts and non-functional grafts. No apparent differences were detected between homo- and heterografts, showing similar tissue development. At 10 days after grafting, the cell walls of the scion and rootstock in the area of the graft junction were thicker than usual. Undifferentiated cells and new vascular tissue emerged from the pre-existing vasculature. Adventitious roots appeared mainly on the scion, arising from the pre-existing vasculature. At 20 days, more pronounced vascular tissue was visible, along with large areas showing vascular connection. At 210 days, vestiges of the changes undergone in graft development were still visible. Generally, non-functional grafts presented layers of necrotic remains and deposition of cell wall material in the cut edges, impeding the suitable scion-rootstock connection. Our results show that accurate changes in pre-existing vasculature and the cell walls of the adhesion line are crucial to the development of functional grafts.

Tomato Graft Union Failure Is Associated with Alterations in Tissue Development and the Onset of Cell Wall Defense Responses

“…Functional grafts 10 DAG showed callus cell clusters (undifferentiated cells), adhesion between scions and rootstocks, and vascular reconnections (Figure S2), all of which were generating the union callus (Figure S2a-d). These findings are compatible with the previous works about tomato graft histology [12,[25][26][27] and the general hypothesis of callus generation in graft unions [14]. Additionally, they shed light on graft tissues development, especially with regard to how vascular connections are made.…”

“…The formation of callus cell clusters and new vasculature were located close to the pre-existing vasculature (Figure 2d,e,g-i), just like it was identified in functional tomato grafts [12]. The primary origin of callus cell clusters is most likely the pre-existing vasculature.…”

“…Furthermore, the most distal extremes of the pre-existing vasculature remained inert, playing no active part in the response to grafting (Figure 2e,g-i). These three methods of vasculature development have also been described in functional tomato grafts [12], suggesting that vascular regeneration depends strongly on the pre-existing vasculature. Auxins play a key role in the new vascular tissue formation [33][34][35].…”

“…The seeds of commercial varieties "Minibel" (Mascarel-Semillas S.L.) and "Marmande VR" (Semillas Batlle S.A.) were sown in individual recipients with 170 ± 10 mL of peat substrate and placed on plastic trays, as previously reported [12]. Hoagland nutritive solution was used for watering and the substrate was maintained near to the field capacity.…”

“…During grafting, there is an abrupt interruption in the exchange of substances between rootstock and scion, resulting in an asymmetry that affects the tissue development and physiological status of both graft partners. The success of the graft will depend on an orderly process of regeneration involving: (1) tissue damage response, cell adhesion, and callus formation; (2) tissue cohesion and vascular cell differentiation; and, finally, (3) vascular reconnection [7][8][9][10][11][12][13][14]. The success of the graft is also dependent on factors such as grafting method, graft healing conditions (especially adequate conditions of humidity and temperature), previous physiological status of the plants, etc.…”