Plant grafting: insights into tissue regeneration

Abstract

For millennia, people have cut and joined different plants together through a process known as grafting. The severed tissues adhere, the cells divide and the vasculature differentiates through a remarkable process of regeneration between two genetically distinct organisms as they become one. Grafting is becoming increasingly important in horticulture where it provides an efficient means for asexual propagation. Grafting also combines desirable roots and shoots to generate chimeras that are more vigorous, more pathogen resistant and more abiotic stress resistant. Thus, it presents an elegant and efficient way to improve plant productivity in vegetables and trees using traditional techniques. Despite this horticultural importance, we are only beginning to understand how plants regenerate tissues at the graft junction. By understanding grafting better, we can shed light on fundamental regeneration pathways and the basis for self/non-self recognition. We can also better understand why many plants efficiently graft whereas others cannot, with the goal of improving grafting so as to broaden the range of grafted plants to create even more desirable chimeras. Here, I review the latest findings describing how plants graft and provide insight into future directions in this emerging field.

Keywords: Arabidopsis; callus; grafting; horticulture; regeneration; vasculature; wounding.

Figure 1

Grafting in nature and in horticulture. (A), (B) Ficus virens trees whose branches and stems naturally self‐graft through contact and fusion of tissues. (C) Two varieties of Prunus grafted together. (D) Two varieties of Tilia grafted together. (E) Grafts between a colorful cactus on top (Gymnocalycium) and a green cactus in the stalk (species vary).

Figure 2

Stem anatomy and grafting tools. (A) The arrangement of cell types in the Arabidopsis root and hypocotyl; the latter tissue is commonly grafted in science. Tissues including the cortex, cambium, endodermis, and pericycle, are thought to divide and differentiate in various plant species upon cutting to heal the graft junction (Jeffree & Yeoman, 1983; Melnyk et al., 2015). Wound‐induced callus is found throughout the cut surface. Adapted from Melnyk, 2016 with permission from Wiley. (B) In vitro grafting assays place short segments of stock and scion together to observe the nutrient and hormone requirements for successful connection (Parkinson & Yeoman, 1982). Nutrients and hormones are placed in agar at either end of the plate. The in vitro grafting system is a useful tool for identifying and characterizing factors that act systemically to promote or inhibit graft formation.

Figure 3

Grafting in tomato and Arabidopsis. (A), (B), (C) Callus forms between the stock and scion of tomato grafts 19 or 75 days after grafting (DAG). Stems were cut and stained with toluidine blue which stains xylem light blue (B), (C) (images by Matt Jacobs). The triangle or dashed lines denote the graft junction. (D)−(G) Less callus forms at Arabidopsis grafts and instead, the vascular tissues expand and divide (G). (F), (G) Scions expressing green fluorescent protein are grafted to rootstocks expressing tomato fluorescent protein. The triangle or dashed lines denote the graft junction. (H), (I), (J), (K) New xylem elements (denoted by white triangles) form across the graft junction (dashed lines) between 5 and 6 DAG in Arabidopsis thaliana. Image reprinted from Melnyk et al. (2015) with permission from Elsevier.

Figure 4

A framework for graft formation in Arabidopsis thaliana. Upon cutting, auxin, cytokinin, and sugar transport are blocked at the graft junction and a wound healing pathway is activated. The WIND1 pathway activates and produces callus whereas auxin response proteins (TIR1, AFBs, AXR1, ALF4) below the graft junction perceive scion‐derived auxin to promote vascular connection (Iwase et al., 2011; Melnyk et al., 2015). Vascular tissues divide and differentiate to reconnect the vascular tissues by 8 days after grafting (Melnyk et al., 2015). Wound‐induced callus is found throughout the cut surface. Time‐course data taken from Yin et al. (2012) and Melnyk et al. (2015).