Plant grafting and graft incompatibility: A review from the grapevine perspective

Grafting is a method for plant propagation and improvement. In the European viticulture, grafting is the sole control strategy against Phylloxera injuries and thus is of crucial importance for sustainable grape production. Despite its benefits, grafting is also a source for disease dissemination and graft incompatibility sometimes detected long after grafting can result in propagation losses. However, the physiology of grafting, such as compatibility factors, callus formation and healing processes, and the signaling components exchanged between scion and rootstock, are still not well understood by the scientific community. Recent insights in grafting research hint at a complex scion-rootstock communication. Molecules exchanged between rootstock and scion, such as hormones, metabolites, proteins, and RNAs coordinate the grafted plant parts and are suspected to modulate the healing of the union and to facilitate the regeneration of the vascular tissues. Such processes result in a successful grafted plant but in many cases, the graft develops distress symptoms, which can appear in early or late stages of plant growth eventually leading to a so-called ‘graft incompatibility’. It is not known whether the cause of incompatibility is based on a rejection of the opposing partners, on growth differences, or on the stress induced by grafting itself. One potential factor leading to graft incompatibility can be based on a deficiency of the recognition of graft-transmissible RNA signals shown to coordinate developmental and environmental shoot-root responses. This review summarizes the current knowledge on grafting from the perspective of viticulture, discusses the hypotheses behind graft incompatibility, addresses the molecular effects of grafting, and novel research perspectives that might help to unveil this millenary mystery.