Abstract
Grafting is a technique used for millennia for vegetative propagation, especially in perennial fruit crops. This method, used on woody and herbaceous plants, can improve several agronomic characteristics, such as yield or vigor, as well as tolerance to biotic and abiotic stresses. However, some scion/rootstock combinations suffer from poor graft compatibility, i.e., they are unable to form and/or sustain a successful graft union. Identifying symptoms of graft incompatibility is difficult because they are not always present in the first years after grafting and in most cases the causes of incompatibility are still poorly understood. Studies of changes in transcript abundance during graft union formation indicate that grafting responses are similar to responses to wounding and include the differential expression of genes related to hormone signaling, oxidative stress, formation of new vascular vessels, cell development, and secondary metabolites, in particular polyphenols. This review summarizes current knowledge of the changes in transcript abundance, redox status and metabolites accumulation during graft union formation and in cases of graft incompatibility. The goal of this review is to discuss the possibility of identifying marker transcripts, enzyme activities and/or metabolites of grafting success and graft compatibility which could be used to score grafting success for genetic research and in breeding programs. We highlight gaps in current knowledge and potential research directions in this field.
Highlights
Grafting is a traditional horticultural technique that manipulates plant wound healing mechanisms to join together two genotypes to form a composite plant
It is known that graft incompatibility is under genetic control (Salesses and Al Kaï, 1985; Salesses and Bonnet, 1992) and the parentage of a given genotype is frequently a likely indicator of its compatibility (Cordeau, 1998)
To begin to overcome these problems, recently graft union formation was assessed at 1 month and 1 year after grafting by scoring the necrotic line, and wood and bark discontinuity, and cellular arrangements at the interface in a bi-parental F1 apricot scion population grafted onto a plum rootstock (Irisarri et al, 2019)
Summary
Grafting is a traditional horticultural technique that manipulates plant wound healing mechanisms to join together two genotypes to form a composite plant. The first stage of graft union formation is the initial mechanical injury response (i.e., cellular damage and the disruption of the protective layers), which requires rapid wound closure to prevent water loss and pathogen entry. Polymerized phenolic compounds such as suberin and Molecular Markers of Grafting Success lignin accumulate to act as a physical and antimicrobial barrier at the site of wounds. There is the differentiation of the cambial cells into vascular vessels, which begins with the formation of phloem vessels in some herbaceous plants [from 3 days after grafting (DAG)] and xylem vessels (Trinchera et al, 2013; Melnyk et al, 2015) and allows the connection between scion and rootstock (Figure 1C)
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