New review on molecular interactions of grafting
A successful graft depends on the compatibility between the rootstock and scion and the reconnection of the vasculature to maintain normal water and nutrient transportation. Previous work has explored the complex mechanisms involved in the formation of a successful graft union, and plant hormones have been given a pivotal role. Now a new review published in the journal New Phytologist has explored the molecular mechanisms underlying grafting.
The heritability of graft-induced phenotypic changes suggests that the regulatory processes underlying the scion–rootstock communication must also involve a genetic component. There a wide array of mechanism by which the transportation of genetic materials may occur across the graft site. The review, authored by a team from The Centre for Computational Biology at Beijing Forestry University explores these mechanisms using examples from the literature. These include horizontal gene transfer; epigenetic modifications such as DNA methylation or RNA gene silencing; and messenger RNAs (mRNAs), which may act as a regulatory signal or produce functional proteins in the scion or conversely the rootstock. Proteins have also been demonstrated to pass across the graft site acting as a chaperone of mRNA from source to sink tissues or as regulators of plant hormones.
Vascular reconnection was found to be a poorly understood area, although it is clear that hormones play a significant role. Both auxin and cytokines are reported by the authors to be the major hormones involved, as each plays a pivotal role in the promotion of wound healing and leaf vascular formation and development. The authors also highlight two transcription factors ALTERED PHLOEM DEVELOPMENT (APL) and OCTOPUS (OPS), which are reportedly involved in the induction of phloem differentiation. This is just the tip of the iceberg and there are likely multiple gene pathways involved in the vascular reconnection process.
“Identifying additional regulatory pathways and modifications of transcription factors during vascular reconnection may be challenging but will help us to better understand the mysteries of grafting,” say the authors.
As part of the review the team proposes a conceptual framework to model and analyze how these regulators interact and coordinate to reconnect and regenerate vasculature after grafting. They suggest comparing the differences in gene expression of scion–rootstock interactions between two sharply contrasting genotypes (A and B) which are self-grafted onto their own rootstocks and that of the reciprocal genotype to give four combinations (A × A, A × B, B × B, B × A). Comparing the level of change between the un-grafted (A × A, B × B) with the grafted (A × B, B × A) can be used to interpret the way the scion interacts with the rootstock to obtain the optimal fitness of each tissue, and will shed light on the evolution of the rooting system driven by the scion.
Looking at future research directions the authors highlight some key areas, including exploring the possibility of long distance transfer of DNA; achieving a better understanding of the regulation of hormone levels via trafficking events; understanding whether these mobile genetic elements interact with one another; and monitoring the transport of molecules at different post-graft time points.
Horticultural Science contains over 23,000 records on grafting, and over 655 results on the genetics of grafting.
Source: Horticultural Science