Abstract
BackgroundGrafting has been extensively used to enhance the performance of horticultural crops. Since Charles Darwin coined the term “graft hybrid” meaning that asexual combination of different plant species may generate products that are genetically distinct, highly discrepant opinions exist supporting or against the concept. Recent studies have documented that grafting enables exchanges of both RNA and DNA molecules between the grafting partners, thus providing a molecular basis for grafting-induced genetic variation. DNA methylation is known as prone to alterations as a result of perturbation of internal and external conditions. Given characteristics of grafting, it is interesting to test whether the process may cause an alteration of this epigenetic marker in the grafted organismal products.Methodology/Principal FindingsWe analyzed relative global DNA methylation levels and locus-specific methylation patterns by the MSAP marker and locus-specific bisulfite-sequencing in the seed plants (wild-type controls), self- and hetero-grafted scions/rootstocks, selfed progenies of scions and their seed-plant controls, involving three Solanaceae species. We quantified expression of putative genes involved in establishing and/or maintaining DNA methylation by q-(RT)-PCR. We found that (1) hetero-grafting caused extensive alteration of DNA methylation patterns in a locus-specific manner, especially in scions, although relative methylation levels remain largely unaltered; (2) the altered methylation patterns in the hetero-grafting-derived scions could be inherited to sexual progenies with some sites showing further alterations or revisions; (3) hetero-grafting caused dynamic changes in steady-state transcript abundance of genes encoding for a set of enzymes functionally relevant to DNA methylation.Conclusions/SignificanceOur results demonstrate that inter-species grafting in plants could produce extensive and heritable alterations in DNA methylation. We suggest that these readily altered, yet heritable, epigenetic modifications due to interspecies hetero-grafting may shed one facet of insight into the molecular underpinnings for the still contentious concept of graft hybrid.
Highlights
Available records testify that grafting was already discovered and used in ancient China more than 2,000 years ago, and since the practice has been widely employed by horticulturists all over the world [1,2]
Relative global DNA methylation levels in both scions and rootstocks remained largely unaltered based on methylation-sensitive amplified polymorphism (MSAP) analysis
We found alterations at frequencies ranging from 1.4% to 8.4% of the four major methylation patterns that can be scored by MSAP [31], i.e., CG-hyper, CHG-hyper, CG-hypo and CHGhypo, occurred in the hetero-grafts eT1-3 and tE1-3 (Figure 2A and B)
Summary
Available records testify that grafting was already discovered and used in ancient China more than 2,000 years ago, and since the practice has been widely employed by horticulturists all over the world [1,2]. Even nowadays, grafting is still an indispensable means for boosting performance of horticultural plants including many important vegetables and woody fruit trees [3]. This method is generally conducted by grafting the shoot part of a plant (scion) onto a root part of another plant (rootstock) often with distinct genetic constitution (different species or genera). When Darwin put forward the concept of ‘‘graft hybrid’’ meaning that asexual combination of different plant species may generate products that contain features of both grafting partners, and are genetically distinct [11], they were met with skepticism, and being perceived as either nonexistence or a kind of chimeras [1]. It is interesting to test whether the process may cause an alteration of this epigenetic marker in the grafted organismal products
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