[Progress in research on plant graft-induced genetic variation]

Intercropping with post-grafting generation of Solanum photeinocarpum decreases cadmium accumulation in soybean (Glycine max)

A pot experiment was designed to explore the effects of different post-grafting generations of Solanum photeinocarpum Nakamura et Odashima intercropping on growth and cadmium (Cd) accumulation in soybeans (varieties: "Zaodou" and "Liaoxian"). Post generation of S. photeinocarpum (ungrafted, grafted on eggplant, potato, and tomato, respectively) were utilized to intercrop with two varieties of soybean in Cd-contaminated soil. Soybean monoculture was employed as a control. Consequently, intercropping with different post-grafting S. photeinocarpum generation, except for tomato rootstock grafts post-generation, could reduce soybean biomass and photosynthetic pigment content. Additionally, all S. photeinocarpum post-grafting generations had the capacity to reduce Cd content in soybean when intercropping, while tomato rootstock grafts post-generation exhibited an adequate ability to accumulate Cd in S. photeinocarpum compared to the ungrafted treatment. In particular, tomato rootstock grafts post-generation could effectively decrease Cd content in soybean organs by 14.09-62.13%, relative to soybean monoculture, but increased shoot Cd content and shoot Cd extraction of S. photeinocarpum by 10.33-13.49% and 10.38-12.03%, respectively, compared to the ungrafted treatment. Thus, tomato rootstock grafting may enhance the ability of post-grafting generation of S. photeinocarpum to remediate Cd-contaminated soil, and this grafting was able to reduce Cd accumulation in soybean.

Effects of mutual grafting on cadmium accumulation characteristics of first post-generations of Bidens pilosa L. and Galinsoga parviflora Cav

We studied the effects of mutual grafting on cadmium (Cd) accumulation characteristics on the first post-generations of the Cd-hyperaccumulator plants Bidens pilosa L. and Galinsoga parviflora Cav. The seeds from scions and rootstocks of B. pilosa and G. parviflora were collected and planted in Cd-contaminated soil in pot and field experiments. In the pot experiment, rootstock treatment increased the shoot biomass of B. pilosa post-grafting generations, compared with ungrafted B. pilosa, but decreased the Cd content in shoots and Cd extraction by shoots of post-grafting generations; scion treatment decreased or had no significant effect. Mutual grafting resulted in no significant differences to the photosynthetic pigment contents in B. pilosa post-grafting generations. Compared with ungrafted G. parviflora, scion treatment increased the shoot biomass, photosynthetic pigment content, and Cd extraction by shoots of G. parviflora post-grafting generations, but rootstock treatment did not lead to significant differences. Mutual grafting resulted in no significant differences to the Cd contents in shoots of G. parviflora post-grafting generations. In the field experiment, only rootstock treatment increased the shoot biomass of B. pilosa post-grafting generations, and only scion treatment increased the shoot biomass and the Cd extraction by shoots of G. parviflora post-grafting generations. Therefore, mutual grafting of scions may enhance the phytoremediation ability of G. parviflora first post-grafting generations.

JA but not JA-Ile is the cell-nonautonomous signal activating JA mediated systemic defenses to herbivory in Nicotiana attenuata

The whole-plant activation of defense responses to wounding and herbivory requires systemic signaling in which jasmonates (JAs) play a pivotal role. To examine the nature of the slower cell-nonautonomous as compared to the rapid cell-autonomous signal in mediating systemic defenses in Nicotiana attenuata, reciprocal stem grafting-experiments were used with plants silenced for the JA biosynthetic gene ALLENE OXIDE CYCLASE (irAOC) or plants transformed to create JA sinks by ectopically expressing Arabidopsis JA-O-methyltransferase (ovJMT). JA-impaired irAOC plants were defective in the cell-nonautonomous signaling pathway but not in JA transport. Conversely, ovJMT plants abrogated the production of a graft-transmissible JA signal. Both genotypes displayed unaltered cell-autonomous signaling. Defense responses (17-hydroxygeranyllinalool diterpene glycosides, nicotine, and proteinase inhibitors) and metabolite profiles were differently induced in irAOC and ovJMT scions in response to graft-transmissible signals from elicited wild type stocks. The performance of Manduca sexta larvae on the scions of different graft combinations was consistent with the patterns of systemic defense metabolite elicitations. Taken together, we conclude that JA and possibly MeJA, but not JA-Ile, either directly functions as a long-distance transmissible signal or indirectly interacts with long distance signal(s) to activate systemic defense responses.

Interspecies rootstocks affect cadmium accumulation in postgrafting generation plants of potential cadmium-hyperaccumulator Solanum photeinocarpum

Pot and field experiments were conducted to study the effects of interspecies rootstocks on cadmium (Cd) accumulation characteristics of the potential Cd-hyperaccumulator Solanum photeinocarpum postgrafting generation plants. Four treatments (ungrafted and S. photeinocarpum seedlings grafted on the rootstocks of eggplant, potato, and tomato) were utilized in the present study. In the 2 pot experiments, eggplant and potato reduced biomass of S. photeinocarpum postgrafting generation plants, whereas tomato increased shoot biomass compared with the control (ungrafted seedlings). Compared with ungrafted seedlings, tomato increased and eggplant decreased Cd contents in shoots of S. photeinocarpum postgrafting generation. Only tomato increased Cd extraction by shoots of S. photeinocarpum postgrafting generation compared with ungrafted seedlings. In the field experiment, only tomato increased shoot Cd content of S. photeinocarpum postgrafting generation by 8.31% and shoot Cd extraction by 9.30% compared with ungrafted seedlings. Therefore, use of tomato as rootstock could enhance the ability of S. photeinocarpum postgrafting generation plants to remediate Cd-contaminated soil. Environ Toxicol Chem 2016;35:2845-2850. © 2016 SETAC.

Inter-species grafting caused extensive and heritable alterations of DNA methylation in Solanaceae plants

BACKGROUND

Grafting 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 FINDINGS

We 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/SIGNIFICANCE

Our 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.