Hybridization by Grafting: A New Perspective?



In addition to whole-genome transfer, there is the matter of mRNA being transported through the sap. This can alter gene expression in a way that may be permanent (e.g., alteration of root growth habit in apple seedlings) or transmitted to seedlings.

Thank you for posting that link Karl. I’m reading the full text now and finding it of great interest.

Very interesting. I wonder if this has any relationship to the growth habits that vary when plants are grafted to different rootstocks, or are own-root?

Occams razor says there is no need to invoke epigemagic to explain the benefits of grafting. In fact it would be last in line behind water and mineral transport, hormone synthesis and transport, physical effects due to morphology…

Moreover, while it may be interesting if true, what practical use can we put graft hybridization (and epigenetics and so forth) to in our classical breeding programs?

Practical use-a rose rosette resistant species rose used as rootstock may/should give some resistance to a non resistant rose.

“Small interfering RNAs (siRNAs) are silencing signals in plants. Virus-resistant transgenic rootstocks developed through siRNA-mediated gene silencing may enhance virus resistance of nontransgenic scions via siRNAs transported from the transgenic rootstocks. However, convincing evidence of rootstock-to-scion movement of siRNAs of exogenous genes in woody plants is still lacking. To determine whether exogenous siRNAs can be transferred, nontransgenic sweet cherry (scions) was grafted on transgenic cherry rootstocks (TRs), which was transformed with an RNA interference (RNAi) vector expressing short hairpin RNAs of the genomic RNA3 of Prunus necrotic ringspot virus (PNRSV-hpRNA). Small RNA sequencing was conducted using bud tissues of TRs and those of grafted (rootstock/scion) trees, locating at about 1.2 m above the graft unions. Comparison of the siRNA profiles revealed that the PNRSV-hpRNA was efficient in producing siRNAs and eliminating PNRSV in the TRs. Furthermore, our study confirmed, for the first time, the long-distance (1.2 m) transfer of PNRSV-hpRNA-derived siRNAs from the transgenic rootstock to the nontransgenic scion in woody plants. Inoculation of nontransgenic scions with PNRSV revealed that the transferred siRNAs enhanced PNRSV resistance of the scions grafted on the TRs. Collectively, these findings provide the foundation for ‘using transgenic rootstocks to produce products of nontransgenic scions in fruit trees’.”


Interesting Henry, but I presume that would not permanently alter the genetics of the grafted portion as epigenetics could, correct?

Should we perhaps be hybridizing for better root stocks? (I have been aiming for roses that stand on their own feet.)

And if breeding for better cultivars, is it problematic that R. setigera crosses readily with multiflora derivatives, providing a common route for introducing the species’ genes into cultivars?

“Roses reported to be resistant to RRV are: R. setigera, R. aricularis, R. arkansana, R. blanda, R. palustris, R. carolina, and R. spinosissima. The interspecific hybrid, ‘Stanwell Perpetual’ (R. spinosissima and R. x damascena) is susceptible to RRV (Bruce Monroe, personal communication). Therefore progeny of crosses made with resistant roses may not be resistant.”

R. multiflora is the species that appears to be most susceptible to RRD. However, many species and selections
of cultivated roses are also highly susceptible, and no cultivars have been proven to be resistant. Although
the native species Rosa setigera is reported to be resistant to the disease, one grower has reported increased
susceptibility to powdery mildew on plants of R. setigera, which could indicate some level of infection by the
RRD pathogen.
“A species called the McCartney rose, which exists as a weed in Texas, is susceptible to RRD but resistant to
feeding by the mites that transmit the disease. It may be possible, through breeding techniques, to incorporate
this mite resistance into cultivated roses in the future. In the meantime, it would be wise to assume that all
cultivated roses are potentially susceptible to the disease and to be on the lookout for symptoms of rose

Plant Mol Biol. 2003 Nov; 53(4): 493-511.
Rootstock effects on gene expression patterns in apple tree scions.
Jensen PJ, Rytter J, Detwiler EA, Travis JW, McNellis TW
Like many fruit trees, apple trees (> Malus pumila> ) do not reproduce true-to-type from seed. Desirable cultivars are clonally propagated by grafting onto rootstocks that can alter the characteristics of the scion. For example, the M.7 EMLA rootstock is semi-dwarfing and reduces the susceptibility of the scion to > Erwinia amylovora> , the causal agent of fire blight disease. In contrast, the M.9 T337 rootstock is dwarfing and does not alter fire blight susceptibility of the scion. This study represents a comprehensive comparison of gene expression patterns in scions of the ‘Gala’ apple cultivar grafted to either M.7 EMLA or M.9 T337. Expression was determined by cDNA-AFLP coupled with silver staining of the gels. > Scions grafted to the M.9 T337 rootstock showed higher expression of a number of photosynthesis-related, transcription/translation-related, and cell division-related genes> , while > scions grafted to the M.7 EMLA rootstock showed increased stress-related gene expression> . The observed differences in gene expression showed a remarkable correlation with physiological differences between the two graft combinations. The roles that the differentially expressed genes might play in tree stature, stress tolerance, photosynthetic activity, fire blight resistance, and other differences conferred by the two rootstocks are discussed.

In this case, the rootstocks were shown to have altered gene expression in the scion. The researchers did not attempt to learn whether the altered gene expression would be passed on to offspring.

Plant Physiology 143(2): 1037-1043 (Feb 2007)
Cross-Species Translocation of mRNA from Host Plants into the Parasitic Plant Dodder
Jeannine K. Roney, Piyum A. Khatibi and James H. Westwood

An intriguing new paradigm in plant biology is that systemically mobile mRNAs play a role in coordinating development. In this process, specific mRNAs are loaded into the phloem transport stream for translocation to distant tissues, where they may impact on developmental processes. However, despite its potential significance for plant growth regulation, mRNA trafficking remains poorly understood and challenging to study. Here, we show that phloem-mobile mRNAs can also traffic between widely divergent species from a host to the plant parasite lespedeza dodder (> Cuscuta pentagona > Engelm.). Reverse transcription-polymerase chain reaction and microarray analysis were used to detect specific tomato (> Lycopersicon esculentum > Mill.) transcripts in dodder grown on tomato that were not present in control dodder grown on other host species. Foreign transcripts included LeGAI, which has previously been shown to be translocated in the phloem, as well as nine other transcripts not reported to be mobile. Dodders are parasitic plants that obtain resources by drawing from the phloem of a host plant and have joint plasmodesmata with host cortical cells. Although viruses are known to move between dodder and its hosts, translocation of endogenous plant mRNA has not been reported. These results point to a potentially new level of interspecies communication, and raise questions about the ability of parasites to recognize, use, and respond to transcripts acquired from their hosts.

Pollen tubes are, in effect, parasites that derive nourishment from the stylar tissue. I think it is likely that they also swap mRNA (and possibly other “species” of RNA) with the host. Furthermore, if pollen tubes from different pollen parents are growing in the same style, there is the possibility of transfer of RNAs between them. This could lead to altered gene expression in the pollen tubes, as well as in the embryos they “father”.

That is, even though only one pollen tube fuses with a single ovum, the resulting embryo/plant could express certain qualities from a different pollen parent. The offspring would carry genes of only two parents, yet gene expression could be modified by a third.

Cook: Hybridizing (1907)
I have several [rose] seedlings, where the pollen was taken from three and four different varieties mixed together, and they are the richest color in red of any I have ever raised.
Untitled Document

Wichura on Hybrids (1866) p. 73
Wichura confirms Gaertner in the assertion that where hybrid pollen is used for the impregnation of simple or complicated hybrids, as also in pure species, there is a great predominance of individual forms, while hybrid ovules impregnated by the pollen of pure species, even in the most complicated combinations, give very uniform products.
Berkeley: Wichura's Hybrids (1866)

These two statements cannot be reconciled with the neo-Mendelist doctrine [heredity = genetics], but make sense if mRNA (or other RNAs) can be shared among pollen tube growing together in the style, thus influencing gene expression [epigenetics].

There are other aspects of heredity that have not been studied using modern techniques of analysis.

Beaton (1861)
In the great bulk of the Scarlet or Horseshoe Geraniums there are but seven stamens, four long ones, one of medium length, but which is often wanting, and two almost sessile like the anthers of Wheat—that is, very short indeed, and opening at the bottom face to face. These two are they which reduce a whole family to beggary; first to dwarfs or Tom Thumbs, or better still, to minimums, or the smallest of that kind consistent with vigour sufficient to become a useful plant in cultivation, and, lastly, to the brink of ruin, and drive that race out of existence altogether, if there were not other means provided to arrest the decline, or keep it from manifesting itself at all in a state of Nature.
Beaton: Pollen of the same flower (1861)

Anderson-Henry (1861)
Mr. Beaton as the first, perhaps, to find out, and certainly the first so far as I know, to announce this strange discovery, is entitled to its full merit. Its full value has not yet been sufficiently tested. For although I have produced the tiny things in the Rhododendron family which he has done with Pelargonium, inquiry should not stop here. And for my part I did not limit my aim merely to produce by them more dwarfish plants than the parents. Regarding as I did, the pollen of these small anthers as of finer particles than the pollen of the longer and larger ones, I used it as a provision of Nature’s own suggesting, in preference to the latter in crossing the smaller species whose pollen-tubes I feared might not admit the grosser globules of these larger anthers. And when the two dwarf stamens failed, I used the smallest and shortest of the remaining stamens.
Anderson-Henry: Variegation, Muling, Short-stamens (1861)

Bradley: Hippeastrums (1906)
Seeing that the top division of the perianth is always the largest and best coloured, I generally use the anther, the filament of which is adnate to this division; whether this be the reason or not I do not know, but the progeny generally have more equal divisions to the perianth, and the bottom division is greatly improved.
On the other hand, with a view to getting as white a bloom as possible, I use the bottom division (generally all white) from the white red-striped varieties; and in the seedlings the flowers have much less colour; but the shape of the bloom is spoilt, the divisions being narrow.
Bradley: Hybridising at the Antipodes (1906)

It is worth noting that Beaton’s dwarf pelargoniums also had narrower petals.

I see no plausible reason to suppose that pollen from the short stamens would be uniformly, genetically different from pollen in the long stamens of the same flowers. Nevertheless, the offspring are different, and the differences are then passed along, at least in part, to the next generation.

Then there is the matter of “Physiological Predetermination”.

Kidd & West (1918):
IT may not always be fully realised to what a degree the developmental capacity of plants is pre-determined by the action of environmental conditions during the earliest stages of their life-histories. During the course of germination and in the seedling stage, or even earlier during the sojourn of the seed upon the parent and in the dormant period, the “potentialities” of plants may be affected by actions which only subsequently produce visible results. These results appear during the later stages of development, without reference to the conditions then existing. In this way adverse conditions in the later stages of development may not suppress a vigour of growth which has been pre-determined under favourable conditions during the ripening of the seed in the previous year; or favourable conditions during summer may fail to increase the yield owing to adverse conditions which have previously operated during the period of germination. It is such effects, namely those which are to be traced to the environmental conditions which have operated in the past stage of the plant’s life, that we may term effects of physiological pre-determination in order to mark their distinction from those which are due to hereditary causes.

Kidd and West did not study the influence of pollen from different stamens, but I think the results already mentioned can be included under “Physiological Predetermination”.

Ripeness of seed has also been shown to influence the qualities of plants grown from them. Van Mons (1835) collected unripe pears from forest trees, allowed the fruit to ferment, then sowed the seeds in his nursery. Wild pears began fruiting in 10-12 years. After 5 generations of trees raised according to Van Mons’ methods, there were trees flowering and fruiting in 3-4 years from seed. He also worked his “magic” on wild roses, similarly reducing the time from seed to flower.

Goff (1892) had similar results with tomatoes.

Tayler: Germination of the Morning Glory (1906)
“… plants grown in a green-house from immature green seeds blossomed earlier, had shorter stems and produced fewer seed-pods by about one-half than did those raised under the same conditions from seeds having no chlorophyll in the embryo. When the plants so grown from immature green seeds had ceased to blossom, those raised from mature colorless seeds were thrifty and still forming buds and maturing flowers and fruit. Both kinds of seeds were planted at the same time.”

Taylor, however, did not continue the experiment to a second generation.

Tourney: Seeding and Planting (1916)
In the spring of 1904, the author gathered 100 fruits of the silver maple 7 days before maturity and planted them at once. A week later an equal number of the mature fruits was gathered from the the same tree and planted. On the eighth day after planting 79 per cent of the immature seeds germinated, while 92 per cent of the mature seeds germinated on the sixth day after planting. The seedlings from the mature seeds were more robust and made more rapid growth. Two weeks after the mature seeds germinated the seedlings averaged more than twice as large as those from the immature ones.

Tourney did not discuss the age of maturity of his silver maple seedlings. Nor did he raise a second generation. He was only concerned with forest trees. However, if a city dweller wanted a silver maple as a yard tree, one grown from immature seed might be a better choice. And if the effect is cumulative, as it is in pears and tomatoes, one might even breed dwarf silver maples.

Again, I see no plausible genetic explanation for this repeatedly verified phenomenon. That seems to leave epigenetics. Maybe someone will get around to comparing the gene expression of tomatoes or morning glories raised from unripe vs. ripe seeds.

Philip, the last reference that you cite (link) that states no resistant species have been found is dated 2012. Your first link (although undated) appears to be very recent.
Concerning your question: “Should we perhaps be hybridizing for better root stocks? (I have been aiming for roses that stand on their own feet.)”

I suggest that research is needed to see if a modern rose grafted to one of the reported resistant species rose is resistant.

The first paper, “Observations on Rose Rosette Disease” cites “Gillett-Kaufman, J. 2014. Rose Rosette Virus (RRV)- Confirmed in Florida! - UF/IFAS Pest Alert” and was itself cited on November 11, 2014 by Elizabeth Lamb, “Roses are red but rose rosette virus will make you blue.”
rose rosette disease – Think IPM Blog

I think it is safe to say that the paper was published in 2014.

It would certainly be interesting to learn whether the alleged resistance of some species can be expressed in grafted scions, but maybe that is missing the point.

In the paper I cited previously, Jensen, et al. (2003), the M.7 EMLA rootstock altered expression of certain “stress genes” in the ‘Gala’ apple which gave that variety increased resistance to fire blight. This was not a case of specialized “fire blight resisting genes”, but a more generalized stress resistance.

One test that might be explored would be to cross species that are reputed to be resistant to RRD with susceptible type, backcross to the resistant parent (if it is really resistant) and look for segregation of resistance/susceptibility. One could then compare the siblings, looking for gene or gene-expression differences.

But before embarking on any such program, or a mass grafting program, it would be desirable to determine whether any species or cultivar is genuinely resistant. And if so, determine whether the resistance is directed at the virus or at the mite that spreads the virus.

Maybe the solution is as easy as developing a mite-repellant spray. Or identifying a predator that would gobble up the little vermin before they can spread the virus.