By Scott H. Smith

Now and again discoveries happen that change the field of horticulture. In a recent study by Reeves et al. (2022, p.286) it was concluded that “all seed-bearing plant lineages have the competency to graft.” Until now this had never been proven and was widely believed to be impossible.

Dicotyledonous plants (dicots) and monocotyledonous plants (monocots) have many key differences. Among the differences are the fact that dicots have long been grafted in various ways throughout history. On the other hand, monocots have widely been believed to be impossible to graft using any method. Prior to the study by Reeves et al. (2022), vascular cambium was seen as the critical factor to grafting success (Gautier et al., 2018). This belief was so much so, that Professor Julian Hibberd describes the discovery of successful monocot grafting as “science at its best, where you find something out even though everyone says it’s not possible,” (Wilkins, 2021, [online]). So why is the discovery of successful monocot grafting important? How was it discovered? What real world applications does it yield?..

Importance of monocots

With monocot grain crops accounting for more than 50% of the world’s daily calorie consumption (Murphy & Zerbe, 2020), the importance of the discovery of monocot grafting spans beyond horticulture and may assist with United Nations predicted requirement to increase food production by 70% to meet the global population of 2050 (Population Council, 2009). This is largely because monocot grafting harbours the possibility of breeding tolerance in monocot plant crops to some of the current disease and environmental issues that they face. By breeding against such issues, crop yields can, potentially, be increased exponentially.

Successful monocot grafting discovered

The latest techniques used in the Reeves et al (2022) study, grafts the plants at very early stages in their life cycle in comparison to older methods unsuccessfully trialled. Several different methods were used in the study by Reeves et al. (2022) in order to secure successful grafts. The method used among members of the grass family (Poaceae) was by grafting the plumule of one type e.g., wheat (Triticum aestivum) to the radicle of another type e.g., oats (Avena sativa) whilst still at an embryonic, pre-germination seed stage. This forms a new plant that successfully has a graft union at the primary stem between plumule and radicle known as the hypocotyl (or mesocotyl in grasses), and allows for xylem and phloem exchange as per a normal plant and as per a typical successful dicot graft.

The study found however, that monocots outside of the grass family that produce seed can also be grafted successfully using the same method, with the exception that the graft is performed after germination (for some plants this was days after, for other slower growing plants this was weeks after) because most monocots outside of the grass family required time for the hypocotyl to grow long enough to sufficiently perform the graft. In plants that do not produce seeds readily, such as cultivated cavendish bananas (Musa acuminata ‘Dwarf Cavendish’), microscopic cavendish banana shoots (scions) grown as a lab culture were grafted to the severed embryonic roots (rootstocks) of wild banana (Musa acuminata) seed, and thereafter a normal banana plant was grown sharing the characteristics of both the scion and the rootstock. Reeves et al. (2022) have proven that the lack of vascular cambium no longer makes it an impossibility to graft monocots. By grafting with mature embryonic tissue of plumule and radicle at the hypocotyl/mesocotyl, which reprograms itself via meristematic tissue to form the connective xylem and phloem structures characteristic of a successful union (Turnbull & Carrington, 2022), monocot grafting is possible.

Proof of successful grafts came by using a fluorescent dye that meant it was possible to trace the dye moving across the plant from root to shoot and vice versa, in essence proving the graft had worked and that a vascular connection was established. Furthermore, not only was one species of monocot successfully grafted, but at least one species from each of the eleven orders that make up the monocot clade. In total, 37 different species of monocot plant were successfully grafted. This includes commercial crop plants such as pineapple (Ananas comosus), onion (Allium cepa) and wild leek (Allium ampeloprasum). It also includes ornamental plants valuable in production horticulture such as Indian shot (Canna indica), sweet flag (Acorus calamus) and New Zealand flax (Phormium tenax). Describing his inspiration for the study, Dr Greg Reeves said “I read back over decades of research papers on grafting and everybody said it couldn’t be done in monocots. I was stubborn enough to keep going, for years, until I proved them wrong,” (University of Cambridge, 2021, [online]).

Real world application: possibilities and limitations

Monocot grafting holds the potential to solve real, present day economic issues. One such issue within horticulture that it may solve, is alleviating issues from Panama disease (Fusarium oxysporum forma specialis cubense) in banana crops (University of Cambridge, 2021). With the world banana industry reliant on a single variety of banana, Musa acuminata ‘Dwarf Cavendish’, (Royal Botanic Gardens Kew, (n.d.)), the impacts of the study could assist in alleviating the fear of a global shortage of bananas due to the increasingly rapid spread of ‘Tropical Race 4’, an aggressive version of Panama disease (University of Cambridge, 2021). Due to the cavendish banana only reproducing via cloning, it means the crops are genetically identical and therefore especially vulnerable to disease (Wilkins, 2021). ‘Tropical Race 4’ currently threatens 80% of global production of bananas due to there being no known cure and no current banana varieties being resistant (Australian Government- Department of Agriculture, Water and the Environment, 2020). With the latest discovery by Reeves et al. (2022) however, the possibility to graft cavendish banana scions onto the disease resistant stems of wild banana was successfully accomplished thereby offering a potent potential solution to the threat of Panama disease (Turnbull & Carrington, 2022). This graft is therefore extremely relevant to the banana industry and to any member of public that indulges in a banana as part of their diet.

Successful monocot grafting also opens many doors of opportunity botanically, with part of the study investigating the gene expression of monocots during grafting i.e., how the plants communicate across the graft with hormone exchange. The study (Reeves et al., 2022) looked into how this worked and also used externally added hormones to some grafts, which in turn altered the scion growth. Real world applications for this includes scope to modify plant heights, increasing or decreasing of branching and the ability to accelerate when flowering begins (Turnbull & Carrington, 2022). This in itself hosts a wealth of possibilities for horticulture, with it being possible to modify monocot plant structure like never before and the theoretical possibility to increase plant production via earlier flowering and improving crop yield by restriction or addition of branching. Limitations of the discovery naturally exist for certain monocots however, with it being unfeasible to graft each individual seed of main crop staples such as wheat and oat prior to sowing. This is especially true given the millions of hectares dedicated to these crops globally (Turnbull & Carrington, 2022). Further research would be needed to make the United Nations goal of increasing crop yield to feed the growing population feasible (Population Council, 2009).

The discovery of successful monocot grafting is only the beginning of a journey which, with further research, has the potential to improve worldwide crop yields. As such, with theoretical hindsight monocot grafting has the potential to lessen global hunger, improve the economic performance of monocot crop producing countries and assist with the preservation of threatened cultivated crops such as the cavendish banana.

Scott H. Smith MCIHort, DipHort(RHS)

Head Gardener Pitmedden Garden

& Haddo House (National Trust for Scotland)

E:ssmith_101@hotmail.co.uk

Main image: Banana tree (Image:Piero Di Maria, Pixabay)

References

Australian Government- Department of Agriculture, Water and the Environment, (2020) Panama disease Tropical Race 4. [Online] Available at:   https://www.awe.gov.au/biosecurity-trade/pests-diseases-weeds/plant/panama-disease-tropical-race-4 (Accessed 16 April 2022)

Ersek, K., (2012) Monocots Vs Dicots: What You Need To Know. [Online] Available at: https://www.holganix.com/blog/monocots-vs-dicots-what-you-need-to-know
(Accessed 06 May 2022)

Gautier, A. T. et al., (2018) Merging genotypes: graft union formation and scion–rootstock interactions, Journal of Experimental Botany, November 2018,70 (3): 747-755. Available at: https://academic.oup.com/jxb/article/70/3/747/5210403?login=true

Murphy, K. M. & Zerbe, P., (2020) Specialized diterpenoid metabolism in monocot crops: Biosynthesis and chemical diversity, Phytochemistry, April 2020, 172(112289): 1-11. Available at: https://www.sciencedirect.com/science/article/pii/S0031942219312038

Population Council, (2009) FAO’s Director-General on How to Feed the World in 2050, [no place]: Population and Development Review. [online] Available at:  https://www.fao.org/fileadmin/templates/wsfs/docs/expert_paper/How_to_Feed_the_World_in_2050.pdf (Accessed 26 April 2022)

Reeves, G. et al., (2022) Monocotyledonous plants graft at the embryonic root–shoot interface, Nature, February 2022, 602 (7896): 280-286. Available at:     https://www.nature.com/articles/s41586-021-04247-y 

Royal Botanic Gardens Kew, (n.d.) Musa acuminata Cavendish banana. [Online] Available at: https://www.kew.org/plants/cavendish-banana (Accessed 16 April 2022)

Turnbull, C. & Carrington, S., (2022) A hard graft problem solved for key global food crops, Nature, 25 January, pp. 214-215. Available at: https://www-nature-com-s.caas.cn/articles/d41586-022-00050-5?error=cookies_not_supported&code=707bf7b6-18ac-4f17-a717-005d323c80c0 (Accessed 19 April 2022)

University of Cambridge, (2021) New grafting technique could combat the disease threatening Cavendish bananas. [Online] Available at:          https://www.cam.ac.uk/research/news/new-grafting-technique-could-combat-the-disease-threateningcavendish%20bananas#:~:text=Grafting%20is%20the%20technique%20of,vascular%20cambium%2C%20in%20their%20stem (Accessed 28 March 2022)

Wilkins, A., (2021) ‘Near impossible’ plant-growing technique could revolutionise farming. [Online]
Available at: https://www.newscientist.com/article/2302939-near-impossible-plant-growing-technique-could-revolutionise-farming/ (Accessed 16 April 2022)