Have you noticed how news of discoveries in animal research, whether in genetics, neuroscience or pharmacology, is usually reported from studies in mice, flies, frogs, or fish? These are considered by researchers as some of the “model organisms” or “model systems” for scientific studies. Basically, they choose an organism for their experiments as a standard or reference, or even as a substitute. Now, heated arguments about animal testing or mammalian experiments are well known, but have you ever wondered about model plants used in experiments for botanical research? It sure is surprising how so little limelight seems to fall on the silent plants which have ultimately led to meet many of our present-day needs and comforts, such as improved food crops, modern medicines, and biofuel sources, to name a few.
If you visit a park or a garden or depending on where you live, just look outside your window, you would easily appreciate the vast diversity of plants. From mosses spreading over a brick wall to the tallest trees among bushes and grasses, plants greatly differ from each other in size, shape, spread, lifespan, life processes, defense mechanisms, and whatnot. So obviously, one plant species does not fill the bill as a model for every kind of experiment. Different organisms are used as model systems for different research topics. Some of the criteria considered in selecting a model plant includes – how easy is it to grow and maintain? how well does it suit the conditions of this study? how long does it take to produce the next generation? and so on. So, depending on the aim of their research, a considerable variety of species have been used in understanding various aspects of plant life, while new ones are suggested from time to time. Yet, these special plants are rarely mentioned outside of scientific publications. Do you recognize any of them in the list below? (Hint: Some of them grow almost everywhere like weeds.)
- Common liverwort or umbrella liverwort (Marchantia polymorpha)
Common liverworts can easily be spotted forming a green to greenish-purple mat on a moist surface almost anywhere in the world. It has been used as a Bryophyte (or “primitive plants”) model system for nearly two centuries. The details of how plants moved from living underwater (as aquatic plants) to above the ground (as terrestrial plants), and later evolved and diversified into “higher plants” (with well-developed veins and vessels for transporting fluids, that is to say, a ‘vascular system’ or ‘vasculature’) are inspected by some scientists in these damp green patches of liverwort.
- Spikemoss (Selaginella moellendorffii)
Spikemoss is one of the oldest surviving ‘cosmopolitans’ on the planet. Staying alive in cold and dry alpine environments as well as hot and moist tropics, these fern-like beings belong to a group called ‘lycophytes’ which were leaders in developing the first leafy structures (called ‘microphylls’) in land plants. Having a clearly defined “plant feature” in their profile has led to spikemoss being assigned as a model system for studying the evolutionary steps leading to vascular development in land plants.
- C-Fern® (Ceratopteris richardii)
C-Fern® is not a common fern. It is a specially derived variety of an unusual tropical water fern (Ceratopteris richardii) found in wet areas like ponds, ditches, rivers, and rice fields. People grow and eat the naturally growing, wild Ceratopteris in some southeast Asian countries, but perhaps more famously, it is known as ‘water sprite’ in aquariums! C-Fern® however, was specifically selected for scientific use as a model for investigating the unique nature of life cycles in terrestrial plants (and some algae) called “alternation of generations” (which are cycles of distinct sexual and asexual divisions).
- Thale cress or mouse-ear cress (Arabidopsis thaliana)
Arabidopsis thaliana (yes, this one happens to be more commonly referred by its scientific name rather than its “common name”) is a flowering plant with a short life cycle and small body. It has been found growing in all kinds of terrain across Eurasia, North America, and Africa. It is a member of the mustard family, which includes mustard (obviously!), cabbage, cauliflower, as well as garden cress (the “baby greens” used in salads and sandwiches). Its resemblance to many crop plants and great overall features for laboratory use have undoubtedly made Arabidopsis thaliana the model system of choice for a large majority of plant biology research. Besides, recognizing the usefulness of some peculiarities among various Arabidopsis relatives has prompted the emergence of several different Arabidopsis-models. For example, Arabidopsis halleri is used for studying heavy metal tolerance, and Arabidopsis lyrata for understanding self-incompatibility in plant sexual reproduction. In fact, Arabidopsis is so popular among plant-scientists that the number of academic publications per year based on studies using Arabidopsis beats those from all other model plants, fungi, and non-mammalian animals. In short, “Arabidopsis” is one plant worth remembering and identifying.
- Barrelclover or barrel medic (Medicago truncatula)
Barrelclover is a legume plant, which is often the kind of plants that form tiny wart-like nodules in their roots to host friendly soil bacteria. The plants and bacteria help each other out; the plants give the bacteria their favorite foods (organic acids), and the bacteria fix nitrogen gas in the atmosphere into a nutritious form for the plants. This fascinating relationship is important to understand in as much detail as possible because not all plants have this special ability to form a healthy friendship with their bacterial neighbors. If they did, then farmers would not need to spend so much time and money on compost and fertilizers for crops, especially cereal crops. So barrelclover, a relative of the alfalfa bean plant, with its small size, production of many seeds per plant, and of course the ability to form beneficial bacteria-housing nodules, has been selected as a model organism to study ‘biological nitrogen fixation’.
- Purple false brome (Brachypodium distachyon)
Purple false brome is a grass species native to southern Europe, northern Africa, and southwestern Asia to the east of India, though now found to be widely spread by human activity. It follows a photosynthetic mechanism called ‘C3 photosynthesis’ similar to rice, wheat, and Arabidopsis. A major hurdle for genetic research with cereal crops is their humungous genome sizes. With a much smaller genome as well as other Arabidopsis-like suitability for laboratory handling, purple false brome is considered an ideal substitute for rice or wheat-related research.
- Green foxtail (Setaria viridis)
Green foxtail is another model grass species native to Eurasia but now widely spread as an invasive weed. This grass follows a photosynthetic mechanism similar to the corn plant, called ‘C4 photosynthesis’, which is a specialty of plants growing in mostly hot and dry conditions. Also with a small genome size, green foxtail is used as a substitute for corn-related research, and as a model for studying the adaptations and life processes of desert grasses or C4 plants as well.
- Common duckweed (Lemna minor)
Common duckweed is a tiny aquatic plant growing on the surface of ponds, lakes, and canals. They form the wide, green mats commonly seen on stagnant water bodies that have been left undisturbed for a long time. Being easy to grow in large quantities, it serves as a model system for looking into the potential of freshwater plants as a biofuel source. Additionally, since the little plants are tender and highly sensitive to environmental toxins, they have been used for ecological studies on industrial effluents.
- Black cottonwood or California poplar (Populus trichocarpa)
California poplar is well-known because of being a fast-growing tree (~5 years), and so a valuable source of timber. In addition to a short growing and maturing time (for a tree!), the saplings are also surprisingly adaptable for laboratory procedures and have been used as a model for understanding things about trees, like wood formation, response to soil salinity, and synthesis of special biochemicals that are toxic to harmful insects and microbes (which is termed as ‘phytochemical defense strategies’).
- Flooded gum (Eucalyptus grandis)
This hardwood Eucalyptus tree species, primarily found in Australia, is a model for tree-related genetic studies, or to be precise – ‘comparative angiosperm tree genomics’. Research about unique oils and other chemical compounds specially produced by some trees is also investigated in this species.
In conclusion, the natural world is home to a great number of plant species which fall into many categories, and so the models for their study are many and varied. Among them, these are ten uniquely interesting plants that draw the attention of plant-scientists all over the world. Whether these established models will continue holding their focus, or be replaced by better models as research progresses, or else be completely abandoned in favor of exclusively crop-based methods, only time will tell. For now, it is worth noting that these particular plants have contributed immensely to humanity by aiding scientific research and are continuing to do so. And this is over and above the fact that in general, plants make it possible for nearly all life on earth to exist.
a Photograph by Vivienne Palmer https://bugwomanlondon.com/2016/08/03/wednesday-weed-star-headed-liverwort/
b Photograph by Gribskov, distributed under a CC0 1.0 license
c Image adopted from C-Fern Web Manual by Leslie G. Hickok and Thomas R. Warne
Copyright © 1998 http://www1.biologie.uni-hamburg.de/b-online/library/cfern/cfern.bio.utk.edu/manual/manual.html
d Photograph by Frost Museum, distributed under a CC BY 2.0 license
e Photograph by Ninjatacoshell, distributed under a CC BY-SA 3.0 license
f Photograph by Neil Harris, distributed under a CC BY-SA 4.0 license
g Photograph by Wendy VanDyk Evans, Bugwood.org, distributed under a CC BY-NC 3.0 US license
h Photograph by phrakt, distributed under a CC BY-NC 2.0 license
i Image adopted from Joint Genome Institute news releases https://jgi.doe.gov/signatures-selection-inscribed-poplar-genomes/
j Photograph by Wrigley, J.W., image credit to Australian National Botanic Gardens