The precious pods of the vanilla orchid

By H. Zell licensed under CC BY-SA 3.0  via Wikimedia Commons.

By H. Zell licensed under CC BY-SA 3.0 via Wikimedia Commons.

At the mention of “vanilla”, the first things I think of are ice cream, cake and other tasty foods. Next, I think of the little bottles of vanilla extract or flavouring that I often use when baking. In turn, that makes me think of the brown, shrivelled vanilla pods I have occasionally used in particular recipes. Vanilla is a popular ingredient in many foods and perfumes, but where does it come from?

The answer lies in an orchid called Vanilla planifola (also known as flat-leaved vanilla), which originates from Mexico. The Totonac people of Mexico were the first to cultivate vanilla. In their mythology, this plant was born when Princess Xanat—whose father had forbidden her from marrying a mortal—fled to the forest with her human lover. The lovers were captured and killed, and it is said that the vine of the first vanilla plant grew from the ground where their blood landed (2).

This myth reflects how highly vanilla was prized in the Totonac culture. Flat-leaved vanilla is a climbing vine and its delicate greenish-yellow flowers give rise to vanilla pods that contain hundreds of tiny seeds (2). The pods take about 6-9 months to mature on the vine, and then they are harvested, dried and fermented to produce the distinctive vanilla flavour (1), so it easy to understand how the pods became precious to the Totonacs.

Vanilla also became popular with the the Aztecs after they invaded Totonac lands. When the Spanish arrived in South America and reached the Aztec leader Montezuma, the Aztecs believed that their creator had returned and presented gifts, including their favourite drink, which was made from cocoa beans (chocolatl) (1). This drink contained vanilla and other various flavours that were previously unknown to the Spanish. Not surprisingly, vanilla soon became popular across Europe.

The Spanish kept tight control of vanilla production until the early 1800s, when French entrepreneurs shipped vanilla plants to the Islands of Réunion and Mauritious in the Southern Indian Ocean (2). The early attempts to grow flat-leaved vanilla outside of South America were not successful because the vanilla flowers, which only last one day, were missing their natural pollinators. In 1841, a 12-year old slave called Edmond Albius developed a simple and efficient hand-pollination method using a small pointed stick (2). By 1898, Madagascar, Réunion, and the Cooris Islands produced about 80% of the world’s vanilla (2).

Two other species of vanilla orchid—V. tahitensis and V. pompona—are also cultivated to produce vanilla, but the majority of the world’s supply comes from flat-leaved vanilla (3). Today, the main producers are Madagascar and other islands in the Indian Ocean, and Indonesia. The flowers are still pollinated by hand and, since the production process is so labour-intensive, vanilla is the second most expensive spice in the world behind saffron (2).

Vanilla contains more than 250 active ingredients, but the main one that is responsible for its distinctive flavour is called vanillin. Vanilla is so expensive–and the flavour is so popular–that many of the vanilla-flavoured food products we eat are made from artificially-made vanillin. Currently, most vanillin is produced by converting a naturally occurring organic compound called guaiacol, but researchers are also developing methods to produce vanillin from other sources, for example, from a structural molecule found in most plants called lignin. Another possibility may be to use a soil bacterium to convert a compound found in sugar beet pulp, wheat/maize bran and other agricultural waste products into vanillin (4). Unfortunately, I’ve struggled to find out more about this research because much of the published literature appears to be behind paywalls (grr!).

Whatever methods we use to make vanillin, the demand for “real” vanilla in premium foods is likely to sustain the livelihoods of the people that cultivate flat-leaved vanilla for some time yet!

Flat-leaved vanilla is the Organism of the Month. Apologies for the delay in publishing this post, it has taken me longer to finish than planned due to a combination of a family holiday, illness and the demands of my growing garden!

  1. Laws, B (2010) Fifty plants that changed the course of history. David and Charles
  2. Wikipedia: Vanilla https://en.wikipedia.org/wiki/Vanilla (retrieved 16/07/15)
  3. Schmitt et al. (2015) Highly selective generation of vanillin by anodic degradation of lignin: a combined approach of electrochemistry and product isolation by adsorption. Beilstein Journal of Organic Chemistry. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4419547/
  4. Greener Industry Vanillin http://www.greener-industry.org.uk/pages/vanillin/8Vanillin_PM3.htm (retrieved 16/07/15)

Negative but not useless: the results of a GM wheat field trial

Image by by Bluemoose (CC-BY-SA-3.0) via Wikimedia Commons

Image by by Bluemoose (CC-BY-SA-3.0) via Wikimedia Commons

Last week it was reported that a GM wheat variety that was found to deter aphids in laboratory tests failed to do the same in field trials (1). Some opponents of GM technology have called the trial a “waste of over £1 million of public money” and said that the trial “confirms the simple fact that when GM tries to outwit nature, nature adapts in response” (2). Are these fair criticisms?

The GM wheat variety—which was developed by researchers at Rothamstead Research in the UK—can make an insect pheromone called (E)-β-farnesene. This pheromone is normally produced by aphids when they are under threat to warn other aphids so they disperse. Some of the natural predators of aphids (e.g. parasitic wasps) are also able to detect this pheromone and are attracted by it.

(E)-β-farnesene and other insect pheromones have the potential to be used in agriculture as an alternative to pesticides, which typically kill beneficial insects too. However, the pheromones evaporate easily at normal temperatures and break down quickly, so application of synthetically made pheromones in fields is costly and technically challenging (1). Therefore, genetically engineering plants to produce the pheromone themselves could be more effective.

To enable the wheat plants to produce (E)-β-farnesene, the researchers introduced the genes that encode two enzymes called farnesyl diphosphate synthase and (E)-β-farnesene synthase, which modify molecules found naturally in the wheat plants. Laboratory tests found that the (E)-β-farnesene produced by these plants deterred three species of aphids and also increased the foraging behavior of a species of parasitic wasp called Aphidius ervi that preys on aphids. Based on these promising results, a field trial was carried out in 2012/3.

Why were the wheat plants able to deter insects in the laboratory but not in the field? The conditions in the environment could have played a role: both summers of the field trial were cold and wet, and researchers observed that the populations of aphids and parasitic wasps in both years were low, well below the threshold levels of infestation that would be reached before farmers would spray the crop with pesticides.

Also, the way in which the wheat plants produced the pheromone is perhaps not ideal. Aphids only produce the pheromone when threatened, but the GM wheat variety produced (E)-β-farnesene all the time. In the field, this could have led to the aphids becoming used to the pheromone—like how fishermen become accustomed to the strong smell of fish—so that the pheromone ceased to act as a deterrent. Therefore, developing different varieties that only make the pheromone at particular points in time (e.g. during aphid attacks) may be more effective.

The researchers have shown that it is possible to produce GM wheat that can make an insect pheromone. Although this variety is ineffective at deterring aphids in the field, it provides useful insights that could be used to develop better varieties in future, or may inform other approaches to deter aphids from attacking crops. “Negative” results are very common in scientific research because it is impossible to be sure of the results of an experiment before you have tried it. Therefore, this field trial was not “a waste of money”, but a normal step in research.

While I agree that “nature adapts” to finds ways around human technologies, this does not mean that we should completely give up on GM technology. After all, we haven’t stopped trying to develop new antibiotics just because some bacteria have developed resistance to some of the existing ones. Our problems with antibiotic resistant bacteria have been exacerbated by the over-use of antibiotics, which have placed bacteria under stronger selection pressures to develop resistance. Similarly, growing a limited number of crop varieties in monocultures places selection pressures on pests to develop resistance to our attempts to deter, or kill them. Using a mixture of different technologies and farming practices is likely to be much more effective in improving crop yields in the long-term. Even if they never make it to market, GM crop varieties are useful for scientific research, which will inform our efforts to improve agriculture in future.

References:

  1. Bruce, TJA, Aradottir, BGI, Smart, LE, Martin, JL, Caulfield, JC, Doherty, A, Sparkers, CA, Woodcock, CM, Birkett, MA, Napier, JA, Jones HD and Pickett, JA (2015) The first crop plant geneticallyengineered to release an insectpheromone for defence. Scientific Reports.
  2. BBC News: UK GM wheat ‘does not repel pests’ by Claire Marshall http://www.bbc.co.uk/news/science-environment-33262885

Dog rose: more mother than father

Image by Anemone Projectors via Wikimedia Commons (CC BY-SA 2.0)

Image by Anemone Projectors via Wikimedia Commons (CC BY-SA 2.0)

In the UK at this time of year, you may well see pink or white flowers that belong to a prickly shrub called the dog rose (Rosa canina) peeking out of a hedgerow. Later in the summer these flowers will be replaced with orange-red fruits called rosehips, which can be used to make a variety of food products including syrup, tea and marmalade. Rosehips are a rich source of vitamin C, and during WWII, British schoolchildren were actively encouraged to forage for them (1).

The dog rose—which is native to Europe, North Africa and western Asia—also has medicinal properties and was used in many traditional remedies. The seeds contain a diuretic, its leaves contain a laxative, and the plant was even used to treat rabid dog bites. Roses also feature in other aspects of human culture. For example, the flowers of the dog rose and other wild roses inspired the stylized roses in medieval heraldry (e.g. the Tudor rose). The pleasant scent of rose flowers means they have been popular ingredients in perfumes for centuries, and cultivated “ornamental” roses are found in many gardens.

Cuttings of ornamental roses are often grafted onto established dog rose root systems to generate healthy and uniform-looking plants for gardens, therefore there is a lot of interest in understanding dog rose genetics. Like our genetic information, the DNA of the dog rose is arranged into structures called chromosomes. We have two sets of 23 chromosomes (one set inherited from each parent when an egg and sperm cell fuse). Dog roses, on the other hand, have five sets of seven chromosomes (35 in total) and only one set comes from the male parent (Figure 1). This is possible because dog roses and their close relatives make egg and sperm cells in a slightly different way to most other organisms.

Egg and sperm cells are made in a type of cell division called meiosis. During meiosis in humans and most other organisms, each chromosome pairs up with the matching (but not necessarily identical) chromosome from the other set. These chromosome pairs are separated before the cell divides to make daughter cells with half the number of chromosomes.

However, since dog roses have 5 sets of chromosomes, it is not possible for all of them to pair up. Instead, only two sets of chromosomes (14 in total) form pairs (Figure 1; reference 2). These “bivalent” chromosomes are then divided equally between the daughter cells. In the female part of the flower, the remaining 21 “univalent” chromosomes join one of the sets of bivalent chromosomes to make an egg cell with 28 chromosomes. However, in the pollen, the 21 chromosomes are left out of the daughter cells to make sperm that only have 7 bivalent chromosomes.

Figure 1: Meiosis in dog roses. Sperm cells with 7 chromosomes are produced in the pollen and egg cells with 28 chromosomes are produced in the female part of the flower. Fusion of an egg and sperm cell produces an embryo with the full number of chromosomes (35). The chromosomes that form pairs (bivalent) are highlighted in pink. Image by Ritz et al. (2011) licensed under CC BY 2.0.

Figure 1: Meiosis in dog roses. Sperm cells with 7 chromosomes are produced in the pollen and egg cells with 28 chromosomes are produced in the female part of the flower. Fusion of an egg and sperm cell produces an embryo with the full number of chromosomes (35). The chromosomes that form pairs (bivalent) are highlighted in pink. Image by Ritz et al. (2011) licensed under CC BY 2.0.

Research suggests that it is usually the same two sets of chromosomes that pair up during meiosis (3). This might be because the other chromosomes are not similar enough to be able to form pairs. It is also possible that these chromosomes are capable of pairing up, but are prevented from doing so by a control system within the cell. Therefore, over many generations, the genes contained within the univalent chromosomes are likely to become more different to their gene copies on the bivalent chromosomes—which are mixed and rearranged when they pair up during meiosis.

The chromosomes in dog roses appear to have originated from different ancestral species of rose. This suggests that the extra sets of chromosomes are the result of the production of hybrids (when members of two different species reproduce). Hybrids are often sterile (i.e. mules) because the genetic material they inherit from the two parent species is too different to allow the different sets of chromosomes to pair up properly in meiosis. However, this problem can be avoided by duplicating all of the chromosomes so those from each species can pair up separately. For example, durum wheat—which has 28 chromosomes—originated from two grass species that normally have two sets of seven chromosomes each.

Although many plant species have more than two sets of chromosomes, the dog rose and its close relatives are some of the only plants known to inherit unequal numbers of chromosomes from their male and female parents. The next challenge for researchers is to understand how this process works at a molecular level. In the meantime, it gives me another reason to admire the dog rose as I pass by…

References:

  1. Laws (2010) Fifty plants that changed the course of history. David and Charles.
  2. Ritz, CM, Köhhnen, I, Groth, M, Theiβen, and Wissemann, V (2011) To be or not to be the odd one out – Allele-specific transcription in pentaploid dogroses (Rosa L sect. Caninae (DC.) Ser) BMC Plant Biology http://www.biomedcentral.com/1471-2229/11/37
  3. Lim, KY, Werlemark, G, Matyasek, R, Bringloe, JB, Sieber, V, El Mokadem, H, Meynet, J, Hemming, J, Leithc, AR and Roberts, AV (2005) Evolutionary implciations of permanent odd polyploidy in the stable sexual, pentaploid of Rosa canina L. Heridity http://www.nature.com/hdy/journal/v94/n5/full/6800648a.html

The surprise potatoes

One of the potato plants I found in my garden. Two shoots and plenty of roots are growing from the tuber. Growing from the stems are several long white structures called stolens from which new tubers will grow later in the year.

A few weeks ago, I found some potato plants growing in the garden of my new home. The plants were completely surrounded by weeds but—after I nearly stabbed one with a garden fork—I realized what they must be when I noticed that the stems were growing from buried potatoes. Apparently, the previous occupants missed some of their crop when they harvested their potatoes last year.

Potato (Solanum tuberosum) is the world’s fourth most important food crop (behind rice, maize and wheat) and is grown across the globe in areas with temperate and sub-tropical climates. It belongs to the nightshade family of flowering plants, which also includes other crop species such as tomato, aubergine (also called eggplant) and chilli pepper (1).

Potatoes produce flowers that are either white, pink, red purple or blue, depending on the variety. These flowers can go on to produce small green fruits that look a bit like cherry tomatoes and each contain about 300 seeds (2). However, these fruits—like most of the rest of the plant—are poisonous because they contain the toxin solanine and several other glycoalkaloids.

The part of the plant that we call “a potato” is a structure known as a tuber, which is produced from the stems of the plant to store starch and other nutrients underground. The tubers of cultivated potatoes are safe to eat because they contain very little solanine compared to the rest of the plant. Solanine levels increase if the potatoes start to turn green, which can happen in response to light exposure, physical damage and as the tubers age (2).

The tubers allow potato plants to survive for several years (known as perennial). At the end of the growing season, the stems die back leaving just the tubers in the soil, which can produce new stems the next year. Since each individual tuber is able to produce a fully independent plant that can produce many more tubers, over successive years a single potato plant can effectively produce an army of clones. We exploit this ability in agriculture and most of the potatoes we eat are grown from tubers.

Finding the potato plants in my garden presented me with a bit of a dilemma: I was reluctant to throw them away, but I had no idea how to care for them. Aided by some advice from friends I decided to move the plants to “potato bags” (plastic bag designed for growing potatoes in that you fill with compost). This freed up the vegetable patch for the vegetables I had originally planned to grow and should make it easier for me to harvest the new tubers at the end of the season.

The potatoes survived their move and are now thriving in the bags.

The potatoes survived their move and are now thriving in the bags.

Some of my new neighbours spotted my potato bags and I told them about how I had found the plants. They have an allotment and—concerned that my surprise plants may not produce many potatoes—they popped around a few days later to give me a few of their own seed potatoes (Thank you!). Seed potatoes are tubers that are grown with the intention of using them to grow potatoes as opposed to being eaten. They are carefully grown and regularly monitored to be sure that they are disease free. In the UK, most seed potatoes are grown in Scotland where the cold winters kill pests and the wind deters aphids from attacking the plants and spreading plant viruses.

So, I now have two small sets of potato plants to care for over the summer and into autumn. When the foliage starts to turn yellow they will be ready to harvest, and hopefully I will have some tasty homegrown potatoes to eat this winter.

Potato is the Organism of the Month on Plant Scientist. To see the organisms featured in previous months click here.

References:

1. Kew: Solanum tuberosum (potato) http://www.kew.org/science-conservation/plants-fungi/solanum-tuberosum-potato (retrieved 03/06/15)

2. Wikipedia: potato http://en.wikipedia.org/wiki/Potato (retrieved 03/06/15)

The natural history of a “model” plant

Arabidopsis thaliana. Image by Bahamut Chao (CC BY-NC-ND 2.0).

Arabidopsis thaliana. Image by Bahamut Chao (CC BY-NC-ND 2.0).

The flowering plant Arabidopsis thaliana (also known as thale cress) is a small weed belonging to the Brassicaceae family that originates from Western Eurasia. Other members of this family include crops such as cabbage and oil seed rape, but A. thaliana itself has no uses in agriculture. Despite this, it is commonly used in research as a “model organism” to study a range of processes in plants from seedling growth to plant defence against disease.

A. thaliana has many characteristics that make it easy to use in experiments, including a small genome, short life cycle (as little as 6-8 weeks) and the ability to reproduce by self-fertilization. However, these characteristics also make it quite unusual compared to other flowering plants and even amongst its closest relatives in the Arabidopsis genus.

To understand more about the origins of A. thaliana’s unusual features we need to take a look at its natural history. It is a rapidly growing plant that can colonize disturbed soil and happily grows in stony or sandy soils that are low in nutrients. Its closest relatives (e.g. A. lyrata and A. halleria) are also found in bare rocky habitats, but grow more slowly with longer lifespans as biannuals (2 years) or perennials, which can live for many years.

A. thaliana is smaller in size than most of its closest relatives. Each plant produces many more seeds, which are smaller and can survive in the soil for longer periods of time than those of other Arabidopsis species. A. thaliana seeds are mostly the product of self-fertilization, whereas other Arabidopsis species reproduce by mostly or exclusively “out-crossing” (where pollen from one individual fertilizes the egg cells of another). Altogether, A. thaliana’s characteristics enable it to thrive and reproduce in various environments, and to quickly take advantage of new opportunities before other plants can establish themselves. During my PhD, I frequently found unwanted A. thaliana plants growing in my experiments (see image below).

A. thaliana growing as a weed in my experiment with Medicago truncatula plants that were only sown 4-weeks before.

A. thaliana (front) growing as a weed in my experiment with Medicago truncatula plants that had only been sown 4-weeks before.

Why has A. thaliana adopted such a different survival strategy to its closest relatives? It is thought that A. thaliana split from the common ancestor of other Arabidopsis species around 5 million years ago. This coincides with a transition in the Earth’s climate from a relatively warm period to much a colder climate with several ice ages. In these conditions, A. thaliana’s lifestyle would have had many advantages and is likely to have allowed it to live in a wider geographic area and climatic range than its closest relatives. In the last 100 years, A. thaliana has continued to expand its range by migrating to and across North America, southward in Africa and more recently to East Asia in association with human activities.

Some of A. thaliana’s characteristics can impose limitations on research. For example, much of our molecular knowledge about how plant roots grow comes from studies in A. thaliana even though it has a much simpler root structure than many other species. While this makes it easier to carry out experiments on the roots, it also means that our understanding of how plant roots grow is based on an exception, instead of the “norm”. Therefore, it is also important to translate our knowledge of the inner workings of A. thaliana into other plant species. This may involve carrying out experiments on a crop plant of interest, or using another “model” plant that is more closely related to it, for example, a grass called Brachypodium distachyon is often used as a model for wheat.

Studying A. thaliana alongside other members of the Arabidopsis genus may help us to understand the molecular basis of some of the characteristics A. thaliana is missing. Also, it may shed new light on how the loss of these traits has affected the natural history of this famous weed.

This post is based on an article by Ute Krämer published in the journal eLife as part of a special series called the “Natural History of Model Organisms”. Other species covered in this series include maize, the bacterium E.coli and the zebrafish.

Reference:

Krämer (2015) The natural history of model organisms: Planting molecular functions in an ecological context with Arabidopsis thaliana. eLife 4:e06100

Dandelion: a useful weed?

Common dandelion (Taraxacum officinale). Image by Franz Eugen Köhler, Köhler's Medizinal-Pflanzen (public domain).

Common dandelion (Taraxacum officinale). Image by Franz Eugen Köhler, Köhler’s Medizinal-Pflanzen (public domain).

I have recently moved house and I now have a small garden to take care of. I’m pretty excited by this prospect and have been trying to decide what to try and grow in it. However, my little garden is currently somewhat overgrown with weeds—most of which are dandelions.

The common dandelion (Taraxacum officinale) belongs to the Daisy family (Compositae/Asteraceae) of flowering plants alongside more popular garden plants such as sunflowers. It is native to Eurasia but has spread to many other parts of the world including North America. The name dandelion derives from the French ‘dent de lion’, which means ‘lion’s tooth’ and refers to the deeply toothed leaves (1).

A common cause of frustration to gardeners, the common dandelion has several features that enable it to be a vigorous weed in most types of soil. Each flower head produces around 2000 seeds in white tufted fruits that form a puffy ball on the plant (so-called “dandelion clocks”). The seeds are dispersed by the wind and can survive dormant in the soil for many years until the conditions are right for them to germinate. Also, the plants produce a taproot that can grow up to 10-15 ft deep into the soil (2). Whole plants can regrow from a small fragment of taproot, which means that it can be very difficult to remove them from your garden.

When I was a child growing up in London, there was a house near me where, rumour had it, a hermit lived by eating the dandelions growing in his back garden. This story is, admittedly, probably just the imaginings of local children, but it isn’t quite as far-fetched as you might think. Dandelions are indeed edible: the leaves can be served raw or cooked and the flowers are used to make drinks including wine or cordial. Even the root is edible, and in the UK it was commonly ground up and to make a coffee substitute during the Second World War (1).

The common dandelion has other useful qualities. It is a diuretic and a laxative and has been traditionally used in medicine as a tonic, for rheumatic problems, and to purify the blood (1). Also, dandelions produce a milky-coloured sap that contains latex (rubber). The demand for latex is fueling research efforts to harvest it from dandelions. These efforts are focussing on the Russian dandelion (Taraxacum kok-saghyz), which produces sap with higher latex content than the common dandelion (3, 4).

So, a plant that is considered to be an annoying weed by many can be considered useful to others. The plant scientist in me does admire dandelions, but that isn’t going to stop me from trying to remove most of them from my flowerbeds. Let battle commence.

Common dandelions in bloom in my garden (Norwich, UK)

Common dandelions in bloom in my garden (Norfolk, UK)

Quite by accident Plant Scientist appears to be having a weed-themed month. Stay-tuned for more posts about plants that often find themselves in the “wrong” places.

In other news, this is the 100th post on the blog. Hooray!

References:

  1. Kew – Taraxacum officinale (dandelion) (retrieved 11/05/15)
  2. UC IPM – How to manage pests: Dandelions (retrieved 11/05/15)
  3. Science Daily – Rubber from Dandelions: Key components in formation of rubber identified. (retrieved 11/05/15)
  4. Science Daily – Making rubber from dandelion juice (retrieved 11/05/15)

Guest post. The private life of a common garden weed

By Guru V Radhakrishnan (@guru_vighnesh)

Gametophyte of Marchantia polymorpha subsp. ruderalis in Brandenberg, Germany. Splash cups are visible on some of the thalli.

Marchantia polymorpha subsp. ruderalis growing in Brandenberg, Germany. Splash cups are visible on some of the thalli. Image by Franz Mattuschka (CC BY-SA3.0) via Wikimedia Commons

The umbrella liverwort (Marchantia polymorpha) is a noxious weed. It grows in most gardens, paths and glasshouses, and is found in almost all parts of the world – from tropical to arctic climates. There are several subspecies of this plant that are found in damp habitats at varying altitudes. The plants themselves are of little economic importance, but in ancient times, liverworts were believed to be a cure for liver diseases. This ancient idea was part of the “doctrine of signatures” [1]. This doctrine suggested that plants resembling a certain body part could be used to treat ailments of that body part. Although an interesting idea, there is no scientific evidence to support this claim.

The genus name Marchantia comes from the name of a French botanist Nicholas Marchant. And the species name polymorpha is Greek for many forms (polymorph). This name comes from the ability of these plants to take up different sizes, shapes and shades of green depending on the conditions they are grown in.

These liverworts, along with hornworts and mosses, form a group of plants known as the bryophytes. Of all the groups of plants that are alive today, the bryophytes are suggested to be the most ancient. The earliest ancestor of the bryophytes is thought to be ancestral to all other land plants [2]. Because of the evolutionary position of the liverworts, scientists are using M. polymorpha to understand how early land plants evolved.

Like all other land plants, liverworts also have two alternating stages in their life cycle, known as the sporophyte and the gametophyte. In ferns, seed plants and other higher plants, the structure we would recognise as being “the plant” is the sporophyte, and the gametophyte stage is short-lived. For example, in seed plants the gametophytes are the egg and sperm cells within the flowers or cones on the sporophyte. However, in bryophytes the gametophyte stage as the dominant part of their life cycle. In M. polymorpha, the gametophyte structure is known as the thallus – which is the green, flat plant body (see photo). The small sporophyte forms within the female sexual structures of the gametophyte, which is an umbrella-shaped structure called the archegonial head (see figure below).

M.polymorpha is dioecious, which means that there are separate male and female plants. The male plants only produce male sexual structures, which are known as antheridial heads, and female plants produce only female structures. When water falls onto the male plant, sperm can be released from the antheridial head and swim through the water to reach the archegonial head on a nearby female plant. Here, the sperm cells can fertilize egg cells leading to the production of millions of spores on the archegonial head. These spores are minute and can be transported by wind. Once the spores settle on a suitable part of the ground, they germinate into individual gametophytes.

Life cycle of M. polymorpha. The gametophyte is the dominant stage with the sporophytes forming in the archegonial heads of female gameophytes. Image by Lady of Hats released into public domain.

Life cycle of liverworts. The gametophyte is the dominant stage with the sporophytes forming in the archegonial heads of female gameophytes. Image by Lady of Hats (public domain) via Wikimedia Commons.

In addition to sexual reproduction, there is also asexual reproduction in liverworts. Here, specific cells separate themselves from the plant and start forming vegetative buds called gemmae. These gemmae originate from single cells and undergo multiple cell divisions. They accumulate in a specialised structure called a splash cup (or a gemma cup; see figure). These cups contain about a hundred gemmae. When water splashes onto these cups, the gemmae are catapulted out of the splash cups and fall on to the ground. They then continue to grow and become new plants.

These vigorous and efficient methods of reproduction help these plants to quickly colonise a patch of land after a fire. This is also why it is so hard to get rid of these plants from our gardens!

About the author: Guru is a PhD student at the John Innes Centre, UK studying how plants evolved the ability to interact with microbes. Follow him on twitter (@guru_vighnesh).

The umbrella liverwort is the Organism of the Month.

Links:

  1. https://en.wikipedia.org/wiki/Doctrine_of_signatures
  2. https://en.wikipedia.org/wiki/Evolution_of_plants