Parasitic plants: Stealing food and genes

 Is it a mushroom? Is it a toadstool? No! It’s Helosis cayennensis, a plant!

Helosis is not your average plant.

Instead of photosynthesising to produce its own food, Helosis grows as a parasite on the roots of trees, stealing water and nutrients from its host. It spends most of its life underground, erupting only to flower in the mushroom-like inflorescences you can see above.

Helosis cayennensis is just one of around 4100 species of parasitic plant. They all produce haustoria, special threadlike organs that penetrate the host plant like a fungus to steal its sap.

There are two main types of parasitic plant:

  • Facultative parasites that are able to photosynthesise and live independently without a host, but can exploit other plants when available.
  • Obligate parasites like Helosis, which typically lack chlorophyll and exploit their host plants for the nutrients they need for survival.

Obligate parasites are the freakier of the bunch. They often don’t have leaves or roots, just a stem, their parasitic haustorium and eventually some flowers.

Parasitic plants steal DNA

The ‘corpse flower’ Rafflesia arnoldii is famous for producing the world’s largest flowers, which stink of rotting meat. It’s also an obligate parasite with no leaves, roots or stem, absorbing everything it needs to make these gigantic 1m flowers from its host, the Tetrastigma vine.

Rafflesia

Parasitic plant Rafflesia arnoldii produces the world’s largest flower, which smells of rotting meat! Image credit: Wikimedia

But that’s not all it steals.

In 2012, a team studying Rafflesia cantleyi and its host Tetrastigma rafflesiae found something strange in their DNA. Just looking at the two plants is enough to show they are distantly related, but for certain genes Rafflesia‘s DNA sequence closely resembled its host. The only possible explanation was that Rafflesia had stolen Tetrastigma‘s genes.

The researchers found that Rafflesia had acquired 49 genes from its host in a process called ‘horizontal gene transfer’. This is best known in bacteria, which can pass useful genes, for example those encoding antibiotic resistance, to other individuals in a colony.

Horizontal gene transfer is very rare in multi-cellular organisms. Interestingly, Tetrastigma did not have any of Rafflesia‘s genes. This suggests that the parasite benefits from stealing its host’s DNA, possibly by camouflaging it from the host plant which would otherwise mount a defensive immune response against it. This has also been seen in a malaria parasite (Plasmodium vivax), which has acquired human genes involved in the immune system (e.g. Interleukin-1 genes) that might help it to manipulate or evade the immune response and persist in the body.

Click here to read the Rafflesia paper (open access).

Parasitic plants and hosts exchange mRNA

When a cell wants to express a particular gene, the DNA sequence is transcribed into a messenger RNA (mRNA) sequence before being translated into its corresponding protein. mRNA was believed to be highly unstable, but recent research has found that it can be exchanged between parasitic plants and their hosts as a possible means of communication.

The team investigated the obligate parasitic plant dodder (Cuscata) growing on two hosts, Arabidopsis (the lab rat of the plant kingdom) and tomato. Dodder produces a network of tangled yellow stems that wrap around host plants and can cause significant damage to crops if left untreated.

Amazingly, the researchers found that huge amounts of mRNA was exchanged between dodder and its hosts. Almost half of the expressed Arabiodopsis genome was found in the parasite! Unlike the Rafflesia gene stealing above, around a quarter of dodder’s mRNA sequences were also found in Arabidopsis, showing for the first time that large amounts of mRNA could be exchanged between species in both directions.

Why is this important? Well, it’s known that mRNAs are able to transmit information within a single plant by moving between cells, so the researchers suggest that parasites could use these molecules to monitor the host plant or manipulate it for its own gain.

It’s possible that the mRNAs move passively from host to parasite, but the fact that parasite mRNAs are moving into the host cell too suggests that they are actively moved by the plants for meaningful communication. What are these plants talking about? With around 45% of Arabidopsis‘ mRNA sequences present in dodder it’s hard to decode the message, but hopefully science will soon be able to answer that question.

(Original paper is here, behind a paywall).


If you enjoyed this post, please feel free to leave a comment below or on Twitter (you can find me at @JoseSci)! Thanks!

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Arabidopsis, the darling of plant science

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Arabidopsis thaliana, the lab rat of plants

Plant scientists are obsessed with a little weed called Arabidopsis.

On the surface it’s a strange plant to study; it’s not grown for food nor to feed animals, it’s not ecologically important and its flowers are, frankly, boring. Yet tens of thousands of researchers around the world dedicate their careers to discovering everything there is to know about thale cress, Arabidopsis thaliana.

This little weed is more than it seems. Like rats, E. coli and fruit flies, Arabidopsis is a model organism. Scientists study model organisms to learn their basic biology, which can be extrapolated into other related species. Arabidopsis can tell us a lot about crop plants, many of which are notoriously difficult to study. It’s even given us an insight into human diseases!

What makes Arabidopsis a good model plant?

Arabidopsis is a weed, which is great for research because it’ll pretty much grow anywhere, from soil to Petri dishes to liquid nutrient solutions. The mature plants are only about 5-10cm in diameter, so you can grow hundreds of them in a typical growth chamber. Best of all, it’s fast growing, going from seed to (irritatingly small) seed in just 6 weeks.

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Arabidopsis seeds are tiny!

What really separates Arabidopsis from the crowd is its tiny genome. It has around 27,500 genes encoded within a genome of 125 million bases (letters) of DNA. Contrast this with the huge genomes of crop plants like barley (30,000 genes, 5.1 billion bases) and wheat (96,000 genes, 17.1 billion bases) and it’s easy to understand how Arabidopsis was the first plant to have its genome sequenced, way back in 2000 (a year before the Human Genome Project was completed). Since then, fantastic genetic resources have been developed in Arabidopsis, leading to a revolution in plant science.

Arabidopsis’ impact

By deciding to focus on Arabidopsis, scientists around the world were able to share knowledge and rapidly build upon new ideas. Many people, including myself, believe that this had led to a much broader and deeper understanding of plant biology than we could ever have achieved otherwise.

One way to understand the function of a gene is to stop it from working properly and look at the effect on the plant. The Arabidopsis genome sequence has allowed scientists to develop huge libraries of “mutants”, plants deficient in every single known gene. We knew the basic processes of plants from work done on crops over 100 years ago, but didn’t really understand the details. By working backwards from the mutants, we are able to discover which genes are responsible.

dna alignment

Genes from different species have some changes. The less closely related they are, the more different they are likely to be. Image produced in ApE.

The next stage is to translate the knowledge gained in weedy little Arabidopsis into more useful plants, like crop plants. Even though they look so different, almost all the genes you find in Arabidopsis are present in all flowering plants.

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Barley in a growth cabinet

Using the new genome sequences of plants like wheat and barley, it’s easy to search for Arabidopsis genes in these crops. It’s a bit like Google. Stick the letters in and it’ll give you the best matches. Different plants usually have a few changes in the sequence of a gene, but the best match is usually the gene you’re looking for.

From there it’s a matter of silencing the gene you have identified in your chosen crop plant to see if it has the same impact as it did in Arabidopsis. If so, you’ve just improved our understanding of an economically important crop species.

The future of Arabidopsis

Genome sequencing is becoming increasingly cheap and easy. Will we still need Arabidopsis in a world of species-specific genomes? I’d argue yes, at least for a decade or two yet. It is still comparatively very difficult to work with crop plants directly. We don’t yet have full libraries of mutants so Arabidopsis plays a vital first step in gene identification. A full understanding of genes in the model plant would make it a lot more easy to investigate their functions in crop species. Future research areas like synthetic plant biology are predicted to be developed in Arabidopsis too.

Arabidopsis thaliana is the backbone of plant science. Its genetic resources have led to great leaps in our understanding of plant biology, focussing research and enabling translation into crop plants.

Weeds are flowers too, once you get to know them – A. A. Milne

Do plants get cancer?

Spend all day in the sun and you run the risk of developing skin cancer, yet plants seem to bask in UV rays without a care.

Truth is, plants can develop tumours. Unregulated cell division occurs in plants just like animals, so why don’t we ever hear of a plant dying of cancer?

Types of plant cancer

Infections can cause tumours in plants, in a similar way to the HPV virus causing cervical cancer in humans.

The bacterium Agrobacterium tumefaciens causes tumours called crown galls in many species of plant. The bacterium inserts its own DNA into the plant and messes up its growth hormones (auxin and cytokinin), creating a Agrobacterium-friendly tumour where it can live happily ever after. It’s rarely fatal but can cause some yield loss in perennial crops like fruit trees because gall production steals energy that could have been used to make more delicious apples or cherries.

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The corn smut fungus is a delicacy in Mexico. Source: wikipedia.

Fungi like Ustilago maydis can produce tumours in a similar way. U. maydis causes corn smut, turning ears of corn into the strangely grey and deformed Mexican delicacy known as huitlacoche.

Geminiviruses cause tumours by directly interfering with cell replication in plants.

Although it’s uncommon, certain types of plants are quite prone to spontaneous tumours too. In a fun twist of fate, tobacco (Nicotiana) plants are particularly prone to developing cancer; when two species cross-breed with each other, the resulting offspring often develop tumours because of hormone regulation problems.

How to grow a tumour

Plant tumours share some similarities with human cancers. Plant tumours are disorganised lumps of cells, similar to human cancers. They are often caused by problems with levels of the hormones auxin and cytokinin. Fluctuations in hormones like oestrogen can lead to cancer in humans too.

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Agrobacterium tumefaciens causes crown galls to develop. Source: Wikipedia.

Cell replication is strictly regulated in animals and plants by genes that are amazingly similar in both groups. Auxin and cytokinins, as well as human hormones like oestrogen, can interact with these cell cycle genes. When the hormones are out of balance, cells can start to multiply out of control.

These cell cycle genes can mutate and stop functioning properly, causing cancers in animals. Plants are less likely to fall victim to these random mutations because they have many copies of most cell cycle genes, so another version can take over if one is put out of action.

Plants also have a few other fail safes to protect themselves from potential cancerous cells:

  • Regeneration. Brain tumours are one of the deadliest human cancers, with an 85% mortality rate after five years. The brain is such a vital organ that we cannot survive without it, but plants can regrow any damaged organs reducing the impact of a tumour.
  • Totipotency. Plant cells are totipotent, which means they can develop into any cell type. If too many cells are produced in a leaf they can be incorporated into a normal structure. Each cell will be smaller than normal to maintain roughly the correct leaf shape overall.
  • Containment. Plant cells are contained within a cell wall. Cancer cells can’t squeeze into neighbouring tissues, so the tumour is restricted to one area. Plant veins are different to humans too; only water and things dissolved within it can move through the vascular system. Tumour cells can’t cause new tumours elsewhere.

There are really two main stages to developing cancer; the cells go wrong then spread throughout the body. Infections, unstable hormones and plain old mutations can cause plant cells to override regulation and begin to divide, but cell walls and a dynamic body plan means plants are able to stop tumours from spreading uncontrollably or doing too much damage.