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.
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).
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