'Talking' Plants

They may not seem particularly chatty, but plants are constantly communicating with each other and with their surroundings using chemical signals. Researchers from Virginia Tech have found out that parasitic plants have their own special way of communicating with their hosts. Their paper, published in the journal Science, showed that proteins very similar to DNA, can actually move between the two plants, possibly allowing them to send and receive messages. Ginny Smith spoke to Jim Westwood about his research, and started by asking him what we mean when we talk about a parasitic plant.

Jim -   So basically, there's one plant that is attacking another plant and physically attaching to it in order to withdraw water and nutrients from this, what we call the host plant.  And it gives nothing back.  It's just taking its resources, everything it needs to live from that host.

Ginny -   What is it you found out that's new?

Jim -   The host and the parasite are exchanging messenger RNAs and what is unique about this I guess is that they're exchanging a lot of them and that it's going both ways, that some of the parasite RNA is also going back into the host.

Ginny -   What is a messenger RNA?

Jim -   I imagine this as like a factory where the executive office, the DNA, writes a memo, that is the messenger RNA, sends to the production floor and where it's turned into a product and directs when something is produced and what exactly is produced.

Ginny -   So, is it quite unusual to find these being sent actually out of the plant that produces them into something else?

Jim -  Well, we know that plants use these within their system.  So, they will be sent around from one cell to another cell in the plant to transmit information say, from a leaf, down to a root or something like that.  And so, it can be used as a method of internal communication, but the idea that they are being taken or escaping to another organism is quite new.

Ginny -   So, how do they get from one plant to the other?

Jim -   The parasite has a structure called a haustorium and that invades the host tissue and forms connections to the vascular system, the tubes where there's water and sugars.  Basically, what you get is a cell to cell connection where one cell of the parasite is right up next to the cell of the host and they are exchanging information through the walls basically.

Ginny -   Do you think this is just accidental leakage as the parasite is trying to get access to nutrients or do you think the plants have actually evolved to use this for some process?

Jim -   Well, that is a great question.  We don't know that they are actually used as information, but RNA is an information molecule.  It is very possible that it is using them.

Ginny -   What kind of things could they be using them for?

Jim -   Going back to the factory model, if the parasite is intercepting these internal memos from the host plant in reading them, it could get information on the status of the host and know whether the host cells are under stress or going to keep producing more food.  Even more interesting is the idea that the parasite is sending these messages into the host.  In that case, you could imagine that the parasite is actually telling a host through these memos basically to well, make more food or stand down security and treat me like a friend.

Ginny -   So, if there is this kind of backwards and forwards signalling, is there any way we could sort of hijack this system to get rid of these parasitic plants where they're affecting our crops for example?

Jim -   If you could engineer a host plant that is making a specific - we call a silencing RNA molecule that can move into the parasite and shutdown some potentially critical process in the parasite.  This could be a very elegant way to get at control of parasitic plants.

Ginny -   How big a problem are parasitic plants for agriculture?

Jim -   Well, it depends where you're talking.  In the developing world, the parasitic plants are a huge problem.  In places of sub-Saharan Africa, there is a parasite called witch weed which is devastating to cereal crops and on some of the world's poorest land and affecting the world's poorest farmers.

Ginny -  This idea of information being passed from one organism to another, we know that in bacteria, they can actually do this horizontal gene transfer and actually, take part of each other's DNA and start using them.  Do you think anything like that could be going on here?

Jim -   It is possible.  The horizontal gene transfer occurs very rarely between plants, but it seems that it is more likely where there is a physical association, as in with parasitic plants.  We don't really have evidence right now that DNA is moving, but there is at least one example that it's been reported by another group where it looks like a horizontal gene transfer event.   It could be traced back to RNA intermediate where the RNA would've been converted to DNA in the parasite and inserted into the genome.

Ginny -   Could you just talk me through how that mechanism would work?

Jim -   There is a process called reverse transcription where you take an RNA molecule and it's translated back into a DNA.  And then if that would be inserted into the genome and incorporated, then it  could persist. And especially, if it would have a useful function, then it could be selected for and retained.