The Trouble with Sapsuckers
Just as we fall victim to biting insects that want to drink our blood, plants are assailed by sap suckers. And in the same way that mosquitoes give us malaria, some of these plant attacking insects infect their hosts with bacterial and viral disease too. Saskia Hogenhout is from The John Innes Centre in Norwich.
Chris - So, what sorts of insects do you study?
Saskia - I work on sap feeding insects. These are aphids, leaf hoppers, plant hoppers, psyllids, and they all have like stinging mouth parts that can reach through the phloem and to the vascular system of the plant and drink the sap of the plant like mosquitoes do.
Chris - Yeah, so it's directly analogous. They're sending a part of their mouthparts into the flesh and then into the vessels of the plant.
Saskia - Yes and these vessels carry the sap stream of the plant so the sugars that are transported to the leaves for metabolism.They are the bloodstream of the plant and so, the insects feed on this like mosquitoes do from blood.
Chris - Indeed and just as I mentioned, in the same way that mosquitoes can transmit things like malaria and viruses in us, these organisms pouncing on one plant and then another can spread diseases.
Saskia - Yes, so the insects, they spread the disease and some of the viruses, maybe most of the viruses, they are completely dependent on the insects for spread in nature and some bacteria as well.
Chris - So, what sorts of viruses are these?
Saskia - They are in the plants and they carry it around by the insect, but they don't infect the insect. Other viruses, they circulate in the insect, but they don't amplify in the insects but the insect can still carry them around.
Chris - When you say amplify, you mean as in, they get into the insect and then they grow and increase their number in the insect, so the infectious dose in the insect goes up.
Saskia - Yes and other viruses, they amplify even more. So, they really use the insect also as a host for replication and for multiplication, and colonisation.
Chris - That's extraordinary because this means you have a virus which is capable of infecting an insect's cells and growing in the insect, and infecting the cells of a plant which is a very, very different beast, if that's the right word.
Saskia - Yes, that's right. So, they're very different hosts and maybe the plant and the insects are much more different than for example mosquito and a human.
Chris - So, is it quite literally like malaria where the sap sucker will land on the plant, drink some fluids, and because the virus is circulating in the plant fluids, some gets into the insect, and it then departs, goes to another plant, and when it drinks from that second plant, it just happens to pass the virus on to the new recipient.
Saskia - Yes, so depending on the virus, this insect can acquire the virus and then transmit it immediately. Sometimes it takes longer. So, sometimes it takes 10 or 20 days or longer.
Chris - What do the plants do about this because given how widespread this sort of herbivory and plant parasitism is by sap suckers and things? Do the plants have a sort of immune response to stop this happening?
Saskia - Yeah, that's an interesting question because the majority of the viruses but also insects are specialised. So, they can only really infect certain plant species. And so, it's often a very specialised interaction, but only a small fraction like 10% of the insects species that will feed only on a plant can feed on many different plant hosts and those are also often the most efficient factors of viruses because they can transmit a virus from one plant species to another and often from weeds that surround the crop fields to the crop.
Chris - What about in winter time? Where do the viruses go because if you're living in an insect that disappears in winter and a plant that withers and dies in winter, how do the viruses persist?
Saskia - So, sometimes they stay in the plants like the weeds that surround crops. So, the crops that are maybe not there anymore, but the weeds stay around and sometimes they stay in the insect vector as well because the insect can last the winter and they stay quiet and they don't do much, but they still carry the virus. And when the insect appears again in spring then it can transmit it to new plants.
Chris - Do any of these viruses get into the genetic material of the plants because in the same way, we see there are some viruses that lurk inside the genomes of humans? Can they lurk inside the plant genome and then come out again periodically when they want to?
Saskia - Not many plant viruses do that, no. Not many plant viruses. There are, as far as I know, not any, but there are bacteria that can do that. So, fragments of the bacteria can lurk inside the plant, but these are not transmitted by insects.
Chris - Indeed, but there are bacteria that are transmitted by insects in the same way as we've outlined these viruses aren't there?
Saskia - Yeah, there are several species of bacteria and some are also replicating or amplifying, colonising the insects. And some are only carried by the insect.
Chris - What is the impact on the plants of catching these bugs?
Saskia - They actually severely damage so they have stenting chlorosis as streaks, they don't form seeds, they don't form flowers, and when oil seed rape for example in the UK, can be infected by several viruses and they think it actually reduces yield by 30%.
Chris - Do they also render the plants susceptible to other infections? I keep bringing you along human analogies, but obviously, we can catch HIV. This damages our immune system and render us susceptible to infection by a whole lot of other low grade things that wouldn't normally bother us. Does the same thing happen with plants, with these sorts of bugs?
Saskia - Yes, actually, there's not many examples of viruses, but there are examples of bacteria and some of which I've worked on, that suppress the immune response of the plants and this plant then becomes then more susceptible to insect vectors which then can also then acquire those fired plasmas, so the bacteria again and then spread them around.
Chris - And just to finish, what about the converse because there's evidence that if we pick up certain virus infections, paradoxically, we can end up with a better immune system? Do any of these viruses or bacteria enhance the health of the plant as a sort of payback for having been infected with them?
Saskia - Yes, there are also examples in which they transmit the infected viruses and these viruses induce certain immune responses that are then inhibiting infection of other pathogens and bacteria. Or maybe even sometimes if the virus is not transmitted by the insect, they can also inhibit herbivory.