Question of the Week

Do plants have immunity?

Sat, 5th Sep 2009

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Mike, Oregon asked:

Do plants have immunity to viruses and bacteria?


John Carr from the Department of Plant and Sciences at the University of Cambridge:

John -    Most microbes like bacteria, fungi, and viruses can’t infect the plant.  But some through evolution, have gained the ability to break down the initial barriers to infection such as cell walls and so on and these can cause disease.  Now in response the plants have evolved the ability to respond to and recognize particular types of pathogens.  So, that’s why some plants have resistance genes and these is a sort of genetic mechanism of allowing them to pass on the ability to fight off particular diseases.  Now when this occurs, you might find that the cells which are initially infected with a virus or a bacteria or fungus actually commit suicide.  And this is one way of creating a kind of a scorch earth against the pathogen but also it’s a way of creating signals, lots more interesting chemicals that float out through the plant tissue.  Sometimes plants will produce salicylic acid, it is the parent compound of aspirin and it is a very, very powerful inducer of resistance.  So if plants are producing salicylic acid, they are better able to fight off perhaps the first pathogen to attack them unremarkably they’re able to fight off possibly lots of other types of pathogen as well.  So salicylic acid itself aspirin like compound can give rise to something they call methyl salicylate and this can float off to other plants and influence other plants so they become more resistant.

Jonathan Jones, Sainsbury Laboratory, Norwich:

Jonathan -   Hi, I am Jonathan Jones.  I worked at Sainsbury Laboratory in Norwich.  Humans have two kinds of immune system, they’ve got the innate immune system, which recognize molecules that pathogens can help making like flagellum of bacteria for example.  And they’ve got the adaptive immune system which involves antibodies and that’s what is triggered when you immunize against viruses for example.  Plants and many others sort of less sophisticated organisms have only an innate immune system.  They can recognize molecules and pathogens and activate defense.  The defense components involve making a sort of bleach - an active oxygen cocktail that inhibits microbes and can culminate in cell death.  They also in plants make a lot of anti-microbial proteins that inhibit growth of microbes but also many pathogens squirt proteins into plants cells, to shut down that immune system.  And then there’s another immune system involving proteins inside the plant cell that recognizes when these molecules show up inside the plant cell and activate defense.


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Completely unqualified answer:
Yes and no.
Plants do have an immune system though not like our own and it is far from perfect so they do not have immunity to everything.
A plant's immune system is usually one of two types:  (for lack of enough knowledge, I am making these terms up) Defensive and non-reactive.
Defenseive is slightly similar to our own immune system in that it destroys the infecting bacteria etc. but it generally does not do it in a targeted way like our immune system does.  The plant's defenses are often a chemical or physical element that makes it hostile to the invading baddies.  This is often accomplished by poisons.
In a non-reactive immunity, a plant will prevent infection by not having the receptors or features that make it infectable.  Many viruses, bacteria, and the like require specific receptors to bond to to infect the cells.  By eliminating those or not having them in the first place, a plant can be immune to those specific baddies.
Databit, Tue, 4th Aug 2009

The first line of defense that plants have against bacterial infection is performed by the stomata, the same structures on the leaf surface which are mostly associated with regulating gas exchanges between the plant and the atmosphere.
Maeli Melotto, from the University of Texas at Arlington, studies how the stomata close when faced with a potential bacterial infection, and how bacteria counter that. I wrote about it for Pesquisa, a Brazilian magazine. It has been translated to English here: mariaguimaraes, Wed, 5th Aug 2009

Plants can also kill their cells by apoptosis - programmed cell death - so I suspect that they will use this as an anti-viral strategy - if cells surrounding a site of infection all self-destruct then there will be nowhere for the virus to replicate next... chris, Thu, 6th Aug 2009

In humans, L-ascorbic acid seem to have antiviral properties ( or Although human and plant is nowhere near identical in terms immune systems, the Vitamin C in fruit plants may fight of infections.

A second possibility (I could be wrong here). Plants are pleased with sunlight, however, intruding bacteria for instance might already be fought off the plants by the the small percentage of UVC rays thrown at it from the sun. DrChemistry, Fri, 14th Aug 2009

if that were true, then any microorganism exposed to the rays would die way earlier than reaching the plant. glovesforfoxes, Fri, 14th Aug 2009

if that were true, then any microorganism exposed to the rays would die way earlier than reaching the plant.

A helping hand from the sun to our plants. Plants do not have an active defense system like human do and depend on other chemical or physical mechanisms to take place within or outside the plant, one probability could be apoptosis as Chris pointed out. Other ways of preventing bacterial or viral infections in plants could include the root net, or as I mentioned, the L-ascorbic acid in some fruit plants. The roots are in fact very complex structures. We have the primary root, lateral roots and the root hairs. Specifically 'trained' if you like, to absorb only what's needed for plant growth. Soil contain loads of bacteria, and hence plants had to find a way to keep them out. Bacteria is usually of a sizer bigger than what the roots can absorb, and are hence left out. However, if I remember right, I once read an article by Department of Agriculture in which they had supposedly spotted that some sort of bacterium had evolved shocking capabilities of getting absorbed by some specific roots. Whether this is the actual case I have never heard a conclusion on (surely something worth investigating). However, if this actually is the case, the bacteria or microorganism in question would not die prior entering the plant.

Edit: Spelling mistakes, inaccuracies, punctuation etc. DrChemistry, Fri, 14th Aug 2009

isn't sap part of the immune system of a tree? Although this is not the case for all plants, most do not have sap. techmatt, Mon, 17th Aug 2009

I have a distant memory of my A level biology teacher also mentioning that plants can also selectively block off some of their conducting vessels in order to prevent the spread of microorganisms via that route. The mechanism of blockage was the secretion of callose, a carbohydrate, into the phloem sieve plates.

Chris chris, Mon, 17th Aug 2009

I was just wondering plant such as chili that produce capsaicin does that act as an defensive agent against bacterial as well? hau, Tue, 18th Aug 2009

@Chris: I've heard something similar. The callose forms a wall that will block most structures attempting to pass through. This way they can block some pathogens. As far as I know, some microorganisms have the ability to break down the Callose wall, sadly.

@hau: According to this fact sheet It is mentioned that capsaicin is believed to protect the seeds against particular pathogens.

I think plants have an immune system we have a hard time to imagine  . The responses available against a pathogen attack depends on the plant and the compounds it hold. Not long ago I read in a Science Magazine that the plants enzymes and proteins that can disintegrade the cell wall of the attacking microorganism. Hopefully someone can confirm this  . DrChemistry, Tue, 18th Aug 2009

Forgive me I had to:

Plants have immunity only when the DA accepts their plea bargain.

( runs and hides) Edster, Thu, 20th Aug 2009

You should read this recent article in Science Magazine's Origins blog: It explains at the beginning that plants don't have an adaptive immune system (mainly because of the lack of a circulatory system) but they do have an innate one. The article then carries on to present the state of the art on why the innate immune system of plants and animals is so similar. mudd1, Mon, 24th Aug 2009

Salicylic acid (aspirin) is used as a signaling molecule in plants that leads to a immune-like response.  One of the resulting responses is cell death, as mentioned previously. "Systemic Aquired Resistance" is the term to look up to learn more.  It is not specific to a particular pathogen, however, so it is more like our non-specific immune responses, not like the B-cell mediated immunity.

There was some research a while back that also plants can signal each other through a hormone, jasmonate, to warn of a pathogen!

Jessica H, Mon, 7th Sep 2009

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