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Rice diseaseIt Depends on How You Define “Immune System”

Plants get sick. That is, they can be infected by pathogens.

But after hundreds of millions of years of pathogen attacks, plants are still here. So, they must have ways to get well after being sick.

Plants can defend themselves against disease-causing organisms (pathogens) such as viruses, bacteria, and fungi.

They do so by producing physical barriers (e.g., plant cell walls), some antibiotic compounds (e.g., phytoalexins), and even enzymes that perturb pathogens.

In a broad sense, these are all part of a plant’s immune response, that is, biological processes that an organism uses to defend itself against disease.

But do plants have an immune system similar to that in animals? One that can “remember” exposure to specific pathogens?

Recognizing (and Remembering) Self from Non-Self

Humans, along with most other vertebrates, have a multifaceted immune system called an adaptive immune system, which is the culmination of complex interactions at the biochemical, genetic and cellular levels.

Restricted Key parts of this adaptive system are the organism’s ability to (1) biochemically distinguish between it’s own cells (self) and foreign (non-self) entities AND (2) “remember” specific features of the foreigner.

All pathogens – from viruses to fungi – have so-called macromolecules on their surfaces that distinguish them.

Adaptive immune systems (AIS) use these macromolecules as antigens. That is, the immune system uses these characteristic surface features as a way to specifically identify foreign (non-self) entities.

The AIS uses the antigens to generate specific antibodies, which are used to tag the “foreigner” for destruction by specialized blood cells called lymphocytes. These specific antibodies then allow for the rapid detection of subsequent infections with a particular pathogen, which allows for relatively quick defensive responses.

Although plants don’t possess such a sophisticated AIS, there are instances of self/non-self recognition in plants, mainly having to do with issues of self-pollination. (A topic for another time.)

JasminePlants Have an Innate (Passive) Immune System

A more generic, non-specific response to infection characterizes a plant’s immune system.

This type of response is called an innate immune system, in contrast to AIS.

Plants don’t have antibodies or special cells that search for and destroy pathogens.

Plants do, however, have cell-surface receptors to identify certain patterns characteristic of pathogens.

Such receptors, when activated, trigger the production of chemical signals, such as methyl jasmonate (think jasmine perfume or jasmine tea) that may elicit both local and systemic defense responses.

Local defensive responses included the so-called “hypersensitive response” characterized by the self-destruction of the plant cells in a localized area around the site of infection. (See Refs. 2 & 3 below for recent news regarding this.)

Plants also possess inducible systemic defense responses when locally infected by pathogens (e.g., see Ref. 4 below). That is, a single, localized infection may elicit defensive responses throughout the plant.

Bottom Line: Although plants do have the ability to defend themselves against disease-causing organisms (sort of a rudimentary immune system), plants don’t have an immune system as complex as humans.

References

1. Jones, J. D. G and J. L. Dangl (2006) “The plant immune system.”, Nature, Vol. 444, pp. 323-329. (Full Text)

2. Bhattacharjee, S., M. K. Halane, S. H. Kim and W. Gassmann (2011) “Pathogen effectors target Arabidopsis EDS1 and alter its interactions with immune regulators.” Science, Vol. 334, pp. 1405-1408. (Abstract)

3. Heidrich, K., L. Wirthmueller, C. Tasset, C. Pouzet, L. Deslandes and J. E. Parker (2011) “Arabidopsis EDS1 connects pathogen effector recognition to cell compartment–specific immune responses.” Science, Vol. 334, pp. 1401-1404. (Abstract)

4. Jung, H. W., et al. (2009) “Priming in systemic plant immunity.” Science, Vol. 324, p. 89-91. (Abstract)

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