“If you know the enemy and know yourself, you need not fear the result of a hundred battles.”
– Sun Tzu, The Art of War
Who Are You?
How do plants distinguish “unfriendly” (a.k.a., pathogenic) microbes from “friendly” microbes (with which to form mutually beneficial partnerships, e.g.)?
How do flowering plants choose their mating partners in order to promote outcrossing?
There is evidence that some plants can distinguish the roots of related (“kin”) from non-related neighboring plants. How does this “kin recognition” work in plants?
All of the above situations involve the concept of biological “self” versus “non-self” perception.
It turns out that how plants distinguish “self” from “non-self” is critically important for plant defense, mutualistic symbioses, successful sexual reproduction in many flowering plants, and maybe even root-root interactions among different plant species.
Pattern Recognition in Plant Defense and Symbiosis
One of the most important defense mechanisms against microbial pathogens is a plant’s innate immune system. But what triggers the plant’s immune system to switch the plant from growth and development into a defense mode?
“The ability to distinguish ‘self’ from ‘nonself’ is the most fundamental aspect of any immune system. The evolutionary solution in plants to the problems of perceiving and responding to pathogens involves surveillance of nonself, damaged-self and altered-self as danger signals.” (From Ref. 1 below)
As mentioned in the previous post, these “danger signals” are distinct, small fragments of cells, either from the plant itself (“self”) or from another biological entity (“non-self”). Plant cells may have an array of different cell-surface receptors, each one activated by a specific cellular fragment.
In other words, “The first line of defense in plants is the recognition of conserved molecules characteristic of many microbes. These elicitors are also known as microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs). MAMPs are essential structures for the microbes and are for that reason conserved both among pathogens, non-pathogenic and saprophytic microorganisms. MAMPs are recognized by pattern recognition receptors (PRRs), which are localized on the surface of plant cells…” (From Ref. 2 below.)
Recognition of pathogen-derived molecules by pattern recognition receptors (PRRs) is a common feature of both animal and plant innate immune systems. In plants, PRR signaling is initiated at the cell surface by kinase complexes, resulting in the activation of immune responses that repel microbes.
“PRRs were first discovered in plants. Since that time many plant PRRs have been predicted by genomic analysis (370 in rice; 47 in Arabidopsis). Unlike animal PRRs, which associated with intracellular kinases via adaptor proteins, plant PRRs are composed of an extracellular domain, transmembrane domain, juxtamembrane domain and intracellular kinase domain as part of a single protein.” (From Wikipedia)
Interestingly, some of these PRRs also mediate symbiotic signaling (see Ref. 3 below). That is, small, unique molecules from “friendly” microbes, such as Rhizobium bacteria or mycorrhizal fungi, may specifically interact with some plant PRRs, which, in turn, may set in motion “symbiotic” or “mutualistic” responses in the plant.
Recent findings suggest that, in some instances, the immune response may contribute to the process of symbiotic partner choice in plants (see Ref. 4).
“In short, emerging data suggest that ‘know thyself’, or at least ‘know thy partner’, may be as essential for development of beneficial infections as it is in defence against disease.” (From Ref. 4 below)
To Be Continued….Self-incompatibilty and Kin Recognition
1. Sanabria, N. M., J.-C. Huang and I. A. Dubery (2010) “Self/non-self perception in plants in innate immunity and defense.” Self/Nonself, Vol. 1, pp. 40-54. (Full Text)
2. Newman, M.-A., T. Sundelin, J. T. Nielsen, and G. Erbs (2013) “MAMP (microbe-associated molecular pattern) triggered immunity in plants.” Frontiers in Plant Science, 4:139. doi: 10.3389/fpls.2013.00139 (Full Text)
3. Antolín-Llovera, M., et al. (2014) “Knowing your friends and foes – plant receptor-like kinases as initiators of symbiosis or defence.” New Phytologist, Vol. 204, pp. 791–802. (Full Text)
4. Saffo, M. B. (2014) “Mutualistic Symbioses.” In: eLS. John Wiley & Sons, Ltd: Chichester. DOI: 10.1002/9780470015902.a0003281.pub2 (Download PDF for Full Text)
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