The Selfish Plant

Looking Out For Number One

In a previous post, way back last December, regarding the notion of plant “pain”, I acknowledged the subject of “damaged-self recognition” in plants, but I didn’t want to elaborate on it at that time.

Well, it looks like now’s the time….

“Damaged-self recognition” in plants has to do with how a plant senses that it has been physically injured.

For example, how does a plant become aware that it’s been physically damaged – by a hailstorm, for instance, or by a herbivore? And can it tell the difference between the two?

It’s well-known that plants can, and do, respond at the whole-plant (systemic) level when a part of the plant has been physically damaged.

And recent experimental evidence tends to support the idea that plants can indeed distinguish different kinds of wounding – for example, caterpillar-chewing versus cattle-grazing – and respond differently to each.

If so, then what are the cellular and whole-plant mechanisms that enable plants to distinguish and to defend themselves against different kinds of wounding?

Perhaps the chief proponent (evangelist?) of the concept of “damaged-self recognition” in plants is Dr. Martin Heil. (See also an interview with Martin Heil.)

In his words, here is a partial definition:
Most elicitors of plant responses to folivory represent, or contain, parts of plant-derived molecules that are degraded, digested or localized outside their original cell compartment. To the plant, these
elicitors indicate the ‘damaged self’. Such elicitors are released from disrupted cells and are probably perceived by receptors that monitor the extracellular chemistry. Thus, the information on the ‘damaged self’ is transported into the inner compartments of intact and metabolically active cells, which react through metabolic responses such as the synthesis of systemic signals and defence compounds.
” (From: Ref. 1 below)

Briefly put, when a leaf, for example, is grazed by a herbivore (“folivore”), this produces broken pieces of plant cells and plant cell walls. These cellular fragments (“elicitors”) diffuse to surrounding intact cells, which may contain specific receptors for some of these elicitors.

When these elicitor-receptors are activated, they trigger a chain of cellular processes that lead to the production of chemical “wound signals” that can diffuse throughout the plant, alerting it to the damage and initiating various defensive responses.

PAMPs, MAMPs, DAMPs and HAMPs

Multicellular organisms suffer injury and serve as hosts for microorganisms. Therefore, they require mechanisms to detect injury and to distinguish the self from the non-self and the harmless non-self (microbial mutualists and commensals) from the detrimental non-self (pathogens).” (From Ref. 3 below)

The concept that animals and plants achieve the above by detecting specific molecules produced by, or on the surface of, microbes has been around a long time.

Immunologists were likely among the first to identify and characterize such molecules. They discovered that cells of the immune system were activated by specific molecules associated with groups of pathogens. They called these elicitors of the immune response pathogen-associated molecular patterns (PAMPs).

Since then, this concept has expanded and diversified well beyond the field of immunology, even into the realm of plant science.

So, for example, today we now have MAMPs (microbe-associated molecular patterns) that are recognized by the plant innate immune systems pattern recognition receptors (PRRs).

In more general terms, we also have damage-associated molecular patterns (DAMPs), also known as “danger-associated molecular patterns” or simply “danger signals”. For example, these may be molecular pieces of plant cells themselves that can act as “elicitors”, as mentioned above. (Please see Ref. 3 below.)

And, finally, we have “herbivore-associated molecular patterns” (HAMPs) that include specific molecules derived from the regurgitate of feeding caterpillars and also plant-derived protein fragments that are formed specifically during insect feeding. Interestingly, a plant’s defensive responses elicited by HAMPs may differ from those triggered by DAMPs.

Recent research (see Ref. 4 below) indicates that plants may also use a combination of DAMPs and HAMPs as cues in order to deploy a more fine-tuned response to specific wounding.

Thus, a plant may possess quite sophisticated mechanisms – as yet to be fully understood – that allows it to not only perceive “damaged-self” but also damage by “non-self”.

The Selfish Plant

Delving into the subject of “damaged-self recognition” in plants got me to thinking about other ways plants display an awareness of self. (Dare I say “selfishness”?)

No, I’m not going to investigate the subject of self-aware plants as it relates to plant “consciousness”. (For that, I’ll refer you to Ref. 5 below.)

Instead, over the next few posts, lets’s explore the nature of what I’ll call “the selfish plant”.

Next-Up: Self- and Non-Self Recognition in Plants

References

1. Heil, M. (2009) “Damaged-self recognition in plant herbivore defence.” Trends in Plant Science, Vol. 14, pp. 356-363. (Abstract)

2. Heil, M., et al. (2012) “How Plants Sense Wounds: Damaged-Self Recognition Is Based on Plant-Derived Elicitors and Induces Octadecanoid Signaling.” PLoS ONE 7(2): e30537. doi:10.1371/journal.pone.0030537. (Full Text)

3. Heil, M. and W. G. Land (2014) “Danger signals – damaged-self recognition across the tree of life.” Frontiers in Plant Science, Vol. 5, article 578. DOI: 10.3389/fpls.2014.00578. (Full Text)

4. Duran-Flores, D. and M. Heil (2016) “Sources of specificity in plant damaged-self recognition.” Current Opinion in Plant Biology, Vol. 32, pp. 77-87. (Abstract)

5. Trewavas, A. J. and F. Baluška (2011) “The ubiquity of consciousness.” EMBO Reports, Vol. 12, pp. 1221-1225. (Full Text)

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