Plants and their shocking immune response

Most plants can't move, so they can't escape by running away when under attack.  Which is not to say they can't defend themselves -- they do have thorns and poisons, they can emit noxious odors or taste unpleasant, they have tough bark or leaves, and so forth.  And when pathogens descend, plants can mount a quick response.  Most have evolved gene-for-gene pathogen-specific responses, involving the products of the plant's resistance (R) genes with proteins coded by the pathogen's avirulence (Avr) genes. We discussed this in our book The Mermaid's Tale, because it shows how very different species have somewhat similar ways to combat a similar threat.

Foxglove produces chemicals that can be toxic if consumed; Wikipedia

It's a fascinating dance of co-evolution, actually.  The plant's R genes code for receptors to the proteins produced by the pathogen's Avr genes: the pathogen releases the Avr product into the plant at the site where it invades, and the Avr product interacts with the product of the R genes, induced in the plant by the presence of Avr proteins.  This in turn activates a cascade of plant defenses, including the oxidative burst, which initiates the hypersensitive response, or rapid cell death at the site of infection.  This is beneficial (to the plant) because when the cells surrounding the site of attack die, the invading pathogen is deprived of nutrients and the opportunity to grow or spread.  It also induces the production of antimicrobial proteins.

Pathogen attack also can induce non-specific responses, that is not associated with R or Avr genes.  The attack may induce the plant to crosslink its already rigid cell walls, chemically toughening them to better resist the invading pathogens, and the oxidative burst and the hypersensitive response it triggers may be non-specific as well.

Plants have also evolved at least one systemic response to pathogenic attack, called systemic acquired resistance (SAR), analogous to the innate immune response in animals.  In SAR, a pathogen attack induces a state of heightened resistance throughout the plant, which helps to protect it against subsequent attack by the same pathogen as well as a wide range of additional potential attackers.  This occurs even if the pathogen is only attacking part of the plant.  Once induced, this heightened state can persist through an entire season, just in case the bastards dare to try again!  SAR involves the accumulation throughout the plant of a set of proteins that confer increased nonspecific resistance to the plant, probably through antimicrobial activity.  The roots of plants are often colonozied by nonpathogenic bacteria, and plants can respond with a process called induced systemic resistance which induces the release of defense-related proteins.  

How do they do that?
It has long been known that plants communicate systemically, sending long-distance signals when under attack.  In that sense the plant is a unified organism, not just a set of cells all spread out.  Now, a new paper on plant immune responses in the 22 August issue of Nature , "GLUTAMATE RECEPTOR-LIKE genes mediate leaf-to-leaf wound signalling," by Mousavi et al., reports on a new understanding of how this is done.

Plant systemic responses rely on a family of regulatory lipids called jasmonates which are known to quickly build up in wounded tissue.  But how do they then send danger signals?  Plasma membrane depolarization --  a chemoelectric change -- has been observed in various plants when a tissue is wounded, and it's known that this can stimulate the expression of jasmonate-regulated genes.  So, Mousavi et al. monitored the electric response of Arabidopsis plants (mustard plants) under varying conditions.

On herbivore attack, levels of the plant hormone jasmonate increase, triggering defence responses. Mousavi et al. show that leaf injury, caused by herbivory or mechanical wounding, induces the transmission of electrical signals that are generated by the activity of glutamate-receptor-like (GLR) ion channels. These signals induce the formation of jasmonate at local and distant sites in the plant. Source: Christmann and Grill, Nature, 21 Aug 2013

And it turns out that plants don't respond when larvae crawl on the leaves, or to simple touch, but they do when the larvae begin to feed.  To determine whether the response is to the chemical elicitors the larvae inject into the leaf, rather than the simple fact of wounding, Mousavi et al. wounded the leaves mechanically, and this, too, elicited a response, which they call "wound-activated surface potential changes (WASPs)".  And, the response was rapid, traveling at a top speed of 9 cm per minute, and induced expression of JAZ10, an indicator that the jasmonate pathway has been activated.  Further, the authors tested for a direct link between the jasmonate pathway and electrical activity by wiring up a leaf, sending current through it, and monitoring the result -- surface potential changes away from the site of injection. 

The authors determined that hundreds of genes were up-regulated upon wounding of the plant (or current injection), and genes that code for proteins involved in ion transport through the cell membrane, such as ion channels or pumps, have been previously found to affect jasmonate signalling.  The GLUTAMATE RECEPTOR-LIKE (GLR) genes are ion channel genes known to be involved in various aspects of plant development and responses to stress, as well as ion channel transport.  Arabidopsis have 20 GLR genes; Mousavi et al. found that mutations in several of these genes reduced the surface potential changes upon wounding, and thus expression of jasmonate regulator genes, but how GLRs are activated by these mechanical insults isn't well understood.

Mousavi et al. conclude that they've identified ion channel genes, GLRs,  involved in plant defense through electrical signaling.  Further,

Our results now show that GLRs control the distal wound-stimulated expression of several key jasmonate-inducible regulators of jasmonate signalling (JAZ genes) in the adult-phase plant. Finally, GLRs are related to ionotropic glutamate receptors (iGluRs) that are important for fast excitatory synaptic transmission in the vertebrate nervous system⁴². They and their plant relatives may control signalling mechanisms that existed before the divergence of animals and plants⁴³. If so, a deeply conserved function for these genes might be to link damage perception to distal protective responses.

The animal/plant link
The plant signal goes from the affected leaf to others, much as (and using similar chemoelectric mechanisms) neuronal and other inter-cellular signals in animals. There are two classes of glutamate receptor in animals; ionotropic glutamate receptors, that form ion channels that operate when the receptor binds to glutamate, and metabotropic GRs, that indirectly activate ion channels when the receptor binds to glutamate (glutamate is an amino acid).  Glutamate receptors in animals are primarily found in the central nervous system and are involved in neurotransmission and, it seems, response to stress.  

The points here are many.  That we have these same receptors shows deep evolutionary connections between plants and animals.  These are old mechanisms involving similar and hence very old genes and cellular activities.  The information transferred by these sorts of means is highly varied.  Ion channels are adjustable pores in the surfaces of cells, that open or close depending on conditions, to adjust the ion concentrations (that is, of charged molecules like calcium or potassium) between the outside and inside of the cell. The changes can adjust the ionic difference within the cell compared to the outside, or the signal can travel along the cell (as in muscle and nerves in animals).  In plants there are no muscles or nerves, but the relevant mechanisms are used to adjust the cell behavior--and to transmit the information about the threat to help other, as yet unaffected tissues in the plant, so they can anticipate an attack and defend themselves.

The authors speculate that "a deeply conserved function for these genes might be to link damage perception to distal protective responses."  But we are observing the mechanism countless millions of years since it arose, and there's really no way to know from today's world what the original function was.  However, the general usages of ion channels suggest, to us, that the original use was 'simple' inside-outside adjustments, and that the warning usage came later.  And plants have their separate mechanisms for immune defense, that we described at the top of this post.

The plant behaves as an integrated organism, but communication from one part of the plant to another is not the same as having a central nervous system.  We might try to imagine what it feels like to be a plant (normally or when under attack), but even if the plant is a unitary creature, its sensory awareness doesn't have to be 'conscious' or centralized for it to be an example of the integration of many parts into a whole.

No matter how you might want to interpret it, this is but one more instance of similar widespread phenomena with diverse uses.