Tuesday, February 4, 2014

"Roots": the saga of (plant) families caring for each other

We did a post a few weeks ago about plant intelligence.  We wrote primarily about a recent Michael Pollan piece in The New Yorker that discussed the issue at length, we thought in a nicely thought-provoking way.  A piece in The Scientist, "Plant Talk" by Dan Cossins, pushes the issue a bit further.

Cossins describes an experiment that demonstrates that plants communicate through their root systems.  They send warning signals about insect infestations, and share nutrients.  A PhD student in Scotland, Zdenka Babikova, tested the role of mycorrhizal fungi in this communication system, and published the results in Ecology Letters in July.  She planted 5 bean plants in 8 different pots; one was a 'donor' plant, and the other four were 'receiver' plants.  One formed root and mycorrhizal contact with the donor, another only mycorrhizal contact, and two had neither.  Cossins writes:
Once the mycorrhizal networks were well established, Babikova infested the donor plants with aphids and sealed each plant in a separate plastic bag that allowed for the passage of carbon dioxide, water, and water vapor but blocked larger molecules, such as the VOCs [volatile organic compounds] used for airborne communication.
Four days later, Babikova placed individual aphids or parasitoid wasps in spherical choice chambers to see how they reacted to the VOC bouquets collected from receiver plants. Sure enough, only plants that had mycorrhizal connections to the infested plant were repellent to aphids and attractive to wasps, an indication that the plants were in fact using their fungal symbionts to send warnings.
Figure 1. Experimental mesocosm (30 cm diameter; n = 8) showing the donor plant, which was colonised by aphids, and four aphid-free receiver plants. All plants were grown in the mycorrhizal condition but one plant was prevented from forming mycelial connections to donor plants (0.5 μm mesh), another was allowed to form connections initially but the connections were snapped after additions of aphids to the donor (rotated 40 μm mesh), and two other plants were allowed to form shared mycorrhizal fungal networks (non-rotated 40 μm mesh allowing fungal contact only; no barrier allowing fungal and root contact) with the donor plant for the duration of the experiment. Ecology Letters, Babikova et al., 2013

Cossins adds, with respect to plant communication, "Moreover, plants can “talk” in several different ways: via airborne chemicals, soluble compounds exchanged by roots and networks of threadlike fungi, and perhaps even ultrasonic sounds. Plants, it seems, have a social life that scientists are just beginning to understand."

A lot of the work on communication between plants was dismissed by most botanists until recently. Now botanists acknowledge that there may well be signaling going on, but some still are not willing to consider plants to be social organisms, or altruistic, and see all this signaling as an offshoot of a plant's ability to alert healthy cells of insect attack from damaged cells, or of a plant's response to drought, and so forth.  Is that just a bias because plants don't do it the same way we do?

Plus, the inter-plant communication doesn't seem to do the sender of the signal any good, so some wonder how this could have evolved.  The same debate has been going on with respect to animals for a very long time, and the same kinds of answers are coming up -- plants seem to most effectively receive signals from related plants, so they may be protecting copies of their own genes when they signal to neighboring plants, because related plants share many of their genotypes.  Kin selection is controversial especially if it involves risk to the helping organism, such as a plant when it helps another.

This is all interesting for its own sake.  But there are possible important applications.  For example, Babikova et al. note that it has been found that some commercially bred maize no longer produces the VOCs induced by insect infestation. That is, a natural early warning system may be being bred out of crop plants, and thus increasing the need for pesticides. As Babikova et al. say, "...our data suggest a pressing need to determine the extent to which manipulation of common mycorrhizal mycelial networks can provide sustainable solutions to manage insect pests. The role of mycorrhizal fungi in mediating multitrophic interactions in agricultural ecosystems has largely been overlooked, but our findings suggest that there may be potential to develop fungal treatments to enhance crop protection." So indeed learning about inter-plant communication is of more than whimsical interest. 

As for how apparent altruism could have evolved in plants, or whether these communication mechanisms are helpful to other plants purely by accident, this is only a problem when the assumption is that evolution is about competition rather than cooperation.  The argument can be made -- we made it in our book "The Mermaid's Tale" and elsewhere, but more and more evolutionary biologists are making the argument as well -- that cooperation is more fundamental to life and to evolution than competition.

This is a serious problem if one individual makes a big reproductive sacrifice that serves to help another's reproduction.  It is that aspect of altruism that has attracted so much theoretical attention over the years (in animals).  But, again, it is a serious problem mainly if one views each organism as an independent actor, ignores mutuality and things like recombination and population change over time, and ignores the almost automatic kinship among nearby individuals in many species.

Whether human culture-derived words like kin or altruism should be applied to plants in a way that skirts so close to anthropomorphizing, this depends, we think, on whether you think of humans as exceptions, or more representative of the rest of nature than we generally like to accept. Or if you think that our kind of wet-ware (neurons and interconnections that form our unitary sense of existence) is the only one that counts.  But that's somewhat like saying trains or planes are not vehicles, because they don't work like cars do.  The same sorts of questions can be asked of consciousness -- are we really the only organisms that have self-awareness?

No comments: