Friday, March 7, 2014

New evidence that plants have decision-making capabilities based on the perception of risk

Plants are amazing.  That is a fact.  As a plant scientist, that is the answer I've always given to the typical question: "why do you even like plants?  They're so boring!"  NO!  Plants are unbelievably complex, animals are boring!
Sidebar: I am so into plants that this will likely be the first of a series of posts on how cool plants are.  Prepare yourselves.

Animal behaviour and adaptation is based on movement.  Stressful environment?  Cool, I'll just migrate.  But plants can't move around, so they've had to develop all these super amazing and complex strategies for surviving and thriving in stressful and changing environments.  Case: wild tobacco plants in the U.S. were pollinated by a type of moth that would also lay its eggs on the plants.  This attracted caterpillars that started eating the plants, so what did the plants do?  The plants stopped producing the chemicals that attract its usual pollinator and started flowering during the day instead of at night, to attract a different kind of pollinator!  That would be like a woman being able to actively decide when she ovulates.  It's bananas!  See, plants are cool!

Plants have a really complex secondary metabolism.  Every living creature has primary metabolic pathways, which are necessary for life.  This is the metabolic network that allows organisms to grow and reproduce, maintain structures, and respond to the environment.  Secondary metabolism refers to the network that also helps organisms survive, but they're not absolutely necessary for survival.  Secondary metabolism is what makes plants so awesome!  In plants, secondary metabolism supports primary metabolism by keeping the primary metabolic network working.  It's also extremely important for plant defense.  A lot of the volatile compounds (a fancy way of saying compounds that vapourize at room temperature) are produced during secondary metabolism.  Volatile compounds are what makes herbs and spices smell and taste good.  They're used to deter herbivores, and are often used in signalling from one plant to another.  One really cool defense mechanism that plants have is through methyl jasmonate, which is a compound that plants release when they're being eaten by caterpillars.  This heightens the plants own defenses, but it also warns the plants around it that there are herbivores around, so they might want to heighten their own defenses.  There are other insects that prey on these caterpillars that have adapted to recognize methyl jasmonate.  They then sweep in and lay their eggs inside the caterpillars.  But because plants aren't wasteful, jasmonates are also used as a signal for flowering and aging.

Anyway, that's my attempt to convince you, dear reader, that plants are pretty amazing.  This post is actually about a new paper that outlines the seed abortion decision-making process in plants that is based on both internal and external cues.  Seed abortion in this case basically means that plants are selectively deciding whether or not a seed will develop.  In the context of this paper, that means that if there is a high amount of parasite infestations in the surrounding environment, then producing a seed, and the fruit to protect the seed and help in its dispersal, is energetically too costly for the probability that the seed will survive.

The authors used Tephritid fruit flies, which lay their eggs in the fruit of the European Barberry.  Larvae eat the fruit and seeds.  There is usually only one larva per fruit, and each fruit has two seeds.  That means that the larvae probably need more than one seed to survive.  So the plant can let both seeds die, or it can selectively abort one of the seeds, leaving the other to develop.  This increases the likelihood that one of the seeds will survive and grow into a plant.

What the authors found was that this selective abortion of seeds was strategic.  Seed abortion was higher when there were two viable seeds per fruit (neither was damaged), increasing the odds that one would survive.  It was also higher when water constraints and predation by the fruit flies occurred together; exposing a plant to one stressor decreases its likelihood of survival, exposing it to two stressors is even worse.  Basically, the plants are strategically deciding, based on environmental cues, what the likelihood of offspring survival is, and how best to increase the odds.

This has led a lot of people to describe the results in the context of plants "thinking", which ends up in a philosophical debate about thinking that derails the conversation about a really interesting discovery.  If we think of human thought as a process that allows human beings to reflect on and interpret their environment and to make plans and predictions about the world, then can we really say that this process in plants is any different?  Just some food for thought.

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