Associative Learning in Plants

Pea plant, Pisum sativum. Ink and watercolor by W. Giglioli. Hulton Fine Art Collection

Yes. That's right. You heard me. LEARNING IN PLANTS! And not just in some crazy special plant, but in the lowly garden pea! Although everybody else is talking about the feathery baby dinosaur tail discovered preserved in amber, I just desperately need to talk about learning in plants.  The paper is in Nature Scientific Reports led by Monica Gagliano from the University of Western Australia.

But first let's talk about learning in general. In animal behavior there are two categories of learning: associative learning, and non-associative learning. Associative learning is the formation of an association between a stimulus to which you previously had no response (called the conditioned stimulus, or CS) and a stimulus to which you have an innate response (the unconditioned stimulus, or US). The classic example is the work of a Russian physiologist Ivan Pavlov, who together with his assistant, Ivan Tolochinov, developed the concept of associative conditioning in 1901. The Ivans' experiments showed that when you present a dog with a dish of food (the US) they will salivate (called the unconditioned response, or UR). When the presentation of the US is consistently paired with another stimulus, such as a sound (apocryphally the ringing of a bell in the Ivans' experiments, the CS), eventually the dogs will salivate at the sound of the bell. They have formed an association. Non-associative learning includes other forms of learning such as habituation and sensitization. Habituation is when an animal decreases it's response to a stimulus over repeated exposure (you stop jumping at every firework explosion after listening to them for twenty minutes) and sensitization is an increase in response over repeated exposure. 

A 2014 paper also from Monica Gagliano demonstrated habituation in Mimosa pudica, which is often called "the sensitive plant". When touched, Mimosa folds its leaves down. There is a pretty silly video of people tickling these plants here. It has long been known that if you repeatedly touch a Mimosa plant it will stop responding, which is highly parallel with habituation learning in animals.

But associative learning??! In this new paper Gagliano and colleagues used garden peas, Pisum sativum. The unconditioned response was that plants grow towards light. The conditioned stimulus was the breeze from a fan. They put a little pea seedling in a split PVC pipe y-maze (see below). In one treatment the pea plant would have the fan blow on it from one side of the y-maze for 60 minutes, and then a light would come on on that same side. In a second treatment the fan would blow for 60 minutes on one side and then the light would come on on the opposite side. In both treatments the location of the fan is a predictable indictor of where the light is going to be (the same side as the fan, or the opposite side). They switched the sides that the stimuli were presented on for each training session. To test the plants they just gave them the fan from one side. 

"Figure 1. Training and testing protocol for associative learning in pea seedlings. (A) During training seedlings were exposed to the fan [F] and light [L] on either the same arm (i) or on the opposite arm (ii) of the Y-maze. The fan served as the conditioned stimulus (CS), light as the unconditioned stimulus (US). During testing with exposure to the fan alone two categories of responses were distinguished. Correct response: Seedlings growing into the arm of the maze where the light was “predicted” by the fan to occur [green arrow; iii (corresponding to scenario i) and iv (corresponding to scenario ii)]; Incorrect response: Seedlings growing into the arm of the maze where the light was not “predicted” by the fan to occur (black arrow; iii and iv). (B) Seedlings received training for three consecutive days before testing. Each training day consisted of three 2-h training sessions separated by 1-h intervals. The 90-min CS preceded the 60-min US by 60minutes so that there was a 30-min overlap. (i). During the 1-day testing session, seedlings were exposed to the fan alone for three 90-min sessions (ii). Seedlings of the control group were left undisturbed (no fan, no light; iii)." From Gagliano et al. 2016 .

The majority of plants grew in the direction they had been trained (toward the fan if the fan had been paired with light and away from the fan if the fan had been in the opposite arm from the light).  In a second experiment, Gagliano and colleagues showed that plants are only capable of this associative learning if they are trained during the "day" part of a light/dark cycle. That is, they can only learn to pair light with the fan cue during the time of day when they would normally be exposed to light. 

My mind is a bit boggled by this paper. In particular I am really curious about what the mechanism is. Also I wonder if we need some different vocabulary? I am generally for maintaining simplicity, and I think that using the same vocabulary across diverse systems can highlight interesting comparisons and similarities, but does it make sense to discuss learning, or even behavior, in an animal with no brain or even neurons?