Do Plants Think?

Arabidopsis thaliana

A recent article on plant chemical behavior has received press attention because the researchers have shown that plant cells can effectively carry out basic arithmetic calculations. The article–by Scialdone et al., in press in the journal e-Life–describes experimental results confirming what researchers have theoretically expected, based on mathematical calculations about biochemical behavior. Basically, it appears that after a few hundred millions of years of terrestrial plant evolution, genes in the cells of leaves produce different proteins that keep track of how much starch photosynthesis has produced in each cell during the day AND how long it’s recently been dark at night over the past few days. Starch catabolism is key for the plant at night, since energy from the starch fuels the organism’s physiological maintenance and growth. And the pace of starch degradation and mobilization appears to be critical. Plants appear to have the capacity to use available starch during the nighttime, so that starch runs out almost exactly at dawn. Regardless of how plants achieve this, the authors note, “[i]n the model plant Arabidopsis thaliana this phenomenon is essential for productivity: mutants with defects in either the accumulation or the degradation of starch have reduced productivity and exhibit symptoms of starvation.” Scialdone et al. (in press) combine formal mathematical models with laboratory experiments in which they control Arabidopsis plants’ day-night cycles. The result is that plants are able to update circadian clock information about changes in night-time duration, while using up starch at a rate so that–as long as the laboratory scientists don’t drastically change the duration of nightime from day to day–their energy source runs out right around dawn. The neat chemical trick is that some proteins are sensitive to the amount of starch, carrying out reactions in the cells at a rate directly proportional to starch granule availability, day and night; other proteins are sensitive to the duration of daylight and nighttime. The enzymes that directly facilitate starch degradation interact with circadian rhythm enzymes, so that starch degredation enzyme concentration is influenced by current starch quantity, but also limited by the circadian-rhythm enzymes, whose concentrations relate to time to dawn.

This is an analog way of carrying out a continuously updated division problem: starch is consumed at a rate that equals current starch availability level divided by how much time is expected to be left until dawn. Consumption = Starch Amount ÷ Time Left Till Dawn.

The authors state, “Our analysis here has underlined the utility of analog chemical kinetics in performing arithmetic computations in biology. Importantly, we have for the first time provided a concrete example of a biological system where such a computation is of fundamental importance.” It is very likely the case that activity in networks of neurons are maintained and modulated by similar analog biochemical systems. Thus, many cognition researchers would agree that the way we store information in our brains about what’s happened in the past, so that it can modify bodily responses in the future, involves chemical feedback systems that are shaped by–and thus represent–environmental stimulus patterns from the recent past (see Chemero 2009). In fact, chemical concentrations of different kinds of biomolecules–from neurotransmitters to hormones to a range of other cellular enzymes and DNA transcription factors–can mutually influence each other in our bodies, thus storing information that is distributed not just in our central nervous system but throughout our bodies. No one would argue that plants are intentionally carrying out division problems. And in fact, we’d be much worse than plants if we had to measure our own food-levels consciously and constantly, and also measure information about how long that food will have to last in the near future, so that we could slowly but constantly sip away at that food until we can reasonably expect to get more. Natural selection on DNA over a huge time period has favored a robust chemical system in plants, adapting their metabolic cycle to day-night rhythms in the environment. But the demonstration of how chemical concentration feedback systems can store and apply information in Arabidopsis plants gives us a tantalizing hint about the more complex web of cellularly, physiologically, and anatomically structured biochemical feedback systems underlying embodied human thought and consciousness. Thus, the difference between human consciousness and plant starch utilization is not entirely night-and-day.

REFERENCES

Chemero, A. (2009). Radical embodied cognitive science. Cambridge, Mass.: MIT Press.

Scialdone, A., Mugford, S. T., Feike, D., Skeffington, A., Borrill, P., Graf, A., … Howard, M. (2013). Arabidopsis plants perform arithmetic division to prevent starvation at night. eLife, 2. doi:10.7554/eLife.00669