Plants Can See (March 2011)

I’m a fourth-year biology major. I love learning about what makes the living world tick.

One of my favorite biology sub-disciplines is plant physiology. As I write this, I’m finishing up my honors thesis on plant signal transduction and stress response.

But in high school, plants were the last thing I ever wanted to touch, let alone learn about. The thought of studying plants for my thesis would have been as preposterous as Frosh Week without Gaels.

What changed?

Simply put, I finally wised up to the genius of how plants evolved.

Plant evolution is radically different from human/animal evolution. Whereas evolution endowed animals with legs, plants were given the gift of biochemistry. Biochemistry that allows the plant to feed, protect, and reproduce (often with itself!), as well as humiliate, leech, and even kill.

The maple tree outside your window can survive day in, day out in the -20 ice-cube of winter, whereas brave, all-mighty humans cannot do so without artificial heating.

Orchid flowers can mimic female wasp scents so well that they not only attract male wasp pollinators, but drive them into such a promiscuous frenzy that the males start humping each other.

I work next door to a lab that works with castor beans, home of one of the world’s most deadly chemicals, ricin. To put things into perspective, 8 beans are enough to do the trick.

Why do we even eat plants? Because our feeble bodies are physically incapable of synthesizing the compounds that plants pump out regularly and heartily.

And to really kick the dead horse, can we photosynthesize and make our own food? No. Plants? Duh!

You can see why I find plants awesome (sometimes even more awesome than some of my fellow humans)!

But what fascinates me the most about plants is something that most of us take for granted in ourselves. Many of us would bet our grungy Kingston bicycles against this fact, but like us humans, plants can see.

Yep, it’s true. Plants can see.

Here’s why. (Prepare to have your world uprooted, so to speak!)

Plants, like animals, can detect light and respond appropriately.

Just like the light-sensing chemicals in the retinas of our eyes, plants also have chemicals that perform this vital service.

One critical light-sensing chemical in plants is a beast known as phytochrome. It’s found in most tissues in the plant, during most stages of development.

Similar to how our retinal chemicals only detect light of certain wavelengths, phytochrome also detects light at 2 specific wavelengths: red, and far-red. Red light is the light that plants receive when they are exposed to the sun. Far-red light is the light received when the plants are shaded, either by the soil covering the seed, or by the leaves of other plants above. So you can think of phytochrome as the plant’s shade-sensor, allowing the plant to see when it is being covered by something that blocks sunlight.

Why is sensing blockage from sunlight important? Because plants get virtually all their energy from the sun! So when things are blocking its energy source, the plant, via phytochrome, sees this, and does a lot of different things in response. What the plant does will depend on how old it is.

For instance, if small seeds (like lettuce seeds) are covered by too much soil, phytochrome in the seeds senses the blockage of sunlight and stops the seeds from germinating. This is quite clever, when you think about it. Unlike bigger, bean-like seeds, small seeds don’t have a lot of carbohydrates and other energy stores built up inside the seed. So when small seeds germinate, the seedling needs an immediate source of energy (sunlight) in order to keep growing. But if the sunlight is chocked off by a concrete-like soil layer, phytochrome is there to stop the seeds from sprouting stupidly into the dark, effectively preventing seed suicide.

In newly-sprouted seeds, the role of phytochrome is different. If a new sprout is exposed to sunlight, its phytochromes see this, and cause the sprout to develop its first green, photosynthetic leaves. (Woot, food!) But if the sprout grows in darkness, as perceived by the all-seeing phytochrome, then the sprout doesn’t spend its energy making photosynthetic chemicals. Instead, it remains ghostly white in color. Its stem shoots up, like a growth-spurting teenager, becoming taller and taller until hopefully it breaks through the annoying canopy blocking its sun. If the phytochromes eventually see sunlight, the greening party is on! But if there is still no sun...we get our juicy bean sprouts. (How else did you think bean sprouts are grown?)

These are just a sample of the many ways phytochrome allows the plant to see and respond to sunlight, or lack thereof. Phytochrome also controls other light-induced functions, from day-night leaf movements to plant flowering. The amazing thing is, along with phytochrome, plants also have many other types of light-sensing chemicals, which can see other light wavelengths. These visual pigments can even pick up wavelengths we can’t see, like ultraviolet rays. How’s that for putting us meager humans in our place?

Think about it. The lowly plant: bottom of the food chain, trampled over, pooped on, and worst, shunned by biology students. But how lowly are they, in the grand scheme of things? After all, they too have been blessed with vision. And their form of vision is one that we can only dream of, and at best, computer-simulate.

In sight, and in many other ways, plants are actually more inventive than we are, certainly more than we give them credit for. So the next time you eat your salad greens, remember to thank the phytochrome molecules inside for your meal. But even if you forget, no worries! You’ll surely be seeing your lettuce in a new light.