Green is a dominant color on our planet. It’s the most common color of plant life we see. Take the MS Windows XP ‘Bliss’ photo where the green is the color that really pops off the screen. It’s one of the first things I see.
The plants we see out in the world are a sea of green. It’s easy to just say green = photosynthesis and be done with it. Even though there’s a diversity of green shading in the plant world (and beyond in the diversity of photosynthesizing organisms), it belies the underlying complexity that is plant life on the Earth.
We see plants as green because that’s the wavelength of visible light not absorbed, enriched for that middle part of the visible spectrum of light.
The light that’s absorbed in the red and blue part of the spectrum doesn’t just go to photosynthesis though that’s part of it.
Plants have a series of informational light sensing proteins (coded by genes) that help them maximize their growth and environmental responses to take most advantage of the sun.
These informational photoreceptors fall into several types defined by the wavelength of light they recognize: The red/far-red light perceiving phytochromes, the blue light detectors cryptochromes & phototropins, and even detectors of UV light. And one that’s been in the news recently that some algae, hornworts and ferns have called neochrome that is a combination of a phytochrome and a blue light receptor photoropin.
These informational receptors allow plants to read their light environment and set off responses within individual plant cells to cause chloroplasts to reposition themselves, stems to elongate or stop growing, and even when to germinate when they’re just seeds buried in the Earth.
One function of phytochromes is neighbor detection. Plants read the ratio of red to far red light (and have a phytochrome that sees each of those wavelengths best). Red light largely gets absorbed by leaves and so if a plant sees a sudden spike in far-red light, it knows there must be a leaf in the way somewhere. Phototropins are responsible for directional growth of plants, usually towards the light.
There were two articles by Ed Yong and Carl Zimmer about the origins of the fern neochrome based on a paper recently published in The Proceedings of The National Academy of Sciences (PNAS) by Fay-Wei Li et al. In it, the researchers show that the neochrome gene that’s in some present fern lineages was the result of what is called horizontal gene transfer; something bacteria do all the time, but is less common in complex multi-cellular life. The fern neochrome gene originated in an early kind of land plant called a hornwort. As Carl Zimmer points out, this kind of cross-species gene transfer is exactly what scientists do today to create some genetically modified organisms. However, nature in all its messiness did this one the old fashioned way. I would be curious to know if there was any more of the hornwort genome transferred into ferns as nature is rarely neat in it’s processes. The neochrome gene, in one protein, helps ferns both detect their neighbors and cause directional growth towards a more optimal light source, helping them adapt to life in the forest floor; think Endor from Return of The Jedi (that is actually the redwood forest in Northern California whose ferns do have the neochrome gene and they are common). Even without neochrome, plants have small populations of phytochromes and phototropins that are associated with one another as this PNAS paper demonstrates (technical paper, freely available).
There is also evidence that plants, most likely through the photoreceptors outlined above use green light as a source of information too, but it’s not enough to not have a majority of green light pass through for our eyes to see. Plants can’t afford to not get all the light information they can.
There’s a whole lot more going on in a plant than it just being green. Not all plants have adapted to use light in the same way; the information’s the same and even the receptors are similar, but the outcomes can vary.
Whenever you look at anything more deeply than the surface, details emerge that reveal the full, deep and different story. It’s something we learn in science over and over again and yet humans seem to be hard wired to often not see beyond the surface of things. And even if we can in one area, we have a hard time doing it in another; especially now that we live in a sea of information.
Diversity is something I feel I’ve learned a lot more about in the last 6 months or so– not that I wasn’t aware of it as a term, but learning about studies like this one on bias and hiring/grad school acceptance– it’s a lesson we can take from the green plants we see; utilize the diverse spectra of information (or the full array of humanity) available to help survive and thrive in what can be a harsh world.
This post was written as part of the April berry go round blog carnival. Plants aren’t the only photosynthetic organisms that use color beautifully and efficiently.
Postdoc Street 
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