WHAT PLANTS KNOW (Chapter 1)

Chamovitz, Daniel. What a Plant Knows: A Field Guide to the Senses. New York: Farrar, Straus and Giroux, 2012.

This book is 228 pages in length. As noted in the prologue, the author was motivated to take on the study of parallels between plants and humans in terms of how they respond to light, among other things, after discovering that both organisms share genes that regulate responses to light. The unique aspect of plants is that, unlike animals, they are immobile and unable to choose their environments and therefore must develop adaptations in terms of complex sensory and regulatory systems to survive and thrive where they are.  As he noted in the prologue, “a head of lettuce has to know if there are ravenous aphids abut to eat it up so that it can protect itself by making poisonous chemicals to kill the pests”. In each chapter, the author uses latest research information to describe how plants demonstrate “senses” that allow them to take in light, chemical, and temperature information and adjust and respond to their environment.

Even though the book is about plant biology, I added it to my reading list because I suspect that plant “senses” ultimately have a biochemical basis.  It also may contain interesting vignettes that I can share with my class which most of the time has a number of biology majors who have taken or will be taking botany.

From the book’s section on “A Note about the Author”
Daniel Chamovitz, Ph.D., is the director of the Manna Center for Plant Biosciences at Tel Aviv University. He grew up in Aliquippa, Pennsylvania, and studied at Columbia University before receiving his Ph.D. in genetics from the Hebrew University of Jerusalem. He has been a visiting scientist at Yale University and at the Fred Hutchinson Cancer Research Center in Seattle, and has lectured at universities worldwide. His research on plants and fruitflies has appeared in leading scientific journals.


CHAPTER ONE: WHAT A PLANT SEES
In this chapter, the author describes how plants detect light, dark, and colors and regulate functions in response to light changes.  The author begins this chapter by explaining what it means to have sight and to detect not just light and dark but color as well.  He spends a few paragraphs describing visible light and how human retina contains the photoreceptor proteins in the form of rods and cones that allow us to detect and interpret different wavelengths of light as different colors. The human retina contains about 125 million rods and 6 million cones; this number of light detectors is equivalent to a digital camera resolution of 130 megapixels! Rods, containing the protein rhodopsin, are the more sensitive of the two and they are what allow us to see under low light conditions at night.  Cones, containing photopsin proteins, on the other hand, allow us to differentiate bright light into colors.

One of the earliest researchers of plant response to light was Charles Darwin who stated in his final book “The Power of Movement” that most plants move toward light in the process now called phototropism.  In 1864, Julius von Sachs discovered that, in particular, they bend toward blue light but not others. Experiments carried out and published in 1880 by Darwin and his son indicated that the plant’s light sensor is on the tip of the plant’s shoot, demonstrating what the author called “rudimentary sight” in plants.

In the next section, the author describes the phenomenon of the Maryland Mammoth, a strain of tobacco plant first observed in 1906 that “just kept growing” to a height of 15 feet, all leaves and no flowers. While the abundant leaves were a boon to the farmers, no flowers means that they cannot harvest seed for re-planting for the next season. Subsequent experiments in 1918 showed that when exposed to an “artificial” short days of light, these “mammoths” stopped growing and started flowering in a phenomenon now called as photoperiodism. This phenomenon pertains to plants’ ability to measure light they take in and respond accordingly as ‘long-day” (irises and barley) or “short-day” (chrysanthemums and soybeans) plants when it comes to flowering.

The initial observation of photoperiodism provided a basis for scientists to control the flowering process in plants simply by artificially changing the amount of light they receive.  The flowering process in short day plants on short days can be suppressed simply by shining light on them even for a few minutes in the middle of the night.  The reverse can be done for the long-day plants during winter. These experiments suggested that plants measure the amount of continuous darkness and not the amount of light they are exposed to. Further probing revealed that plants only record red light shone on them at night to measure the length of continuous darkness and ignores blue and green flashes of light.  Thus, blue for bending and red for measuring the length of darkness. It got even more interesting when scientists in the early 1950’s found out that a longer wavelength of red light cancels any effect of red light on the flowering process, in alternating ways as if longer wavelength red light is an off switch for flowering and shorter wavelength red light is an on switch for flowering for long-day plants. By the 1960’s, further experiments revealed that phytochrome receptors are responsible for the plant’s response to red light and longer-wavelength red light (far red light).  This process acts as a way for the plant to measure the amount of darkness based on how long ago it last saw red light. Further experiments showed that shining the light on a single leaf is sufficient for the entire signal to get propagated  to the entire plant and to induce the effect but shining the light on the tip or stem had no effect.

Humans have 5 photoreceptors: rhodopsin for light and shadows, red, blue, and green photopsins, and cryptochrome for regulating internal clocks.  Plants have directional blue light receptors (called a phototropin) and red and far-red light phytochrome receptors but later genetic experiments on Arabidopsis thaliana showed that it had at least 11 different photoreceptors, each one regulating a function (e.g., when to flower, when to bend to light, etc.).

Because plants are immobile, they need to grow toward their food (sunlight). In order to survive and thrive, they have developed fine tuning methods to regulate their functions as a function of light detected, the amount and quality of which change depending on the time of day, season of the year, and their local environments.

Plant photoreceptors can detect a wider range of electromagnetic light, both shorter and longer beyond the tight mostly visible spectrum that humans can detect.  Plants, however, do not have a nervous system that allows them to “see” but rather they use the light detection process to gain signals for regulating function. As the author notes, “sight is the ability not only to detect electromagnetic waves but also the ability to respond to these waves”.  Using this definition, one can then say that plants have “sight” as well, as they respond to light and translate these signals into “physiologically recognizable instruction”.  While human red photopsin and plant phytochrome both absorb red light, they have different functions and chemistries.  Structurally, they are similar in that they are both proteins to which a chemical dye is attached.  Both humans and plants also have the blue-light receptors cryptochromes that control both human and plant internal (circadian) clocks using light signals but with different function responses. An interesting example of this is the evolutionary development of circadian clock regulation of cell division to only occur at night, possibly to the damaging effects of UV rays in sunlight.