Being light-powered lifeforms, most green plants seek out the sunlight and avoid the shade.
But being sessile organisms, plants can not uproot themselves and mosey from the shade into the sun. Because they are stuck in the same spot all of their lives, plants have apparently evolved ways to maximize their light exposure.
One of these ways is to grow towards the light. And when young stems or shoots do it, we call this positive phototropism. (For an enlightening time-lapse movie showing phototropism in corn seedlings, please see “worshiping the light”.)
The phototropic movement of plants has certainly fascinated people for millennia. (Please see Ref. 1 for an excellent historical perspective on the study of phototropism.) But it’s only been in the past few years that the cellular mechanisms of plant phototropism have been coming into focus. (For a current review of this subject, please see Ref. 2 below.)
Despite these recent advances, significant gaps in our understanding of phototropism still remain.
Intriguing clues toward solving one of the biggest mysteries of how plant phototropism works has recently been provided by some surprising results reported in the 7 November 2013 Science Express (see Ref. 3 below).Blue-Light Special
It’s been known for over 100 years that blue light is the most effective color of light in positive phototropism in plants. And the search for the blue-light photoreceptors primarily involved in phototropism lasted for decades, until the mid-1990’s. It was then that researchers discovered photoreceptor proteins that we now call phototropins, the light-receptors inside plant cells responsive to directional light. (Please see here for a nice overview of blue light sensing in plants.)
We now know that phototropins are involved in a range of responses that serve to optimize the photosynthetic efficiency of plants. These include not only phototropism, but also light-induced stomatal opening, leaf enlargement, and chloroplast movements in response to changes in light intensity. (Phototropins have been previously mentioned here in connection with cytoplasmic streaming.)
One of the biggest missing pieces of the puzzle of phototropism is how phototropins work.
We know that phototropins can act as protein kinases, that is, they are “…enzymes that modify other proteins by chemically adding phosphate groups to them (phosphorylation). Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins.”We also know that phototropin-signaling is initiated when these photoreceptor proteins are activated by blue light. But the main question has been, especially with regard to phototropism, what are the targets of the phototropins.
It now appears that at least one of the targets is an enzyme that modifies the plant cell cytoskeleton.
Named after the Japanese sword katana, katatin uses the energy from ATP to break and disassemble stable microtubules, components of the cytoskeleton. These researchers determined that this katatin activity caused the generation of large numbers of microtubules that are perpendicular to pre-existing microtubules, which, in turn, resulted in a reorientation of parts of the plant cell cytoskeleton.
OK. That’s nice. But what has this got to do with phototropism?
The reorientation of the cytoskeleton may affect (1) plant cell wall synthesis and (2) the transport and localization of the plant growth hormone auxin. Both of these factors certainly affect plant cell elongation, which is an integral part of the process of phototropism.
Although at the present time we don’t know precisely how, the phototropin-stimulated reorientation of the cytoskeletons of plant cells on the illuminated side of growing stems may be an important contributor to phototropism.
1. Whippo, C. W. and R. P. Hangarter (2006) “Phototropism: Bending towards Enlightenment.” Plant Cell, Vol. 18, pp. 1110-1119. (Full Text)
2. Goyal, A., B. Szarzynska and C. Fankhauser (2013) “Phototropism: at the crossroads of light-signaling pathways.” Trends in Plant Science, Vol. 18, pp. 393-402. (PDF)
3. Lindeboom, J. J., et al. (2013) “A mechanism for reorientation of cortical microtubule arrays driven by microtubule severing.” Science, DOI: 10.1126/science.1245533. (Abstract)
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