Molecular Biology and Evolution of Phytochrome Systems
Photoreceptors are critical molecules that function at the interface between organism and environment. Plants use specific light signals, perceived via photoreceptors, to determine their place in time and space, allowing them to synchronize their growth, metabolism and development with fluctuating environmental conditions. This ability is crucial for the survival of plants because it allows them to time their development and position themselves in space in a way that maximizes photosynthesis and reproductive success.
Light cues in the blue, red, and far-red region of the visible spectrum are particularly important. Plants have two photoreceptor systems for blue light, the cryptochromes and phototropins, and one system for red and far-red light, the phytochromes. A hallmark of the phytochromes is their capacity to exist in two molecular forms—one that absorbs red light and one that absorbs far-red light—and to interconvert between them. Absorption of red light changes the red-absorbing form to the far-red absorbing form; conversely, absorption of far-red light changes the far-red absorbing form to the red absorbing form. This allows phytochromes to function as switches that induce processes when red light is absorbed and inhibit those same processes when far-red light is absorbed.
Additionally, this dual sensing system allows plants to monitor changes in the ratio of red to far-red light in the environment. Because vegetation absorbs red light but transmits far-red light, light under a canopy or reflected from neighboring stems has a lower ratio of red to far-red light than does direct sunlight. Thus, a low ratio of red to far-red light is a signal that neighbors are near. Plants respond to this signal by enhancing elongation growth, suppressing branching growth, and flowering early, essentially trying to outgrow their competitors or to complete their life cycle before their access to light is reduced.
Figure 1. Evolutionary tree of five species, A-3
A central question in evolutionary biology concerns the mechanisms underlying change and diversification. Since light cues are so critical to plant survival, it is likely that evolution in photoreceptors has involved adaptive change. One framework for evaluating whether change was adaptive or neutral, as well as for thinking clearly about how species differ from one another, is an evolutionary tree, a graph that represents a hypothesis of how species are related to each other. For example, the tree in Figure 1 suggests that C and D are more closely related to one another than they are to E, and that C, D, and E shared a more recent common ancestor with each other than they did with A and B. Evolutionary trees are inferred from many different types of data, including both the outward characteristics and DNA sequences of organisms.
We use the phytochrome system both as a source of DNA sequence data from which to infer evolutionary trees and to investigate how proteins change through evolutionary time and in plants with different lifestyles (for example, in parasitic plants, many of which do not require light for a part or all of their life cycle). These studies lay the groundwork to address the question of how changes in light-sensing mechanisms have influenced the ability of plants to survive and diversify. Current projects in the lab focus on phytochrome evolution at several points during the evolutionary history of plants, including
- during the origin and radiation of flowering plants, which rapidly achieved ecological dominance and came to outnumber all other land plants combined;
- in the flowering plant family Orobanchaceae, which comprises root parasites that are partially or wholly parasitic on their host plants and have responses to light that differ from those of non-parasitic plants;
- in seed plants, 80 percent of which are now extinct and which now comprise just five lines, the flowering plants, conifers, ginkgos, cycads, and gnetophytes; and
- in Arabidopsis thaliana, the genetic model system of plant biologists.
To learn more about this research, please visit Sarah Mathews's Lab website http://www.huh.harvard.edu/research/mathews-lab/index.html.