Phytochrome
Introduction
Phytochrome is a type of photoreceptor found in plants that is sensitive to light in the red and far-red regions of the visible spectrum. These photoreceptors play a crucial role in plant development, regulating processes such as seed germination, stem elongation, leaf expansion, and the timing of flowering.
Structure and Function
Phytochromes are large proteins with a molecular mass of about 120 kDa. They consist of a protein part, known as the apoprotein, and a non-protein part, the chromophore. The chromophore is a linear tetrapyrrole compound, which absorbs light and undergoes a conformational change, triggering a signal transduction pathway in the plant cell.
The apoprotein part of the phytochrome molecule is encoded by a small family of genes, known as PHY genes. In Arabidopsis, a model plant species, there are five PHY genes (PHYA to PHYE), each encoding a different phytochrome protein.
Phytochromes exist in two different forms: Pr and Pfr. The Pr form absorbs red light and converts to the Pfr form, which absorbs far-red light and converts back to the Pr form. This reversible conversion between Pr and Pfr allows plants to respond to changes in the light environment.
Role in Plant Development
Phytochromes play a key role in regulating plant development in response to light conditions. They are involved in a wide range of processes, including:
- Seed Germination: Phytochromes regulate the germination of seeds in response to light. In many plant species, light is required for seed germination, and it is the Pfr form of phytochrome that triggers the germination process.
- Stem Elongation: Phytochromes also control the elongation of plant stems. In low light conditions, the Pr form of phytochrome promotes stem elongation, allowing the plant to grow towards the light.
- Leaf Expansion: The expansion of leaves is another process regulated by phytochromes. The Pfr form of phytochrome promotes leaf expansion, enabling the plant to maximize light capture for photosynthesis.
- Flowering Time: Phytochromes regulate the timing of flowering in many plant species. The Pfr form of phytochrome promotes flowering in long-day plants, while the Pr form inhibits flowering in short-day plants.
Phytochrome Signaling
The signaling pathway triggered by phytochrome activation is complex and involves a number of different proteins. When a phytochrome molecule absorbs light and converts from the Pr form to the Pfr form, it undergoes a conformational change that allows it to interact with a class of proteins known as phytochrome-interacting factors (PIFs). This interaction leads to the degradation of the PIFs, altering the expression of a wide range of genes and leading to changes in plant development.
In addition to PIFs, phytochromes also interact with other proteins, such as constitutive photomorphogenic 1 (COP1) and suppressor of phytochrome A-105 (SPA1), which are involved in the regulation of light-responsive genes.
Evolution of Phytochromes
Phytochromes are thought to have evolved in early land plants, and they are found in all major groups of plants, from mosses and ferns to flowering plants. They are also found in some groups of bacteria, known as cyanobacteria, suggesting that the phytochrome system may have originated in these photosynthetic bacteria.
The diversity of PHY genes in different plant species reflects the evolutionary history of the phytochrome family. In Arabidopsis, the five PHY genes are thought to have arisen through a series of duplication events, followed by divergence and specialization of function.