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Light plays a significant role in regulating the growth and development of plants. In addition to providing energy for photosynthesis, light provides other important cues to regulate a range of developmental and physiological responses in plants.

What Is a Photoreceptor?

Plants respond to light using a unique set of light-sensitive proteins called photoreceptors. Photoreceptors contain photopigments, which consist of a protein component bound to a non-protein, light-absorbing pigment called the chromophore. There are several different types of photoreceptors, which vary in their amino acid sequences and the type of chromophore present. These types maximally respond to different specific wavelengths of light, ranging from ultraviolet B (280-315 nanometers) to far-red (700-750 nanometers). The chromophore's absorption of light elicits structural changes in the photoreceptor, triggering a series of signal transduction events that result in gene expression changes.

The Phytochrome System

Many types of photoreceptors are present in plants. Phytochromes are a class of photoreceptors that sense red and far-red light. The phytochrome system acts as a natural light switch, allowing plants to respond to the intensity, duration, and color of environmental light.

The phytochrome system plays a significant role in photomorphogenesis—the growth and development of plants in response to light. Bright sunlight contains more red light than far-red light. Chlorophyll strongly absorbs red light, so shaded plant regions receive more far-red light than red light.

Plants use phytochromes to adapt their growth in response to red and far-red light. Exposure to far-red light in shaded regions triggers the elongation of stems and petioles in search of light. On the other hand, exposure to red wavelengths from unfiltered sunlight enhances lateral growth and branching.