Phytochrome is a special class of photoreceptor proteins found in plants, acting much like a plant’s internal eye. By detecting specific wavelengths of light, phytochrome helps plants regulate numerous aspects of their growth and development. This sophisticated system enables plants to adapt to changing light cues, influencing their life cycle from germination to flowering.
How Phytochrome Senses Light
Phytochrome exists in two interconvertible forms, each sensitive to different wavelengths of light. The red light-absorbing form, known as Pr, is the inactive state of the protein. When Pr absorbs red light (around 667 nanometers), it undergoes a molecular change. This converts Pr into Pfr, the far-red light-absorbing form, which is the biologically active state of phytochrome.
The Pfr form absorbs far-red light (around 730 nanometers), which then converts it back to the inactive Pr state. This reversible process, called photoconversion, functions like a precise light switch for the plant. In direct sunlight, which contains a high proportion of red light, Pr rapidly converts to Pfr, signaling ample light conditions. Conversely, in shaded environments or darkness, where far-red light may be more prevalent or red light is scarce, Pfr either absorbs far-red light or slowly reverts to Pr, indicating lower light availability. This interconversion allows plants to monitor their light environment.
Guiding Early Plant Life
Phytochrome plays a role in the initial stages of a plant’s development, particularly in seed germination and de-etiolation. For many plant species, a brief exposure to light is necessary to trigger germination. When red light is present, the inactive Pr form of phytochrome converts to the active Pfr form, initiating germination. If seeds are exposed to far-red light, Pfr reverts to Pr, which can inhibit germination, ensuring the seed only sprouts when conditions are suitable.
Once a seedling emerges, phytochrome continues its guidance through a process called de-etiolation. Seedlings grown in complete darkness often appear etiolated, characterized by a pale, elongated stem, underdeveloped leaves, and a closed apical hook. Upon reaching sunlight, the active Pfr form of phytochrome signals a shift in growth. This signal triggers the seedling to reduce stem elongation, begin producing chlorophyll to turn green, and expand its cotyledons or true leaves to maximize light capture for photosynthesis. This transformation allows the seedling to become a light-harvesting, photosynthetic organism.
Shaping the Mature Plant
Beyond the seedling stage, phytochrome continues to influence the architecture and competitive strategies of a mature plant through its role in the shade avoidance response. Plant leaves absorb most red light for photosynthesis but allow an amount of far-red light to pass through or be reflected. Consequently, a plant growing under the canopy of other plants experiences a lower ratio of red light to far-red light compared to direct sunlight. This altered light quality is perceived by the phytochrome system as an indicator of neighboring vegetation.
In response to this low red-to-far-red ratio, the active Pfr form of phytochrome decreases, signaling the plant to initiate shade avoidance. The plant then prioritizes stem and petiole elongation to grow taller and extend its leaves, aiming to outcompete its neighbors for access to unfiltered sunlight. This strategic growth adjustment helps the plant avoid being overshadowed, allowing it to capture more red light for photosynthesis and enhance its survival and reproduction in dense plant communities.
Setting the Plant’s Internal Clock
Phytochrome helps plants perceive the passage of time, particularly the length of day and night, a phenomenon known as photoperiodism. This system allows plants to anticipate seasonal changes and adjust their development accordingly. The active Pfr form of phytochrome slowly reverts back to the inactive Pr form during periods of darkness. By measuring how much Pfr remains at dawn, the plant can effectively gauge the duration of the preceding night.
This measurement of night length is important for processes like flowering. For instance, short-day plants flower when nights are long. Conversely, long-day plants flower when nights are short. Phytochrome’s interaction with the plant’s internal circadian rhythms refines this time-keeping mechanism, synchronizing physiological activities with the 24-hour daily cycle.