Photomorphogenesis in response to red and far red light is regulated by :

Question

Accepted Answer

The correct option is 3: Phytochrome

Photomorphogenesis, which is the process where light regulates plant growth and development (such as seed germination, stem elongation, and flowering), is principally controlled by **Phytochrome** in response to red (R) and far-red (FR) light. Phytochrome acts as a dimeric photoreceptor protein that exists in two photo-interconvertible forms: a red-light absorbing form (Pr, or inactive form) and a far-red-light absorbing form (Pfr, or biologically active form). When the plant is exposed to red light (about 660 nm), Pr is rapidly converted to Pfr, which then initiates the developmental response. Conversely, far-red light (about 730 nm) converts the active Pfr back to the inactive Pr, reversing the response. This unique, reversible switch allows the plant to accurately sense the quality of light—specifically the Red/Far-Red ratio—which is vital for responses like shade avoidance. Therefore, the mechanism of R/FR photoreversibility and the subsequent developmental changes define the essential role of phytochrome in photomorphogenesis.

The incorrect option is 1: Universal chlorophyll

Universal chlorophyll, specifically chlorophyll *a*, is indeed the primary photosynthetic pigment found in virtually all photosynthetic organisms. Its primary function, however, is to absorb light energy—predominantly in the blue and red regions of the spectrum—and convert it into chemical energy (ATP and NADPH) to power photosynthesis. Chlorophyll is directly involved in energy metabolism and sugar production, not in acting as a signaling molecule to regulate developmental switches in response to the R/FR ratio. While the ratio of red to far-red light is affected by the absorption of red light by chlorophyll in the canopy, chlorophyll itself is the machinery for energy capture, not the sensor for morphogenesis. Therefore, it does not regulate photomorphogenesis in the sense that a photoreceptor does.

The incorrect option is 2: Cryptochrome

Cryptochromes are another major class of photoreceptors in plants, but they are primarily responsible for sensing **blue and UV-A light** (320-500 nm). These proteins are critical for regulating various aspects of plant development and the circadian clock. Cryptochromes help regulate stem elongation (de-etiolation), cotyledon expansion, and the timing of flowering in response to blue light conditions. Crucially, they do not exhibit the characteristic photoreversibility between red and far-red light, which is the defining mechanism of the process described in the question. Thus, while they are important for photomorphogenesis, their light spectrum specificity makes them the incorrect choice here.

The incorrect option is 4: Phycobilins

Phycobilins are accessory pigments that are mainly found in cyanobacteria and red algae, as well as in some cryptomonads. Their biological role is to capture light energy in the green-to-yellow spectrum and efficiently transfer it to the main photosynthetic pigments, like chlorophyll. They are highly effective light-harvesting components of the phycobilisome antenna structure. Although they are pigments, they are not present as the primary R/FR photoreceptor in the vast majority of higher plants, and their function is focused on enhancing light absorption for photosynthesis, not on acting as the developmental signal transducer for photomorphogenesis. The phytochrome molecule, on the other hand, uses a covalently linked chromophore called phytochromobilin, which is a type of bilin, but the entire functional protein, Phytochrome, is the regulator.