Light Perception and Transduction

Plants utilize light not only for photosynthesis but also as an environmental signal for many developmental processes. Plants are highly sensitive to seasonal, daily, and moment-to-moment changes in solar radiation. They are thus capable of perceiving the wavelength, intensity, direction, duration, and other attributes of light to bring about appropriate physiological and developmental changes. These light-triggered growth and developmental responses are known as photomorphogenic responses (Box 13.1). Thus, photomorphogenesis may be defined as the developmental response of an organism to the information in light, which may be its quantity, quality (i.e., wavelength), and the direction or relative length of day and night (photoperiod). The importance of light in plant development can be most dramatically illustrated in the case of early seedling growth. A dark-grown seedling is said to be etiolated. In etiolation, the embryonic stem (the hypocotyl in dicots and the epicotyl in monocots) of seedlings exhibits extremely rapid and extensive elongation of the internodes. There is no cotyledon/leaf expansion, and the seedlings appear pale as there is no chloroplast development. So, etiolated plants are pale yellow and the hypocotyl remains "hooked" at the apex. Curving of the hypocotyl is believed to protect the apical meristem from damage during seedling growth through the soil. On exposure to light, hypocotyl elongation slows down, and the cotyledons and leaves expand and become green. The apical hook straightens. In monocots, the etiolated coleoptile exhibits extended growth, which gets decelerated (slows down) in light and the developing leaves pierce its tip (Fig. 13.1). In contrast to skotomorphogenesis, seedlings grown in light exhibit photomorphogenesis and have a relatively embryonic stem, lose the apical hook, develop expanded green cotyledons, and exhibit rapid initiation of leaf development at the shoot meristem. Upon sensing light, a seedling emerging from the soil switches from skotomorphogenesis to photomorphogenesis. This process is known as de-etiolation, and it involves the coaction of a red (R)/far-red (FR) light-absorbing phytochrome and a blue light-sensitive cryptochrome.