Photoreceptors and Plant Responses to Light

When a seed germinates, the roots extend into the soil and the shoot towards the light. Plants also modify their growth to maximize light exposure. How do plants sense the direction and quality of light? 

Light-sensitive receptors, also called photoreceptors, mediate a plant’s response to light. Photoreceptors contain a protein component bound to a light-absorbing pigment called the chromophore. 

Plants may possess multiple families and variants of photoreceptors. Together, they respond to wavelengths of light ranging from ultraviolet to the far-red regions of the light spectrum. 

Each chromophore in a specific photoreceptor absorbs light of a particular wavelength, causing a structural change in the receptor. Activation of the photoreceptor triggers a signaling cascade within the plant cells. This, in turn, can lead to gene expression affecting plant growth and morphology.

For example, plants alter their growth in response to shade using the family of phytochrome photoreceptors. Each phytochrome has two distinct, interconvertible forms: the physiologically inactive Pr and the active Pfr. 

Pr absorbs red light and is quickly converted to the active Pfr form. Pfr absorbs far-red light and is converted back to the inactive Pr form.  The successive conversion of Pr and Pfr reaches a dynamic equilibrium in daylight.

Taller plants can filter the red light out of the light spectrum, leaving plants underneath with more far-red light than direct sunlight. The phytochrome system allows the plant to sense the ratio of red to far-red light and adjust its growth.