Phytochromes are a type of protein that is sensitive to light. They can be found in plants, bacteria, and fungi. There are two types of red-sensitive phytochromes. Type I is sensitive to far-red light. Type II is sensitive to red light.[1]
The way they work is to control the plant's development. There are other receptors which respond to blue and ultraviolet light. Their pigments are also phytochromes.[2]