The chlorophyll of green leaves absorbs blue as well as red light; therefore, the light under a plant canopy usually contains lower fluences of blue light than is present in sunlight. Sun-adapted plants can respond to the lower fluences of blue light by initiating a shade avoidance response. Such shade avoidance responses to low fluences of blue light have been shown to be regulated by the cryptochromes.

It is likely that crytochrome regulates shade avoidance by interacting directly with phytochrome. Studies have shown that cryptochromes 1 and 2 (CRY1 and 2) interact with phytochrome (PHYA and PHYB) during the regulation of flowering (see textbook Chapter 20). Both CRY1 and CRY2 interact with PHYA in vitro and can be phosphorylated in a PHYA-dependent manner. Phosphorylation of CRY1 has also been demonstrated to occur in vivo in a red light–dependent manner.

In addition to cryptochrome, both auxin and brassinosteroids (BR) are required for hypocotyl elongation in seedlings under low-blue-light conditions. Members of the XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE (XTH) gene family are important cell wall modifying enzymes that are regulated by blue light. Different subsets of XTH enzymes are regulated by auxin and BR. Experiments in which auxin- or BR-signaling was blocked using mutants or chemical inhibitors showed that hypocotyl elongation in response to low fluences of blue light depended on both auxin and BR, and that the two hormones functioned non-redundantly and non-synergistically in shade avoidance responses.