Photomorphogenesis is critical for seedling development. Buried seed germinates and develops into etiolated seedling with long hypocotyl, closed cotyledons and curved apical hook. Once seedling reaches soil surface, the elongation of hypocotyl is quickly inhibited, and cotyledons are open and expand in response to light.

Plants have evolved sensitive signal transduction systems to fine-tune photomorphogenesis in response to the changes of light environments. Among them, the COP1–HY5 module is the central hub for integrating diverse light signals.

In a study published in The Plant Cell, a research group led by Dr. LIU Hongtao from CAS Center for Excellence in Molecular Plant Sciences of the Chinese Academy of Sciences identified that two new proteins, named COR27 and COR28, act as key regulators in the COP1–HY5 regulatory hub by ensuring proper skotomorphogenic growth in the dark and photomorphogenic development in the light.

COLD REGULATED (COR) genes, COR27 and COR28, are involved in the diverse biological processes such as circadian clock, freezing tolerance, and flowering time. It has been shown that light and temperature coordinately regulate their expression. Interestingly, blue light has been also found to stabilize the COR27 and COR28 at protein level. However, its underlying mechanism is unknown.

The researchers in this study showed that COR27 and COR28 physically interact with COP1 and this interaction is responsible for their degradation in the dark.

Functional analysis indicated that COR27 and COR28 promote hypocotyl elongation and act as negative regulators of photomorphogenesis. COR27 interacts with HY5, and directly associates with the promoters of the HY5 target genes.

Consistently, genome-wide gene expression analysis showed that HY5, COR27, and COR28 co-regulate many common genes.

These findings demonstrated that light promotes the accumulation of HY5 and COR27, while COR27 physically interacts with HY5 to inhibit its transcriptional activity and fine-tune skotomorphogenesis development in the dark and photomorphogenic development in the light.