A new study by scholars at Southern University of Science and Technology (SUSTech) has found how plants use their circadian clock to regulate their growth cycles, based on their exposure to light.

On 30^th July, Long-Term Distinguished Visiting Chair Professor Xingwang Deng (Biology) led his research team to publish a paper in the high-impact academic journal, Plant Cell (IF=9.618). Their article, entitled “COLD-REGULATED GENE 27 integrates signals from light and the circadian rhythm to promote hypocotyl growth in Arabidopsis”, showed a novel regulation of seedlings photomorphogenesis by the integration of light signal and the circadian clock.

As the essential external or internal cue, light and the circadian clock are critical for the survival and development of plants. While germinating under the soil in the absence of light signals, seedlings undergo skotomorphogenesis. It is characterized by accelerated hypocotyl elongation, closed and etiolated cotyledon, and apical hook. This development process enables seedlings to receive the light once breaking the surface of the soil. When exposed to light, seedlings undergo photomorphogenesis. This process features inhibited hypocotyl elongation, open and greening cotyledon, and disappeared apical hook.

COP1-SPA complex is the crucial repressor of light signaling. It functions as an E3 ligase to promote downstream substrates ubiquitination and degradation. A group of transcription factors plays a vital role in the regulation of photomorphogenesis. bZIP type transcription factor HY5 and bHLH type transcription factor PIF4 regulate photomorphogenesis. HY5acts as a positive regulator of light signaling, while PIF4 represses photomorphogenic development.

The researchers in this study identified a new repressor of light signaling, COR27. COR27 integrated light and the circadian clock is signaling to modulate the growth and development in Arabidopsis. When exposed to light, the accumulated COR27 protein repressed the DNA-binding capacity of HY5 through direct protein-protein interaction.

COR27 could directly bind to the promoter region of PIF4 to promote the expression of PIF4 and its target genes, specifically in the afternoon. Consequently, COR27 promoted hypocotyl elongation, at least in part through these two characterized molecular regulatory mechanisms. In the dark, the COP1-SPA complex promoted the degradation of COR27 via the 26S proteasome system. This study revealed a novel mechanism underlying the regulation of photomorphogenesis by COR27-mediated integration of light and the circadian clock signals (Fig. 1).

Fig. 1 A working model depicting the regulation of photomorphogenesis by COR27-mediated integration of light and circadian clock signaling

Senior Research Assistant Wei Zhu and Hua Zhou (Institute of Plant and Food Science) are the co-first authors. Professor Xingwang Deng at SUSTech and Professor Dongqing Xu at Nanjing Agricultural University are the co-corresponding authors.

This research received support from SUSTech, the National Key R&D Program of China, the National Natural Science Foundation of China, Peking-Tsinghua Center for Life Sciences, Nanjing Agricultural University, Nanjing Science and Technology Innovation Program for Overseas Students, and the Jiangsu Collaborative Innovation Center for Modern Crop Production.

Link to paper: http://www.plantcell.org/content/early/2020/07/30/tpc.20.00192