The phytohormone jasmonate is a signaling molecule that is essential for regulating a variety of physiological processes. JASMONATE ZIM-DOMAIN (JAZ) repressors negatively regulate jasmonate signaling by interacting with and inhibiting multiple transcription factors. CONSTANS (CO) is a master flowering-time regulator that integrates photoperiodic and circadian signals in Arabidopsis thaliana. CO is expressed in multiple tissues, including young leaves and seedling roots. However, it remains unclear about the roles and underlying mechanisms of CO in mediating physiological responses other than flowering. 

In a study published in The Plant Cell, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences tried to reveal the role and mechanism of CO in jasmonate signaling. They revealed that CO is a negative regulator of jasmonate-imposed root growth inhibition and anthocyanin accumulation. It functions together with JAZ repressors and basic helix-loop-helix (bHLH) subgroup IIId factors in jasmonate signaling. 

The researchers firstly confirmed the spatial pattern of CO expression and detected its transcripts in young leaves and seedling roots. They observed that the CO transcription level decreases in seedlings treated with methyl jasmonate (MeJA). Afterwards, the researchers conducted phenotypic and genetic analysis.  

Phenotypic analyses indicated that CO negatively regulated jasmonate-induced root growth inhibition and anthocyanin accumulation. Compared to the wild-type control, loss-of-function co mutants exhibited greater MeJA-induced root growth inhibition and anthocyanin accumulation. Conversely, CO overexpression decreased the sensitivity of transgenic seedlings to MeJA. CO was able to interact with several JAZ repressors of jasmonate signaling. 

Genetic analyses indicated that CO functions in a CORONATINE-INSENSITIVE1 (COI1)-dependent manner to modulate jasmonate responses. CO physically associated with bHLH subgroup IIId transcription factors and acted cooperatively in suppressing jasmonate signaling, but JAZ proteins interfered with their transcriptional functions and physical interaction. 

"This study reveals the crucial regulatory effects of CO on mediating jasmonate responses. Our data may be useful for clarifying how CO functions together with JAZ repressors and bHLH 5 subgroup IIId transcription factors to precisely control jasmonate signaling," said HU Yanru of XTBG.