Carbon availability acts via cytokinins to promote gemma cup formation in Marchantia polymorpha

Liverworts can clonally propagate by producing compact shoot structures called gemmae, which develop within basket-like structures known as gemma cups. It was previously reported in Marchantia nepalensis that carbon availability promotes gemma cup formation. However, the mechanisms by which carbon availability controls this process remains largely unexplored. To address this knowledge gap, we investigated how carbon promotes gemma cup formation using Marchantia polymorpha as a model species. Through a series of pharmacological and genetic experiments, we found that carbon availability promotes gemma cup formation by inducing the cytokinin pathway, thereby increasing the expression of MpGCAM1 and MpSTG, which encode two transcription factors involved in forming the basal floor of gemma cups. Indeed, our data show that cytokinins accumulate in marchantia thallus in response to sucrose and to high light treatments. In addition, constitutive induction of cytokinin signalling could overcome the repressive effect of low sucrose on gemma cup formation, whereas suppression of this hormonal pathway led to inhibition of sucrose-induced gemma cup formation. Furthermore, our results indicate that sucrose can induce gemma cup formation independently of KAI2A and MAX2, two molecular components of karrikin signalling known to control this developmental process by inducing cytokinin synthesis. Interestingly, in flowering plants, carbon availability also promotes cytokinin accumulation to induce axillary bud outgrowth, a process involving the transcription factors AtRAX and AtLOF1, the Arabidopsis thaliana orthologues of MpGCAM1 and MpSTG, respectively. Collectively, these observations indicate that the interactions between carbon and cytokinins are critical for the developmental plasticity of land plants in response to their environment.