Cytokinin Signaling Is Essential for Organ Formation in Marchantia polymorpha. (28th May 2019) - Aki, Shiori S
Abstract: Cytokinins are known to regulate various physiological events in plants. Cytokinin signaling is mediated by the phosphorelay system, one of the most ancient mechanisms controlling hormonal pathways in plants. The liverwort Marchantia polymorpha possesses all components necessary for cytokinin signaling; however, whether they respond to cytokinins and how the signaling is fine-tuned remain largely unknown . Here, we report cytokinin function in Marchantia development and organ formation. Our measurement of cytokinin species revealed that cis -zeatin is the most abundant cytokinin in Marchantia . We reduced the endogenous cytokinin level by overexpressing the gene for cytokinin oxidase, Mp CKX, which inactivates cytokinins, and generated overexpression and knockout lines for type-A (Mp RRA ) and type-B (Mp RRB ) response regulators to manipulate the signaling. The overexpression lines of Mp CKX and Mp RRA, and the knockout lines of Mp RRB, shared phenotypes such as inhibition of gemma cup formation, enhanced rhizoid formation and hyponastic thallus growth. Conversely, the knockout lines of Mp RRA produced more gemma cups and exhibited epinastic thallus growth. Mp RRA expression was elevated by cytokinin treatment and reduced by knocking out Mp RRB, suggesting that Mp RRA is upregulated by the MpRRB-mediated cytokinin signaling, which is antagonized by MpRRA. Our findings indicate that when plants moved onto land they already deployed the negative feedback loop of cytokinin signaling, which has an indispensable role in organogenesis.