Type-B ARABIDOPSIS RESPONSE REGULATORs Specify the Shoot Stem Cell Niche by Dual Regulation of WUSCHEL

Cytokinin signalling regulates organ identity via AHK4 receptor in Arabidopsis

Mutual interactions of the phytohormones cytokinins and auxin determine root or shoot identity during postembryonic de novo organogenesis in plants. However, our understanding to the role of hormonal metabolism and perception during early stages of cell fate reprograming is still elusive.

In the hypocotyl explant assay, auxin activated root formation while cytokinins mediated early loss of the root identity, primordia disorganization and initiation of shoot development. Exogenous but also endogenous cytokinins influenced the initiation of newly formed organs as well as the pace of organ developmental sequence. The process of de novo shoot apical meristem establishment was accompanied by accumulation of endogenous cytokinins, differential regulation of genes for individual cytokinin receptors, strong activation of AHK4-mediated signalling and induction of shoot-specific homeodomain regulator WUSCHEL. The latter associated with upregulation of isopentenyladenine-type cytokinins, revealing higher shoot-forming potential when compared with trans-zeatin. Moreover, AHK4-controlled cytokinin signalling negatively regulated root stem cell organizer WUSCHEL RELATED HOMEOBOX 5 in the root quiescent centre. We propose an important role of endogenous cytokinin biosynthesis and AHK4-mediated cytokinin signalling in the control of de novo induced organ identity.

DNA METHYLTRANSFERASE1-mediated shoot regeneration is regulated by cytokinin-induced cell cycle in Arabidopsis

DNA methylation plays a critical role in diverse biological processes of plants. Arabidopsis DNA METHYLTRANSFERASE1 (MET1) represses shoot regeneration by inhibiting WUSCHEL (WUS) expression, which is essential for shoot initiation. However, the upstream signals regulating MET1 expression during this process are unclear. We analyzed the signals regulating MET1 expression using a number of established strategies, such as genetic analysis, confocal microscopy, quantitative real-time PCR and chromatin immunoprecipitation. MET1 expression patterns underwent dynamic changes with the initiation of WUS during shoot regeneration. The cell cycle regulator E2FA was characterized as an upstream factor directly promoting MET1 expression. Moreover, cytokinin promoted MET1 expression partially by enhancing CYCD3 expression. Our findings reveal that MET1-mediated shoot regeneration is regulated by the cytokinin-induced cell cycle, and provide new insights into the regulation of DNA methylation in shoot regeneration.

Type B Response Regulators Act As Central Integrators in Transcriptional Control of the Auxin Biosynthesis Enzyme TAA1

During embryogenesis and organ formation, establishing proper gradient is critical for auxin function, which is achieved through coordinated regulation of both auxin metabolism and transport. Expression of auxin biosynthetic genes is often tissue specific and is regulated by environmental signals. However, the underlying regulatory mechanisms remain elusive. Here, we investigated the transcriptional regulation of a key auxin biosynthetic gene, l-Tryptophan aminotransferase of Arabidopsis1 (TAA1). A canonical and a novel Arabidopsis (Arabidopsis thaliana) response regulator (ARR) binding site were identified in the promoter and the second intron of TAA1, which were required for its tissue-specific expression. C-termini of a subset of the type B ARRs selectively bind to one or both cis elements and activate the expression of TAA1 We further demonstrated that the ARRs not only mediate the transcriptional regulation of TAA1 by cytokinins, but also mediate its regulation by ethylene, light, and developmental signals. Through direct protein-protein interactions, the transcriptional activity of ARR1 is enhanced by ARR12, DELLAs, and ethylene-insenstive3 (EIN3). Our study thus revealed the ARR proteins act as key node that mediate the regulation of auxin biosynthesis by various hormonal, environmental, and developmental signals through transcriptional regulation of the key auxin biosynthesis gene TAA1.

Type-B ARABIDOPSIS RESPONSE REGULATORs Directly Activate WUSCHEL

The WUSCHEL (WUS) gene is necessary for the maintenance of stem cells in the shoot apical meristem. Four recent reports show that cytokinin responsive type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) directly activate WUS expression, providing a long-awaited explanation for how cytokinin influences the maintenance of the stem cell niche.