Structural basis for the complex DNA binding behavior of the plant stem cell regulator WUSCHEL.

Stem cells are one of the foundational evolutionary novelties that allowed the independent emergence of multicellularity in the plant and animal lineages. In plants, the homeodomain (HD) transcription factor WUSCHEL (WUS) is essential for the maintenance of stem cells in the shoot apical meristem. WUS has been reported to bind to diverse DNA motifs and to act as transcriptional activator and repressor. However, the mechanisms underlying this remarkable behavior have remained unclear. Here, we quantitatively delineate WUS binding to three divergent DNA motifs and resolve the relevant structural underpinnings. We show that WUS exhibits a strong binding preference for TGAA repeat sequences, while retaining the ability to weakly bind to TAAT elements. This behavior is attributable to the formation of dimers through interactions of specific residues in the HD that stabilize WUS DNA interaction. Our results provide a mechanistic basis for dissecting WUS dependent regulatory networks in plant stem cell control.

Control of plant cell fate transitions by transcriptional and hormonal signals.

Plant meristems carry pools of continuously active stem cells, whose activity is controlled by developmental and environmental signals. After stem cell division, daughter cells that exit the stem cell domain acquire transit amplifying cell identity before they are incorporated into organs and differentiate. In this study, we used an integrated approach to elucidate the role of HECATE (HEC) genes in regulating developmental trajectories of shoot stem cells in Arabidopsis thaliana. Our work reveals that HEC function stabilizes cell fate in distinct zones of the shoot meristem thereby controlling the spatio-temporal dynamics of stem cell differentiation. Importantly, this activity is concomitant with the local modulation of cellular responses to cytokinin and auxin, two key phytohormones regulating cell behaviour. Mechanistically, we show that HEC factors transcriptionally control and physically interact with MONOPTEROS (MP), a key regulator of auxin signalling, and modulate the autocatalytic stabilization of auxin signalling output.

ML3 is a NEDD8- and ubiquitin-modified protein.

NEDD8 (NEURAL PRECURSOR CELL-EXPRESSED, DEVELOPMENTALLY DOWN-REGULATED PROTEIN8) is an evolutionarily conserved 8-kD protein that is closely related to ubiquitin and that can be conjugated like ubiquitin to specific lysine residues of target proteins in eukaryotes. In contrast to ubiquitin, for which a broad range of substrate proteins are known, only a very limited number of NEDD8 target proteins have been identified to date. Best understood, and also evolutionarily conserved, is the NEDD8 modification (neddylation) of cullins, core subunits of the cullin-RING-type E3 ubiquitin ligases that promote the polyubiquitylation of degradation targets in eukaryotes. Here, we show that Myeloid differentiation factor-2-related lipid-recognition domain protein ML3 is an NEDD8- as well as ubiquitin-modified protein in Arabidopsis (Arabidopsis thaliana) and examine the functional role of ML3 in the plant cell. Our analysis indicates that ML3 resides in the vacuole as well as in endoplasmic reticulum (ER) bodies. ER bodies are Brassicales-specific ER-derived organelles and, similar to other ER body proteins, ML3 orthologs can only be identified in this order of flowering plants. ML3 gene expression is promoted by wounding as well as by the phytohormone jasmonic acid and repressed by ethylene, signals that are known to induce and repress ER body formation, respectively. Furthermore, ML3 protein abundance is dependent on NAI1, a master regulator of ER body formation in Arabidopsis. The regulation of ML3 expression and the localization of ML3 in ER bodies and the vacuole is in agreement with a demonstrated importance of ML3 in the defense to herbivore attack. Here, we extend the spectrum of ML3 biological functions by demonstrating a role in the response to microbial pathogens.