The cold-induced factor CBF3 mediates root stem cell activity, regeneration, and developmental responses to cold

Plant growth and development involve the specification and regeneration of stem cell niches (SCNs). Although plants are exposed to disparate environmental conditions, how environmental cues affect developmental programs and stem cells is not well understood. Root stem cells are accommodated in meristems in SCNs around the quiescent center (QC), which maintains their activity. Using a combination of genetics and confocal microscopy to trace morphological defects and correlate them with changes in gene expression and protein levels, we show that the cold-induced transcription factor (TF) C-REPEAT BINDING FACTOR 3 (CBF3), which has previously been associated with cold acclimation, regulates root development, stem cell activity, and regeneration. CBF3 is integrated into the SHORT-ROOT (SHR) regulatory network, forming a feedback loop that maintains SHR expression. CBF3 is primarily expressed in the root endodermis, whereas the CBF3 protein is localized to other meristematic tissues, including root SCNs. Complementation of cbf3-1 using a wild-type CBF3 gene and a CBF3 fusion with reduced mobility show that CBF3 movement capacity is required for SCN patterning and regulates root growth. Notably, cold induces CBF3, affecting QC activity. Furthermore, exposure to moderate cold around 10°C-12°C promotes root regeneration and QC respecification in a CBF3-dependent manner during the recuperation period. By contrast, CBF3 does not appear to regulate stem cell survival, which has been associated with recuperation from more acute cold (∼4°C). We propose a role for CBF3 in mediating the molecular interrelationships among the cold response, stem cell activity, and development.

Quantitative regeneration: Skoog and Miller revisited

In 1957, Skoog and Miller published their seminal work on the effects of hormones upon plant growth. By varying the concentrations of auxin and cytokinin, they observed dramatic differences in shoot and root growth from tobacco stem cultures. Their finding that quantitative differences in hormone concentrations could dramatically alter the fate of developing organs provided a foundation for understanding organ formation and tissue regeneration. Their in vitro assays established plant propagation techniques that were critical for regenerating transgenic plants. Here, I discuss their original paper, what led to their findings and its impact on our understanding of hormone interactions, how plants regenerate and in vitro tissue culture techniques.

Theoretical approaches to understanding root vascular patterning: a consensus between recent models

The root vascular tissues provide an excellent system for studying organ patterning, as the specification of these tissues signals a transition from radial symmetry to bisymmetric patterns. The patterning process is controlled by the combined action of hormonal signaling/transport pathways, transcription factors, and miRNA that operate through a series of non-linear pathways to drive pattern formation collectively. With the discovery of multiple components and feedback loops controlling patterning, it has become increasingly difficult to understand how these interactions act in unison to determine pattern formation in multicellular tissues. Three independent mathematical models of root vascular patterning have been formulated in the last few years, providing an excellent example of how theoretical approaches can complement experimental studies to provide new insights into complex systems. In many aspects these models support each other; however, each study also provides its own novel findings and unique viewpoints. Here we reconcile these models by identifying the commonalities and exploring the differences between them by testing how transferable findings are between models. New simulations herein support the hypothesis that an asymmetry in auxin input can direct the formation of vascular pattern. We show that the xylem axis can act as a sole source of cytokinin and specify the correct pattern, but also that broader patterns of cytokinin production are also able to pattern the root. By comparing the three modeling approaches, we gain further insight into vascular patterning and identify several key areas for experimental investigation.

Regulation of MIR165/166 by class II and class III homeodomain leucine zipper proteins establishes leaf polarity

A defining feature of plant leaves is their flattened shape. This shape depends on an antagonism between the genes that specify adaxial (top) and abaxial (bottom) tissue identity; however, the molecular nature of this antagonism remains poorly understood. Class III homeodomain leucine zipper (HD-ZIP) transcription factors are key mediators in the regulation of adaxial-abaxial patterning. Their expression is restricted adaxially during early development by the abaxially expressed microRNA (MIR)165/166, yet the mechanism that restricts MIR165/166 expression to abaxial leaf tissues remains unknown. Here, we show that class III and class II HD-ZIP proteins act together to repress MIR165/166 via a conserved cis-element in their promoters. Organ morphology and tissue patterning in plants, therefore, depend on a bidirectional repressive circuit involving a set of miRNAs and its targets.

Juicy stories on female reproductive tissue development: coordinating the hormone flows

In the past 20-30 years, developmental biologists have made tremendous progress in identifying genes required for the specification of individual cell types of an organ and in describing how they interact in genetic networks. In comparison, very little is known about the mechanisms that regulate tissue polarity and overall organ patterning. Gynoecia and fruits from members of the Brassicaceae family of flowering plants provide excellent model systems to study organ patterning and tissue specification because they become partitioned into distinct domains whose formation is determined by polarity establishment both at a cellular and whole tissue level. Interactions among key regulators of Arabidopsis gynoecium and fruit development have revealed a network of upstream transcription factor activities required for such tissue differentiation. Regulation of the plant hormone auxin is emerging as both an immediate downstream output and input of these activities, and here we aim to provide an overview of the current knowledge regarding the link between auxin and female reproductive development in plants. In this review, we will also demonstrate how available data can be exploited in a mathematical modeling approach to reveal and understand the feedback regulatory circuits that underpin the polarity establishment, necessary to guide auxin flows.