An auxin maximum is positioned along the xylem axis of the Arabidopsis root tip. The pattern depends on mutual feedback between auxin and cytokinins mediated by the PIN class of auxin efflux ...transporters and AHP6, an inhibitor of cytokinin signalling. This interaction has been proposed to regulate the size and the position of the hormones' respective signalling domains and specify distinct boundaries between them. To understand the dynamics of this regulatory network, we implemented a parsimonious computational model of auxin transport that considers hormonal regulation of the auxin transporters within a spatial context, explicitly taking into account cell shape and polarity and the presence of cell walls. Our analysis reveals that an informative spatial pattern in cytokinin levels generated by diffusion is a theoretically unlikely scenario. Furthermore, our model shows that such a pattern is not required for correct and robust auxin patterning. Instead, auxin-dependent modifications of cytokinin response, rather than variations in cytokinin levels, allow for the necessary feedbacks, which can amplify and stabilise the auxin maximum. Our simulations demonstrate the importance of hormonal regulation of auxin efflux for pattern robustness. While involvement of the PIN proteins in vascular patterning is well established, we predict and experimentally verify a role of AUX1 and LAX1/2 auxin influx transporters in this process. Furthermore, we show that polar localisation of PIN1 generates an auxin flux circuit that not only stabilises the accumulation of auxin within the xylem axis, but also provides a mechanism for auxin to accumulate specifically in the xylem-pole pericycle cells, an important early step in lateral root initiation. The model also revealed that pericycle cells on opposite xylem poles compete for auxin accumulation, consistent with the observation that lateral roots are not initiated opposite to each other.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The regulation of signalling capacity, combined with the spatiotemporal distribution of developmental signals themselves, is pivotal in setting developmental responses in both plants and animals
. ...The hormone auxin is a key signal for plant growth and development that acts through the AUXIN RESPONSE FACTOR (ARF) transcription factors
. A subset of these, the conserved class A ARFs
, are transcriptional activators of auxin-responsive target genes that are essential for regulating auxin signalling throughout the plant lifecycle
. Although class A ARFs have tissue-specific expression patterns, how their expression is regulated is unknown. Here we show, by investigating chromatin modifications and accessibility, that loci encoding these proteins are constitutively open for transcription. Through yeast one-hybrid screening, we identify the transcriptional regulators of the genes encoding class A ARFs from Arabidopsis thaliana and demonstrate that each gene is controlled by specific sets of transcriptional regulators. Transient transformation assays and expression analyses in mutants reveal that, in planta, the majority of these regulators repress the transcription of genes encoding class A ARFs. These observations support a scenario in which the default configuration of open chromatin enables a network of transcriptional repressors to regulate expression levels of class A ARF proteins and modulate auxin signalling output throughout development.
Tree architecture has evolved to support a top-heavy above-ground biomass, but this integral feature poses a weight-induced challenge to trunk stability. Maintaining an upright stem is expected to ...require vertical proprioception through feedback between sensing stem weight and responding with radial growth. Despite its apparent importance, the principle by which plant stems respond to vertical loading forces remains largely unknown. Here, by manipulating the stem weight of downy birch (Betula pubescens) trees, we show that cambial development is modulated systemically along the stem. We carried out a genetic study on the underlying regulation by combining an accelerated birch flowering program with a recessive mutation at the ELIMÄKI locus (EKI), which causes a mechanically defective response to weight stimulus resulting in stem collapse after just 3 months. We observed delayed wood morphogenesis in eki compared with WT, along with a more mechanically elastic cambial zone and radial compression of xylem cell size, indicating that rapid tissue differentiation is critical for cambial growth under mechanical stress. Furthermore, the touch-induced mechanosensory pathway was transcriptionally misregulated in eki, indicating that the ELIMÄKI locus is required to integrate the weight-growth feedback regulation. By studying this birch mutant, we were able to dissect vertical proprioception from the gravitropic response associated with reaction wood formation. Our study provides evidence for both local and systemic responses to mechanical stimuli during secondary plant development.
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•Trees stems adjust their radial growth allometry in response to a weight increase•Fast-forward genetics by accelerated flowering reveals a Mendelian trait in birch•Vertical proprioception response in radial growth is linked to the ELIMÄKI locus•Timely tissue differentiation is critical for cambial growth under mechanical stress
Alonso-Serra et al. use birch trees to explore vertical proprioception, the growth response of stems to changes in aerial body weight. They identify a genetic locus, which is required for this developmental response, and study the consequences of cambial growth under mechanical stress.
Higher plant vasculature is characterized by two distinct developmental phases. Initially, a well-defined radial primary pattern is established. In eudicots, this is followed by secondary growth, ...which involves development of the cambium and is required for efficient water and nutrient transport and wood formation. Regulation of secondary growth involves several phytohormones, and cytokinins have been implicated as key players, particularly in the activation of cell proliferation, but the molecular mechanisms mediating this hormonal control remain unknown. Here we show that the genes encoding the transcription factor AINTEGUMENTA (ANT) and the D-type cyclin CYCD3;1 are expressed in the vascular cambium of Arabidopsis roots, respond to cytokinins and are both required for proper root secondary thickening. Cytokinin regulation of ANT and CYCD3 also occurs during secondary thickening of poplar stems, suggesting this represents a conserved regulatory mechanism.
The cytokinin class of plant hormones plays key roles in regulating diverse developmental and physiological processes 1. Arabidopsis perceives cytokinins with three related and partially redundant ...receptor histidine kinases (HKs): CRE1 (the same protein as WOL and AHK4), AHK2, and AHK3 (CRE-family receptors) 2–5. It is suggested that binding of cytokinins induces autophosphorylation of these HKs and subsequent transfer of the phosphoryl group to a histidine phosphotransfer protein (HPt) and then to a response regulator (RR), ultimately regulating downstream signaling events 6, 7. Here we demonstrate that, in vitro and in a yeast system, CRE1 is not only a kinase that phosphorylates HPts in the presence of cytokinin but is also a phosphatase that dephosphorylates HPts in the absence of cytokinin. To explore the roles of these activities in planta, we replaced CRE1 with mutant versions of the gene or with AHK2. Replacing CRE1 with CRE1(T278I), which lacks cytokinin binding activity 8 and is locked in the phosphatase form, decreased cytokinin sensitivity. Conversely, replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity. These results indicate that in the presence of cytokinins, cytokinin receptors feed phosphate to phosphorelay-integrating HPt proteins. In the absence of cytokinins, CRE1 removes phosphate from HPt proteins, decreasing the system phosphoload.
Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is ...shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch (
Betula pendula
). Using a candidate gene approach, we identified a bushy
kanttarelli
variant with a loss-of-function mutation in the
BpMAX1
gene required for strigolactone (SL) biosynthesis. While
kanttarelli
is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied
kanttarelli
in transgenic
BpMAX1::RNAi
birch lines. SL profiling confirmed that both
kanttarelli
and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and
BpMAX1::RNAi
. In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture.
With the tremendous progress of the past decades, molecular plant science is becoming more unified than ever. We now have the exciting opportunity to further connect subdisciplines and understand ...plants as whole organisms, as will be required to efficiently utilize them in natural and agricultural systems to meet human needs. The subfields of photosynthesis, plant developmental biology and plant stress are used as examples to discuss how plant science can become better integrated. The challenges, strategies and rich opportunities for the integration of the plant sciences are discussed. In recent years, more and more overlap between various subdisciplines has been inadvertently discovered including tradeoffs that may occur in plants engineered for biotechnological applications. Already important, bioinformatics and computational modelling will become even more central to structuring and understanding the ever growing amounts of data. The process of integrating and overlapping fields in plant biology research is advancing, but plant science will benefit from dedicating more effort and urgency to reach across its boundaries.
Hormonal signalling plays a pivotal role in almost every aspect of plant development, and of high priority has been to identify the receptors that perceive these hormones. In the past seven months, ...the receptors for the plant hormones auxin, gibberellins and abscisic acid have been identified. These join the receptors that have previously been identified for ethylene, brassinosteroids and cytokinins. This review therefore comes at an exciting time for plant developmental biology, as the new findings shed light on our current understanding of the structure and function of the various hormone receptors, their related signalling pathways and their role in regulating plant development.
The response of plants to cold exposure provides them with a way to align their flowering in spring. Because daily temperatures fluctuate, long-term rather than short-term cold is the key signal ...required for a flowering cue; prolonged cold followed by warmer weather provides an indication that winter has ended, bringing the advent of spring and flowering time. Zhao etal. conducted experiments to examine the degradation rate of NTL8, and found that it was stable in warm and cold conditions. ...a temperature-dependent change in the degradation rate could not be the reason NTL8 accumulates slowly. ...during cold weather, NTL8 levels in cells would be expected to increase gradually (Fig. 1).
Pattern formation is typically controlled through the interaction between molecular signals within a given tissue. During early embryonic development, roots of the model plant
have a radially ...symmetric pattern, but a heterogeneous input of the hormone auxin from the two cotyledons forces the vascular cylinder to develop a diarch pattern with two xylem poles. Molecular analyses and mathematical approaches have uncovered the regulatory circuit that propagates this initial auxin signal into a stable cellular pattern. The diarch pattern seen in
is relatively uncommon among flowering plants, with most species having between three and eight xylem poles. Here, we have used multiscale mathematical modelling to demonstrate that this regulatory module does not require a heterogeneous auxin input to specify the vascular pattern. Instead, the pattern can emerge dynamically, with its final form dependent upon spatial constraints and growth. The predictions of our simulations compare to experimental observations of xylem pole number across a range of species, as well as in transgenic systems in
in which we manipulate the size of the vascular cylinder. By considering the spatial constraints, our model is able to explain much of the diversity seen in different flowering plant species.
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