Vascular tissues in plants are crucial to provide physical support and to transport water, sugars and hormones and other small signalling molecules throughout the plant. Recent genetic and molecular ...studies have identified interconnections among some of the major signalling networks that regulate plant vascular development. Using Arabidopsis thaliana as a model system, these studies enable the description of vascular development from the earliest tissue specification events during embryogenesis to the differentiation of phloem and xylem tissues. Moreover, we propose a model for how oriented cell divisions give rise to a three-dimensional vascular bundle within the root meristem.
Optimal plant growth is hampered by deficiency of the essential macronutrient phosphate in most soils. Plant roots can, however, increase their root hair density to efficiently forage the soil for ...this immobile nutrient. By generating and exploiting a high-resolution single-cell gene expression atlas of
roots, we show an enrichment of TARGET OF MONOPTEROS 5/LONESOME HIGHWAY (TMO5/LHW) target gene responses in root hair cells. The TMO5/LHW heterodimer triggers biosynthesis of mobile cytokinin in vascular cells and increases root hair density during low-phosphate conditions by modifying both the length and cell fate of epidermal cells. Moreover, root hair responses in phosphate-deprived conditions are TMO5- and cytokinin-dependent. Cytokinin signaling links root hair responses in the epidermis to perception of phosphate depletion in vascular cells.
Apical growth in plants initiates upon seed germination, whereas radial growth is primed only during early ontogenesis in procambium cells and activated later by the vascular cambium
. Although it is ...not known how radial growth is organized and regulated in plants, this system resembles the developmental competence observed in some animal systems, in which pre-existing patterns of developmental potential are established early on
. Here we show that in Arabidopsis the initiation of radial growth occurs around early protophloem-sieve-element cell files of the root procambial tissue. In this domain, cytokinin signalling promotes the expression of a pair of mobile transcription factors-PHLOEM EARLY DOF 1 (PEAR1) and PHLOEM EARLY DOF 2 (PEAR2)-and their four homologues (DOF6, TMO6, OBP2 and HCA2), which we collectively name PEAR proteins. The PEAR proteins form a short-range concentration gradient that peaks at protophloem sieve elements, and activates gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by the HD-ZIP III proteins, well-known polarity transcription factors
-the expression of which is concentrated in the more-internal domain of radially non-dividing procambial cells by the function of auxin, and mobile miR165 and miR166 microRNAs. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, and thereby establish a negative-feedback loop that forms a robust boundary that demarks the zone of cell division. Taken together, our data establish that during root procambial development there exists a network in which a module that links PEAR and HD-ZIP III transcription factors integrates spatial information of the hormonal domains and miRNA gradients to provide adjacent zones of dividing and more-quiescent cells, which forms a foundation for further radial growth.
Individual proteins often function as part of a protein complex. The identification of interacting proteins is therefore vital to understand the biological role and function of the studied protein. ...Here we describe a method for the in vivo identification of nuclear, cytoplasmic, and membrane-associated protein complexes from plant tissues using a strategy of immunoprecipitation followed by tandem mass spectrometry. By performing quantitative mass spectrometry measurements on biological triplicates, relative abundance of proteins in GFP-tagged complexes compared to background controls can be statistically evaluated to identify high-confidence interactors. We detail the entire workflow of this approach.
During the exploration of the soil by plant roots, uptake of water and nutrients can be greatly fostered by a regular spacing of lateral roots (LRs). In the Arabidopsis root, a regular branching ...pattern depends on oscillatory gene activity to create prebranch sites, patches of cells competent to form LRs. Thus far, the molecular components regulating the oscillations still remain unclear. Here, we show that a local auxin source in the root cap, derived from the auxin precursor indole-3-butyric acid (IBA), modulates the oscillation amplitude, which in turn determines whether a prebranch site is created or not. Moreover, transcriptome profiling identified novel and IBA-regulated components of root patterning, such as the MEMBRANE-ASSOCIATED KINASE REGULATOR4 (MAKR4) that converts the prebranch sites into a regular spacing of lateral organs. Thus, the spatiotemporal patterning of roots is fine-tuned by the root cap-specific conversion pathway of IBA to auxin and the subsequent induction of MAKR4.
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•The amplitude of oscillatory gene expression is essential for LR formation•A root cap-specific IBA-to-IAA conversion contributes to the oscillation amplitude•MAKR4 acts downstream of gene oscillations to transform prebranch sites into LRs
The regular spacing of lateral organs along the longitudinal axis of the root is guaranteed by a pre-patterning mechanism involving oscillatory gene expression in a sub-apical part of the root. Xuan et al. reveal that a root cap-derived auxin source regulates this process by influencing the amplitude of oscillatory gene expression.
Recent identification of several different types of RNA editing factors in plant organelles suggests complex RNA editosomes within which each factor has a different task. However, the precise protein ...interactions between the different editing factors are still poorly understood. In this paper, we show that the E⁺-type pentatricopeptide repeat (PPR) protein SLO2, which lacks a C-terminal cytidine deaminase-like DYW domain, interacts in vivo with the DYW-type PPR protein DYW2 and the P-type PPR protein NUWA in mitochondria, and that the latter enhances the interaction of the former ones. These results may reflect a protein scaffold or complex stabilization role of NUWA between E⁺-type PPR and DYW2 proteins. Interestingly, DYW2 and NUWA also interact in chloroplasts, and DYW2-GFP overexpressing lines show broad editing defects in both organelles, with predominant specificity for sites edited by E⁺-type PPR proteins. The latter suggests a coordinated regulation of organellar multiple site editing through DYW2, which probably provides the deaminase activity to E⁺ editosomes.
•Oriented cell divisions shape the vascular cell lineage throughout development.•The genetic and hormonal pathways controlling vascular proliferation are slowly being interconnected, resulting in a ...detailed framework for vascular proliferation.•A major challenge is to understand how these molecular factors control the actual orientation of cell divisions.
The plant vascular system develops from a handful of provascular initial cells in the early embryo into a whole range of different cell types in the mature plant. In order to account for such proliferation and to generate this kind of diversity, vascular tissue development relies on a large number of highly oriented cell divisions. Different hormonal and genetic pathways have been implicated in this process and several of these have been recently interconnected. Nevertheless, how such networks control the actual division plane orientation and how they interact with the generic cell cycle machinery to coordinate these divisions remains a major unanswered question.
Plants have a remarkable potential for sustained (indeterminate) postembryonic growth. Following their specification in the early embryo, tissue-specific precursor cells first establish tissues and ...later maintain them postembryonically. The mechanisms underlying these processes are largely unknown. Here we define local control of oriented, periclinal cell division as the mechanism underlying both the establishment and maintenance of vascular tissue. We identify an auxin-regulated basic helix-loop-helix (bHLH) transcription factor dimer as a critical regulator of vascular development. Due to a loss of periclinal divisions, vascular tissue gradually disappears in bHLH-deficient mutants; conversely, ectopic expression is sufficient for triggering periclinal divisions. We show that this dimer operates independently of tissue identity but is restricted to a small vascular domain by integrating overlapping transcription patterns of the interacting bHLH proteins. Our work reveals a common mechanism for tissue establishment and indeterminate vascular development and provides a conceptual framework for developmental control of local cell divisions.
► A TMO5/LHW bHLH heterodimer controls plant vascular development ► The dimer is necessary and sufficient for triggering periclinal cell division ► Overlapping transcription patterns localize TMO5/LHW activity ► Vascular tissue formation and indeterminate growth share a common genetic basis
De Rybel et al. show that gene regulation of two interacting bHLH transcription factors defines a small population of cells and triggers the oriented cell divisions that underlie plant vascular tissue development. The complex is required for both early specification and sustained growth in the mature plant.
The plant root cap, surrounding the very tip of the growing root, perceives and transmits environmental signals to the inner root tissues. In Arabidopsis thaliana, auxin released by the root cap ...contributes to the regular spacing of lateral organs along the primary root axis. Here, we show that the periodicity of lateral organ induction is driven by recurrent programmed cell death at the most distal edge of the root cap. We suggest that synchronous bursts of cell death in lateral root cap cells release pulses of auxin to surrounding root tissues, establishing the pattern for lateral root formation. The dynamics of root cap turnover may therefore coordinate primary root growth with root branching in order to optimize the uptake of water and nutrients from the soil.
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