Diffusible repression of cytokinin signalling produces endodermal symmetry and passage cells

Nitrate transporter protein NPF5.12 and major latex-like protein MLP6 are important defense factors against Verticillium longisporum

Plant defense responses to the soil-borne fungus Verticillium longisporum causing stem stripe disease on oilseed rape (Brassica napus) are poorly understood. In this study, a population of recombinant inbred lines (RILs) using the Arabidopsis accessions Sei-0 and Can-0 was established. Composite interval mapping, transcriptome data, and T-DNA mutant screening identified the NITRATE/PEPTIDE TRANSPORTER FAMILY 5.12 (AtNPF5.12) gene as being associated with disease susceptibility in Can-0. Co-immunoprecipitation revealed interaction between AtNPF5.12 and the MAJOR LATEX PROTEIN family member AtMLP6, and fluorescence microscopy confirmed this interaction in the plasma membrane and endoplasmic reticulum. CRISPR/Cas9 technology was applied to mutate the NPF5.12 and MLP6 genes in B. napus. Elevated fungal growth in the npf5.12 mlp6 double mutant of both oilseed rape and Arabidopsis demonstrated the importance of these genes in defense against V. longisporum. Colonization of this fungus depends also on available nitrates in the host root. Accordingly, the negative effect of nitrate depletion on fungal growth was less pronounced in Atnpf5.12 plants with impaired nitrate transport. In addition, suberin staining revealed involvement of the NPF5.12 and MLP6 genes in suberin barrier formation. Together, these results demonstrate a dependency on multiple plant factors that leads to successful V. longisporum root infection.

The Arabidopsis SGN3/GSO1 receptor kinase integrates soil nitrogen status into shoot development

The Casparian strip is a barrier in the endodermal cell walls of plants that allows the selective uptake of nutrients and water. In the model plant Arabidopsis thaliana, its development and establishment are under the control of a receptor-ligand mechanism termed the Schengen pathway. This pathway facilitates barrier formation and activates downstream compensatory responses in case of dysfunction. However, due to a very tight functional association with the Casparian strip, other potential signaling functions of the Schengen pathway remain obscure. In this work, we created a MYB36-dependent synthetic positive feedback loop that drives Casparian strip formation independently of Schengen-induced signaling. We evaluated this by subjecting plants in which the Schengen pathway has been uncoupled from barrier formation, as well as a number of established barrier-mutant plants, to agar-based and soil conditions that mimic agricultural settings. Under the latter conditions, the Schengen pathway is necessary for the establishment of nitrogen-deficiency responses in shoots. These data highlight Schengen signaling as an essential hub for the adaptive integration of signaling from the rhizosphere to aboveground tissues.

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

Suberin deficiency and its effect on the transport physiology of young poplar roots

The precise functions of suberized apoplastic barriers in root water and nutrient transport physiology have not fully been elucidated. While lots of research has been performed with mutants of Arabidopsis, little to no data are available for mutants of agricultural crop or tree species. By employing a combined set of physiological, histochemical, analytical, and transport physiological methods as well as RNA-sequencing, this study investigated the implications of remarkable CRISPR/Cas9-induced suberization defects in young roots of the economically important gray poplar. While barely affecting overall plant development, contrary to literature-based expectations significant root suberin reductions of up to 80-95% in four independent mutants were shown to not evidently affect the root hydraulic conductivity during non-stress conditions. In addition, subliminal iron deficiency symptoms and increased translocation of a photosynthesis inhibitor as well as NaCl highlight the involvement of suberin in nutrient transport physiology. The multifaceted nature of the root hydraulic conductivity does not allow drawing simplified conclusions such as that the suberin amount must always be correlated with the water transport properties of roots. However, the decreased masking of plasma membrane surface area could facilitate the uptake but also leakage of beneficial and harmful solutes.

A developmentally controlled cellular decompartmentalization process executes programmed cell death in the Arabidopsis root cap

Programmed cell death (PCD) is a fundamental cellular process crucial to development, homeostasis, and immunity in multicellular eukaryotes. In contrast to our knowledge on the regulation of diverse animal cell death subroutines, information on execution of PCD in plants remains fragmentary. Here, we make use of the accessibility of the Arabidopsis (Arabidopsis thaliana) root cap to visualize the execution process of developmentally controlled PCD. We identify a succession of selective decompartmentalization events and ion fluxes as part of the terminal differentiation program that is orchestrated by the NO APICAL MERISTEM, ARABIDOPSIS THALIANA ACTIVATING FACTOR, CUP-SHAPED COTYLEDON (NAC) transcription factor SOMBRERO. Surprisingly, the breakdown of the large central vacuole is a relatively late and variable event, preceded by an increase of intracellular calcium levels and acidification, release of mitochondrial matrix proteins, leakage of nuclear and endoplasmic reticulum lumina, and release of fluorescent membrane reporters into the cytosol. In analogy to animal apoptosis, the plasma membrane remains impermeable for proteins during and after PCD execution. Elevated intracellular calcium levels and acidification are sufficient to trigger cell death execution specifically in terminally differentiated root cap cells, suggesting that these ion fluxes act as PCD-triggering signals. This detailed information on the cellular processes occurring during developmental PCD in plants is a pivotal prerequisite for future research into the molecular mechanisms of cell death execution.

Making watercress (Nasturtium officinale) cropping sustainable: genomic insights into enhanced phosphorus use efficiency in an aquatic crop

Watercress (Nasturtium officinale) is a nutrient-dense salad crop with high antioxidant capacity and glucosinolate concentration and with the potential to contribute to nutrient security as a locally grown outdoor aquatic crop in northern temperate climates. However, phosphate-based fertilizers used to support plant growth contribute to the eutrophication of aquatic habitats, often pristine chalk streams, downstream of farms, increasing pressure to minimize fertilizer use and develop a more phosphorus-use efficient (PUE) crop. Here, we grew genetically distinct watercress lines selected from a bi-parental mapping population on a commercial watercress farm either without additional phosphorus (P-) or under a commercial phosphate-based fertilizer regime (P+), to decipher effects on morphology, nutritional profile, and the transcriptome. Watercress plants sustained shoot yield in P- conditions, through enhanced root biomass, but with shorter stems and smaller leaves. Glucosinolate concentration was not affected by P- conditions, but both antioxidant capacity and the concentration of sugars and starch in shoot tissue were enhanced. We identified two watercress breeding lines, with contrasting strategies for enhanced PUE: line 60, with highly plastic root systems and increased root growth in P-, and line 102, maintaining high yield irrespective of P supply, but less plastic. RNA-seq analysis revealed a suite of genes involved in cell membrane remodeling, root development, suberization, and phosphate transport as potential future breeding targets for enhanced PUE. We identified watercress gene targets for enhanced PUE for future biotechnological and breeding approaches enabling less fertilizer inputs and reduced environmental damage from watercress cultivation.

A dirigent protein complex directs lignin polymerization and assembly of the root diffusion barrier

Functionally similar to the tight junctions present in animal guts, plant roots have evolved a lignified Casparian strip as an extracellular diffusion barrier in the endodermis to seal the root apoplast and maintain nutrient homeostasis. How this diffusion barrier is structured has been partially defined, but its lignin polymerization and assembly steps remain elusive. Here, we characterize a family of dirigent proteins (DPs) essential for both the localized polymerization of lignin required for Casparian strip biogenesis in the cell wall and for attachment of the strip to the plasma membrane to seal the apoplast. We reveal a Casparian strip lignification mechanism that requires cooperation between DPs and the Schengen pathway. Furthermore, we demonstrate that DPs directly mediate lignin polymerization as part of this mechanism.

GreenGate 2.0: Backwards compatible addons for assembly of complex transcriptional units and their stacking with GreenGate

Molecular cloning is a crucial technique in genetic engineering that enables the precise design of synthetic transcriptional units (TUs) and the manipulation of genomes. GreenGate and several other modular molecular cloning systems were developed about ten years ago and are widely used in plant research. All these systems define grammars for assembling transcriptional units from building blocks, cloned as Level 0 modules flanked by four-base pair overhangs and recognition sites for a particular Type IIs endonuclease. Modules are efficiently assembled into Level 1 TUs in a hierarchical assembly process, and Level 2 multigene constructs are assembled by stacking Level 1 TUs. GreenGate is highly popular but has three main limitations. First, using ad-hoc overhangs added by PCR and classical restriction/ligation prevents the efficient use of a one-pot, one-step reaction to generate entry clones and domesticate internal sites; second, a Level 1 TU is assembled from a maximum of six modules, which may be limiting for applications such as multiplex genome editing; third, the generation of Level 2 assemblies is sequential and inefficient. GreenGate 2.0 (GG2.0) expands GreenGate features. It introduces additional overhangs, allowing for the combination of up to 12 Level 0 modules in a Level 1 TU. It includes a Universal Entry Generator plasmid (pUEG) to streamline the generation of Level 0 modules. GG2.0 introduces GreenBraid, a convenient method for stacking transcriptional units iteratively for multigene assemblies. Importantly, GG2.0 is backwards compatible with most existing GreenGate modules. Additionally, GG2.0 includes Level 0 modules for multiplex expression of guide RNAs for CRISPR/Cas9 genome editing and pre-assembled Level 1 vectors for dexamethasone-inducible gene expression and ubiquitous expression of plasma membrane and nuclear fluorescent markers. GG2.0 streamlines and increases the versatility of assembling complex transcriptional units and their combination.

Spatiotemporal control of root immune responses during microbial colonization

The entire evolutionary trajectory of plants towards large and complex multi-cellular organisms has been accompanied by incessant interactions with omnipresent unicellular microbes. This led to the evolution of highly complex microbial communities, whose members display the entire spectrum of pathogenic to mutualistic behaviors. Plant roots are dynamic, fractally growing organs and even small Arabidopsis roots harbor millions of individual microbes of diverse taxa. It is evident that microbes at different positions on a root surface could experience fundamentally different environments, which, moreover, rapidly change over time. Differences in spatial scales between microbes and roots compares to humans and the cities they inhabit. Such considerations make it evident that mechanisms of root-microbe interactions can only be understood if analyzed at relevant spatial and temporal scales. This review attempts to provide an overview of the rapid recent progress that has been made in mapping and manipulating plant damage and immune responses at cellular resolution, as well as in visualizing bacterial communities and their transcriptional activities. We further discuss the impact that such approaches will have for a more predictive understanding of root-microbe interactions.

Selective autophagy regulates chloroplast protein import and promotes plant stress tolerance

Chloroplasts are plant organelles responsible for photosynthesis and environmental sensing. Most chloroplast proteins are imported from the cytosol through the translocon at the outer envelope membrane of chloroplasts (TOC). Previous work has shown that TOC components are regulated by the ubiquitin-proteasome system (UPS) to control the chloroplast proteome, which is crucial for the organelle's function and plant development. Here, we demonstrate that the TOC apparatus is also subject to K63-linked polyubiquitination and regulation by selective autophagy, potentially promoting plant stress tolerance. We identify NBR1 as a selective autophagy adaptor targeting TOC components, and mediating their relocation into vacuoles for autophagic degradation. Such selective autophagy is shown to control TOC protein levels and chloroplast protein import and to influence photosynthetic activity as well as tolerance to UV-B irradiation and heat stress in Arabidopsis plants. These findings uncover the vital role of selective autophagy in the proteolytic regulation of specific chloroplast proteins, and how dynamic control of chloroplast protein import is critically important for plants to cope with challenging environments.

Bacterial community and root endodermis: a complementary relationship

There are feedforward and feedback loops along the microbiota-root-shoot axis to maintain plant growth or defense under environmental stresses. Here, we highlight a reciprocal interaction between the endodermis and the plant-bacterial community, which stabilizes the diffusion barriers to maintain nutrient homeostasis under nutritional stress.

Cell type-specific mapping of ion distribution in Arabidopsis thaliana roots

Cell type-specific mapping of element distribution is critical to fully understand how roots partition nutrients and toxic elements with aboveground parts. In this study, we developed a method that combines fluorescence-activated cell sorting (FACS) with inductively coupled plasma mass spectrometry (ICP-MS) to assess the ionome of different cell populations within Arabidopsis thaliana roots. The method reveals that most elements exhibit a radial concentration gradient increasing from the rhizodermis to inner cell layers, and detected previously unknown ionomic changes resulting from perturbed xylem loading processes. With this approach, we also identify a strong accumulation of manganese in trichoblasts of iron-deficient roots. We demonstrate that confining manganese sequestration in trichoblasts but not in endodermal cells efficiently retains manganese in roots, therefore preventing toxicity in shoots. These results indicate the existence of cell type-specific constraints for efficient metal sequestration in roots. Thus, our approach opens an avenue to investigate element compartmentation and transport pathways in plants.

Effect of ipt Gene Induction in Transgenic Tobacco Plants on Hydraulic Conductance, Formation of Apoplastic Barriers and Aquaporin Activity under Heat Shock

Cytokinins are known to keep stomata open, which supports gas exchange and correlates with increased photosynthesis. However, keeping the stomata open can be detrimental if the increased transpiration is not compensated for by water supply to the shoots. In this study, we traced the effect of ipt (isopentenyl transferase) gene induction, which increases the concentration of cytokinins in transgenic tobacco plants, on transpiration and hydraulic conductivity. Since water flow depends on the conductivity of the apoplast, the deposition of lignin and suberin in the apoplast was studied by staining with berberine. The effect of an increased concentration of cytokinins on the flow of water through aquaporins (AQPs) was revealed by inhibition of AQPs with HgCl₂. It was shown that an elevated concentration of cytokinins in ipt-transgenic plants increases hydraulic conductivity by enhancing the activity of aquaporins and reducing the formation of apoplastic barriers. The simultaneous effect of cytokinins on both stomatal and hydraulic conductivity makes it possible to coordinate the evaporation of water from leaves and its flow from roots to leaves, thereby maintaining the water balance and leaf hydration.

GOLVEN peptides regulate lateral root spacing as part of a negative feedback loop on the establishment of auxin maxima

Lateral root initiation requires the accumulation of auxin in lateral root founder cells, yielding a local auxin maximum. The positioning of auxin maxima along the primary root determines the density and spacing of lateral roots. The GOLVEN6 (GLV6) and GLV10 signaling peptides and their receptors have been established as regulators of lateral root spacing via their inhibitory effect on lateral root initiation in Arabidopsis. However, it remained unclear how these GLV peptides interfere with auxin signaling or homeostasis. Here, we show that GLV6/10 signaling regulates the expression of a subset of auxin response genes, downstream of the canonical auxin signaling pathway, while simultaneously inhibiting the establishment of auxin maxima within xylem-pole pericycle cells that neighbor lateral root initiation sites. We present genetic evidence that this inhibitory effect relies on the activity of the PIN3 and PIN7 auxin export proteins. Furthermore, GLV6/10 peptide signaling was found to enhance PIN7 abundance in the plasma membranes of xylem-pole pericycle cells, which likely stimulates auxin efflux from these cells. Based on these findings, we propose a model in which the GLV6/10 signaling pathway serves as a negative feedback mechanism that contributes to the robust patterning of auxin maxima along the primary root.

An ABA-serotonin module regulates root suberization and salinity tolerance

Suberin in roots acts as a physical barrier preventing water/mineral losses. In Arabidopsis, root suberization is regulated by abscisic acid (ABA) and ethylene in response to nutrient stresses. ABA also mediates coordination between microbiota and root endodermis in mineral nutrient homeostasis. However, it is not known whether this regulatory system is common to plants in general, and whether there are other key molecule(s) involved. We show that serotonin acts downstream of ABA in regulating suberization in rice and Arabidopsis and negatively regulates suberization in rice roots in response to salinity. We show that ABA represses transcription of the key gene (OsT5H) in serotonin biosynthesis, thus promoting root suberization in rice. Conversely, overexpression of OsT5H or supplementation with exogenous serotonin represses suberization and reduces tolerance to salt stress. These results identify an ABA-serotonin regulatory module controlling root suberization in rice and Arabidopsis, which is likely to represent a general mechanism as ABA and serotonin are ubiquitous in plants. These findings are of significant importance to breeding novel crop varieties that are resilient to abiotic stresses and developing strategies for production of suberin-rich roots to sequestrate more CO₂ , helping to mitigate the effects of climate change.

Cytokinin and abiotic stress tolerance -What has been accomplished and the way forward?

More than a half-century has passed since it was discovered that phytohormone cytokinin (CK) is essential to drive cytokinesis and proliferation in plant tissue culture. Thereafter, cytokinin has emerged as the primary regulator of the plant cell cycle and numerous developmental processes. Lately, a growing body of evidence suggests that cytokinin has a role in mitigating both abiotic and biotic stress. Cytokinin is essential to defend plants against excessive light exposure and a unique kind of abiotic stress generated by an altered photoperiod. Secondly, cytokinin also exhibits multi-stress resilience under changing environments. Furthermore, cytokinin homeostasis is also affected by several forms of stress. Therefore, the diverse roles of cytokinin in reaction to stress, as well as its interactions with other hormones, are discussed in detail. When it comes to agriculture, understanding the functioning processes of cytokinins under changing environmental conditions can assist in utilizing the phytohormone, to increase productivity. Through this review, we briefly describe the biological role of cytokinin in enhancing the performance of plants growth under abiotic challenges as well as the probable mechanisms underpinning cytokinin-induced stress tolerance. In addition, the article lays forth a strategy for using biotechnological tools to modify genes in the cytokinin pathway to engineer abiotic stress tolerance in plants. The information presented here will assist in better understanding the function of cytokinin in plants and their effective investigation in the cropping system.

The making of suberin

Outer protective barriers of animals use a variety of bio-polymers, based on either proteins (e.g. collagens), or modified sugars (e.g. chitin). Plants, however, have come up with a particular solution, based on the polymerisation of lipid-like precursors, giving rise to cutin and suberin. Suberin is a structural lipophilic polyester of fatty acids, glycerol and some aromatics found in cell walls of phellem, endodermis, exodermis, wound tissues, abscission zones, bundle sheath and other tissues. It deposits as a hydrophobic layer between the (ligno)cellulosic primary cell wall and plasma membrane. Suberin is highly protective against biotic and abiotic stresses, shows great developmental plasticity and its chemically recalcitrant nature might assist the sequestration of atmospheric carbon by plants. The aim of this review is to integrate the rapidly accelerating genetic and cell biological discoveries of recent years with the important chemical and structural contributions obtained from very diverse organisms and tissue layers. We critically discuss the order and localisation of the enzymatic machinery synthesising the presumed substrates for export and apoplastic polymerisation. We attempt to explain observed suberin linkages by diverse enzyme activities and discuss the spatiotemporal relationship of suberin with lignin and ferulates, necessary to produce a functional suberised cell wall.

Killing two birds with a single stone-genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

The development of high-yielding, bio-fortified, stress-tolerant crop cultivars is the need of the hour in the wake of increasing global food insecurity, abrupt climate change, and continuous shrinking of resources and landmass suitable for agriculture. The cytokinin group of phytohormones positively regulates seed yield by simultaneous regulation of source capacity (leaf senescence) and sink strength (grain number and size). Cytokinins also regulate root-shoot architecture by promoting shoot growth and inhibiting root growth. Cytokinin oxidase/dehydrogenase (CKX) are the only enzymes that catalyze the irreversible degradation of active cytokinins and thus negatively regulate the endogenous cytokinin levels. Genetic manipulation of CKX genes is the key to improve seed yield and root-shoot architecture through direct manipulation of endogenous cytokinin levels. Downregulation of CKX genes expressed in sink tissues such as inflorescence meristem and developing seeds, through reverse genetics approaches such as RNAi and CRISPR/Cas9 resulted in increased yield marked by increased number and size of grains. On the other hand, root-specific expression of CKX genes resulted in decreased endogenous cytokinin levels in roots which in turn resulted in increased root growth indicated by increased root branching, root biomass, and root-shoot biomass ratio. Enhanced root growth provided enhanced tolerance to drought stress and improved micronutrient uptake efficiency. In this review, we have emphasized the role of CKX as a genetic factor determining yield, micronutrient uptake efficiency, and response to drought stress. We have summarised the efforts made to increase crop productivity and drought stress tolerance in different crop species through genetic manipulation of CKX family genes.

Using positional information to provide context for biological image analysis with MorphoGraphX 2.0

Positional information is a central concept in developmental biology. In developing organs, positional information can be idealized as a local coordinate system that arises from morphogen gradients controlled by organizers at key locations. This offers a plausible mechanism for the integration of the molecular networks operating in individual cells into the spatially coordinated multicellular responses necessary for the organization of emergent forms. Understanding how positional cues guide morphogenesis requires the quantification of gene expression and growth dynamics in the context of their underlying coordinate systems. Here, we present recent advances in the MorphoGraphX software (Barbier de Reuille et al., 2015⁠) that implement a generalized framework to annotate developing organs with local coordinate systems. These coordinate systems introduce an organ-centric spatial context to microscopy data, allowing gene expression and growth to be quantified and compared in the context of the positional information thought to control them.

Using CRISPR-Kill for organ specific cell elimination by cleavage of tandem repeats

CRISPR/Cas has been mainly used for mutagenesis through the induction of double strand breaks (DSBs) within unique protein-coding genes. Using the SaCas9 nuclease to induce multiple DSBs in functional repetitive DNA of Arabidopsis thaliana, we can now show that cell death can be induced in a controlled way. This approach, named CRISPR-Kill, can be used as tool for tissue engineering. By simply exchanging the constitutive promoter of SaCas9 with cell type-specific promoters, it is possible to block organogenesis in Arabidopsis. By AP1-specific expression of CRISPR-Kill, we are able to restore the apetala1 phenotype and to specifically eliminate petals. In addition, by expressing CRISPR-Kill in root-specific pericycle cells, we are able to dramatically reduce the number and the length of lateral roots. In the future, the application of CRISPR-Kill may not only help to control development but could also be used to change the biochemical properties of plants.

How double strand breaks (DSBs) are repaired within the plant 45S rDNA repeats is unclear. Here, the authors show that Cas9-mediated DSBs in 45S rDNA are mainly repaired by cNHEJ and describe CRISPR-Kill as a tool for organ-specific cell elimination by targeting functional repetitive DNA in Arabidopsis.

Extracellular vesiculo-tubular structures associated with suberin deposition in plant cell walls

Suberin is a fundamental plant biopolymer, found in protective tissues, such as seed coats, exodermis and endodermis of roots. Suberin is deposited in most suberizing cells in the form of lamellae just outside of the plasma membrane, below the primary cell wall. How monomeric suberin precursors, thought to be synthesized at the endoplasmic reticulum, are transported outside of the cell, for polymerization into suberin lamellae has remained obscure. Using electron-microscopy, we observed large numbers of extracellular vesiculo-tubular structures (EVs) to accumulate specifically in suberizing cells, in both chemically and cryo-fixed samples. EV presence correlates perfectly with root suberization and we could block suberin deposition and vesicle accumulation by affecting early, as well as late steps in the secretory pathway. Whereas many previous reports have described EVs in the context of biotic interactions, our results suggest a developmental role for extracellular vesicles in the formation of a major cell wall polymer.

Suberizing plant cells export suberin monomers outside of the cell to form a hydrophobic barrier. Here the authors propose a role for extracellular vesiculo-tubular structures in the deposition of suberin monomers.

Microtubule-based perception of mechanical conflicts controls plant organ morphogenesis

Precise coordination between cells and tissues is essential for differential growth in plants. During lateral root formation in Arabidopsis thaliana, the endodermis is actively remodeled to allow outgrowth of the new organ. Here, we show that microtubule arrays facing lateral root founder cells display a higher order compared to arrays on the opposite side of the same cell, and this asymmetry is required for endodermal remodeling and lateral root initiation. We identify that MICROTUBULE ASSOCIATED PROTEIN 70-5 (MAP70-5) is necessary for the establishment of this spatially defined microtubule organization and endodermis remodeling and thus contributes to lateral root morphogenesis. We propose that MAP70-5 and cortical microtubule arrays in the endodermis integrate the mechanical signals generated by lateral root outgrowth, facilitating the channeling of organogenesis.

The hidden half comes into the spotlight: Peeking inside the black box of root developmental phases

Efficient use of natural resources (e.g., light, water, and nutrients) can be improved with a tailored developmental program that maximizes the lifetime and fitness of plants. In plant shoots, a developmental phase represents a time window in which the meristem triggers the development of unique morphological and physiological traits, leading to the emergence of leaves, flowers, and fruits. Whereas developmental phases in plant shoots have been shown to enhance food production in crops, this phenomenon has remained poorly investigated in roots. In light of recent advances, we suggest that root development occurs in three main phases: root apical meristem appearance, foraging, and senescence. We provide compelling evidence suggesting that these phases are regulated by at least four developmental pathways: autonomous, non-autonomous, hormonal, and periodic. Root developmental pathways differentially coordinate organ plasticity, promoting morphological alterations, tissue regeneration, and cell death regulation. Furthermore, we suggest how nutritional checkpoints may allow progression through the developmental phases, thus completing the root life cycle. These insights highlight novel and exciting advances in root biology that may help maximize the productivity of crops through more sustainable agriculture and the reduced use of chemical fertilizers.

Physiological roles of Casparian strips and suberin in the transport of water and solutes

The formation of Casparian strips (CS) and the deposition of suberin at the endodermis of plant roots are thought to limit the apoplastic transport of water and ions. We investigated the specific role of each of these apoplastic barriers in the control of hydro-mineral transport by roots and the consequences on shoot growth. A collection of Arabidopsis thaliana mutants defective in suberin deposition and/or CS development was characterized under standard conditions using a hydroponic system and the Phenopsis platform. Mutants altered in suberin deposition had enhanced root hydraulic conductivity, indicating a restrictive role for this compound in water transport. In contrast, defective CS directly increased solute leakage and indirectly reduced root hydraulic conductivity. Defective CS also led to a reduction in rosette growth, which was partly dependent on the hydro-mineral status of the plant. Ectopic suberin was shown to partially compensate for defective CS phenotypes. Altogether, our work shows that the functionality of the root apoplastic diffusion barriers greatly influences the plant physiology, and that their integrity is tightly surveyed.

Building and breaking of a barrier: Suberin plasticity and function in the endodermis

Plant cells coated with hydrophobic compounds constitute a protective barrier to control movement of materials through plant tissues. In roots, the endodermis develops two barriers: the Casparian strips establish an apoplastic barrier and suberin lamellae prevent diffusion through the plasma membrane. Suberin is a complex biopolymer and its deposition is highly responsive to the environment. While the enzymatic framework involved in suberin biosynthesis is well characterized, subsequent steps in suberin formation and regulation remained elusive. Recent publications, studying suberin from a cell biological perspective, have enriched our knowledge on suberin transport and polymerization in the cell wall. These studies have also elucidated the molecular mechanisms controlling suberin biosynthesis and regulation as well as its physiological role in plant abiotic and biotic interactions.

Visualizing polymeric components that define distinct root barriers across plant lineages

Hydrophobic cell wall depositions in roots play a key role in plant development and interaction with the soil environment, as they generate barriers that regulate bidirectional nutrient flux. Techniques to label the respective polymers are emerging, but are efficient only in thin roots or sections. Moreover, simultaneous imaging of the barrier constituents lignin and suberin remains problematic owing to their similar chemical compositions. Here, we describe a staining method compatible with single- and multiphoton confocal microscopy that allows for concurrent visualization of primary cell walls and distinct secondary depositions in one workflow. This protocol permits efficient separation of suberin- and lignin-specific signals with high resolution, enabling precise dissection of barrier constituents. Our approach is compatible with imaging of fluorescent proteins, and can thus complement genetic markers or aid the dissection of barriers in biotic root interactions. We further demonstrate applicability in deep root tissues of plant models and crops across phylogenetic lineages. Our optimized toolset will significantly advance our understanding of root barrier dynamics and function, and of their role in plant interactions with the rhizospheric environment.

Casting the Net-Connecting Auxin Signaling to the Plant Genome

Auxin represents one of the most potent and most versatile hormonal signals in the plant kingdom. Built on a simple core of only a few dedicated components, the auxin signaling system plays important roles for diverse aspects of plant development, physiology, and defense. Key to the diversity of context-dependent functional outputs generated by cells in response to this small molecule are gene duplication events and sub-functionalization of signaling components on the one hand, and a deep embedding of the auxin signaling system into complex regulatory networks on the other hand. Together, these evolutionary innovations provide the mechanisms to allow each cell to display a highly specific auxin response that suits its individual requirements. In this review, we discuss the regulatory networks connecting auxin with a large number of diverse pathways at all relevant levels of the signaling system ranging from biosynthesis to transcriptional response.

Convergent evolution of gene regulatory networks underlying plant adaptations to dry environments

Plants transitioned from an aquatic to a terrestrial lifestyle during their evolution. On land, fluctuations on water availability in the environment became one of the major problems they encountered. The appearance of morpho-physiological adaptations to cope with and tolerate water loss from the cells was undeniably useful to survive on dry land. Some of these adaptations, such as carbon concentrating mechanisms (CCMs), desiccation tolerance (DT) and root impermeabilization, appeared in multiple plant lineages. Despite being crucial for evolution on land, it has been unclear how these adaptations convergently evolved in the various plant lineages. Recent advances on whole genome and transcriptome sequencing are revealing that co-option of genes and gene regulatory networks (GRNs) is a common feature underlying the convergent evolution of these adaptations. In this review, we address how the study of CCMs and DT has provided insight into convergent evolution of GRNs underlying plant adaptation to dry environments, and how these insights could be applied to currently emerging understanding of evolution of root impermeabilization through different barrier cell types. We discuss examples of co-option, conservation and innovation of genes and GRNs at the cell, tissue and organ levels revealed by recent phylogenomic (comparative genomic) and comparative transcriptomic studies.

Suberin plasticity to developmental and exogenous cues is regulated by a set of MYB transcription factors

Suberin is a hydrophobic biopolymer that can be deposited at the periphery of cells, forming protective barriers against biotic and abiotic stress. In roots, suberin forms lamellae at the periphery of endodermal cells where it plays crucial roles in the control of water and mineral transport. Suberin formation is highly regulated by developmental and environmental cues. However, the mechanisms controlling its spatiotemporal regulation are poorly understood. Here, we show that endodermal suberin is regulated independently by developmental and exogenous signals to fine-tune suberin deposition in roots. We found a set of four MYB transcription factors (MYB41, MYB53, MYB92, and MYB93), each of which is individually regulated by these two signals and is sufficient to promote endodermal suberin. Mutation of these four transcription factors simultaneously through genome editing leads to a dramatic reduction in suberin formation in response to both developmental and environmental signals. Most suberin mutants analyzed at physiological levels are also affected in another endodermal barrier made of lignin (Casparian strips) through a compensatory mechanism. Through the functional analysis of these four MYBs, we generated plants allowing unbiased investigation of endodermal suberin function, without accounting for confounding effects due to Casparian strip defects, and were able to unravel specific roles of suberin in nutrient homeostasis.

Suberin plasticity to developmental and exogenous cues is regulated by a set of MYB transcription factors

Suberin is a hydrophobic biopolymer that can be deposited at the periphery of cells, forming protective barriers against biotic and abiotic stress. In roots, suberin forms lamellae at the periphery of endodermal cells where it plays crucial roles in the control of water and mineral transport. Suberin formation is highly regulated by developmental and environmental cues. However, the mechanisms controlling its spatiotemporal regulation are poorly understood. Here, we show that endodermal suberin is regulated independently by developmental and exogenous signals to fine-tune suberin deposition in roots. We found a set of four MYB transcription factors (MYB41, MYB53, MYB92, and MYB93), each of which is individually regulated by these two signals and is sufficient to promote endodermal suberin. Mutation of these four transcription factors simultaneously through genome editing leads to a dramatic reduction in suberin formation in response to both developmental and environmental signals. Most suberin mutants analyzed at physiological levels are also affected in another endodermal barrier made of lignin (Casparian strips) through a compensatory mechanism. Through the functional analysis of these four MYBs, we generated plants allowing unbiased investigation of endodermal suberin function, without accounting for confounding effects due to Casparian strip defects, and were able to unravel specific roles of suberin in nutrient homeostasis.

Plant Cell Identity in the Era of Single-Cell Transcriptomics

High-throughput single-cell transcriptomic approaches have revolutionized our view of gene expression at the level of individual cells, providing new insights into their heterogeneity, identities, and functions. Recently, technical challenges to the application of single-cell transcriptomics to plants have been overcome, and many plant organs and tissues have now been subjected to analyses at single-cell resolution. In this review, we describe these studies and their impact on our understanding of the diversity, differentiation, and activities of plant cells. We particularly highlight their impact on plant cell identity, including unprecedented views of cell transitions and definitions of rare and novel cell types. We also point out current challenges and future opportunities for the application and analyses of single-cell transcriptomics in plants. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Root engineering in maize by increasing cytokinin degradation causes enhanced root growth and leaf mineral enrichment

Root-specific expression of a cytokinin-degrading CKX gene in maize roots causes formation of a larger root system leading to higher element content in shoot organs. The size and architecture of the root system is functionally relevant for the access to water and soil nutrients. A great number of mostly unknown genes are involved in regulating root architecture complicating targeted breeding of plants with a larger root system. Here, we have explored whether root-specific degradation of the hormone cytokinin, which is a negative regulator of root growth, can be used to genetically engineer maize (Zea mays L.) plants with a larger root system. Root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene of Arabidopsis caused the formation of up to 46% more root dry weight while shoot growth of these transgenic lines was similar as in non-transgenic control plants. The concentration of several elements, in particular of those with low soil mobility (K, P, Mo, Zn), was increased in leaves of transgenic lines. In kernels, the changes in concentration of most elements were less pronounced, but the concentrations of Cu, Mn and Zn were significantly increased in at least one of the three independent lines. Our data illustrate the potential of an increased root system as part of efforts towards achieving biofortification. Taken together, this work has shown that root-specific expression of a CKX gene can be used to engineer the root system of maize and alter shoot element composition.

A Novel Signaling Pathway Required for Arabidopsis Endodermal Root Organization Shapes the Rhizosphere Microbiome

The Casparian strip (CS) constitutes a physical diffusion barrier to water and nutrients in plant roots, which is formed by the polar deposition of lignin polymer in the endodermis tissue. The precise pattern of lignin deposition is determined by the scaffolding activity of membrane-bound Casparian Strip domain proteins (CASPs), but little is known of the mechanism(s) directing this process. Here, we demonstrate that Endodermis-specific Receptor-like Kinase 1 (ERK1) and, to a lesser extent, ROP Binding Kinase1 (RBK1) are also involved in regulating CS formation, with the former playing an essential role in lignin deposition as well as in the localization of CASP1. We show that ERK1 is localized to the cytoplasm and nucleus of the endodermis and that together with the circadian clock regulator, Time for Coffee (TIC), forms part of a novel signaling pathway necessary for correct CS organization and suberization of the endodermis, with their single or combined loss of function resulting in altered root microbiome composition. In addition, we found that other mutants displaying defects in suberin deposition at the CS also display altered root exudates and microbiome composition. Thus, our work reveals a complex network of signaling factors operating within the root endodermis that establish both the CS diffusion barrier and influence the microbial composition of the rhizosphere.

The endodermal passage cell - just another brick in the wall?

The endodermis surrounds and protects the vasculature partly by depositing hydrophobic suberin in the cell walls. Yet, some cells remain unsuberised. These historically termed 'passage cells' are assumed to provide a low-resistance pathway to the xylem. Only recently have we started to gain molecular insights into these cells, which allow us to probe how roots coordinate communication with the environment across barriers with single-cell precision. Increased understanding of root physiology at a high-resolution is intriguing, as it is likely to provide us with new tools to improve overall plant health. With this in mind, we here provide a brief overview of passage cells, their presence across plant species, as well as a molecular update and future directions for passage cell-related research.

Two chemically distinct root lignin barriers control solute and water balance

Lignin is a complex polymer deposited in the cell wall of specialised plant cells, where it provides essential cellular functions. Plants coordinate timing, location, abundance and composition of lignin deposition in response to endogenous and exogenous cues. In roots, a fine band of lignin, the Casparian strip encircles endodermal cells. This forms an extracellular barrier to solutes and water and plays a critical role in maintaining nutrient homeostasis. A signalling pathway senses the integrity of this diffusion barrier and can induce over-lignification to compensate for barrier defects. Here, we report that activation of this endodermal sensing mechanism triggers a transcriptional reprogramming strongly inducing the phenylpropanoid pathway and immune signaling. This leads to deposition of compensatory lignin that is chemically distinct from Casparian strip lignin. We also report that a complete loss of endodermal lignification drastically impacts mineral nutrients homeostasis and plant growth.

Function of the pseudo phosphotransfer proteins has diverged between rice and Arabidopsis

The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type-B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type-A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the rice PHP genes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three rice PHPs fail to complement an Arabidopsis php mutant (aphp1/ahp6). Disruption of the three rice PHPs results in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type-A RR and cytokinin oxidase genes. The triple php mutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the rice PHPs does not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.

Physcomitrium patens: A Single Model to Study Oriented Cell Divisions in 1D to 3D Patterning

Development in multicellular organisms relies on cell proliferation and specialization. In plants, both these processes critically depend on the spatial organization of cells within a tissue. Owing to an absence of significant cellular migration, the relative position of plant cells is virtually made permanent at the moment of division. Therefore, in numerous plant developmental contexts, the (divergent) developmental trajectories of daughter cells are dependent on division plane positioning in the parental cell. Prior to and throughout division, specific cellular processes inform, establish and execute division plane control. For studying these facets of division plane control, the moss Physcomitrium (Physcomitrella) patens has emerged as a suitable model system. Developmental progression in this organism starts out simple and transitions towards a body plan with a three-dimensional structure. The transition is accompanied by a series of divisions where cell fate transitions and division plane positioning go hand in hand. These divisions are experimentally highly tractable and accessible. In this review, we will highlight recently uncovered mechanisms, including polarity protein complexes and cytoskeletal structures, and transcriptional regulators, that are required for 1D to 3D body plan formation.

GDSL-domain proteins have key roles in suberin polymerization and degradation

Plant roots acquire nutrients and water while managing interactions with the soil microbiota. The root endodermis provides an extracellular diffusion barrier through a network of lignified cell walls called Casparian strips, supported by subsequent formation of suberin lamellae. Whereas lignification is thought to be irreversible, suberin lamellae display plasticity, which is crucial for root adaptative responses. Although suberin is a major plant polymer, fundamental aspects of its biosynthesis and turnover have remained obscure. Plants shape their root system via lateral root formation, an auxin-induced process requiring local breaking and re-sealing of endodermal lignin and suberin barriers. Here, we show that differentiated endodermal cells have a specific, auxin-mediated transcriptional response dominated by cell wall remodelling genes. We identified two sets of auxin-regulated GDSL lipases. One is required for suberin synthesis, while the other can drive suberin degradation. These enzymes have key roles in suberization, driving root suberin plasticity.

Single-cell profiling lights different cell trajectories in plants

The molecular mechanism of the maintenance and differentiation of plant stem cells is an eternal theme in studies on plant growth and development. Recent advances in single-cell RNA sequencing (scRNA-seq) methods have completely changed the understanding of cell heterogeneity and cell function, allowing research precision to identify the differentiation trajectory of stem cells maintained and differentiated at the cellular level. This review aimed to mainly discuss the novel insights provided by scRNA-seq for the maintenance and initiation of plant stem cells, cell differentiation, cell response to environmental changes, and improvement strategies for scRNA-seq. In addition, it highlighted additional perspectives beyond scRNA-seq, such as spatial transcriptomes, epigenomes, and single-cell multiomics, for a renewed understanding of stem cell maintenance and cell differentiation, thus providing potential targets and theoretical foundations for crop improvement.

Targeted expression of the arsenate reductase HAC1 identifies cell type specificity of arsenic metabolism and transport in plant roots

High Arsenic Concentration 1 (HAC1), an Arabidopsis thaliana arsenate reductase, plays a key role in arsenate [As(V)] tolerance. Through conversion of As(V) to arsenite [As(III)], HAC1 enables As(III) export from roots, and restricts translocation of As(V) to shoots. To probe the ability of different root tissues to detoxify As(III) produced by HAC1, we generated A. thaliana lines expressing HAC1 in different cell types. We investigated the As(V) tolerance phenotypes: root growth, As(III) efflux, As translocation, and As chemical speciation. We showed that HAC1 can function in the outer tissues of the root (epidermis, cortex, and endodermis) to confer As(V) tolerance, As(III) efflux, and limit As accumulation in shoots. HAC1 is less effective in the stele at conferring As(V) tolerance phenotypes. The exception is HAC1 activity in the protoxylem, which we found to be sufficient to restrict As translocation, but not to confer As(V) tolerance. In conclusion, we describe cell type-specific functions of HAC1 that spatially separate the control of As(V) tolerance and As translocation. Further, we identify a key function of protoxylem cells in As(V) translocation, consistent with the model where endodermal passage cells, above protoxylem pericycle cells, form a 'funnel' loading nutrients and potentially toxic elements into the vasculature.

Spatially Restricted Immune Responses Are Required for Maintaining Root Meristematic Activity upon Detection of Bacteria

Plants restrict immune responses to vulnerable root parts. Spatially restricted responses are thought to be necessary to avoid constitutive responses to rhizosphere microbiota. To directly demonstrate the importance of spatially restricted responses, we expressed the plant flagellin receptor (FLS2) in different tissues, combined with fluorescent defense markers for immune readouts at cellular resolution. Our analysis distinguishes responses appearing cell autonomous from apparently non-cell-autonomous responses. It reveals lignification as a general immune response, contrasting suberization. Importantly, our analysis divides the root meristem into a central zone refractory to FLS2 expression and a cortex that is sensitized by FLS2 expression, causing meristem collapse upon stimulation. Meristematic epidermal expression generates super-competent lines that detect native bacterial flagellin and bypass the weak or absent response to commensals, providing a powerful tool for studying root immunity. Our manipulations and readouts demonstrate incompatibility of meristematic activity and defense and the importance of cell-resolved studies of plant immune responses.

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

ETHYLENE RESPONSE FACTOR 115 integrates jasmonate and cytokinin signaling machineries to repress adventitious rooting in Arabidopsis

Adventitious root initiation (ARI) is a de novo organogenesis program and a key adaptive trait in plants. Several hormones regulate ARI but the underlying genetic architecture that integrates the hormonal crosstalk governing this process remains largely elusive. In this study, we use genetics, genome editing, transcriptomics, hormone profiling and cell biological approaches to demonstrate a crucial role played by the APETALA2/ETHYLENE RESPONSE FACTOR 115 transcription factor. We demonstrate that ERF115 functions as a repressor of ARI by activating the cytokinin (CK) signaling machinery. We also demonstrate that ERF115 is transcriptionally activated by jasmonate (JA), an oxylipin-derived phytohormone, which represses ARI in NINJA-dependent and independent manners. Our data indicate that NINJA-dependent JA signaling in pericycle cells blocks early events of ARI. Altogether, our results reveal a previously unreported molecular network involving cooperative crosstalk between JA and CK machineries that represses ARI.

High-order mutants reveal an essential requirement for peroxidases but not laccases in Casparian strip lignification

Lignin is a defining polymer of vascular plants and of great physiological, ecological, and economical importance. Yet, its polymerization in the cell wall is still not understood. Lignin polymerizing enzymes, laccases and peroxidases, exist in vast numbers in plant genomes. By focusing on a specific lignin structure, the ring-like Casparian strips (CSs), we reduced candidate numbers and abolished essentially all laccases with detectable endodermal expression. Yet, not even slight defects in CS formation were detected. By contrast, a quintuple peroxidase mutant displayed a complete absence of CS. Our findings suggest that cells lignify differently depending on whether lignin is localized or ubiquitous and whether cells stay alive during and after lignification, as well as the composition of the cell wall.

Lignin has enabled plants to colonize land, grow tall, transport water within their bodies, and protect themselves against various stresses. Consequently, this polyphenolic polymer, impregnating cellulosic plant cell walls, is the second most abundant polymer on Earth. Yet, despite its great physiological, ecological, and economical importance, our knowledge of lignin biosynthesis in vivo, especially the polymerization steps within the cell wall, remains vague—specifically, the respective roles of the two polymerizing enzymes classes, laccases and peroxidases. One reason for this lies in the very high numbers of laccases and peroxidases encoded by 17 and 73 homologous genes, respectively, in Arabidopsis. Here, we have focused on a specific lignin structure, the ring-like Casparian strips (CSs) within the root endodermis. By reducing candidate numbers using cellular resolution expression and localization data and by boosting stacking of mutants using CRISPR-Cas9, we mutated the majority of laccases in Arabidopsis in a nonuple mutant—essentially abolishing laccases with detectable endodermal expression. Yet, we were unable to detect even slight defects in CS formation. By contrast, we were able to induce a complete absence of CS formation in a quintuple peroxidase mutant. Our findings are in stark contrast to the strong requirement of xylem vessels for laccase action and indicate that lignin in different cell types can be polymerized in very distinct ways. We speculate that cells lignify differently depending on whether lignin is localized or ubiquitous and whether cells stay alive during and after lignification, as well as the composition of the cell wall.

Root-specific expression of chickpea cytokinin oxidase/dehydrogenase 6 leads to enhanced root growth, drought tolerance and yield without compromising nodulation

Cytokinin group of phytohormones regulate root elongation and branching during post-embryonic development. Cytokinin-degrading enzymes cytokinin oxidases/dehydrogenases (CKXs) have been deployed to investigate biological activities of cytokinin and to engineer root growth. We expressed chickpea cytokinin oxidase 6 (CaCKX6) under the control of a chickpea root-specific promoter of CaWRKY31 in Arabidopsis thaliana and chickpea having determinate and indeterminate growth patterns, respectively, to study the effect of cytokinin depletion on root growth and drought tolerance. Root-specific expression of CaCKX6 led to a significant increase in lateral root number and root biomass in Arabidopsis and chickpea without any penalty to vegetative and reproductive growth of shoot. Transgenic chickpea lines showed increased CKX activity in root. Soil-grown advanced chickpea transgenic lines exhibited higher root-to-shoot biomass ratio and enhanced long-term drought tolerance. These chickpea lines were not compromised in root nodulation and nitrogen fixation. The seed yield in some lines was up to 25% higher with no penalty in protein content. Transgenic chickpea seeds possessed higher levels of zinc, iron, potassium and copper. Our results demonstrated the potential of cytokinin level manipulation in increasing lateral root number and root biomass for agronomic trait improvement in an edible legume crop with indeterminate growth habit.

Pluripotent Pericycle Cells Trigger Different Growth Outputs by Integrating Developmental Cues into Distinct Regulatory Modules

During post-embryonic development, the pericycle specifies the stem cells that give rise to both lateral roots (LRs) and the periderm, a suberized barrier that protects the plant against biotic and abiotic stresses. Comparable auxin-mediated signaling hubs regulate meristem establishment in many developmental contexts; however, it is unknown how specific outputs are achieved. Using the Arabidopsis root as a model, we show that while LR formation is the main auxin-induced program after de-etiolation, plants with age become competent to form a periderm in response to auxin. The establishment of the vascular cambium acts as the developmental switch required to trigger auxin-mediated periderm initiation. Moreover, distinct auxin signaling components and targets control LR versus periderm formation. Among the periderm-specific-promoting transcription factors, WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNAT1/BREVIPEDICELLUS (BP) stand out as their specific overexpression in the periderm results in an increased number of periderm layers, a trait of agronomical importance in breeding programs targeting stress tolerance. These findings reveal that specificity in pericycle stem cell fate is achieved by the integration of developmental cues into distinct regulatory modules.

Coping With Water Limitation: Hormones That Modify Plant Root Xylem Development

Periods of drought, that threaten crop production, are expected to become more prominent in large parts of the world, making it necessary to explore all aspects of plant growth and development, to breed, modify and select crops adapted to such conditions. One such aspect is the xylem, where influencing the size and number of the water-transporting xylem vessels, may impact on hydraulic conductance and drought tolerance. Here, we focus on how plants adjust their root xylem as a response to reduced water availability. While xylem response has been observed in a wide array of species, most of our knowledge on the molecular mechanisms underlying xylem plasticity comes from studies on the model plant Arabidopsis thaliana. When grown under water limiting conditions, Arabidopsis rapidly adjusts its development to produce more xylem strands with altered identity in an abscisic acid (ABA) dependent manner. Other hormones such as auxin and cytokinin are essential for vascular patterning and differentiation. Their balance can be perturbed by stress, as evidenced by the effects of enhanced jasmonic acid signaling, which results in similar xylem developmental alterations as enhanced ABA signaling. Furthermore, brassinosteroids and other signaling molecules involved in drought tolerance can also impact xylem development. Hence, a multitude of signals affect root xylem properties and, potentially, influence survival under water limiting conditions. Here, we review the likely entangled signals that govern root vascular development, and discuss the importance of taking root anatomical traits into account when breeding crops for enhanced resilience toward changes in water availability.