Talking through walls: mechanisms of lateral root emergence in Arabidopsis thaliana

Nitrate Starvation Induces Lateral Root Organogenesis in Triticum aestivum via Auxin Signaling

The lateral root (LR) is an essential component of the plant root system, performing important functions for nutrient and water uptake in plants and playing a pivotal role in cereal crop productivity. Nitrate (NO3-) is an essential nutrient for plants. In this study, wheat plants were grown in 1/2 strength Hoagland's solution containing 5 mM NO3- (check; CK), 0.1 mM NO3- (low NO3-; LN), or 0.1 mM NO3- plus 60 mg/L 2,3,5-triiodobenzoic acid (TIBA) (LNT). The results showed that LN increased the LR number significantly at 48 h after treatment compared with CK, while not increasing the root biomass, and LNT significantly decreased the LR number and root biomass. The transcriptomic analysis showed that LN induced the expression of genes related to root IAA synthesis and transport and cell wall remodeling, and it was suppressed in the LNT conditions. A physiological assay revealed that the LN conditions increased the activity of IAA biosynthesis-related enzymes, the concentrations of tryptophan and IAA, and the activity of cell wall remodeling enzymes in the roots, whereas the content of polysaccharides in the LRP cell wall was significantly decreased compared with the control. Fourier-transform infrared spectroscopy and atomic microscopy revealed that the content of cell wall polysaccharides decreased and the cell wall elasticity of LR primordia (LRP) increased under the LN conditions. The effects of LN on IAA synthesis and polar transport, cell wall remodeling, and LR development were abolished when TIBA was applied. Our findings indicate that NO3- starvation may improve auxin homeostasis and the biological properties of the LRP cell wall and thus promote LR initiation, while TIBA addition dampens the effects of LN on auxin signaling, gene expression, physiological processes, and the root architecture.

GmHXK2 promotes the salt tolerance of soybean seedlings by mediating AsA synthesis, and auxin synthesis and distribution

BACKGROUND

Salt is an important factor that affects crop productivity. Plant hexokinases (HXKs) are key enzymes in the glycolytic pathway and sugar signaling transduction pathways of plants. In previous studies, we identified and confirmed the roles of GmHXK2 in salt tolerance.

RESULTS

In this study, we analyzed the tissue-specific expression of GmHXK2 at different growth stages throughout the plant's life cycle. The results showed that GmHXK2 was expressed significantly in all tissues at vegetative stages, including germination and seedling. However, no expression was detected in the pods, and there was little expression in flowers during the later mature period. Arabidopsis plants overexpressing the GmHXK2 (OE) had more lateral roots. The OE seedlings also produced higher levels of auxin and ascorbic acid (AsA). Additionally, the expression levels of genes PMM, YUC4/YUC6/YUC8, and PIN/LAX1,LAX3, which are involved respectively in the synthesis of AsA and auxin, as well as polar auxin transport, were upregulated in OE plants. This upregulation occurred specifically under exogenous glucose treatment. AtHKT1, AtSOS1, and AtNHX1 were up-regulated in OE plants under salt stress, suggesting that GmHXK2 may modulate salt tolerance by maintaining ion balance within the cells and alleviating damage caused by salt stress. Additionally, we further confirmed the interaction between GmHXK2 and the protein GmPMM through yeast two-hybridization and bimolecular fluorescence complementation assays, respectively.

CONCLUSION

The expression of GmHXK2 gene in plants is organ-specific and developmental stage specific. GmHXK2 not only regulates the synthesis of AsA and the synthesis and distribution of auxin, but also promotes root elongation and induces lateral root formation, potentially enhancing soil water absorption. This study reveals the crosstalk between sugar signaling and hormone signaling in plants, where GmHXK2 acts as a glucose sensor through its interaction with GmPMM, and sheds light on the molecular mechanism by which GmHXK2 gene is involved in salt tolerance in plants.

Auxin-mediated stress relaxation in pericycle and endoderm remodeling drives lateral root initiation

Plant development relies on the precise coordination of cell growth, which is influenced by the mechanical constraints imposed by rigid cell walls. The hormone auxin plays a crucial role in regulating this growth by altering the mechanical properties of cell walls. During the postembryonic formation of lateral roots, pericycle cells deep within the main root are triggered by auxin to resume growth and divide to form a new root. This growth involves a complex interplay between auxin, growth, and the resolution of mechanical conflicts with the overlying endodermis. However, the exact mechanisms by which this coordination is achieved are still unknown. Here, we propose a model that integrates tissue mechanics and auxin transport, revealing a connection between the auxin-induced relaxation of mechanical stress in the pericycle and auxin signaling in the endodermis. We show that the endodermis initially limits the growth of pericycle cells, resulting in a modest initial expansion. However, the associated stress relaxation is sufficient to redirect auxin to the overlying endodermis, which then actively accommodates the growth, allowing for the subsequent development of the lateral root. Our model uncovers that increased pericycle turgor and decreased endodermal resistance license expansion of the pericycle and how the topology of the endodermis influences the formation of the new root. These findings highlight the interconnected relationship between mechanics and auxin flow during lateral root initiation, emphasizing the vital role of the endodermis in shaping root development through mechanotransduction and auxin signaling.

Transcriptome Analysis of Early Lateral Root Formation in Tomato

Lateral roots (LRs) receive signals from the inter-root environment and absorb water and nutrients from the soil. Auxin regulates LR formation, but the mechanism in tomato remains largely unknown. In this study, 'Ailsa Craig' tomato LRs appeared on the third day and were unevenly distributed in primary roots. According to the location of LR occurrence, roots were divided into three equal parts: the shootward part of the root (RB), the middle part of the root (RM), and the tip part of the root (RT). Transverse sections of roots from days 1 to 6 revealed that the number of RB cells and the root diameter were significantly increased compared with RM and RT. Using roots from days 1 to 3, we carried out transcriptome sequencing analysis. Identified genes were classified into 16 co-expression clusters based on K-means, and genes in four associated clusters were highly expressed in RB. These four clusters (3, 5, 8, and 16) were enriched in cellulose metabolism, microtubule, and peptide metabolism pathways, all closely related to LR development. The four clusters contain numerous transcription factors linked to LR development including transcription factors of LATERAL ORGAN BOUNDRIES (LOB) and MADS-box families. Additionally, auxin-related genes GATA23, ARF7, LBD16, EXP, IAA4, IAA7, PIN1, PIN2, YUC3, and YUC4 were highly expressed in RB tissue. Free IAA content in 3 d RB was notably higher, reaching 3.3-5.5 ng/g, relative to RB in 1 d and 2 d. The LR number was promoted by 0.1 μM of exogenous IAA and inhibited by exogenous NPA. We analyzed the root cell state and auxin signaling module during LR formation. At a certain stage of pericycle cell development, LR initiation is regulated by auxin signaling modules IAA14-ARF7/ARF19-LBD16-CDKA1 and IAA14-ARF7/ARF19-MUS/MUL-XTR6/EXP. Furthermore, as a key regulatory factor, auxin regulates the process of LR initiation and LR primordia (LRP) through different auxin signaling pathway modules.

AtHD2D is involved in regulating lateral root development and participates in abiotic stress response in Arabidopsis

Roots are essential to terrestrial plants, as their growth and morphology are crucial for plant development. The growth of the roots is affected and regulated by several internal and external environmental signals and metabolic pathways. Among them, chromatin modification plays an important regulatory role. In this study, we explore the potential roles of the histone deacetylase AtHD2D in root development and lay the foundation for further research on the biological processes and molecular mechanisms of AtHD2D in the future. Our study indicates that AtHD2D affects the root tip microenvironment homeostasis by affecting the gene transcription levels required to maintain the root tip microenvironment. In addition, we confirmed that AtHD2D is involved in regulating Arabidopsis lateral root development and further explained the possible role of AtHD2D in auxin-mediated lateral root development. AtHD2D can effectively enhance the resistance of Arabidopsis thaliana to abiotic stress. We believe that AtHD2D is involved in coping with abiotic stress by promoting the development of lateral roots. Overexpression of AtHD2D promotes the accumulation of reactive oxygen species (ROS) in roots, indicating that AtHD2D is also involved in developing lateral roots mediated by ROS. Previous studies have shown that the overexpression of AtHD2D can effectively enhance the resistance of Arabidopsis thaliana to abiotic stress. Based on our data, we believe that AtHD2D participates in the response to abiotic stress by promoting the development of lateral roots. AtHD2D-mediated lateral root development provides new ideas for studying the mechanism of HDAC protein in regulating root development.

A newly evolved rice-specific gene JAUP1 regulates jasmonate biosynthesis and signalling to promote root development and multi-stress tolerance

Root architecture and function are critical for plants to secure water and nutrient supply from the soil, but environmental stresses alter root development. The phytohormone jasmonic acid (JA) regulates plant growth and responses to wounding and other stresses, but its role in root development for adaptation to environmental challenges had not been well investigated. We discovered a novel JA Upregulated Protein 1 gene (JAUP1) that has recently evolved in rice and is specific to modern rice accessions. JAUP1 regulates a self-perpetuating feed-forward loop to activate the expression of genes involved in JA biosynthesis and signalling that confers tolerance to abiotic stresses and regulates auxin-dependent root development. Ectopic expression of JAUP1 alleviates abscisic acid- and salt-mediated suppression of lateral root (LR) growth. JAUP1 is primarily expressed in the root cap and epidermal cells (EPCs) that protect the meristematic stem cells and emerging LRs. Wound-activated JA/JAUP1 signalling promotes crosstalk between the root cap of LR and parental root EPCs, as well as induces cell wall remodelling in EPCs overlaying the emerging LR, thereby facilitating LR emergence even under ABA-suppressive conditions. Elevated expression of JAUP1 in transgenic rice or natural rice accessions enhances abiotic stress tolerance and reduces grain yield loss under a limited water supply. We reveal a hitherto unappreciated role for wound-induced JA in LR development under abiotic stress and suggest that JAUP1 can be used in biotechnology and as a molecular marker for breeding rice adapted to extreme environmental challenges and for the conservation of water resources.

H2 supplied via ammonia borane stimulates lateral root branching via phytomelatonin signaling

A reliable and stable hydrogen gas (H2) supply will benefit agricultural laboratory and field trials. Here, we assessed ammonia borane (AB), an efficient hydrogen storage material used in the energy industry, and determined its effect on plant physiology and the corresponding mechanism. Through hydroponics and pot experiments, we discovered that AB increases tomato (Solanum lycopersicum) lateral root (LR) branching and this function depended on the increased endogenous H2 level caused by the sustainable H2 supply. In particular, AB might trigger LR primordia initiation. Transgenic tomato and Arabidopsis (Arabidopsis thaliana) expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only accumulated higher endogenous H2 and phytomelatonin levels but also displayed pronounced LR branching. These endogenous H2 responses achieved by AB or genetic manipulation were sensitive to the pharmacological removal of phytomelatonin, indicating the downstream role of phytomelatonin in endogenous H2 control of LR formation. Consistently, extra H2 supply failed to influence the LR defective phenotypes in phytomelatonin synthetic mutants. Molecular evidence showed that the phytomelatonin-regulated auxin signaling network and cell-cycle regulation were associated with the AB/H2 control of LR branching. Also, AB and melatonin had little effect on LR branching in the presence of auxin synthetic inhibitors. Collectively, our integrated approaches show that supplying H2 via AB increases LR branching via phytomelatonin signaling. This finding might open the way for applying hydrogen storage materials to horticultural production.

The chickpea WIP2 gene underlying a major QTL contributes to lateral root development

Lateral roots are a major component of root system architecture, and lateral root count (LRC) positively contributes to yield under drought in chickpea. To understand the genetic regulation of LRC, a biparental mapping population derived from two chickpea accessions having contrasting LRCs was genotyped by sequencing, and phenotyped to map four major quantitative trait loci (QTLs) contributing to 13-32% of the LRC trait variation. A single- nucleotide polymorphism tightly linked to the locus contributing to highest trait variation was located on the coding region of a gene (CaWIP2), orthologous to NO TRANSMITTING TRACT/WIP domain protein 2 (NTT/WIP2) gene of Arabidopsis thaliana. A polymorphic simple sequence repeat (SSR) in the CaWIP2 promoter showed differentiation between low versus high LRC parents and mapping individuals, suggesting its utility for marker-assisted selection. CaWIP2 promoter showed strong expression in chickpea apical root meristem and lateral root primordia. Expression of CaWIP2 under its native promoter in the Arabidopsis wip2wip4wip5 mutant rescued its rootless phenotype to produce more lateral roots than the wild-type plants, and led to formation of amyloplasts in the columella. CaWIP2 expression also induced the expression of genes that regulate lateral root emergence. Our study identified a gene-based marker for LRC which will be useful for developing drought-tolerant, high-yielding chickpea varieties.

Illuminating the path to shoot meristem regeneration: Molecular insights into reprogramming cells into stem cells

Plant cells possess the ability to dedifferentiate and reprogram into stem cell-like populations, enabling the regeneration of new organs. However, the maintenance of stem cells relies on specialized microenvironments composed of distinct cell populations with specific functions. Consequently, the regeneration process necessitates the orchestrated regulation of multiple pathways across diverse cellular populations. One crucial pathway involves the transcription factor WUSCHEL HOMEOBOX 5 (WOX5), which plays a pivotal role in reprogramming cells into stem cells and promoting their conversion into shoot meristems through WUSCHEL (WUS). Additionally, cell and tissue mechanics, including cell wall modifications and mechanical stress, critically contribute to de novo shoot organogenesis by regulating polar auxin transport. Furthermore, light signaling emerges as a key regulator of plant regeneration, directly influencing expression of meristem genes and potentially influencing aforementioned pathways as well.

ERF1 inhibits lateral root emergence by promoting local auxin accumulation and repressing ARF7 expression

Lateral roots (LRs) are crucial for plants to sense environmental signals in addition to water and nutrient absorption. Auxin is key for LR formation, but the underlying mechanisms are not fully understood. Here, we report that Arabidopsis ERF1 inhibits LR emergence by promoting local auxin accumulation with altered distribution and regulating auxin signaling. Loss of ERF1 increases LR density compared with the wild type, whereas ERF1 overexpression causes the opposite phenotype. ERF1 enhances auxin transport by upregulating PIN1 and AUX1, resulting in excessive auxin accumulation in the endodermal, cortical, and epidermal cells surrounding LR primordia. Furthermore, ERF1 represses ARF7 transcription, thereby downregulating the expression of cell-wall remodeling genes that facilitate LR emergence. Together, our study reveals that ERF1 integrates environmental signals to promote local auxin accumulation with altered distribution and repress ARF7, consequently inhibiting LR emergence in adaptation to fluctuating environments.

Lateral root branching: evolutionary innovations and mechanistic divergence in land plants

The root system architecture in plants is a result of multiple evolutionary innovations over time in response to changing environmental cues. Dichotomy and endogenous lateral branching in the roots evolved in lycophytes lineage but extant seed plants use lateral branching instead. This has led to the development of complex and adaptive root systems, with lateral roots playing a key role in this process exhibiting conserved and divergent features in different plant species. The study of lateral root branching in diverse plant species can shed light on the orderly yet distinct nature of postembryonic organogenesis in plants. This insight provides an overview of the diversity in lateral root (LR) development in various plant species during the evolution of root system in plants.

Memory or acclimation of water stress in pea rely on root system's plasticity and plant's ionome modulation

INTRODUCTION

Peas, as legume crops, could play a major role in the future of food security in the context of worldwide human nutrient deficiencies coupled with the growing need to reduce consumption of animal products. However, pea yields, in terms of quantity and quality (i.e. grain content), are both susceptible to climate change, and more specifically to water deficits, which nowadays occur more frequently during crop growth cycles and tend to last longer. The impact of soil water stress on plant development and plant growth is complex, as its impact varies depending on soil water availability (through the modulation of elements available in the soil), and by the plant's ability to acclimate to continuous stress or to memorize previous stress events.

METHOD

To identify the strategies underlying these plant responses to water stress events, pea plants were grown in controlled conditions under optimal water treatment and different types of water stress; transient (during vegetative or reproductive periods), recurrent, and continuous (throughout the plant growth cycle). Traits related to water, carbon, and ionome uptake and uses were measured and allowed the identification typical plant strategies to cope with water stress.

CONCLUSION

Our results highlighted (i) the common responses to the three types of water stress in shoots, involving manganese (Mn) in particular, (ii) the potential implications of boron (B) for root architecture modification under continuous stress, and (iii) the establishment of an "ecophysiological imprint" in the root system via an increase in nodule numbers during the recovery period.

Molecular hydrogen positively influences lateral root formation by regulating hydrogen peroxide signaling

Although a previous study discovered that exogenous molecular hydrogen (H₂) supplied with hydrogen-rich water (HRW) can mediate lateral root (LR) development, whether or how endogenous H₂ influences LR formation is still elusive. In this report, mimicking the induction responses in tomato seedlings achieved by HRW or exogenous hydrogen peroxide (H₂O₂; a positive control), transgenic Arabidopsis that overexpressed the hydrogenase1 gene (CrHYD1) from Chlamydomonas reinhardtii not only stimulated endogenous hydrogen peroxide (H₂O₂) production, but also markedly promoted LR formation. Above H₂ and H₂O₂ responses were abolished by the removal of endogenous H₂O₂. Moreover, the changes in transcriptional patterns of representative cell cycle genes and auxin signaling-related genes during LR development in both tomato and transgenic Arabidopsis thaliana matched with above phenotypes. The alternations in the levels of GUS transcripts driven by the CYCB1 promoter and expression of PIN1 protein further indicated that H₂O₂ synthesis was tightly linked to LR formation achieved by endogenous H₂, and cell cycle regulation and auxin-dependent pathway might be their targets. There results might provide a reference for molecular mechanism underlying the regulation of root morphogenesis by H₂.

Transcriptome Dynamics of Rooting Zone and Leaves during In Vitro Adventitious Root Formation in Eucalyptus nitens

Wood properties and agronomic traits associated with fast growth and frost tolerance make Eucalyptus nitens a valuable forest alternative. However, the rapid age-related decline in the adventitious root (AR) formation (herein, meaning induction, initiation, and expression stages) limits its propagation. We analyzed transcriptomic profile variation in leaves and stem bases during AR induction of microcuttings to elucidate the molecular mechanisms involved in AR formation. In addition, we quantified expressions of candidate genes associated with recalcitrance. We delimited the ontogenic phases of root formation using histological techniques and Scarecrow and Short-Root expression quantification for RNA sequencing sample collection. We quantified the gene expressions associated with root meristem formation, auxin biosynthesis, perception, signaling, conjugation, and cytokinin signaling in shoots harvested from 2- to 36-month-old plants. After IBA treatment, 702 transcripts changed their expressions. Several were involved in hormone homeostasis and the signaling pathways that determine cell dedifferentiation, leading to root meristem formation. In part, the age-related decline in the rooting capacity is attributable to the increase in the ARR1 gene expression, which negatively affects auxin homeostasis. The analysis of the transcriptomic variation in the leaves and rooting zones provided profuse information: (1) To elucidate the auxin metabolism; (2) to understand the hormonal and signaling processes involved; (3) to collect data associated with their recalcitrance.

The Arabidopsis TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE (TTL) family members are involved in root system formation via their interaction with cytoskeleton and cell wall remodeling

Lateral roots (LR) are essential components of the plant edaphic interface; contributing to water and nutrient uptake, biotic and abiotic interactions, stress survival, and plant anchorage. We have identified the TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE 3 (TTL3) gene as being related to LR emergence and later development. Loss of function of TTL3 leads to a reduced number of emerged LR due to delayed development of lateral root primordia (LRP). This trait is further enhanced in the triple mutant ttl1ttl3ttl4. TTL3 interacts with microtubules and endomembranes, and is known to participate in the brassinosteroid (BR) signaling pathway. Both ttl3 and ttl1ttl3ttl4 mutants are less sensitive to BR treatment in terms of LR formation and primary root growth. The ability of TTL3 to modulate biophysical properties of the cell wall was established under restrictive conditions of hyperosmotic stress and loss of root growth recovery, which was enhanced in ttl1ttl3ttl4. Timing and spatial distribution of TTL3 expression is consistent with its role in development of LRP before their emergence and subsequent growth of LR. TTL3 emerged as a component of the root system morphogenesis regulatory network.

Ectopic expression of GmNF-YA8 in Arabidopsis delays flowering via modulating the expression of gibberellic acid biosynthesis- and flowering-related genes and promotes lateral root emergence in low phosphorus conditions

NUCLEAR FACTOR Y subunit alpha (NF-YA), together with NF-YB and NF-YC, regulates plant growth and development, as well as plant responses to biotic and abiotic stresses. Although extensive studies have examined the functions of NF-YAs in Arabidopsis thaliana, the roles of NF- YAs in Glycinme max are poorly understood. In this study, we identified a phosphorus (P) starvation-responsive NF-YA8 in soybean. The expression of GmNF-YA8 is induced by low P or low nitrogen in leaves, but not by potassium or iron starvation, respectively. GmNF-YA8 is localized in the nucleus and plasma membrane. Ectopic expression of GmNF-YA8 inhibits plant growth and delayed flowering in Arabidopsis. Exogenous application of gibberellic acid (GA) rescues the delayed flowering phenotype in Arabidopsis overexpressing GmNF-YA8 lines GmNF-YA8OE-05 and GmNF-YA8OE-20. Moreover, quantitative real time PCR (qRT-PCR) verified that overexpression of GmNF-YA8 downregulates GA20ox2 and GA3ox2 expression, but upregulates GA2ox2 and GA2ox3 that encode enzymes, which inactive bioactive GAs. Consistent with the late flowering phenotype of Arabidopsis trangenic lines that overexpress GmNF-YA8, the transcript levels of flowering-promoting genes AP1, CO, LFY, and SOC1 are reduced. In addition, overexpression of GmNF-YA8 promotes the emergence of lateral root (LR) primordium from epidermis rather than the initiation of LR in low P, and increases the LR density in low nitrogen. Our results provide insights into the roles of GmNF-YA8.

BYPASS1-LIKE regulates lateral root initiation via exocytic vesicular trafficking-mediated PIN recycling in Arabidopsis

Auxin and auxin-mediated signaling pathways are known to regulate lateral root development. Although exocytic vesicle trafficking plays an important role in recycling the PIN-FORMED (PIN) auxin efflux carriers and in polar auxin transport during lateral root formation, the mechanistic details of these processes are not well understood. Here, we demonstrate that BYPASS1-LIKE (B1L) regulates lateral root initiation via exocytic vesicular trafficking-mediated PIN recycling in Arabidopsis thaliana. b1l mutants contained significantly more lateral roots than the wild type, primarily due to increased lateral root primordium initiation. Furthermore, the auxin signal was stronger in stage I lateral root primordia of b1l than in those of the wild type. Treatment with exogenous auxin and an auxin transport inhibitor indicated that the lateral root phenotype of b1l could be attributed to higher auxin levels and that B1L regulates auxin efflux. Indeed, compared to the wild type, C-terminally green fluorescent protein-tagged PIN1 and PIN3 accumulated at higher levels in b1l lateral root primordia. B1L interacted with the exocyst, and b1l showed defective PIN exocytosis. These observations indicate that B1L interacts with the exocyst to regulate PIN-mediated polar auxin transport and lateral root initiation in Arabidopsis.

Toward synthetic plant development

The ability to engineer plant form will enable the production of novel agricultural products designed to tolerate extreme stresses, boost yield, reduce waste, and improve manufacturing practices. While historically, plants were altered through breeding to change their size or shape, advances in our understanding of plant development and our ability to genetically engineer complex eukaryotes are leading to the direct engineering of plant structure. In this review, I highlight the central role of auxin in plant development and the synthetic biology approaches that could be used to turn auxin-response regulators into powerful tools for modifying plant form. I hypothesize that recoded, gain-of-function auxin response proteins combined with synthetic regulation could be used to override endogenous auxin signaling and control plant structure. I also argue that auxin-response regulators are key to engineering development in nonmodel plants and that single-cell -omics techniques will be essential for characterizing and modifying auxin response in these plants. Collectively, advances in synthetic biology, single-cell -omics, and our understanding of the molecular mechanisms underpinning development have set the stage for a new era in the engineering of plant structure.

Nutrient-hormone relations: Driving root plasticity in plants

Optimal plant development requires root uptake of 14 essential mineral elements from the soil. Since the bioavailability of these nutrients underlies large variation in space and time, plants must dynamically adjust their root architecture to optimize nutrient access and acquisition. The information on external nutrient availability and whole-plant demand is translated into cellular signals that often involve phytohormones as intermediates to trigger a systemic or locally restricted developmental response. Timing and extent of such local root responses depend on the overall nutritional status of the plant that is transmitted from shoots to roots in the form of phytohormones or other systemic long-distance signals. The integration of these systemic and local signals then determines cell division or elongation rates in primary and lateral roots, the initiation, emergence, or elongation of lateral roots, as well as the formation of root hairs. Here, we review the cascades of nutrient-related sensing and signaling events that involve hormones and highlight nutrient-hormone relations that coordinate root developmental plasticity in plants.

Oxygen and reactive oxygen species-dependent regulation of plant growth and development

Oxygen and reactive oxygen species (ROS) have been co-opted during evolution into the regulation of plant growth, development, and differentiation. ROS and oxidative signals arising from metabolism or phytohormone-mediated processes control almost every aspect of plant development from seed and bud dormancy, liberation of meristematic cells from the quiescent state, root and shoot growth, and architecture, to flowering and seed production. Moreover, the phytochrome and phytohormone-dependent transmissions of ROS waves are central to the systemic whole plant signaling pathways that integrate root and shoot growth. The sensing of oxygen availability through the PROTEOLYSIS 6 (PRT6) N-degron pathway functions alongside ROS production and signaling but how these pathways interact in developing organs remains poorly understood. Considerable progress has been made in our understanding of the nature of hydrogen peroxide sensors and the role of thiol-dependent signaling networks in the transmission of ROS signals. Reduction/oxidation (redox) changes in the glutathione (GSH) pool, glutaredoxins (GRXs), and thioredoxins (TRXs) are important in the control of growth mediated by phytohormone pathways. Although, it is clear that the redox states of proteins involved in plant growth and development are controlled by the NAD(P)H thioredoxin reductase (NTR)/TRX and reduced GSH/GRX systems of the cytosol, chloroplasts, mitochondria, and nucleus, we have only scratched the surface of this multilayered control and how redox-regulated processes interact with other cell signaling systems.

An undiscovered facet of hydraulic redistribution driven by evaporation-a study from a Populus tomentosa plantation

Maintaining the activity and function of the shallow root system of plants is essential for withstanding drought stress, but the associated mechanism is poorly understood. By investigating sap flow in 14 lateral roots (LRs) randomly selected from trees of a Chinese white poplar (Populus tomentosa) plantation receiving three levels of irrigation, an unknown root water transport mode of simultaneous daytime bi-directional water flow was discovered. This mode existed in five LRs confined to the surface soil without attached sinker roots. In the longer term, the bi-directional water flow was correlated with the soil water content. However, within the day, it was associated with transpiration. Our data demonstrated that bi-directional root sap flow occurred during the day, and was driven by evaporative demand, further suggesting the existence of circumferential water movement in the LR xylem. We named this phenomenon evaporation-driven hydraulic redistribution (EDHR). A soil-root water transport model was proposed to encapsulate this water movement mode. EDHR may be a crucial drought-tolerance mechanism that allows plants to maintain shallow root survival and activity by promoting root water recharge under extremely dry conditions.

FASCICLIN-LIKE 18 Is a New Player Regulating Root Elongation in Arabidopsis thaliana

The plasticity of root development represents a key trait that enables plants to adapt to diverse environmental cues. The pattern of cell wall deposition, alongside other parameters, affects the extent, and direction of root growth. In this study, we report that FASCICLIN-LIKE ARABINOGALACTAN PROTEIN 18 (FLA18) plays a role during root elongation in Arabidopsis thaliana. Using root-specific co-expression analysis, we identified FLA18 to be co-expressed with a sub-set of genes required for root elongation. FLA18 encodes for a putative extra-cellular arabinogalactan protein from the FLA-gene family. Two independent T-DNA insertion lines, named fla18-1 and fla18-2, display short and swollen lateral roots (LRs) when grown on sensitizing condition of high-sucrose containing medium. Unlike fla4/salt overly sensitive 5 (sos5), previously shown to display short and swollen primary root (PR) and LRs under these conditions, the PR of the fla18 mutants is slightly longer compared to the wild-type. Overexpression of the FLA18 CDS complemented the fla18 root phenotype. Genetic interaction between either of the fla18 alleles and sos5 reveals a more severe perturbation of anisotropic growth in both PR and LRs, as compared to the single mutants and the wild-type under restrictive conditions of high sucrose or high-salt containing medium. Additionally, under salt-stress conditions, fla18sos5 had a small, chlorotic shoot phenotype, that was not observed in any of the single mutants or the wild type. As previously shown for sos5, the fla18-1 and fla18-1sos5 root-elongation phenotype is suppressed by abscisic acid (ABA) and display hypersensitivity to the ABA synthesis inhibitor, Fluridon. Last, similar to other cell wall mutants, fla18 root elongation is hypersensitive to the cellulose synthase inhibitor, Isoxaben. Altogether, the presented data assign a new role for FLA18 in the regulation of root elongation. Future studies of the unique vs. redundant roles of FLA proteins during root elongation is anticipated to shed a new light on the regulation of root architecture during plant adaptation to different growth conditions.

Expression Levels of Genes Encoding Proteins Involved in the Cell Wall-Plasma Membrane-Cytoskeleton Continuum Are Associated With the Maturation-Related Adventitious Rooting Competence of Pine Stem Cuttings

Stem cutting recalcitrance to adventitious root formation is a major limitation for the clonal propagation or micropropagation of elite genotypes of many forest tree species, especially at the adult stage of development. The interaction between the cell wall-plasma membrane and cytoskeleton may be involved in the maturation-related decline of adventitious root formation. Here, pine homologs of several genes encoding proteins involved in the cell wall-plasma membrane-cytoskeleton continuum were identified, and the expression levels of 70 selected genes belonging to the aforementioned group and four genes encoding auxin carrier proteins were analyzed during adventitious root formation in rooting-competent and non-competent cuttings of Pinus radiata. Variations in the expression levels of specific genes encoding cell wall components and cytoskeleton-related proteins were detected in rooting-competent and non-competent cuttings in response to wounding and auxin treatments. However, the major correlation of gene expression with competence for adventitious root formation was detected in a family of genes encoding proteins involved in sensing the cell wall and membrane disturbances, such as specific receptor-like kinases (RLKs) belonging to the lectin-type RLKs, wall-associated kinases, Catharanthus roseus RLK1-like kinases and leucine-rich repeat RLKs, as well as downstream regulators of the small guanosine triphosphate (GTP)-binding protein family. The expression of these genes was more affected by organ and age than by auxin and time of induction.

Arabidopsis AZG2 transports cytokinins in vivo and regulates lateral root emergence

Cytokinin and auxin are key regulators of plant growth and development. During the last decade transport mechanisms have turned out to be the key for the control of local and long-distance hormone distributions. In contrast with auxin, cytokinin transport is poorly understood. Here, we show that Arabidopsis thaliana AZG2, a member of the AZG purine transporter family, acts as cytokinin transporter involved in root system architecture determination. Even though purines are substrates for both AZG1 and AZG2, we found distinct transport mechanisms. The expression of AZG2 is restricted to a small group of cells surrounding the lateral root (LR) primordia and induced by auxins. Compared to the wild-type (WT), mutants carrying loss-of-function alleles of AZG2 have higher LR density, suggesting that AZG2 is part of a regulatory pathway in LR emergence. Moreover, azg2 is partially insensitive to exogenous cytokinin, which is consistent with the observation that the cytokinin reporter TCSn_pro :GFP showed lower fluorescence signal in the roots of azg2 compared to the WT. These results indicate a defective cytokinin signalling pathway in the region of LR primordia. The integration of AZG2 subcellular localization and cytokinin transport capacity data allowed us to propose a local cytokinin : auxin signalling model for the regulation of LR emergence.

Recent Advances in Adventitious Root Formation in Chestnut

The genus Castanea includes several tree species that are relevant because of their geographical extension and their multipurpose character, that includes nut and timber production. However, commercial exploitation of the trees is hindered by several factors, particularly by their limited regeneration ability. Regardless of recent advances, there exists a serious limitation for the propagation of elite genotypes of chestnut due to decline of rooting ability as the tree ages. In the present review, we summarize the research developed in this genus during the last three decades concerning the formation of adventitious roots (ARs). Focusing on cuttings and in vitro microshoots, we gather the information available on several species, particularly C. sativa, C. dentata and the hybrid C.sativa × C. crenata, and analyze the influence of several factors on the achievements of the applied protocols, including genotype, auxin treatment, light regime and rooting media. We also pay attention to the acclimation phase, as well as compile the information available about biochemical and molecular related aspects. Furthermore, we considerate promising biotechnological approaches that might enable the improvement of the current protocols.

Short De-Etiolation Increases the Rooting of VC801 Avocado Rootstock

Dark-grown (etiolated) branches of many recalcitrant plant species root better than their green counterparts. Here it was hypothesized that changes in cell-wall properties and hormones occurring during etiolation contribute to rooting efficiency. Measurements of chlorophyll, carbohydrate and auxin contents, as well as tissue compression, histological analysis and gene-expression profiles were determined in etiolated and de-etiolated branches of the avocado rootstock VC801. Differences in chlorophyll content and tissue rigidity, and changes in xyloglucan and pectin in cambium and parenchyma cells were found. Interestingly, lignin and sugar contents were similar, suggesting that de-etiolated branches resemble the etiolated ones in this respect. Surprisingly, the branches that underwent short de-etiolation rooted better than the etiolated ones, and only a slight difference in IAA content between the two was observed. Gene-expression profiles revealed an increase in ethylene-responsive transcripts in the etiolated branches, which correlated with enrichment in xyloglucan hydrolases. In contrast, transcripts encoding pectin methylesterase and pectolyases were enriched in the de-etiolated branches. Taken together, it seems that the short de-etiolation period led to fine tuning of the conditions favoring adventitious root formation in terms of auxin-ethylene balance and cell-wall properties.

A Kinase-Phosphatase-Transcription Factor Module Regulates Adventitious Root Emergence in Arabidopsis Root-Hypocotyl Junctions

Adventitious roots form from non-root tissues as part of normal development or in response to stress or wounding. The root primordia form in the source tissue, and during emergence the adventitious roots penetrate the inner cell layers and the epidermis; however, the mechanisms underlying this emergence remain largely unexplored. Here, we report that a regulatory module composed of the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), the MAP kinases MPK3 and MPK6, and the phosphatase PP2C12 plays an important role in the emergence of junction adventitious roots (J-ARs) from the root-hypocotyl junctions in Arabidopsis thaliana. ABI4 negatively regulates J-AR emergence, preventing the accumulation of reactive oxygen species and death of epidermal cells, which would otherwise facilitate J-AR emergence. Phosphorylation by MPK3/MPK6 activates ABI4 and dephosphorylation by PP2C12 inactivates ABI4. MPK3/MPK6 also directly phosphorylate and inactivate PP2C12 during J-AR emergence. We propose that this "double-check" mechanism increases the robustness of MAP kinase signaling and finely regulates the local programmed cell death required for J-AR emergence.

Two receptor-like protein kinases, MUSTACHES and MUSTACHES-LIKE, regulate lateral root development in Arabidopsis thaliana

Receptor-like protein kinases (RLKs) play key roles in regulating plant growth, development and stress adaptations. There are at least 610 RLKs (including receptor-like cytoplasmic kinases) in Arabidopsis. The functions of the majority of RLKs have not yet been determined. We previously generated promoter::GUS transgenic plants for all leucine-rich repeat (LRR)-RLKs in Arabidopsis and analyzed their expression patterns during various developmental stages. We found the expression of two LRR-RLKs, MUSTACHES (MUS) and MUSTACHES-LIKE (MUL), are overlapped in lateral root primordia. Independent mutants, mus-3 mul-1 and mus-4 mul-2, show a significantly decreased emerged lateral root phenotype. Our analyses indicate that the defects of the double mutant occur mainly at stage I of lateral root development. Exogenous application of auxin can dramatically enhance the transcription of MUS, which is largely dependent on AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19. MUS and MUL are inactive kinases in vitro but are phosphorylated in planta, possibly by an unknown kinase. The kinase activity of MUS is dispensable for its function in lateral root development. Many cell wall related genes are down regulated in mus-3 mul-1. In conclusion, we identified MUS and MUL, two kinase-inactive RLKs, in controlling the early development of lateral root primordia likely via regulating cell wall synthesis and remodeling.

Immunoprofiling of Cell Wall Carbohydrate Modifications During Flooding-Induced Aerenchyma Formation in Fabaceae Roots

Understanding plant adaptation mechanisms to prolonged water immersion provides options for genetic modification of existing crops to create cultivars more tolerant of periodic flooding. An important advancement in understanding flooding adaptation would be to elucidate mechanisms, such as aerenchyma air-space formation induced by hypoxic conditions, consistent with prolonged immersion. Lysigenous aerenchyma formation occurs through programmed cell death (PCD), which may entail the chemical modification of polysaccharides in root tissue cell walls. We investigated if a relationship exists between modification of pectic polysaccharides through de-methyl esterification (DME) and the formation of root aerenchyma in select Fabaceae species. To test this hypothesis, we first characterized the progression of aerenchyma formation within the vascular stele of three different legumes-Pisum sativum, Cicer arietinum, and Phaseolus coccineus-through traditional light microscopy histological staining and scanning electron microscopy. We assessed alterations in stele morphology, cavity dimensions, and cell wall chemistry. Then we conducted an immunolabeling protocol to detect specific degrees of DME among species during a 48-hour flooding time series. Additionally, we performed an enzymatic pretreatment to remove select cell wall polymers prior to immunolabeling for DME pectins. We were able to determine that all species possessed similar aerenchyma formation mechanisms that begin with degradation of root vascular stele metaxylem cells. Immunolabeling results demonstrated DME occurs prior to aerenchyma formation and prepares vascular tissues for the beginning of cavity formation in flooded roots. Furthermore, enzymatic pretreatment demonstrated that removal of cellulose and select hemicellulosic carbohydrates unmasks additional antigen binding sites for DME pectin antibodies. These results suggest that additional carbohydrate modification may be required to permit DME and subsequent enzyme activity to form aerenchyma. By providing a greater understanding of cell wall pectin remodeling among legume species, we encourage further investigation into the mechanism of carbohydrate modifications during aerenchyma formation and possible avenues for flood-tolerance improvement of legume crops.

Calcium and plasma membrane force-gated ion channels behind development

During development, tissues are submitted to high variation of compression and tension forces. The roles of the cell wall, the cytoskeleton, the turgor pressure and the cell geometry during this process have received due attention. In contrast, apart from its role in the establishment of turgor pressure, the involvement of the plasma membrane as a transducer of mechanical forces during development has been under studied. Force-gated (FG) or Mechanosensitive (MS) ion channels embedded in the bilayer represent 'per se' archetypal mechanosensor able to directly and instantaneously transduce membrane forces into electrical and calcium signals. We discuss here how their fine-tuning, combined with their ability to detect micro-curvature and local membrane tension, allows FG channels to transduce mechanical cues into developmental signals.

Cell Death in Cells Overlying Lateral Root Primordia Facilitates Organ Growth in Arabidopsis

Plant organ growth is widely accepted to be determined by cell division and cell expansion, but, unlike that in animals, the contribution of cell elimination has rarely been recognized. We investigated this paradigm during Arabidopsis lateral root formation, when the lateral root primordia (LRP) must traverse three overlying cell layers within the parent root. A subset of LRP-overlying cells displayed the induction of marker genes for cell types undergoing developmental cell death, and their cell death was detected by electron, confocal, and light sheet microscopy techniques. LRP growth was delayed in cell-death-deficient mutants lacking the positive cell death regulator ORESARA1/ANAC092 (ORE1). LRP growth was restored in ore1-2 knockout plants by genetically inducing cell elimination in cells overlying the LRP or by physically killing LRP-overlying cells by ablation with optical tweezers. Our results support that, in addition to previously discovered mechanisms, cell elimination contributes to regulating lateral root emergence.

Auxin abolishes SHI-RELATED SEQUENCE5-mediated inhibition of lateral root development in Arabidopsis

Lateral roots (LRs), which form in the plant postembryonically, determine the architecture of the root system. While negative regulatory factors that inhibit LR formation and are counteracted by auxin exist in the pericycle, these factors have not been characterised. Here, we report that SHI-RELATED SEQUENCE5 (SRS5) is an intrinsic negative regulator of LR formation and that auxin signalling abolishes this inhibitory effect of SRS5. Whereas LR primordia (LRPs) and LRs were fewer and less dense in SRS5ox and Pro35S:SRS5-GFP plants than in the wild-type, they were more abundant and denser in the srs5-2 loss-of-function mutant. SRS5 inhibited LR formation by directly downregulating the expression of LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16) and LBD29. Auxin repressed SRS5 expression. Auxin-mediated repression of SRS5 expression was not observed in the arf7-1 arf19-1 double mutant, likely because ARF7 and ARF19 bind to the promoter of SRS5 and inhibit its expression in response to auxin. Taken together, our data reveal that SRS5 negatively regulates LR formation by repressing the expression of LBD16 and LBD29 and that auxin releases this inhibitory effect through ARF7 and ARF19.

Arabidopsis Lectin EULS3 Is Involved in ABA Signaling in Roots

The Arabidopsis thaliana lectin ArathEULS3 is upregulated in particular stress conditions and upon abscisic acid (ABA) treatment. ABA is a plant hormone important for plant growth and stress responses. During stress ABA is perceived by PYR/PYL/RCAR receptors, inhibiting protein phosphatases PP2Cs thereby enabling SNRK2s kinases to start downstream phosphorylation cascades and signaling. PYL9, one of the ABA receptors was identified as an interacting partner for ArathEULS3. Promoter::GUS activity studies revealed the expression of ArathEULS3 in the central root cylinder and the cells flanking young lateral root primordia, and showed enhanced expression in root tips after ABA treatment. Transcript levels for ArathEULS3 increased after exposure to ABA and osmotic treatments. ArathEULS3 CRISPR KO mutants served as a tool to expand the knowledge on the role of ArathEULS3 in plant development. KO lines revealed a longer root system compared to WT plants, and showed reduced sensitivity to ABA, salt, and osmotic conditions. Additionally it was noted that the KO mutants had more emerged lateral roots when grown in high osmotic conditions. Together these data suggest that ArathEULS3 may be an important player in ABA responses in roots.

Modulatory Role of Reactive Oxygen Species in Root Development in Model Plant of Arabidopsis thaliana

Reactive oxygen species (ROS), a type of oxygen monoelectronic reduction product, have a higher chemical activity than O₂. Although ROS pose potential risks to all organisms via inducing oxidative stress, indispensable role of ROS in individual development cannot be ignored. Among them, the role of ROS in the model plant Arabidopsis thaliana is deeply studied. Mounting evidence suggests that ROS are essential for root and root hair development. In the present review, we provide an updated perspective on the latest research progress pertaining to the role of ROS in the precise regulation of root stem cell maintenance and differentiation, redox regulation of the cell cycle, and root hair initiation during root growth. Among the different types of ROS, O₂ ^•- and H₂O₂ have been extensively investigated, and they exhibit different gradient distributions in the roots. The concentration of O₂ ^•- decreases along a gradient from the meristem to the transition zone and the concentration of H₂O₂ decreases along a gradient from the differentiation zone to the elongation zone. These gradients are regulated by peroxidases, which are modulated by the UPBEAT1 (UPB1) transcription factor. In addition, multiple transcriptional factors, such as APP1, ABO8, PHB3, and RITF1, which are involved in the brassinolide signaling pathway, converge as a ROS signal to regulate root stem cell maintenance. Furthermore, superoxide anions (O₂ ^•-) are generated from the oxidation in mitochondria, ROS produced during plasmid metabolism, H₂O₂ produced in apoplasts, and catalysis of respiratory burst oxidase homolog (RBOH) in the cell membrane. Furthermore, ROS can act as a signal to regulate redox status, which regulates the expression of the cell-cycle components CYC2;3, CYCB1;1, and retinoblastoma-related protein, thereby controlling the cell-cycle progression. In the root maturation zone, the epidermal cells located in the H cell position emerge to form hair cells, and plant hormones, such as auxin and ethylene regulate root hair formation via ROS. Furthermore, ROS accumulation can influence hormone signal transduction and vice versa. Data about the association between nutrient stress and ROS signals in root hair development are scarce. However, the fact that ROBHC/RHD2 or RHD6 is specifically expressed in root hair cells and induced by nutrients, may explain the relationship. Future studies should focus on the regulatory mechanisms underlying root hair development via the interactions of ROS with hormone signals and nutrient components.

Anchorene is a carotenoid-derived regulatory metabolite required for anchor root formation in Arabidopsis

Anchor roots (ANRs) arise at the root-shoot junction and are the least investigated type of Arabidopsis root. Here, we show that ANRs originate from pericycle cells in an auxin-dependent manner and a carotenogenic signal to emerge. By screening known and assumed carotenoid derivatives, we identified anchorene, a presumed carotenoid-derived dialdehyde (diapocarotenoid), as the specific signal needed for ANR formation. We demonstrate that anchorene is an Arabidopsis metabolite and that its exogenous application rescues the ANR phenotype in carotenoid-deficient plants and promotes the growth of normal seedlings. Nitrogen deficiency resulted in enhanced anchorene content and an increased number of ANRs, suggesting a role of this nutrient in determining anchorene content and ANR formation. Transcriptome analysis and treatment of auxin reporter lines indicate that anchorene triggers ANR formation by modulating auxin homeostasis. Together, our work reveals a growth regulator with potential application to agriculture and a new carotenoid-derived signaling molecule.

Auxin regulates adventitious root formation in tomato cuttings

BACKGROUND

Adventitious root (AR) formation is a critical developmental process in cutting propagation for the horticultural industry. While auxin has been shown to regulate this process, the exact mechanism and details preceding AR formation remain unclear. Even though AR and lateral root (LR) formation share common developmental processes, there are exist some differences that need to be closely examined at the cytological level. Tomato stem cuttings, which readily form adventitious roots, represent the perfect system to study the influence of auxin on AR formation and to compare AR and LR organogenesis.

RESULTS

Here we show the progression by which AR form from founder cells in the basal pericycle cell layers in tomato stem cuttings. The first disordered clumps of cells assumed a dome shape that later differentiated into functional AR cell layers. Further growth resulted in emergence of mature AR through the epidermis following programmed cell death of epidermal cells. Auxin and ethylene levels increased in the basal stem cutting within 1 h. Tomato lines expressing the auxin response element DR5pro:YFP showed an increase in auxin distribution during the AR initiation phase, and was mainly concentrated in the meristematic cells of the developing AR. Treatment of stem cuttings with auxin, increased the number of AR primordia and the length of AR, while stem cuttings treated with the pre-emergent herbicide/auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) occasionally developed thick, agravitropic AR. Hormone profile analyses showed that auxin positively regulated AR formation, whereas perturbations to zeatin, salicylic acid, and abscisic acid homeostasis suggested minor roles during tomato stem rooting. The gene expression of specific auxin transporters increased during specific developmental phases of AR formation.

CONCLUSION

These data show that AR formation in tomato stems is a complex process. Upon perception of a wounding stimulus, expression of auxin transporter genes and accumulation of auxin at founder cell initiation sites in pericycle cell layers and later in the meristematic cells of the AR primordia were observed. A clear understanding and documentation of these events in tomato is critical to resolve AR formation in recalcitrant species like hardwoods and improve stem cutting propagation efficiency and effectiveness.

Nitrate-responsive OBP4-XTH9 regulatory module controls lateral root development in Arabidopsis thaliana

Plant root system architecture in response to nitrate availability represents a notable example to study developmental plasticity, but the underlying mechanism remains largely unknown. Xyloglucan endotransglucosylases (XTHs) play a critical role in cell wall biosynthesis. Here we assessed the gene expression of XTH1-11 belonging to group I of XTHs in lateral root (LR) primordia and found that XTH9 was highly expressed. Correspondingly, an xth9 mutant displayed less LR, while overexpressing XTH9 presented more LR, suggesting the potential function of XTH9 in controlling LR development. XTH9 gene mutation obviously alters the properties of the cell wall. Furthermore, nitrogen signals stimulated the expression of XTH9 to promote LRs. Genetic analysis revealed that the function of XTH9 was dependent on auxin-mediated ARF7/19 and downstream AFB3 in response to nitrogen signals. In addition, we identified another transcription factor, OBP4, that was also induced by nitrogen treatment, but the induction was much slower than that of XTH9. In contrast to XTH9, overexpressing OBP4 caused fewer LRs while OBP4 knockdown with OBP4-RNAi or an artificial miRNA silenced amiOBP4 line produced more LR. We further found OBP4 bound to the promoter of XTH9 to suppress XTH9 expression. In agreement with this, both OBP4-RNAi and crossed OBP4-RNAi & 35S::XTH9 lines led to more LR, but OBP4-RNAi & xth9 produced less LR, similar to xth9. Based on these findings we propose a novel mechanism by which OBP4 antagonistically controls XTH9 expression and the OBP4-XTH9 module elaborately sustains LR development in response to nitrate treatment.

Nitrate is not only a nutrient, but also a signal that controls downstream signaling genes at the whole-plant level. In plants, changes in root system architecture in response to nitrate availability represent a notable example of developmental plasticity in response to environmental stimuli. However, the molecular mechanisms underlying nitrate-associated modulation are largely unknown. Here, we identified a nitrogen-responsive signaling module that comprises both xyloglucan endotransglucosylase 9 (XTH9) and the Dof transcription factor OBP4 and controls lateral root (LR) development. We used root gravitropic bending assays to observe the gene expression of group 1 xyloglucan endotransglucosylases (XTHs) involved in LR primordia. The results showed that XTH9 expression patterns were changed and that xth9 knockout mutants displayed altered LR growth. XTH9 was expressed in the LRs and in response to nitrate treatment, and the xth9 mutants were defective in nitrate-promoted LR growth. Moreover, XTH9 overexpression increased LR length and increased tolerance to low-nitrate stress. We found that OBP4 could negatively regulate XTH9 and inhibited root growth. OBP4 and XTH9 worked downstream of ARF7/9. We conclude that OBP4 and XTH9 constitute a regulatory module which contributes to LR growth in response to different environmental nitrate concentration signals.

More than cell wall hydrolysis: orchestration of cellular dynamics for organ separation

Plants' ability to cope with the ever-changing environment is one of the hallmarks that distinguishes plants from animals. Plants stationed in one place have evolved to remodel their architecture in response to the environmental factors by continuously creating new organ systems and removing existing organs through abscission. Herein, I provide insights into developmental plasticity of plants, focusing on the exit strategy (abscission). When plants start developing organs, the elimination tactics are also established in the form of abscission zones (AZ), that is, specialized cell layers for organ separation. Herein, recent advances in understanding the spatial regulatory mechanism of AZ in terms of cellular dynamics, coordination, and reconfiguration of the physical barrier of the cell wall to achieve precise abscission are discussed.

Cytoskeleton Dynamics Are Necessary for Early Events of Lateral Root Initiation in Arabidopsis

How plant cells re-establish differential growth to initiate organs is poorly understood. Morphogenesis of lateral roots relies on the asymmetric cell division of initially symmetric founder cells. This division is preceded by the tightly controlled asymmetric radial expansion of these cells. The cellular mechanisms that license and ensure the coordination of these events are unknown. Here, we quantitatively analyze microtubule and F-actin dynamics during lateral root initiation. Using mutants and pharmacological and tissue-specific genetic perturbations, we show that dynamic reorganization of both microtubule and F-actin networks is necessary for the asymmetric expansion of the founder cells. This cytoskeleton remodeling intertwines with auxin signaling in the pericycle and endodermis in order for founder cells to acquire a basic polarity required for initiating lateral root development. Our results reveal the conservation of cell remodeling and polarization strategies between the Arabidopsis zygote and lateral root founder cells. We propose that coordinated, auxin-driven reorganization of the cytoskeleton licenses asymmetric cell growth and divisions during embryonic and post-embryonic organogenesis.

A MAPK cascade downstream of IDA-HAE/HSL2 ligand-receptor pair in lateral root emergence

Lateral root (LR) emergence is a highly coordinated process involving precise cell-cell communication. Here, we show that MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6, and their upstream MAP-kinase kinases (MAPKKs), MKK4 and MKK5, function downstream of HAESA (HAE)/HAESA-LIKE2 (HSL2) and their ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) during LR emergence. Loss of function of MKK4/MKK5 or MPK3/MPK6 results in restricted passage of the growing lateral root primordia (LRP) through the overlaying endodermal, cortical and epidermal cell layers, leading to reduced LR density. The MKK4/MKK5-MPK3/MPK6 module regulates the expression of cell wall remodelling genes in cells overlaying LRP and therefore controls pectin degradation in the middle lamella. Expression of constitutively active MKK4 or MKK5 driven by the HAE or HSL2 promoter fully rescues the LR emergence defect in the ida and hae hsl2 mutants. In addition, the MKK4/MKK5-MPK3/MPK6 module is indispensable in auxin-facilitated LR emergence. Our study provides insights into the auxin-governed and IDA-HAE/HLS2 ligand-receptor pair-mediated LR emergence process.

Selective auxin agonists induce specific AUX/IAA protein degradation to modulate plant development

Auxin phytohormones control most aspects of plant development through a complex and interconnected signaling network. In the presence of auxin, AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors are targeted for degradation by the SKP1-CULLIN1-F-BOX (SCF) ubiquitin-protein ligases containing TRANSPORT INHIBITOR RESISTANT 1/AUXIN SIGNALING F-BOX (TIR1/AFB). CULLIN1-neddylation is required for SCF^TIR1/AFB functionality, as exemplified by mutants deficient in the NEDD8-activating enzyme subunit AUXIN-RESISTANT 1 (AXR1). Here, we report a chemical biology screen that identifies small molecules requiring AXR1 to modulate plant development. We selected four molecules of interest, RubNeddin 1 to 4 (RN1 to -4), among which RN3 and RN4 trigger selective auxin responses at transcriptional, biochemical, and morphological levels. This selective activity is explained by their ability to consistently promote the interaction between TIR1 and a specific subset of AUX/IAA proteins, stimulating the degradation of particular AUX/IAA combinations. Finally, we performed a genetic screen using RN4, the RN with the greatest potential for dissecting auxin perception, which revealed that the chromatin remodeling ATPase BRAHMA is implicated in auxin-mediated apical hook development. These results demonstrate the power of selective auxin agonists to dissect auxin perception for plant developmental functions, as well as offering opportunities to discover new molecular players involved in auxin responses.

The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation

The root cap surrounding the tip of plant roots is thought to protect the delicate stem cells in the root meristem. We discovered that the first layer of root cap cells is covered by an electron-opaque cell wall modification resembling a plant cuticle. Cuticles are polyester-based protective structures considered exclusive to aerial plant organs. Mutations in cutin biosynthesis genes affect the composition and ultrastructure of this cuticular structure, confirming its cutin-like characteristics. Strikingly, targeted degradation of the root cap cuticle causes a hypersensitivity to abiotic stresses during seedling establishment. Furthermore, lateral root primordia also display a cuticle that, when defective, causes delayed outgrowth and organ deformations, suggesting that it facilitates lateral root emergence. Our results show that the previously unrecognized root cap cuticle protects the root meristem during the critical phase of seedling establishment and promotes the efficient formation of lateral roots.

Cellular dynamics during maturation-related decline of adventitious root formation in forest tree species

Adventitious root formation is a process in which roots are induced, from determined or differentiated cells that have not been specified to develop a root, at positions where they do not normally occur during development. In forest tree species, a decline in the capacity to form adventitious roots from similar cell types in stem cuttings is associated with tree age and maturity. This decline limits the success of vegetative propagation of selected adult trees. The joint action of local signals and a dynamic cascade of regulatory changes in gene expression, resulting in stereotypical cell division patterns, regulate cell fate changes that enable a somatic differentiated cell to reactivate meristem programs toward the induction of an adventitious root meristem.

Root mediated uptake of Salmonella is different from phyto-pathogen and associated with the colonization of edible organs

BACKGROUND

Pre-harvest contamination of fruits and vegetables by Salmonella in fields is one of the causes of food-borne outbreaks. Natural openings like stomata, hydathodes and fruit cracks are known to serve as entry points. While there are reports indicating that Salmonella colonize and enter root through lateral root emerging area, further investigations regarding how the accessibility of Salmonella to lateral root is different from phyto-pathogenic bacteria, the efficacy of lateral root to facilitate entry have remained unexplored. In this study we attempted to investigate the lateral root mediated entry of Salmonella, and to bridge this gap in knowledge.

RESULTS

Unlike phytopathogens, Salmonella cannot utilize cellulose as the sole carbon source. This negates the fact of active entry by degrading plant cellulose and pectin. Endophytic Salmonella colonization showed a high correlation with number of lateral roots. When given equal opportunity to colonize the plants with high or low lateral roots, Salmonella internalization was found higher in the plants with more lateral roots. However, the epiphytic colonization in both these plants remained unaltered. To understand the ecological significance, we induced lateral root production by increasing soil salinity which made the plants susceptible to Salmonella invasion and the plants showed higher Salmonella burden in the aerial organs.

CONCLUSION

Salmonella, being unable to degrade plant cell wall material relies heavily on natural openings. Therefore, its invasion is highly dependent on the number of lateral roots which provides an entry point because of the epidermis remodeling. Thus, when number of lateral root was enhanced by increasing the soil salinity, plants became susceptible to Salmonella invasion in roots and its transmission to aerial organs.

Getting physical: invasive growth events during plant development

Plant cells are enclosed in cell walls that weld them together, meaning that cells rarely change neighbours. Nonetheless, invasive growth events play critical roles in plant development and are often key hubs for the integration of environmental and/or developmental signalling. Here we review cellular processes involved in three such events: lateral root emergence, pollen tube growth through stigma and style tissues, and embryo expansion through the endosperm (Figures 1-3). We consider processes such as regulation of water fluxes and cell turgor (driving growth), cell wall modifications (e.g. cell separation) and cell death (for creating space) within these three contexts with the aim of identifying key mechanisms implicated in providing a chemical and biophysical environments permitting invasive growth events.

Quantitative 3D imaging of cell level auxin and cytokinin response ratios in soybean roots and nodules

Legume-Rhizobium symbiosis results in root nodules where rhizobia fix atmospheric nitrogen into plant usable forms in exchange for plant-derived carbohydrates. The development of these specialized root organs involves a set of carefully orchestrated plant hormone signalling. In particular, a spatio-temporal balance between auxin and cytokinin appears to be crucial for proper nodule development. We put together a construct that carried nuclear localized fluorescence sensors for auxin and cytokinin and used two photon induced fluorescence microscopy for concurrent quantitative 3-dimensional imaging to determine cellular level auxin and cytokinin outputs and ratios in root and nodule tissues of soybean. The use of nuclear localization signals on the markers and nuclei segmentation during image processing enabled accurate monitoring of outputs in 3D image volumes. The ratiometric method used here largely compensates for variations in individual outputs due to sample turbidity and scattering, an inherent issue when imaging thick root and nodule samples typical of many legumes. Overlays of determined auxin/cytokinin ratios on specific root zones and cell types accurately reflected those predicted based on previously reported outputs for each hormone individually. Importantly, distinct auxin/cytokinin ratios corresponded to distinct nodule cell types indicating a key role for these hormones in nodule cell type identity.

Two Endosomal NHX-Type Na+/H+ Antiporters are Involved in Auxin-Mediated Development in Arabidopsis thaliana

In Arabidopsis thaliana, the endosomal-localized Na+/H+ antiporters NHX5 and NHX6 regulate ion and pH homeostasis and are important for plant growth and development. However, the mechanism by which these endosomal NHXs function in plant development is not well understood. Auxin modulates plant growth and development through the formation of concentration gradients in plant tissue to control cell division and expansion. Here, we identified a role for NHX5 and NHX6 in the establishment and maintenance of auxin gradients in embryo and root tissues. We observed developmental impairment and abnormal cell division in embryo and root tissues in the double knockout nhx5 nhx6, consistent with these tissues showing high expression of NHX5 and NHX6. Through confocal microscopy imaging with the DR5::GFP auxin reporter, we identify defects in the perception, accumulation and redistribution of auxin in nhx5 nhx6 cells. Furthermore, we find that the steady-state levels of the PIN-FORMED (PIN) auxin efflux carriers PIN1 and PIN2 are reduced in nhx5 nhx6 root cells. Our results demonstrate that NHX5 and NHX6 function in auxin-mediated plant development by maintaining PIN abundance at the plasma membrane, and provide new insight into the regulation of plant development by endosomal NHX antiporters.

An auxin-induced β-type endo-1,4-β-glucanase in poplar is involved in cell expansion and lateral root formation

PtrGH9A7, a poplar β-type endo-1,4-β-glucanase gene induced by auxin, promotes both plant growth and lateral root development by enhancing cell expansion. Endo-1,4-β-glucanase (EGase) family genes function in multiple aspects of plant growth and development. Our previous study found that PtrCel9A6, a poplar EGase gene of the β subfamily, is specifically expressed in xylem tissue and is involved in the cellulose biosynthesis required for secondary cell wall formation (Yu et al. in Mol Plant 6:1904-1917, 2013). To further explore the functions and regulatory mechanism of β-subfamily EGases, we cloned and characterized another poplar β-type EGase gene PtrGH9A7, a close homolog of PtrCel9A6. In contrast to PtrCel9A6, PtrGH9A7 is predominantly expressed in parenchyma tissues of the above-ground part; in roots, PtrGH9A7 expression is specifically restricted to lateral root primordia at all stages from initiation to emergence and is strongly induced by auxin application. Heterologous overexpression of PtrGH9A7 promotes plant growth by enhancing cell expansion, suggesting a conserved role for β-type EGases in 1,4-β-glucan chains remodeling, which is required for cell wall loosening. Moreover, the overexpression of PtrGH9A7 significantly increases lateral root number, which might result from improved lateral root primordium development due to enhanced cell expansion. Taken together, these results demonstrate that this β-type EGase induced by auxin signaling has a novel role in promoting lateral root formation as well as in enhancing plant growth.