Lateral root organogenesis - from cell to organ

Genetic and epigenetic controls of plant regeneration

Plants have evolved powerful regeneration abilities to recover from damage. Studies on plant regeneration are of high significance as the underlying mechanisms of plant regeneration are not only linking to the fundamental researches in many fields but also to the development of widely used plant biotechnology. Higher plants show three main types of regeneration: tissue regeneration, de novo organogenesis, and somatic embryogenesis. In this review, we summarize recent research on plant regeneration, mainly focusing on Arabidopsis thaliana and moss. New data suggest that plant hormones trigger regeneration and that several key transcription factors respond to hormone signals to determine cell-fate transition. Cell-fate transition requires genome-wide changes in gene expression, which are regulated via epigenetic pathways. Certain epigenetic factors may be recruited by transcription factors to relocate to new loci and regulate gene expression. Cross talk among hormone signaling, transcription factors, and epigenetic factors is involved in different types of plant regeneration, suggesting that elegant and complex regulatory mechanisms control which type of regeneration is triggered in plants under different circumstances. Since regeneration is initiated by wounding, identification of the wound signal is an important objective for future research.

Overexpression of miR160 affects root growth and nitrogen-fixing nodule number in Medicago truncatula

Auxin action is mediated by a complex signalling pathway involving transcription factors of the auxin response factor (ARF) family. In Arabidopsis, microRNA160 (miR160) negatively regulates three ARF genes (ARF10/ARF16/ARF17) and therefore controls several developmental processes, including primary and lateral root growth. Here, we analysed the role of miR160 in root development and nodulation in Medicago truncatula Gaertn. Bioinformatic analyses identified two main mtr-miR160 variants (mtr-miR160abde and mtr-miR160c) and 17 predicted ARF targets. The miR160-dependent cleavage of four predicted targets in roots was confirmed by analysis of parallel analysis of RNA ends (PARE) data and RACE-PCR experiments. Promoter-GUS analyses for mtr-miR160d and mtr-miR160c genes revealed overlapping but distinct expression profiles during root and nodule development. In addition, the early miR160 activation in roots during symbiotic interaction was not observed in mutants of the nodulation signalling or autoregulation pathways. Composite plants that overexpressed mtr-miR160a under two different promoters exhibited distinct defects in root growth and nodulation: the p35S:miR160a construct led to reduced root length associated to a severe disorganisation of the RAM, whereas pCsVMV:miR160a roots showed gravitropism defects and lower nodule numbers. Our results suggest that a regulatory loop involving miR160/ARFs governs root and nodule organogenesis in M. truncatula.

Form matters: morphological aspects of lateral root development

BACKGROUND

The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots, as a major determinant of the root system architecture, mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors.

SCOPE AND CONCLUSIONS

In this review various morphological aspects of lateral branching in roots are analysed. Morphological events occurring during the formation of a typical lateral root are described. This process involves dramatic changes in the geometry of the developing organ that at early stages are associated with oblique cell divisions, leading to breaking of the symmetry of the cell pattern. Several types of defects in the morphology of primordia are indicated and described. Computer simulations show that some of these defects may result from an unstable field of growth rates. Significant changes in both primary and lateral root morphology may also be a consequence of various mutations, some of which are auxin-related. Examples reported in the literature are considered. Finally, lateral root formation is discussed in terms of mechanics. In this approach the primordium is considered as a physical object undergoing deformation and is characterized by specific mechanical properties.

Message in a bottle: small signalling peptide outputs during growth and development

Classical and recently found phytohormones play an important role in plant growth and development, but plants additionally control these processes through small signalling peptides. Over 1000 potential small signalling peptide sequences are present in the Arabidopsis genome. However, to date, a mere handful of small signalling peptides have been functionally characterized and few have been linked to a receptor. Here, we assess the potential small signalling peptide outputs, namely the molecular, biochemical, and morphological changes they trigger in Arabidopsis. However, we also include some notable studies in other plant species, in order to illustrate the varied effects that can be induced by small signalling peptides. In addition, we touch on some evolutionary aspects of small signalling peptides, as studying their signalling outputs in single-cell green algae and early land plants will assist in our understanding of more complex land plants. Our overview illustrates the growing interest in the small signalling peptide research area and its importance in deepening our understanding of plant growth and development.

Notes from the underground: receptor-like kinases in Arabidopsis root development

During plant development, the frequency and context of cell division must be controlled, and cells must differentiate properly to perform their mature functions. In addition, stem cell niches need to be maintained as a reservoir for new cells. All of these processes require intercellular signaling, whether it is a cell relaying its position to other cells, or more mature cells signaling to the stem cell niche to regulate the rate of growth. Receptor-like kinases have emerged as a major component in these diverse roles, especially within the Arabidopsis root. In this review, the functions of receptor-like kinase signaling in regulating Arabidopsis root development will be examined in the areas of root apical meristem maintenance, regulation of epidermal cell fate, lateral root development and vascular differentiation. [Figure: see text] Frans E. Tax (Corresponding author).

Lateral root formation and growth of Arabidopsis is redundantly regulated by cytokinin metabolism and signalling genes

The plant root system is important for the uptake of water and nutrients and the anchoring of plants in the soil. Lateral roots (LRs) contribute considerably to root system architecture. Their post-embryonic formation is regulated by hormones and environmental cues. The hormone cytokinin influences LR formation and growth in Arabidopsis thaliana on different levels by disturbing cell division activity and pattern formation. This includes inhibition of the first formative cell division of pericycle founder cells and inhibition of the outgrowth of young LR primordia. Mutant analysis revealed that the cytokinin biosynthesis genes IPT3 and IPT5 and all three cytokinin receptor genes (AHK2, AHK3, and CRE1/AHK4) act redundantly during LR initiation. Mutation of AHK2 and AHK3 caused increased auxin sensitivity of LR formation, corroborating the functional relevance of auxin-cytokinin interaction during LR formation. In contrast, LR development of cytokinin receptor mutants in response to other hormones was mostly similar to that of the wild type, which is consistent with separate response pathways. A noticeable exception was an increased sensitivity of LR elongation to brassinolide in ahk2 ahk3 mutants indicating antagonistic action of cytokinin and brassinosteroid. It is proposed that the multilevel redundancy of the cytokinin system in modulating LR formation reflects its role in mediating environmental cues.

Strigolactones fine-tune the root system

Strigolactones were originally discovered to be involved in parasitic weed germination, in mycorrhizal association and in the control of shoot architecture. Despite their clear role in rhizosphere signaling, comparatively less attention has been given to the belowground function of strigolactones on plant development. However, research has revealed that strigolactones play a key role in the regulation of the root system including adventitious roots, primary root length, lateral roots, root hairs and nodulation. Here, we review the recent progress regarding strigolactone regulation of the root system and the antagonism and interplay with other hormones.

Callose deposition and symplastic connectivity are regulated prior to lateral root emergence

Root growth is critical for the effective exploitation of the rhizosphere and productive plant growth. Our recent work¹ showed that root architecture was dependent upon the degree of symplastic connectivity between neighboring cells during the specification of lateral root primordia and was affected by genes regulating callose deposition at plasmodesmata (PD). Here we provide additional evidence that both symplastic connectivity and callose are also important during the later phase of lateral root development: emergence. Callose immunolocalization assays indicated that transient symplastic isolation of the primordium occur immediately prior to emergence through the overlaying tissues to produce the mature lateral root.¹ Here we could corroborate these results by analyzing the mobility of a symplastic tracer and the expression of PD genes in lateral roots and in response to auxins. Moreover, we show that altering callose deposition affects the number of emerged lateral roots suggesting that PD regulation is important for emergence.

Symplastic intercellular connectivity regulates lateral root patterning

Cell-to-cell communication coordinates the behavior of individual cells to establish organ patterning and development. Although mobile signals are known to be important in lateral root development, the role of plasmodesmata (PD)-mediated transport in this process has not been investigated. Here, we show that changes in symplastic connectivity accompany and regulate lateral root organogenesis in Arabidopsis. This connectivity is dependent upon callose deposition around PD affecting molecular flux through the channel. Two plasmodesmal-localized β-1,3 glucanases (PdBGs) were identified that regulate callose accumulation and the number and distribution of lateral roots. The fundamental role of PD-associated callose in this process was illustrated by the induction of similar phenotypes in lines with altered callose turnover. Our results show that regulation of callose and cell-to-cell connectivity is critical in determining the pattern of lateral root formation, which influences root architecture and optimal plant performance.

Nitric oxide as a key component in hormone-regulated processes

Nitric oxide (NO) is a small gaseous molecule, with a free radical nature that allows it to participate in a wide spectrum of biologically important reactions. NO is an endogenous product in plants, where different biosynthetic pathways have been proposed. First known in animals as a signaling molecule in cardiovascular and nervous systems, it has turned up to be an essential component for a wide variety of hormone-regulated processes in plants. Adaptation of plants to a changing environment involves a panoply of processes, which include the control of CO2 fixation and water loss through stomatal closure, rearrangements of root architecture as well as growth restriction. The regulation of these processes requires the concerted action of several phytohormones, as well as the participation of the ubiquitous molecule NO. This review analyzes the role of NO in relation to the signaling pathways involved in stomatal movement, plant growth and senescence, in the frame of its interaction with abscisic acid, auxins, gibberellins, and ethylene.

Unraveling root developmental programs initiated by beneficial Pseudomonas spp. bacteria

Plant roots are colonized by an immense number of microbes, referred to as the root microbiome. Selected strains of beneficial soil-borne bacteria can protect against abiotic stress and prime the plant immune system against a broad range of pathogens. Pseudomonas spp. rhizobacteria represent one of the most abundant genera of the root microbiome. Here, by employing a germ-free experimental system, we demonstrate the ability of selected Pseudomonas spp. strains to promote plant growth and drive developmental plasticity in the roots of Arabidopsis (Arabidopsis thaliana) by inhibiting primary root elongation and promoting lateral root and root hair formation. By studying cell type-specific developmental markers and employing genetic and pharmacological approaches, we demonstrate the crucial role of auxin signaling and transport in rhizobacteria-stimulated changes in the root system architecture of Arabidopsis. We further show that Pseudomonas spp.-elicited alterations in root morphology and rhizobacteria-mediated systemic immunity are mediated by distinct signaling pathways. This study sheds new light on the ability of soil-borne beneficial bacteria to interfere with postembryonic root developmental programs.

OsORC3 is required for lateral root development in rice

The origin recognition complex (ORC) is a pivotal element in DNA replication, heterochromatin assembly, checkpoint regulation and chromosome assembly. Although the functions of the ORC have been determined in yeast and model animals, they remain largely unknown in the plant kingdom. In this study, Oryza sativa Origin Recognition Complex subunit 3 (OsORC3) was cloned using map-based cloning procedures, and functionally characterized using a rice (Oryza sativa) orc3 mutant. The mutant showed a temperature-dependent defect in lateral root (LR) development. Map-based cloning showed that a G→A mutation in the 9th exon of OsORC3 was responsible for the mutant phenotype. OsORC3 was strongly expressed in regions of active cell proliferation, including the primary root tip, stem base, lateral root primordium, emerged lateral root primordium, lateral root tip, young shoot, anther and ovary. OsORC3 knockdown plants lacked lateral roots and had a dwarf phenotype. The root meristematic zone of ORC3 knockdown plants exhibited increased cell death and reduced vital activity compared to the wild-type. CYCB1;1::GUS activity and methylene blue staining showed that lateral root primordia initiated normally in the orc3 mutant, but stopped growing before formation of the stele and ground tissue. Our results indicate that OsORC3 plays a crucial role in the emergence of lateral root primordia.

Osmotic stress responses and plant growth controlled by potassium transporters in Arabidopsis

Osmotic adjustment plays a fundamental role in water stress responses and growth in plants; however, the molecular mechanisms governing this process are not fully understood. Here, we demonstrated that the KUP potassium transporter family plays important roles in this process, under the control of abscisic acid (ABA) and auxin. We generated Arabidopsis thaliana multiple mutants for K(+) uptake transporter 6 (KUP6), KUP8, KUP2/SHORT HYPOCOTYL3, and an ABA-responsive potassium efflux channel, guard cell outward rectifying K(+) channel (GORK). The triple mutants, kup268 and kup68 gork, exhibited enhanced cell expansion, suggesting that these KUPs negatively regulate turgor-dependent growth. Potassium uptake experiments using (86)radioactive rubidium ion ((86)Rb(+)) in the mutants indicated that these KUPs might be involved in potassium efflux in Arabidopsis roots. The mutants showed increased auxin responses and decreased sensitivity to an auxin inhibitor (1-N-naphthylphthalamic acid) and ABA in lateral root growth. During water deficit stress, kup68 gork impaired ABA-mediated stomatal closing, and kup268 and kup68 gork decreased survival of drought stress. The protein kinase SNF1-related protein kinases 2E (SRK2E), a key component of ABA signaling, interacted with and phosphorylated KUP6, suggesting that KUP functions are regulated directly via an ABA signaling complex. We propose that the KUP6 subfamily transporters act as key factors in osmotic adjustment by balancing potassium homeostasis in cell growth and drought stress responses.

Root hydrotropism: an update

While water shortage remains the single-most important factor influencing world agriculture, there are very few studies on how plants grow in response to water potential, i.e., hydrotropism. Terrestrial plant roots dwell in the soil, and their ability to grow and explore underground requires many sensors for stimuli such as gravity, humidity gradients, light, mechanical stimulations, temperature, and oxygen. To date, extremely limited information is available on the components of such sensors; however, all of these stimuli are sensed in the root cap. Directional growth of roots is controlled by gravity, which is fixed in direction and intensity. However, other environmental factors, such as water potential gradients, which fluctuate in time, space, direction, and intensity, can act as a signal for modifying the direction of root growth accordingly. Hydrotropism may help roots to obtain water from the soil and at the same time may participate in the establishment of the root system. Current genetic analysis of hydrotropism in Arabidopsis has offered new players, mainly AHR1, NHR1, MIZ1, and MIZ2, which seem to modulate how root caps sense and choose to respond hydrotropically as opposed to other tropic responses. Here we review the mechanism(s) by which these genes and the plant hormones abscisic acid and cytokinins coordinate hydrotropism to counteract the tropic responses to gravitational field, light or touch stimuli. The biological consequence of hydrotropism is also discussed in relation to water stress avoidance.

Phloem development in nematode-induced feeding sites: the implications of auxin and cytokinin

Sedentary plant parasitic nematodes such as root-knot nematodes and cyst nematodes induce giant cells or syncytia, respectively, in their host plant's roots. These highly specialized structures serve as feeding sites from which exclusively the nematodes withdraw nutrients. While giant cells are symplastically isolated and obtain assimilates by transporter-mediated processes syncytia are massively connected to the phloem by plasmodesmata. To support the feeding sites and the nematode during their development, phloem is induced around syncytia and giant cells. In the case of syncytia the unloading phloem consists of sieve elements and companion cells and in the case of root knots it consists exclusively of sieve elements. We applied immunohistochemistry to identify the cells within the developing phloem that responded to auxin and cytokinin. Both feeding sites themselves did not respond to either hormone. We were able to show that in root knots an auxin response precedes the differentiation of these auxin responsive cells into phloem elements. This process appears to be independent of B-type Arabidopsis response regulators. Using additional markers for tissue identity we provide evidence that around giant cells protophloem is formed and proliferates dramatically. In contrast, the phloem around syncytia responded to both hormones. The presence of companion cells as well as hormone-responsive sieve elements suggests that metaphloem development occurs. The implication of auxin and cytokinin in the further development of the metaphloem is discussed.

Positive and negative regulation of cortical cell division during root nodule development in Lotus japonicus is accompanied by auxin response

Nodulation is a form of de novo organogenesis that occurs mainly in legumes. During early nodule development, the host plant root is infected by rhizobia that induce dedifferentiation of some cortical cells, which then proliferate to form the symbiotic root nodule primordium. Two classic phytohormones, cytokinin and auxin, play essential roles in diverse aspects of cell proliferation and differentiation. Although recent genetic studies have established how activation of cytokinin signaling is crucial to the control of cortical cell differentiation, the physiological pathways through which auxin might act in nodule development are poorly characterized. Here, we report the detailed patterns of auxin accumulation during nodule development in Lotus japonicus. Our analyses showed that auxin predominantly accumulates in dividing cortical cells and that NODULE INCEPTION, a key transcription factor in nodule development, positively regulates this accumulation. Additionally, we found that auxin accumulation is inhibited by a systemic negative regulatory mechanism termed autoregulation of nodulation (AON). Analysis of the constitutive activation of LjCLE-RS genes, which encode putative root-derived signals that function in AON, in combination with the determination of auxin accumulation patterns in proliferating cortical cells, indicated that activation of LjCLE-RS genes blocks the progress of further cortical cell division, probably through controlling auxin accumulation. Our data provide evidence for the existence of a novel fine-tuning mechanism that controls nodule development in a cortical cell stage-dependent manner.

The role of PIN auxin efflux carriers in polar auxin transport and accumulation and their effect on shaping maize development

In plants, proper seed development and the continuing post-embryonic organogenesis both require that different cell types are correctly differentiated in response to internal and external stimuli. Among internal stimuli, plant hormones and particularly auxin and its polar transport (PAT) have been shown to regulate a multitude of plant physiological processes during vegetative and reproductive development. Although our current auxin knowledge is almost based on the results from researches on the eudicot Arabidopsis thaliana, during the last few years, many studies tried to transfer this knowledge from model to crop species, maize in particular. Applications of auxin transport inhibitors, mutant characterization, and molecular and cell biology approaches, facilitated by the sequencing of the maize genome, allowed the identification of genes involved in auxin metabolism, signaling, and particularly in polar auxin transport. PIN auxin efflux carriers have been shown to play an essential role in regulating PAT during both seed and post-embryonic development in maize. In this review, we provide a summary of the recent findings on PIN-mediated polar auxin transport during maize development. Similarities and differences between maize and Arabidopsis are analyzed and discussed, also considering that their different plant architecture depends on the differentiation of structures whose development is controlled by auxins.

Control of the plant cell cycle by developmental and environmental cues

Plant morphogenesis relies on cell proliferation and differentiation strictly controlled in space and time. As in other eukaryotes, progression through the plant cell cycle is governed by cyclin-dependent kinases (CDKs) that associate with their activator proteins called cyclins (CYCs), and the activity of CYC-CDK is modulated at both transcriptional and post-translational levels. Compared with animals and yeasts, plants generally possess many more genes encoding core cell cycle regulators and it has been puzzling how their functions are specified or overlapped in development or in response to various environmental changes. Thanks to the recent advances in high-throughput, genome-wide transcriptome and proteomic technologies, we are finally beginning to see how core regulators are assembled during the cell cycle and how their activities are modified by developmental and environmental cues. In this review we will summarize the latest progress in plant cell cycle research and provide an overview of some of the emerging molecular interfaces that link upstream signaling cascades and cell cycle regulation.

The establishment of asymmetry in Arabidopsis lateral root founder cells is regulated by LBD16/ASL18 and related LBD/ASL proteins

In most dicot plants, lateral root (LR) formation, which is important for the construction of the plant root system, is initiated from coordinated asymmetric cell divisions (ACD) of the primed LR founder cells in the xylem pole pericycle (XPP) of the existing roots. In Arabidopsis thaliana, two AUXIN RESPONSE FACTORs (ARFs), ARF7 and ARF19, positively regulate LR formation through activation of the plant-specific transcriptional regulators LATERAL ORGAN BOUNDARIES-DOMAIN 16/ASYMMETRIC LEAVES2-LIKE 18 (LBD16/ASL18) and the other related LBD/ASL genes. The exact biological role of these LBD/ASLs in LR formation is still unknown. Here, we demonstrate that LBD16/ASL18 is specifically expressed in the LR founder cells adjacent to the XPP before the first ACD and that it functions redundantly with the other auxin-inducible LBD/ASLs in LR initiation. The spatiotemporal expression of LBD16/ASL18 during LR initiation is dependent on the SOLITARY-ROOT (SLR)/IAA14-ARF7-ARF19 auxin signaling module. In addition, XPP-specific expression of LBD16/ASL18 in arf7 arf19 induced cell divisions at XPP, thereby restoring the LR phenotype. We also demonstrate that expression of LBD16-SRDX, a dominant repressor of LBD16/ASL18 and its related LBD/ASLs, does not interfere in the specification of LR founder cells with local activation of the auxin response, but it blocks the polar nuclear migration in LR founder cells before ACD, thereby blocking the subsequent LR initiation. Taken together, these results indicate that the localized activity of LBD16/ASL18 and its related LBD/ASLs is involved in the symmetry breaking of LR founder cells for LR initiation, a key step for constructing the plant root system.

A gain-of-function mutation in OsIAA11 affects lateral root development in rice

Lateral roots are important to plants for the uptake of nutrients and water. Several members of the Aux/IAA family have been shown to play crucial roles in lateral root development. Here, a member of the rice Aux/IAA family genes, OsIAA11 (LOC_Os03g43400), was isolated from a rice mutant defective in lateral root development. The gain-of-function mutation in OsIAA11 strictly blocks the initiation of lateral root primordia, but it does not affect crown root development. The expression of OsIAA11 is defined in root tips, lateral root caps, steles, and lateral root primordia. The auxin reporter DR5-GUS (β-glucuronidase) was expressed at lower levels in the mutant than in wild-type, indicating that OsIAA11 is involved in auxin signaling in root caps. The transcript abundance of both OsPIN1b and OsPIN10a was diminished in root tips of the Osiaa11 mutant. Taken together, the results indicate that the gain-of-function mutation in OsIAA11 caused the inhibition of lateral root development in rice.

De novo shoot organogenesis: from art to science

In vitro shoot organogenesis and plant regeneration are crucial for both plant biotechnology and the fundamental study of plant biology. Although the importance of auxin and cytokinin has been known for more than six decades, the underlying molecular mechanisms of their function have only been revealed recently. Advances in identifying new Arabidopsis genes, implementing live-imaging tools and understanding cellular and molecular networks regulating de novo shoot organogenesis have helped to redefine the empirical models of shoot organogenesis and plant regeneration. Here, we review the functions and interactions of genes that control key steps in two distinct developmental processes: de novo shoot organogenesis and lateral root formation.

Hormonal regulation of lateral root development in Arabidopsis modulated by MIZ1 and requirement of GNOM activity for MIZ1 function

Plant organ development is important for adaptation to a changing environment. Genetic and physiological studies have revealed that plant hormones play key roles in lateral root formation. In this study, we show that MIZU-KUSSEI1 (MIZ1), which was identified originally as a regulator of hydrotropism, functions as a novel regulator of hormonally mediated lateral root development. Overexpression of MIZ1 (MIZ1OE) in roots resulted in a reduced number of lateral roots being formed; however, this defect could be recovered with the application of auxin. Indole-3-acetic acid quantification analyses showed that free indole-3-acetic acid levels decreased in MIZ1OE roots, which indicates that alteration of auxin level is critical for the inhibition of lateral root formation in MIZ1OE plants. In addition, MIZ1 negatively regulates cytokinin sensitivity on root development. Application of cytokinin strongly induced the localization of MIZ1-green fluorescent protein to lateral root primordia, which suggests that the inhibition of lateral root development by MIZ1 occurs downstream of cytokinin signaling. Surprisingly, miz2, a weak allele of gnom, suppressed developmental defects in MIZ1OE plants. Taken together, these results suggest that MIZ1 plays a role in lateral root development by maintaining auxin levels and that its function requires GNOM activity. These data provide a molecular framework for auxin-dependent organ development in Arabidopsis (Arabidopsis thaliana).

Recycling, clustering, and endocytosis jointly maintain PIN auxin carrier polarity at the plasma membrane

A combination of super-resolution microscopy in live cells and computational modeling provides new insights into the dynamic and interwoven mechanism that maintains the polar distribution of an important plant cargo.

Semi-quantitative and subdiffraction resolution fluorescence imaging in living plant cells provided unexpected insights into the mechanisms underlying dynamic maintenance of PIN polarity.

These experiments reveal super-polar targeting of PIN proteins to the center of polar domains, presumably by a TGN/endosome guided delivery mechanism.

PIN proteins are recruited to immobile membrane clusters that reduce lateral PIN mobility, and retrieved from the lateral cell side by spatially defined clathrin-dependent endocytosis.

In silico model simulations are consistent with these experimental observations and reveal the individual roles of these cellular processes in the organization of sharply defined polar plasma membrane domains.

Cell polarity reflected by asymmetric distribution of proteins at the plasma membrane is a fundamental feature of unicellular and multicellular organisms. It remains conceptually unclear how cell polarity is kept in cell wall-encapsulated plant cells. We have used super-resolution and semi-quantitative live-cell imaging in combination with pharmacological, genetic, and computational approaches to reveal insights into the mechanism of cell polarity maintenance in Arabidopsis thaliana. We show that polar-competent PIN transporters for the phytohormone auxin are delivered to the center of polar domains by super-polar recycling. Within the plasma membrane, PINs are recruited into non-mobile membrane clusters and their lateral diffusion is dramatically reduced, which ensures longer polar retention. At the circumventing edges of the polar domain, spatially defined internalization of escaped cargos occurs by clathrin-dependent endocytosis. Computer simulations confirm that the combination of these processes provides a robust mechanism for polarity maintenance in plant cells. Moreover, our study suggests that the regulation of lateral diffusion and spatially defined endocytosis, but not super-polar exocytosis have primary importance for PIN polarity maintenance.

The rules of engagement in the legume-rhizobial symbiosis

Rhizobial bacteria enter a symbiotic association with leguminous plants, resulting in differentiated bacteria enclosed in intracellular compartments called symbiosomes within nodules on the root. The nodules and associated symbiosomes are structured for efficient nitrogen fixation. Although the interaction is beneficial to both partners, it comes with rigid rules that are strictly enforced by the plant. Entry into root cells requires appropriate recognition of the rhizobial Nod factor signaling molecule, and this recognition activates a series of events, including polarized root-hair tip growth, invagination associated with bacterial infection, and the promotion of cell division in the cortex leading to the nodule meristem. The plant's command of the infection process has been highlighted by its enforcement of terminal differentiation upon the bacteria within nodules of some legumes, and this can result in a loss of bacterial viability while permitting effective nitrogen fixation. Here, we review the mechanisms by which the plant allows bacterial infection and promotes the formation of the nodule, as well as the details of how this intimate association plays out inside the cells of the nodule where a complex interchange of metabolites and regulatory peptides force the bacteria into a nitrogen-fixing organelle-like state.

NO synthase-generated NO acts downstream of auxin in regulating Fe-deficiency-induced root branching that enhances Fe-deficiency tolerance in tomato plants

In response to Fe-deficiency, various dicots increase their root branching which contributes to the enhancement of ferric-chelate reductase activity. Whether this Fe-deficiency-induced response eventually enhances the ability of the plant to tolerate Fe-deficiency or not is still unclear and evidence is also scarce about the signals triggering it. In this study, it was found that the SPAD-chlorophyll meter values of newly developed leaves of four tomato (Solanum lycocarpum) lines, namely line227/1 and Roza and their two reciprocal F(1) hybrid lines, were positively correlated with their root branching under Fe-deficient conditions. It indicates that Fe-deficiency-induced root branching is critical for plant tolerance to Fe-deficiency. In another tomato line, Micro-Tom, the increased root branching in Fe-deficient plants was accompanied by the elevation of endogenous auxin and nitric oxide (NO) levels, and was suppressed either by the auxin transport inhibitors NPA and TIBA or the NO scavenger cPTIO. On the other hand, root branching in Fe-sufficient plants was induced either by the auxin analogues NAA and 2,4-D or the NO donors NONOate or SNP. Further, in Fe-deficient plants, NONOate restored the NPA-terminated root branching, but NAA did not affect the cPTIO-terminated root branching. Fe-deficiency-induced root branching was inhibited by the NO-synthase (NOS) inhibitor L-NAME, but was not affected by the nitrate reductase (NR) inhibitor NH(4)(+), tungstate or glycine. Taking all of these findings together, a novel function and signalling pathway of Fe-deficiency-induced root branching is presented where NOS-generated rather than NR-generated NO acts downstream of auxin in regulating this Fe-deficiency-induced response, which enhances the plant tolerance to Fe-deficiency.

Salt stress signals shape the plant root

Plants use different strategies to deal with high soil salinity. One strategy is activation of pathways that allow the plant to export or compartmentalise salt. Relying on their phenotypic plasticity, plants can also adjust their root system architecture (RSA) and the direction of root growth to avoid locally high salt concentrations. Here, we highlight RSA responses to salt and osmotic stress and the underlying mechanisms. A model is presented that describes how salinity affects auxin distribution in the root. Possible intracellular signalling pathways linking salinity to root development and direction of root growth are discussed. These involve perception of high cytosolic Na+ concentrations in the root, activation of lipid signalling and protein kinase activity and modulation of endocytic pathways.