SCF(TIR1/AFB)-auxin signalling regulates PIN vacuolar trafficking and auxin fluxes during root gravitropism

Endomembrane trafficking driven by microtubule growth regulates stomatal movement in Arabidopsis

Microtubule-based vesicle trafficking usually relies upon kinesin and dynein motors and few reports describe microtubule polymerisation driving directional vesicle trafficking. Here we show that Arabidopsis END BINDING1b (EB1b), a microtubule plus-end binding protein, directly interacts with SYP121, a SNARE protein that mediates the trafficking of the K+ channel KAT1 and its distribution to the plasma membrane (PM) in Arabidopsis guard cells. Knockout of AtEB1b and its homologous proteins results in a modest but significant change in the distribution of KAT1 and SYP121 in guard cells and consequently delays light-induced stomatal opening. Live-cell imaging reveals that a portion of SYP121-associated endomembrane compartments co-localise with AtEB1b at the growing ends of microtubules, trafficking along with the growth of microtubules for targeting to the PM. Our study reveals a mechanism of vesicle trafficking driven by microtubule growth, which is involved in the redistribution of PM proteins to modulate guard cell movement.

Polarly localized Bro1 domain proteins regulate PIN-FORMED abundance and root gravitropic growth in Arabidopsis

The developmental plasticity of the root system plays an essential role in the adaptation of plants to the environment. Among many other signals, auxin and its directional, intercellular transport are critical in regulating root growth and development. In particular, the PIN-FORMED2 (PIN2) auxin exporter acts as a key regulator of root gravitropic growth. Multiple regulators have been reported to be involved in PIN2-mediated root growth; however, our information remains incomplete. Here, we identified ROWY Bro1-domain proteins as important regulators of PIN2 sorting control. Genetic analysis revealed that Arabidopsis rowy1 single mutants and higher-order rowy1 rowy2 rowy3 triple mutants presented a wavy root growth phenotype. Cell biological experiments revealed that ROWY1 and PIN2 colocalized to the apical side of the plasma membrane in the root epidermis and that ROWYs are required for correct PM targeting of PIN2. In addition, ROWYs also affected PIN3 protein abundance in the stele, suggesting the potential involvement of additional PIN transporters as well as other proteins. A global transcriptome analysis revealed that ROWY genes are involved in the Fe2+ availability perception pathway. This work establishes ROWYs as important novel regulators of root gravitropic growth by connecting micronutrient availability to the proper subcellular targeting of PIN auxin transporters.

Special Issue "Phytohormones: Important Participators in Plant Growth and Development"

The articles published in the IJMS Special Issue "Phytohormones" are devoted to various aspects of hormonal control of plant growth and development promoting adaptation to normal and stress conditions [...].

Multiple feedbacks on self-organized morphogenesis during plant regeneration

Decades of research have primarily emphasized genetic blueprint as the driving force behind plant regeneration. The flow of information from genetics, which manifests as biochemical properties, including hormones, has been extensively implicated in plant regeneration. However, recent advancements have unveiled additional intrinsic modules within this information flow. Here, we explore the three core modules of plant regeneration: biochemical properties, mechanical forces acting on cells, and cell geometry. We debate their roles and interactions during morphogenesis, emphasizing the potential for multiple feedbacks between these core modules to drive pattern formation during regeneration. We propose that de novo organ regeneration is a self-organized event driven by multidirectional information flow between these core modules.

A precise balance of TETRASPANIN1/TORNADO2 activity is required for vascular proliferation and ground tissue patterning in Arabidopsis

The molecular mechanisms guiding oriented cell divisions in the root vascular tissues of Arabidopsis thaliana are still poorly characterised. By overlapping bulk and single-cell transcriptomic datasets, we unveiled TETRASPANIN1 (TET1) as a putative regulator in this process. TET1 is expressed in root vascular cells, and loss-of-function mutants contain fewer vascular cell files. We further generated and characterised a CRISPR deletion mutant and showed, unlike previously described mutants, that the full knock out is additionally missing endodermal cells in a stochastic way. Finally, we show that HA-tagged versions of TET1 are functional in contrast to fluorescent TET1 translational fusions. Immunostaining using HA-TET1 lines complementing the mutant phenotype suggested a dual plasma membrane and intracellular localisation in the root vasculature and a polar membrane localisation in the young cortex, endodermal and initial cells. Taken together, we show that TET1 is involved in both vascular proliferation and ground tissue patterning. Our initial results pave the way for future work to decipher its precise mode of action.

FAB1C, a phosphatidylinositol 3-phosphate 5-kinase, interacts with PIN-FORMEDs and modulates their lytic trafficking in Arabidopsis

PIN-FORMEDs (PINs) are auxin efflux carriers that asymmetrically target the plasma membrane (PM) and are critical for forming local auxin gradients and auxin responses. While the cytoplasmic hydrophilic loop domain of PIN (PIN-HL) is known to include some molecular cues (e.g., phosphorylation) for the modulation of PIN's intracellular trafficking and activity, the complexity of auxin responses suggests that additional regulatory modules may operate in the PIN-HL domain. Here, we have identified and characterized a PIN-HL-interacting protein (PIP) called FORMATION OF APLOID AND BINUCLEATE CELL 1C (FAB1C), a phosphatidylinositol-3-phosphate 5-kinase, which modulates PIN's lytic trafficking. FAB1C directly interacts with PIN-HL and is required for the polarity establishment and vacuolar trafficking of PINs. Unphosphorylated forms of PIN2 interact more readily with FAB1C and are more susceptible to vacuolar lytic trafficking compared to phosphorylated forms. FAB1C also affected lateral root formation by modulating the abundance of periclinally localized PIN1 and auxin maximum in the growing lateral root primordium. These findings suggest that a membrane-lipid modifier can target the cargo-including vesicle by directly interacting with the cargo and modulate its trafficking depending on the cargo's phosphorylation status.

Chemical inhibition of Arabidopsis PIN-FORMED auxin transporters by the anti-inflammatory drug naproxen

The phytohormone auxin plays central roles in many growth and developmental processes in plants. Development of chemical tools targeting the auxin pathway is useful for both plant biology and agriculture. Here we reveal that naproxen, a synthetic compound with anti-inflammatory activity in humans, acts as an auxin transport inhibitor targeting PIN-FORMED (PIN) transporters in plants. Physiological experiments indicate that exogenous naproxen treatment affects pleiotropic auxin-regulated developmental processes. Additional cellular and biochemical evidence indicates that naproxen suppresses auxin transport, specifically PIN-mediated auxin efflux. Moreover, biochemical and structural analyses confirm that naproxen binds directly to PIN1 protein via the same binding cavity as the indole-3-acetic acid substrate. Thus, by combining cellular, biochemical, and structural approaches, this study clearly establishes that naproxen is a PIN inhibitor and elucidates the underlying mechanisms. Further use of this compound may advance our understanding of the molecular mechanisms of PIN-mediated auxin transport and expand our toolkit in auxin biology and agriculture.

Genome-wide analysis of PIN genes in cultivated peanuts (Arachis hypogaea L.): identification, subcellular localization, evolution, and expression patterns

BACKGROUND

Auxin is an important hormone in plants and the PIN-FORMED (PIN) genes are essential to auxin distribution in growth and developmental processes of plants. Peanut is an influential cash crop, but research into PIN genes in peanuts remains limited.

RESULTS

In this study, 16 PIN genes were identified in the genome of cultivated peanut, resolving into four subfamilies. All PIN genes were predicted to be located in the plasma membrane and a subcellular location experiment confirmed this prediction for eight of them. The gene structure, cis-elements in the promoter, and evolutionary relationships were elucidated, facilitating our understanding of peanut PINs and their evolution. In addition, the expression patterns of these PINs in various tissues were analyzed according to a previously published transcriptome dataset and qRT-PCR, which gave us a clear understanding of the temporal and spatial expression of PIN genes in different growth stages and different tissues. The expression trend of homologous genes was similar. AhPIN2A and AhPIN2B exhibited predominant expression in roots. AhPIN1A-1 and AhPIN1B-1 displayed significant upregulation following peg penetration, suggesting a potential close association with peanut pod development. Furthermore, we presented the gene network and gene ontology enrichment of these PINs. Notably, AhABCB19 exhibited a co-expression relationship with AhPIN1A and AhPIN1B-1, with all three genes displaying higher expression levels in peanut pegs and pods. These findings reinforce their potential role in peanut pod development.

CONCLUSIONS

This study details a comprehensive analysis of PIN genes in cultivated peanuts and lays the foundation for subsequent studies of peanut gene function and phenotype.

Dynamic GOLVEN-ROOT GROWTH FACTOR 1 INSENSITIVE signaling in the root cap mediates root gravitropism

Throughout the exploration of the soil, roots interact with their environment and adapt to different conditions. Directional root growth is guided by asymmetric molecular patterns but how these become established or are dynamically regulated is poorly understood. Asymmetric gradients of the phytohormone auxin are established during root gravitropism, mainly through directional transport mediated by polarized auxin transporters. Upon gravistimulation, PIN-FORMED2 (PIN2) is differentially distributed and accumulates at the lower root side to facilitate asymmetric auxin transport up to the elongation zone where it inhibits cell elongation. GOLVEN (GLV) peptides function in gravitropism by affecting PIN2 abundance in epidermal cells. In addition, GLV signaling through ROOT GROWTH FACTOR 1 INSENSITIVE (RGI) receptors regulates root apical meristem maintenance. Here, we show that GLV-RGI signaling in these 2 processes in Arabidopsis (Arabidopsis thaliana) can be mapped to different cells in the root tip and that, in the case of gravitropism, it operates mainly in the lateral root cap (LRC) to maintain PIN2 levels at the plasma membrane (PM). Furthermore, we found that GLV signaling upregulates the phosphorylation level of PIN2 in an RGI-dependent manner. In addition, we demonstrated that the RGI5 receptor is asymmetrically distributed in the LRC and accumulates in the lower side of the LRC after gravistimulation. Asymmetric GLV-RGI signaling in the root cap likely accounts for differential PIN2 abundance at the PM to temporarily support auxin transport up to the elongation zone, thereby representing an additional level of control on the asymmetrical auxin flux to mediate differential growth of the root.

Regulation of PIN-FORMED Protein Degradation

Auxin action largely depends on the establishment of auxin concentration gradient within plant organs, where PIN-formed (PIN) auxin transporter-mediated directional auxin movement plays an important role. Accumulating studies have revealed the need of polar plasma membrane (PM) localization of PIN proteins as well as regulation of PIN polarity in response to developmental cues and environmental stimuli, amongst which a typical example is regulation of PIN phosphorylation by AGCVIII protein kinases and type A regulatory subunits of PP2A phosphatases. Recent findings, however, highlight the importance of PIN degradation in reestablishing auxin gradient. Although the underlying mechanism is poorly understood, these findings provide a novel aspect to broaden the current knowledge on regulation of polar auxin transport. In this review, we summarize the current understanding on controlling PIN degradation by endosome-mediated vacuolar targeting, autophagy, ubiquitin modification and the related E3 ubiquitin ligases, cytoskeletons, plant hormones, environmental stimuli, and other regulators, and discuss the possible mechanisms according to recent studies.

GhROP6 GTPase modulates auxin accumulation in cotton fibers by regulating cell-specific GhPIN3a localization

PIN-FORMED- (PIN) mediated polar auxin transport plays a predominant role in most auxin-triggered organogenesis in plants. Global control of PIN polarity at the plasma membrane contributes to the essential establishment of auxin maxima in most multicellular tissues. However, establishment of auxin maxima in single cells is poorly understood. Cotton fibers, derived from ovule epidermal cells by auxin-triggered cell protrusion, provide an ideal model to explore the underlying mechanism. Here, we report that cell-specific degradation of GhPIN3a, which guides the establishment of the auxin gradient in cotton ovule epidermal cells, is associated with the preferential expression of GhROP6 GTPase in fiber cells. In turn, GhROP6 reduces GhPIN3a abundance at the plasma membrane and facilitates intracellular proteolysis of GhPIN3a. Overexpression and activation of GhROP6 promote cell elongation, resulting in a substantial improvement in cotton fiber length.

Transmembrane kinase 1-mediated auxin signal regulates membrane-associated clathrin in Arabidopsis roots

Clathrin-mediated endocytosis (CME) is the major endocytic pathway in eukaryotic cells that directly regulates abundance of plasma membrane proteins. Clathrin triskelia are composed of clathrin heavy chains (CHCs) and light chains (CLCs), and the phytohormone auxin differentially regulates membrane-associated CLCs and CHCs, modulating the endocytosis and therefore the distribution of auxin efflux transporter PIN-FORMED2 (PIN2). However, the molecular mechanisms by which auxin regulates clathrin are still poorly understood. Transmembrane kinase (TMKs) family proteins are considered to contribute to auxin signaling and plant development; it remains unclear whether they are involved in PIN transport by CME. We assessed TMKs involvement in the regulation of clathrin by auxin, using genetic, pharmacological, and cytological approaches including live-cell imaging and immunofluorescence. In tmk1 mutant seedlings, auxin failed to rapidly regulate abundance of both CHC and CLC and to inhibit PIN2 endocytosis, leading to an impaired asymmetric distribution of PIN2 and therefore auxin. Furthermore, TMK3 and TMK4 were shown not to be involved in regulation of clathrin by auxin. In summary, TMK1 is essential for auxin-regulated clathrin recruitment and CME. TMK1 therefore plays a critical role in the establishment of an asymmetric distribution of PIN2 and an auxin gradient during root gravitropism.

Plants in Microgravity: Molecular and Technological Perspectives

Plants are vital components of our ecosystem for a balanced life here on Earth, as a source of both food and oxygen for survival. Recent space exploration has extended the field of plant biology, allowing for future studies on life support farming on distant planets. This exploration will utilize life support technologies for long-term human space flights and settlements. Such longer space missions will depend on the supply of clean air, food, and proper waste management. The ubiquitous force of gravity is known to impact plant growth and development. Despite this, we still have limited knowledge about how plants can sense and adapt to microgravity in space. Thus, the ability of plants to survive in microgravity in space settings becomes an intriguing topic to be investigated in detail. The new knowledge could be applied to provide food for astronaut missions to space and could also teach us more about how plants can adapt to unique environments. Here, we briefly review and discuss the current knowledge about plant gravity-sensing mechanisms and the experimental possibilities to research microgravity-effects on plants either on the Earth or in orbit.

Whole genome duplication and dispersed duplication characterize the evolution of the plant PINOID gene family across plant species

PINOID is a kinase belonging to the AGCVIII family, which regulates the polar distribution of PIN proteins and plays an important role in plant geotropism. However, the origin and evolutionary history of this gene family is not fully known. In this study, we identified 79 similar sequences across 17 plant species genomes (PINOID, D6PK, PINOID2, "hypothetical kinase"). Our results show that the AGCVIII kinase family may have originated from related "Hypothetical Kinases" that come out sister to the rest of the gene family members. These kinases differentiated their functions are found in different plant classes: D6PK in moss and PINOID and PINOID2 evolving in angiosperms including the pioneer plant Amborella trichopoda. Our study investigates the evolution of PINOID kinases from a phylogenetic perspective giving us insight into how this important plant signal transduction network switch evolved to play a fundamental and important function in plant growth and development. We highlight the importance of whole genome duplications and dispersed duplications as opposed to tandem duplications in the evolution of this gene family.

Constitutive Active CPK30 Interferes With Root Growth and Endomembrane Trafficking in Arabidopsis thaliana

Calcium-dependent protein kinases (CPK) are key components of a wide array of signaling pathways, translating stress and nutrient signaling into the modulation of cellular processes such as ion transport and transcription. However, not much is known about CPKs in endomembrane trafficking. Here, we screened for CPKs that impact on root growth and gravitropism, by overexpressing constitutively active forms of CPKs under the control of an inducible promoter in Arabidopsis thaliana. We found that inducible overexpression of an constitutive active CPK30 (CA-CPK30) resulted in a loss of root gravitropism and ectopic auxin accumulation in the root tip. Immunolocalization revealed that CA-CPK30 roots have reduced PIN protein levels, PIN1 polarity defects and impaired Brefeldin A (BFA)-sensitive trafficking. Moreover, FM4-64 uptake was reduced, indicative of a defect in endocytosis. The effects on BFA-sensitive trafficking were not specific to PINs, as BFA could not induce aggregation of ARF1- and CHC-labeled endosomes in CA-CPK30. Interestingly, the interference with BFA-body formation, could be reverted by increasing the extracellular pH, indicating a pH-dependence of this CA-CPK30 effect. Altogether, our data reveal an important role for CPK30 in root growth regulation and endomembrane trafficking in Arabidopsis thaliana.

Testing the polar auxin transport model with a selective plasma membrane H+ -ATPase inhibitor

Auxin is unique among plant hormones in that its function requires polarized transport across plant cells. A chemiosmotic model was proposed to explain how polar auxin transport is derived by the H⁺ gradient across the plasma membrane (PM) established by PM H⁺ -adenosine triphosphatases (ATPases). However, a classical genetic approach by mutations in PM H⁺ -ATPase members did not result in the ablation of polar auxin distribution, possibly due to functional redundancy in this gene family. To confirm the crucial role of PM H⁺ -ATPases in the polar auxin transport model, we employed a chemical genetic approach. Through a chemical screen, we identified protonstatin-1 (PS-1), a selective small-molecule inhibitor of PM H⁺ -ATPase activity that inhibits auxin transport. Assays with transgenic plants and yeast strains showed that the activity of PM H⁺ -ATPases affects auxin uptake as well as acropetal and basipetal polar auxin transport. We propose that PS-1 can be used as a tool to interrogate the function of PM H⁺ -ATPases. Our results support the chemiosmotic model in which PM H⁺ -ATPase itself plays a fundamental role in polar auxin transport.

Fourteen Stations of Auxin

Auxin has always been at the forefront of research in plant physiology and development. Since the earliest contemplations by Julius von Sachs and Charles Darwin, more than a century-long struggle has been waged to understand its function. This largely reflects the failures, successes, and inevitable progress in the entire field of plant signaling and development. Here I present 14 stations on our long and sometimes mystical journey to understand auxin. These highlights were selected to give a flavor of the field and to show the scope and limits of our current knowledge. A special focus is put on features that make auxin unique among phytohormones, such as its dynamic, directional transport network, which integrates external and internal signals, including self-organizing feedback. Accented are persistent mysteries and controversies. The unexpected discoveries related to rapid auxin responses and growth regulation recently disturbed our contentment regarding understanding of the auxin signaling mechanism. These new revelations, along with advances in technology, usher us into a new, exciting era in auxin research.

Auxin analog-induced Ca2+ signaling is independent of inhibition of endosomal aggregation in Arabidopsis roots

Much of what we know about the role of auxin in plant development derives from exogenous manipulations of auxin distribution and signaling, using inhibitors, auxins, and auxin analogs. In this context, synthetic auxin analogs, such as 1-naphthalene acetic acid (1-NAA), are often favored over the endogenous auxin, indole-3-acetic acid (IAA), in part due to their higher stability. While such auxin analogs have proven instrumental in revealing the various faces of auxin, they display in some cases bioactivities distinct from IAA. Here, we focused on the effect of auxin analogs on the accumulation of PIN proteins in brefeldin A-sensitive endosomal aggregations (BFA bodies), and correlation with the ability to elicit Ca2+ responses. For a set of commonly used auxin analogs, we evaluated if auxin analog-induced Ca2+ signaling inhibits PIN accumulation. Not all auxin analogs elicited a Ca2+ response, and their differential ability to elicit Ca2+ responses correlated partially with their ability to inhibit BFA-body formation. However, in tir1/afb and cngc14, 1-NAA-induced Ca2+ signaling was strongly impaired, yet 1-NAA still could inhibit PIN accumulation in BFA bodies. This demonstrates that TIR1/AFB-CNGC14-dependent Ca2+ signaling does not inhibit BFA body formation in Arabidopsis roots.

Modeling Auxin Signaling in Roots: Auxin Computations

Auxin signaling and patterning is an inherently complex process, involving polarized auxin transport, metabolism, and signaling, its effect on developmental zones, as well as growth rates, and the feedback between all these different aspects. This complexity has led to an important role for computational modeling in unraveling the multifactorial roles of auxin in plant developmental and adaptive processes. Here we discuss the basic ingredients of auxin signaling and patterning models for root development as well as a series of key modeling studies in this area. These modeling studies have helped elucidate how plants use auxin signaling to compute the size of their root meristem, the direction in which to grow, and when and where to form lateral roots. Importantly, these models highlight how auxin, through patterning of and collaborating with other factors, can fulfill all these roles simultaneously.

Spaceflight studies identify a gene encoding an intermediate filament involved in tropism pathways

We performed a series of experiments to study the interaction between phototropism and gravitropism in Arabidopsis thaliana as part of the Seedling Growth Project on the International Space Station. Red-light-based and blue-light-based phototropism were examined in microgravity and at 1g, a control that was produced by an on-board centrifuge. At the end of the experiments, seedlings were frozen and brought back to Earth for gene profiling studies via RNASeq methods. In this paper, we focus on five genes identified in these space studies by their differential expression in space: one involved in auxin transport and four others encoding genes for: a methyltransferase subunit, a transmembrane protein, a transcription factor for endodermis formation, and a cytoskeletal element (an intermediate filament protein). Time course studies using mutant strains of these five genes were performed for blue-light and red-light phototropism studies as well as for gravitropism assays on ground. Interestingly, all five of the genes had some effects on all the tropisms under the conditions studied. In addition, RT-PCR analyses examined expression of the five genes in wild-type seedlings during blue-light-based phototropism. Previous studies have supported a role of both microfilaments and microtubules in tropism pathways. However, the most interesting finding of the present space studies is that NFL, a gene encoding an intermediate filament protein, plays a role in phototropism and gravitropism, which opens the possibility that this cytoskeletal element modulates signal transduction in plants.

A Comprehensive Review on the Heavy Metal Toxicity and Sequestration in Plants

Heavy metal (HM) toxicity has become a global concern in recent years and is imposing a severe threat to the environment and human health. In the case of plants, a higher concentration of HMs, above a threshold, adversely affects cellular metabolism because of the generation of reactive oxygen species (ROS) which target the key biological molecules. Moreover, some of the HMs such as mercury and arsenic, among others, can directly alter the protein/enzyme activities by targeting their -SH group to further impede the cellular metabolism. Particularly, inhibition of photosynthesis has been reported under HM toxicity because HMs trigger the degradation of chlorophyll molecules by enhancing the chlorophyllase activity and by replacing the central Mg ion in the porphyrin ring which affects overall plant growth and yield. Consequently, plants utilize various strategies to mitigate the negative impact of HM toxicity by limiting the uptake of these HMs and their sequestration into the vacuoles with the help of various molecules including proteins such as phytochelatins, metallothionein, compatible solutes, and secondary metabolites. In this comprehensive review, we provided insights towards a wider aspect of HM toxicity, ranging from their negative impact on plant growth to the mechanisms employed by the plants to alleviate the HM toxicity and presented the molecular mechanism of HMs toxicity and sequestration in plants.

PIN-mediated polar auxin transport regulations in plant tropic responses

Tropisms, growth responses to environmental stimuli such as light or gravity, are spectacular examples of adaptive plant development. The plant hormone auxin serves as a major coordinative signal. The PIN auxin exporters, through their dynamic polar subcellular localizations, redirect auxin fluxes in response to environmental stimuli and the resulting auxin gradients across organs underlie differential cell elongation and bending. In this review, we discuss recent advances concerning regulations of PIN polarity during tropisms, focusing on PIN phosphorylation and trafficking. We also cover how environmental cues regulate PIN actions during tropisms, as well as the crucial role of auxin feedback on PIN polarity during bending termination. Finally, the interactions between different tropisms are reviewed to understand plant adaptive growth in the natural environment.

Systematic analysis of specific and nonspecific auxin effects on endocytosis and trafficking

The phytohormone auxin and its directional transport through tissues are intensively studied. However, a mechanistic understanding of auxin-mediated feedback on endocytosis and polar distribution of PIN auxin transporters remains limited due to contradictory observations and interpretations. Here, we used state-of-the-art methods to reexamine the auxin effects on PIN endocytic trafficking. We used high auxin concentrations or longer treatments versus lower concentrations and shorter treatments of natural indole-3-acetic acid (IAA) and synthetic naphthalene acetic acid (NAA) auxins to distinguish between specific and nonspecific effects. Longer treatments of both auxins interfere with Brefeldin A-mediated intracellular PIN2 accumulation and also with general aggregation of endomembrane compartments. NAA treatment decreased the internalization of the endocytic tracer dye, FM4-64; however, NAA treatment also affected the number, distribution, and compartment identity of the early endosome/trans-Golgi network, rendering the FM4-64 endocytic assays at high NAA concentrations unreliable. To circumvent these nonspecific effects of NAA and IAA affecting the endomembrane system, we opted for alternative approaches visualizing the endocytic events directly at the plasma membrane (PM). Using total internal reflection fluorescence microscopy, we saw no significant effects of IAA or NAA treatments on the incidence and dynamics of clathrin foci, implying that these treatments do not affect the overall endocytosis rate. However, both NAA and IAA at low concentrations rapidly and specifically promoted endocytosis of photo-converted PIN2 from the PM. These analyses identify a specific effect of NAA and IAA on PIN2 endocytosis, thus, contributing to its polarity maintenance and furthermore illustrate that high auxin levels have nonspecific effects on trafficking and endomembrane compartments.

The AtCRK5 Protein Kinase Is Required to Maintain the ROS NO Balance Affecting the PIN2-Mediated Root Gravitropic Response in Arabidopsis

The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, we report that this phenotype correlates with lower superoxide anion (O2•-) and hydrogen peroxide (H2O2) levels but a higher nitric oxide (NO) content in the mutant root tips in comparison to the wild type (AtCol-0). The oxidative stress inducer paraquat (PQ) triggering formation of O2•- (and consequently, H2O2) was able to rescue the gravitropic response of Atcrk5-1 roots. The direct application of H2O2 had the same effect. Under gravistimulation, correct auxin distribution was restored (at least partially) by PQ or H2O2 treatment in the mutant root tips. In agreement, the redistribution of the PIN2 auxin efflux carrier was similar in the gravistimulated PQ-treated mutant and untreated wild type roots. It was also found that PQ-treatment decreased the endogenous NO level at the root tip to normal levels. Furthermore, the mutant phenotype could be reverted by direct manipulation of the endogenous NO level using an NO scavenger (cPTIO). The potential involvement of AtCRK5 protein kinase in the control of auxin-ROS-NO-PIN2-auxin regulatory loop is discussed.

AGC kinases and MAB4/MEL proteins maintain PIN polarity by limiting lateral diffusion in plant cells

Polar subcellular localization of the PIN exporters of the phytohormone auxin is a key determinant of directional, intercellular auxin transport and thus a central topic of both plant cell and developmental biology. Arabidopsis mutants lacking PID, a kinase that phosphorylates PINs, or the MAB4/MEL proteins of unknown molecular function display PIN polarity defects and phenocopy pin mutants, but mechanistic insights into how these factors convey PIN polarity are missing. Here, by combining protein biochemistry with quantitative live-cell imaging, we demonstrate that PINs, MAB4/MELs, and AGC kinases interact in the same complex at the plasma membrane. MAB4/MELs are recruited to the plasma membrane by the PINs and in concert with the AGC kinases maintain PIN polarity through limiting lateral diffusion-based escape of PINs from the polar domain. The PIN-MAB4/MEL-PID protein complex has self-reinforcing properties thanks to positive feedback between AGC kinase-mediated PIN phosphorylation and MAB4/MEL recruitment. We thus uncover the molecular mechanism by which AGC kinases and MAB4/MEL proteins regulate PIN localization and plant development.

•

MAB4/MEL proteins are recruited to the plasma membrane by PINs

•

PINs, MAB4/MELs, and AGC kinases directly interact in a multiprotein complex

•

PIN phosphorylation and MAB4/MEL recruitment form a positive feedback loop

•

MAB4/MELs and AGC kinases maintain PIN polarity by limiting PIN lateral diffusion

Auxin and Root Gravitropism: Addressing Basic Cellular Processes by Exploiting a Defined Growth Response

Root architecture and growth are decisive for crop performance and yield, and thus a highly topical research field in plant sciences. The root system of the model plant Arabidopsis thaliana is the ideal system to obtain insights into fundamental key parameters and molecular players involved in underlying regulatory circuits of root growth, particularly in responses to environmental stimuli. Root gravitropism, directional growth along the gravity, in particular represents a highly sensitive readout, suitable to study adjustments in polar auxin transport and to identify molecular determinants involved. This review strives to summarize and give an overview into the function of PIN-FORMED auxin transport proteins, emphasizing on their sorting and polarity control. As there already is an abundance of information, the focus lies in integrating this wealth of information on mechanisms and pathways. This overview of a highly dynamic and complex field highlights recent developments in understanding the role of auxin in higher plants. Specifically, it exemplifies, how analysis of a single, defined growth response contributes to our understanding of basic cellular processes in general.

Modulation of plant root growth by nitrogen source-defined regulation of polar auxin transport

Availability of the essential macronutrient nitrogen in soil plays a critical role in plant growth, development, and impacts agricultural productivity. Plants have evolved different strategies for sensing and responding to heterogeneous nitrogen distribution. Modulation of root system architecture, including primary root growth and branching, is among the most essential plant adaptions to ensure adequate nitrogen acquisition. However, the immediate molecular pathways coordinating the adjustment of root growth in response to distinct nitrogen sources, such as nitrate or ammonium, are poorly understood. Here, we show that growth as manifested by cell division and elongation is synchronized by coordinated auxin flux between two adjacent outer tissue layers of the root. This coordination is achieved by nitrate-dependent dephosphorylation of the PIN2 auxin efflux carrier at a previously uncharacterized phosphorylation site, leading to subsequent PIN2 lateralization and thereby regulating auxin flow between adjacent tissues. A dynamic computer model based on our experimental data successfully recapitulates experimental observations. Our study provides mechanistic insights broadening our understanding of root growth mechanisms in dynamic environments.

Pho-view of Auxin: Reversible Protein Phosphorylation in Auxin Biosynthesis, Transport and Signaling

The phytohormone auxin plays a central role in shaping plant growth and development. With decades of genetic and biochemical studies, numerous core molecular components and their networks, underlying auxin biosynthesis, transport, and signaling, have been identified. Notably, protein phosphorylation, catalyzed by kinases and oppositely hydrolyzed by phosphatases, has been emerging to be a crucial type of post-translational modification, regulating physiological and developmental auxin output at all levels. In this review, we comprehensively discuss earlier and recent advances in our understanding of genetics, biochemistry, and cell biology of the kinases and phosphatases participating in auxin action. We provide insights into the mechanisms by which reversible protein phosphorylation defines developmental auxin responses, discuss current challenges, and provide our perspectives on future directions involving the integration of the control of protein phosphorylation into the molecular auxin network.

Cellular requirements for PIN polar cargo clustering in Arabidopsis thaliana

Cell and tissue polarization is fundamental for plant growth and morphogenesis. The polar, cellular localization of Arabidopsis PIN-FORMED (PIN) proteins is crucial for their function in directional auxin transport. The clustering of PIN polar cargoes within the plasma membrane has been proposed to be important for the maintenance of their polar distribution. However, the more detailed features of PIN clusters and the cellular requirements of cargo clustering remain unclear. Here, we characterized PIN clusters in detail by means of multiple advanced microscopy and quantification methods, such as 3D quantitative imaging or freeze-fracture replica labeling. The size and aggregation types of PIN clusters were determined by electron microscopy at the nanometer level at different polar domains and at different developmental stages, revealing a strong preference for clustering at the polar domains. Pharmacological and genetic studies revealed that PIN clusters depend on phosphoinositol pathways, cytoskeletal structures and specific cell-wall components as well as connections between the cell wall and the plasma membrane. This study identifies the role of different cellular processes and structures in polar cargo clustering and provides initial mechanistic insight into the maintenance of polarity in plants and other systems.

Auxin-Regulated Reversible Inhibition of TMK1 Signaling by MAKR2 Modulates the Dynamics of Root Gravitropism

Plants are able to orient their growth according to gravity, which ultimately controls both shoot and root architecture.¹ Gravitropism is a dynamic process whereby gravistimulation induces the asymmetric distribution of the plant hormone auxin, leading to asymmetric growth, organ bending, and subsequent reset of auxin distribution back to the original pre-gravistimulation situation.1, 2, 3 Differential auxin accumulation during the gravitropic response depends on the activity of polarly localized PIN-FORMED (PIN) auxin-efflux carriers.1, 2, 3, 4 In particular, the timing of this dynamic response is regulated by PIN2,^5,6 but the underlying molecular mechanisms are poorly understood. Here, we show that MEMBRANE ASSOCIATED KINASE REGULATOR2 (MAKR2) controls the pace of the root gravitropic response. We found that MAKR2 is required for the PIN2 asymmetry during gravitropism by acting as a negative regulator of the cell-surface signaling mediated by the receptor-like kinase TRANSMEMBRANE KINASE1 (TMK1).^2,7, 8, 9, 10 Furthermore, we show that the MAKR2 inhibitory effect on TMK1 signaling is antagonized by auxin itself, which triggers rapid MAKR2 membrane dissociation in a TMK1-dependent manner. Our findings suggest that the timing of the root gravitropic response is orchestrated by the reversible inhibition of the TMK1 signaling pathway at the cell surface.

•

MAKR2 is co-expressed with PIN2 and regulates the pace of root gravitropism

•

MAKR2 controls PIN2 asymmetric accumulation at the root level during gravitropism

•

MAKR2 binds to and is a negative regulator of the TMK1 receptor kinase

•

Auxin antagonizes the MAKR2 inhibition of TMK1 by delocalizing MAKR2 in the cytosol

Rice plants respond to ammonium stress by adopting a helical root growth pattern

High levels of ammonium nutrition reduce plant growth and different plant species have developed distinct strategies to maximize ammonium acquisition while alleviating ammonium toxicity through modulating root growth. To date, the mechanisms underlying plant tolerance or sensitivity towards ammonium remain unclear. Rice (Oryza sativa) uses ammonium as its main N source. Here we show that ammonium supply restricts rice root elongation and induces a helical growth pattern, which is attributed to root acidification resulting from ammonium uptake. Ammonium-induced low pH triggers the asymmetric distribution of auxin in rice root tips through changes in auxin signaling, thereby inducing a helical growth response. Blocking auxin signaling completely inhibited this root response. In contrast, this root response is not activated in ammonium-treated Arabidopsis. Acidification of Arabidopsis roots leads to the protonation of indole-3-acetic acid and dampening of the intracellular auxin signaling levels that are required for maintaining root growth. Our study suggests a different mode of action by ammonium on the root pattern and auxin response machinery in rice versus Arabidopsis, and the rice-specific helical root response towards ammonium is an expression of the ability of rice to moderate auxin signaling and root growth to utilize ammonium while confronting acidic stress.

FERONIA mediates root nutating growth

Root nutation indicates the behavior that roots grow in a waving and skewing way due to unequal growth rates on different sides. Although a few developmental and environmental factors have been reported, genetic pathways mediating this process are obscure. We report here that the Arabidopsis CrRLK1L family member FERONIA (FER) is critical for root nutation. Functional loss of FER resulted in enhanced root waviness on tilted plates or roots forming anti-clockwise coils on horizontal plates. Suppressing polar auxin transport, either by pharmacological treatment or by introducing mutations at PIN-FORMED2 (PIN2) or AUXIN RESISTANT1 (AUX1), suppressed the asymmetric root growth (ARG) in fer-4, a null mutant of FER, indicating that FER suppression of ARG depends on polar auxin transport. We further showed by pharmacological treatments that dynamic microtubule organization and Ca²⁺ signaling are both critical for FER-mediated ARG. Results presented here demonstrate a key role of FER in mediating root nutating growth, through PIN2- and AUX1-mediated auxin transport, through dynamic microtubule organization, and through Ca²⁺ signaling.

Directional auxin fluxes in plants by intramolecular domain-domain coevolution of PIN auxin transporters

Morphogenesis and adaptive tropic growth in plants depend on gradients of the phytohormone auxin, mediated by the membrane-based PIN-FORMED (PIN) auxin transporters. PINs localize to a particular side of the plasma membrane (PM) or to the endoplasmic reticulum (ER) to directionally transport auxin and maintain intercellular and intracellular auxin homeostasis, respectively. However, the molecular cues that confer their diverse cellular localizations remain largely unknown. In this study, we systematically swapped the domains between ER- and PM-localized PIN proteins, as well as between apical and basal PM-localized PINs from Arabidopsis thaliana, to shed light on why PIN family members with similar topological structures reside at different membrane compartments within cells. Our results show that not only do the N- and C-terminal transmembrane domains (TMDs) and central hydrophilic loop contribute to their differential subcellular localizations and cellular polarity, but that the pairwise-matched N- and C-terminal TMDs resulting from intramolecular domain-domain coevolution are also crucial for their divergent patterns of localization. These findings illustrate the complexity of the evolutionary path of PIN proteins in acquiring their plethora of developmental functions and adaptive growth in plants.

Strigolactones inhibit auxin feedback on PIN-dependent auxin transport canalization

Directional transport of the phytohormone auxin is a versatile, plant-specific mechanism regulating many aspects of plant development. The recently identified plant hormones, strigolactones (SLs), are implicated in many plant traits; among others, they modify the phenotypic output of PIN-FORMED (PIN) auxin transporters for fine-tuning of growth and developmental responses. Here, we show in pea and Arabidopsis that SLs target processes dependent on the canalization of auxin flow, which involves auxin feedback on PIN subcellular distribution. D14 receptor- and MAX2 F-box-mediated SL signaling inhibits the formation of auxin-conducting channels after wounding or from artificial auxin sources, during vasculature de novo formation and regeneration. At the cellular level, SLs interfere with auxin effects on PIN polar targeting, constitutive PIN trafficking as well as clathrin-mediated endocytosis. Our results identify a non-transcriptional mechanism of SL action, uncoupling auxin feedback on PIN polarity and trafficking, thereby regulating vascular tissue formation and regeneration.

The bZIP53-IAA4 module inhibits adventitious root development in Populus

Adventitious roots (ARs) are important for some plants that depend on clonal propagation. In this study, we demonstrate that a salt-responsive gene module is involved in the negative regulation of AR development in poplar. In this module, the expression of bZIP53 is induced by salt stress and it encodes a transcription factor with transactivation activity. Overexpression or induced expression of bZIP53 in poplar lines resulted in inhibition of AR growth, while heterologous overexpression of bZIP53 in Arabidopsis resulted in a similar phenotype. Results from RNA-seq and RT-qPCR assays predicted IAA4-1 and IAA4-2 to be downstream genes that were regulated by bZIP53. Further investigation of protein-DNA interactions using yeast one-hybrid, electrophoretic mobility shift, dual luciferase reporter, and GUS co-expression assays also showed that IAA4-1/2 were the genes that were directly regulated by bZIP53. Induced-expression IAA4-1/2 transgenic poplar lines also showed inhibited AR growth. In addition, both poplar bZIP53 and IAA4-1/2 showed a response to salt stress. On the basis of these results, we conclude that the bZIP53-IAA4 module is involved in the negative regulation of AR development in poplar.

All Roads Lead to Auxin: Post-translational Regulation of Auxin Transport by Multiple Hormonal Pathways

Auxin is a key hormonal regulator, that governs plant growth and development in concert with other hormonal pathways. The unique feature of auxin is its polar, cell-to-cell transport that leads to the formation of local auxin maxima and gradients, which coordinate initiation and patterning of plant organs. The molecular machinery mediating polar auxin transport is one of the important points of interaction with other hormones. Multiple hormonal pathways converge at the regulation of auxin transport and form a regulatory network that integrates various developmental and environmental inputs to steer plant development. In this review, we discuss recent advances in understanding the mechanisms that underlie regulation of polar auxin transport by multiple hormonal pathways. Specifically, we focus on the post-translational mechanisms that contribute to fine-tuning of the abundance and polarity of auxin transporters at the plasma membrane and thereby enable rapid modification of the auxin flow to coordinate plant growth and development.

Arabidopsis Flippases Cooperate with ARF GTPase Exchange Factors to Regulate the Trafficking and Polarity of PIN Auxin Transporters

Cell polarity is a fundamental feature of all multicellular organisms. PIN auxin transporters are important cell polarity markers that play crucial roles in a plethora of developmental processes in plants. Here, to identify components involved in cell polarity establishment and maintenance in plants, we performed a forward genetic screening of PIN2:PIN1-HA;pin2 Arabidopsis (Arabidopsis thaliana) plants, which ectopically express predominantly basally localized PIN1 in root epidermal cells, leading to agravitropic root growth. We identified the regulator of PIN polarity 12 (repp12) mutation, which restored gravitropic root growth and caused a switch in PIN1-HA polarity from the basal to apical side of root epidermal cells. Next Generation Sequencing and complementation experiments established the causative mutation of repp12 as a single amino acid exchange in Aminophospholipid ATPase3 (ALA3), a phospholipid flippase predicted to function in vesicle formation. repp12 and ala3 T-DNA mutants show defects in many auxin-regulated processes, asymmetric auxin distribution, and PIN trafficking. Analysis of quintuple and sextuple mutants confirmed the crucial roles of ALA proteins in regulating plant development as well as PIN trafficking and polarity. Genetic and physical interaction studies revealed that ALA3 functions together with the ADP ribosylation factor GTPase exchange factors GNOM and BIG3 in regulating PIN polarity, trafficking, and auxin-mediated development.

Transcription factor WRKY23 is involved in ammonium-induced repression of Arabidopsis primary root growth under ammonium toxicity

Although WRKY transcription factors (TFs) are known to be involved in the regulation of plant root development, the mechanisms by which these TFs regulate plant tolerance to ammonium (NH₄⁺) toxicity remain unclear. To identify the molecular mechanisms underlying NH₄⁺-induced repression of primary root growth and NH₄⁺ sensitivity in Arabidopsis, wild-type (Col-0) and mutant (wrky23) plants were treated with 10 mM KNO₃ (control) or 5 mM (NH₄)₂SO₄ (NH₄⁺ toxicity) for 7 days. Under NH₄⁺ toxicity, the fresh weight of wrky23 mutant was significantly lower than that of Col-0 plants, and the NH₄⁺ concentration in wrky23 roots was significantly higher than that in Col-0 roots. However, we observed no significant differences between the two genotypes under the control treatment. Ammonium transporter AMT1;2 expression was induced in wrky23 roots but not in Col-0 roots. The transcript levels of cytosolic glutamine synthetase-encoding genes and activity of glutamine synthetase did not differ significantly between wrky23 and Col-0. Furthermore, the fluorescence and staining patterns of DR5::GFP and DR5::GUS, respectively, were more pronounced under NH₄⁺ toxicity than under the control treatment. Collectively, our results indicate that AMT1;2 expression was induced in the wrky23 mutant in response to NH₄⁺ toxicity, leading to NH₄⁺ accumulation in the roots and primary root growth repression. Under NH₄⁺ toxicity, both auxin transport and distribution were affected, and auxin accumulation in the root tips inhibited primary root growth in the wrky23 mutant. Our study provides important insights into the molecular mechanisms by which WRKY23 TF regulates plant responses to NH₄⁺ toxicity.

The lipid code-dependent phosphoswitch PDK1-D6PK activates PIN-mediated auxin efflux in Arabidopsis

Directional intercellular transport of the phytohormone auxin mediated by PIN-FORMED (PIN) efflux carriers has essential roles in both coordinating patterning processes and integrating multiple external cues by rapidly redirecting auxin fluxes. PIN activity is therefore regulated by multiple internal and external cues, for which the underlying molecular mechanisms are not fully elucidated. Here, we demonstrate that 3'-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1 (PDK1), which is conserved in plants and mammals, functions as a molecular hub that perceives upstream lipid signalling and modulates downstream substrate activity through phosphorylation. Using genetic analysis, we show that the loss-of-function Arabidopsis pdk1.1 pdk1.2 mutant exhibits a plethora of abnormalities in organogenesis and growth due to defective polar auxin transport. Further cellular and biochemical analyses reveal that PDK1 phosphorylates D6 protein kinase, a well-known upstream activator of PIN proteins. We uncover a lipid-dependent phosphorylation cascade that connects membrane-composition-based cellular signalling with plant growth and patterning by regulating morphogenetic auxin fluxes.

Arp2/3 Complex Is Required for Auxin-Driven Cell Expansion Through Regulation of Auxin Transporter Homeostasis

The Arp2/3 complex is an actin nucleator shown to be required throughout plant morphogenesis, contributing to processes such as cell expansion, tissue differentiation or cell wall assembly. A recent publication demonstrated that plants lacking functional Arp2/3 complex also present defects in auxin distribution and transport. This work shows that Arp2/3 complex subunits are predominantly expressed in the provasculature, although other plant tissues also show promoter activity (e.g., cotyledons, apical meristems, or root tip). Moreover, auxin can trigger subunit expression, indicating a role of this phytohormone in mediating the complex activity. Further investigation of the functional interaction between Arp2/3 complex and auxin signaling also reveals their cooperation in determining pavement cell shape, presumably through the role of Arp2/3 complex in the correct auxin carrier trafficking. Young seedlings of arpc5 mutants show increased auxin-triggered proteasomal degradation of DII-VENUS and altered PIN3 distribution, with higher levels of the protein in the vacuole. Closer observation of vacuolar morphology revealed the presence of a more fragmented vacuolar compartment when Arp2/3 function is abolished, hinting a generalized role of Arp2/3 complex in endomembrane function and protein trafficking.

Auxin guides roots to avoid obstacles during gravitropic growth

This article is a Commentary on https://doi.org/10.1111/nph.16076.

Polar expedition: mechanisms for protein polar localization

Most cells show asymmetry in their shape or in the organization of their components that results in poles with different properties. This is a fundamental feature that participates in modulating the development of an organism and its responses to external stimuli. In plants, a number of proteins that are important for developmental and physiological processes have been shown to display polar localization. However, how these polarities are established, maintained, or dynamically modulated is still largely unclear for most of these proteins. In this review we report recent updates on the mechanisms of polar protein localization, focusing on a subset of these proteins that are the focus of current research efforts.

Root Growth Adaptation is Mediated by PYLs ABA Receptor-PP2A Protein Phosphatase Complex

Plant root architecture dynamically adapts to various environmental conditions, such as salt-containing soil. The phytohormone abscisic acid (ABA) is involved among others also in these developmental adaptations, but the underlying molecular mechanism remains elusive. Here, a novel branch of the ABA signaling pathway in Arabidopsis involving PYR/PYL/RCAR (abbreviated as PYLs) receptor-protein phosphatase 2A (PP2A) complex that acts in parallel to the canonical PYLs-protein phosphatase 2C (PP2C) mechanism is identified. The PYLs-PP2A signaling modulates root gravitropism and lateral root formation through regulating phytohormone auxin transport. In optimal conditions, PYLs ABA receptor interacts with the catalytic subunits of PP2A, increasing their phosphatase activity and thus counteracting PINOID (PID) kinase-mediated phosphorylation of PIN-FORMED (PIN) auxin transporters. By contrast, in salt and osmotic stress conditions, ABA binds to PYLs, inhibiting the PP2A activity, which leads to increased PIN phosphorylation and consequently modulated directional auxin transport leading to adapted root architecture. This work reveals an adaptive mechanism that may flexibly adjust plant root growth to withstand saline and osmotic stresses. It occurs via the cross-talk between the stress hormone ABA and the versatile developmental regulator auxin.

Salicylic Acid Targets Protein Phosphatase 2A to Attenuate Growth in Plants

Plants, like other multicellular organisms, survive through a delicate balance between growth and defense against pathogens. Salicylic acid (SA) is a major defense signal in plants, and the perception mechanism as well as downstream signaling activating the immune response are known. Here, we identify a parallel SA signaling that mediates growth attenuation. SA directly binds to A subunits of protein phosphatase 2A (PP2A), inhibiting activity of this complex. Among PP2A targets, the PIN2 auxin transporter is hyperphosphorylated in response to SA, leading to changed activity of this important growth regulator. Accordingly, auxin transport and auxin-mediated root development, including growth, gravitropic response, and lateral root organogenesis, are inhibited. This study reveals how SA, besides activating immunity, concomitantly attenuates growth through crosstalk with the auxin distribution network. Further analysis of this dual role of SA and characterization of additional SA-regulated PP2A targets will provide further insights into mechanisms maintaining a balance between growth and defense.

•

SA modulates root development independently of NPR1-mediated canonical signaling

•

SA attenuates growth through crosstalk with the auxin transport network

•

SA upregulates the phosphorylation status of PIN auxin efflux carriers through PP2A

•

SA directly targets A subunits of PP2A, inhibiting the activity of the complex

NRT1.1-Mediated Nitrate Suppression of Root Coiling Relies on PIN2- and AUX1-Mediated Auxin Transport

Asymmetric root growth (ARG) on tilted plates, or root coiling on horizontally placed plates, is proposed to be a combination of gravitropism, mechanical sensing, and "circumnutation," a word designated by Charles Darwin to describe the helical movement that all plant organs make around the growth direction. ARG is developmentally controlled in which microtubule-regulating proteins and the phytohormone auxin participates. Nutrient deficiency influences ARG. However, it is unclear which nutrients play key roles in regulating ARG, what endogenous components are involved in responding to nutrient deficiency for ARG, and how nutrient deficiency is translated into endogenous responses. We report here that nitrate deficiency resulted in a strong ARG in Arabidopsis. Nitrate deficiency caused root coiling on horizontal plates, which is inhibited by an auxin transport inhibitor, and by mutations in PIN-FORMED2 (PIN2) and AUXIN RESISTANT 1 (AUX1). We further show that suppression of ARG by nitrate is mediated by the nitrate transporter/sensor NRT1.1. In addition, PIN2- and AUX1-mediated auxin transports are epistatic to NRT1.1 in nitrate deficiency-induced ARG. This study reveals a signaling pathway in root growth by responding to exogenous nitrate and relaying it into altered auxin transport.

Brassinosteroid signaling delimits root gravitropism via sorting of the Arabidopsis PIN2 auxin transporter

Arabidopsis PIN2 protein directs transport of the phytohormone auxin from the root tip into the root elongation zone. Variation in hormone transport, which depends on a delicate interplay between PIN2 sorting to and from polar plasma membrane domains, determines root growth. By employing a constitutively degraded version of PIN2, we identify brassinolides as antagonists of PIN2 endocytosis. This response does not require de novo protein synthesis, but involves early events in canonical brassinolide signaling. Brassinolide-controlled adjustments in PIN2 sorting and intracellular distribution governs formation of a lateral PIN2 gradient in gravistimulated roots, coinciding with adjustments in auxin signaling and directional root growth. Strikingly, simulations indicate that PIN2 gradient formation is no prerequisite for root bending but rather dampens asymmetric auxin flow and signaling. Crosstalk between brassinolide signaling and endocytic PIN2 sorting, thus, appears essential for determining the rate of gravity-induced root curvature via attenuation of differential cell elongation.

Remove, Recycle, Degrade: Regulating Plasma Membrane Protein Accumulation

Interactions between plant cells and the environment rely on modulation of protein receptors, transporters, channels, and lipids at the plasma membrane (PM) to facilitate intercellular communication, nutrient uptake, environmental sensing, and directional growth. These functions are fine-tuned by cellular pathways maintaining or reducing particular proteins at the PM. Proteins are endocytosed, and their fate is decided between recycling and degradation to modulate localization, abundance, and activity. Selective autophagy is another pathway regulating PM protein accumulation in response to specific conditions or developmental signals. The mechanisms regulating recycling, degradation, and autophagy have been studied extensively, yet we are just now addressing their regulation and coordination. Here, we (1) provide context concerning regulation of protein accumulation, recycling, or degradation by overviewing endomembrane trafficking; (2) discuss pathways regulating recycling and degradation in terms of cellular roles and cargoes; (3) review plant selective autophagy and its physiological significance; (4) focus on two decision-making mechanisms: regulation of recycling versus degradation of PM proteins and coordination between autophagy and vacuolar degradation; and (5) identify future challenges.

Auxin-mediated statolith production for root gravitropism

Root gravitropism is one of the most important processes allowing plant adaptation to the land environment. Auxin plays a central role in mediating root gravitropism, but how auxin contributes to gravitational perception and the subsequent response are still unclear. Here, we showed that the local auxin maximum/gradient within the root apex, which is generated by the PIN directional auxin transporters, regulates the expression of three key starch granule synthesis genes, SS4, PGM and ADG1, which in turn influence the accumulation of starch granules that serve as a statolith perceiving gravity. Moreover, using the cvxIAA-ccvTIR1 system, we also showed that TIR1-mediated auxin signaling is required for starch granule formation and gravitropic response within root tips. In addition, axr3 mutants showed reduced auxin-mediated starch granule accumulation and disruption of gravitropism within the root apex. Our results indicate that auxin-mediated statolith production relies on the TIR1/AFB-AXR3-mediated auxin signaling pathway. In summary, we propose a dual role for auxin in gravitropism: the regulation of both gravity perception and response.

Evolution of fast root gravitropism in seed plants

An important adaptation during colonization of land by plants is gravitropic growth of roots, which enabled roots to reach water and nutrients, and firmly anchor plants in the ground. Here we provide insights into the evolution of an efficient root gravitropic mechanism in the seed plants. Architectural innovation, with gravity perception constrained in the root tips along with a shootward transport route for the phytohormone auxin, appeared only upon the emergence of seed plants. Interspecies complementation and protein domain swapping revealed functional innovations within the PIN family of auxin transporters leading to the evolution of gravitropism-specific PINs. The unique apical/shootward subcellular localization of PIN proteins is the major evolutionary innovation that connected the anatomically separated sites of gravity perception and growth response via the mobile auxin signal. We conclude that the crucial anatomical and functional components emerged hand-in-hand to facilitate the evolution of fast gravitropic response, which is one of the major adaptations of seed plants to dry land.