Modulation of abscisic acid signaling via endosomal TOL proteins.

The phytohormone abscisic acid (ABA) functions in the control of plant stress responses, particularly in drought stress. A significant mechanism in attenuating and terminating ABA signals involves regulated protein turnover, with certain ABA receptors, despite their main presence in the cytosol and nucleus, subjected to vacuolar degradation via the Endosomal Sorting Complex Required for Transport (ESCRT) machinery. Collectively our findings show that discrete TOM1-LIKE (TOL) proteins, which are functional ESCRT-0 complex substitutes in plants, affect the trafficking for degradation of core components of the ABA signaling and transport machinery. TOL2,3,5 and 6 modulate ABA signaling where they function additively in degradation of ubiquitinated ABA receptors and transporters. TOLs colocalize with their cargo in different endocytic compartments in the root epidermis and in guard cells of stomata, where they potentially function in ABA-controlled stomatal aperture. Although the tol2/3/5/6 quadruple mutant plant line is significantly more drought-tolerant and has a higher ABA sensitivity than control plant lines, it has no obvious growth or development phenotype under standard conditions, making the TOL genes ideal candidates for engineering to improved plant performance.

WAVY GROWTH Arabidopsis E3 ubiquitin ligases affect apical PIN sorting decisions.

Directionality in the intercellular transport of the plant hormone auxin is determined by polar plasma membrane localization of PIN-FORMED (PIN) auxin transport proteins. However, apart from PIN phosphorylation at conserved motifs, no further determinants explicitly controlling polar PIN sorting decisions have been identified. Here we present Arabidopsis WAVY GROWTH 3 (WAV3) and closely related RING-finger E3 ubiquitin ligases, whose loss-of-function mutants show a striking apical-to-basal polarity switch in PIN2 localization in root meristem cells. WAV3 E3 ligases function as essential determinants for PIN polarity, acting independently from PINOID/WAG-dependent PIN phosphorylation. They antagonize ectopic deposition of de novo synthesized PIN proteins already immediately following completion of cell division, presumably via preventing PIN sorting into basal, ARF GEF-mediated trafficking. Our findings reveal an involvement of E3 ligases in the selective targeting of apically localized PINs in higher plants.

Endosomally Localized RGLG-Type E3 RING-Finger Ligases Modulate Sorting of Ubiquitylation-Mimic PIN2.

Intracellular sorting and the abundance of sessile plant plasma membrane proteins are imperative for sensing and responding to environmental inputs. A key determinant for inducing adjustments in protein localization and hence functionality is their reversible covalent modification by the small protein modifier ubiquitin, which is for example responsible for guiding proteins from the plasma membrane to endosomal compartments. This mode of membrane protein sorting control requires the catalytic activity of E3 ubiquitin ligases, amongst which members of the RING DOMAIN LIGASE (RGLG) family have been implicated in the formation of lysine 63-linked polyubiquitin chains, serving as a prime signal for endocytic vacuolar cargo sorting. Nevertheless, except from some indirect implications for such RGLG activity, no further evidence for their role in plasma membrane protein sorting has been provided so far. Here, by employing RGLG1 reporter proteins combined with assessment of plasma membrane protein localization in a rglg1 rglg2 loss-of-function mutant, we demonstrate a role for RGLGs in cargo trafficking between plasma membrane and endosomal compartments. Specifically, our findings unveil a requirement for RGLG1 association with endosomal sorting compartments for fundamental aspects of plant morphogenesis, underlining a vital importance for ubiquitylation-controlled intracellular sorting processes.

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.

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.

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.

TOLs Function as Ubiquitin Receptors in the Early Steps of the ESCRT Pathway in Higher Plants.

Protein abundance and localization at the plasma membrane (PM) shapes plant development and mediates adaptation to changing environmental conditions. It is regulated by ubiquitination, a post-translational modification crucial for the proper sorting of endocytosed PM proteins to the vacuole for subsequent degradation. To understand the significance and the variety of roles played by this reversible modification, the function of ubiquitin receptors, which translate the ubiquitin signature into a cellular response, needs to be elucidated. In this study, we show that TOL (TOM1-like) proteins function in plants as multivalent ubiquitin receptors, governing ubiquitinated cargo delivery to the vacuole via the conserved Endosomal Sorting Complex Required for Transport (ESCRT) pathway. TOL2 and TOL6 interact with components of the ESCRT machinery and bind to K63-linked ubiquitin via two tandemly arranged conserved ubiquitin-binding domains. Mutation of these domains results not only in a loss of ubiquitin binding but also altered localization, abolishing TOL6 ubiquitin receptor activity. Function and localization of TOL6 is itself regulated by ubiquitination, whereby TOL6 ubiquitination potentially modulates degradation of PM-localized cargoes, assisting in the fine-tuning of the delicate interplay between protein recycling and downregulation. Taken together, our findings demonstrate the function and regulation of a ubiquitin receptor that mediates vacuolar degradation of PM proteins in higher plants.

PIN FORMED 2 Modulates the Transport of Arsenite in Arabidopsis thaliana.

Arsenic contamination is a major environmental issue, as it may lead to serious health hazard. The reduced trivalent form of inorganic arsenic, arsenite, is in general more toxic to plants compared with the fully oxidized pentavalent arsenate. The uptake of arsenite in plants has been shown to be mediated through a large subfamily of plant aquaglyceroporins, nodulin 26-like intrinsic proteins (NIPs). However, the efflux mechanisms, as well as the mechanism of arsenite-induced root growth inhibition, remain poorly understood. Using molecular physiology, synchrotron imaging, and root transport assay approaches, we show that the cellular transport of trivalent arsenicals in Arabidopsis thaliana is strongly modulated by PIN FORMED 2 (PIN2) auxin efflux transporter. Root transport assay using radioactive arsenite, X-ray fluorescence imaging (XFI) coupled with X-ray absorption spectroscopy (XAS), and inductively coupled plasma mass spectrometry analysis revealed that pin2 plants accumulate higher concentrations of arsenite in roots compared with the wild-type. At the cellular level, arsenite specifically targets intracellular sorting of PIN2 and thereby alters the cellular auxin homeostasis. Consistently, loss of PIN2 function results in arsenite hypersensitivity in roots. XFI coupled with XAS further revealed that loss of PIN2 function results in specific accumulation of arsenical species, but not the other metals such as iron, zinc, or calcium in the root tip. Collectively, these results suggest that PIN2 likely functions as an arsenite efflux transporter for the distribution of arsenical species in planta.

Auxin signalling in growth: Schrödinger's cat out of the bag.

The phytohormone auxin acts as an amazingly versatile coordinator of plant growth and development. With its morphogen-like properties, auxin controls sites and timing of differentiation and/or growth responses both, in quantitative and qualitative terms. Specificity in the auxin response depends largely on distinct modes of signal transmission, by which individual cells perceive and convert auxin signals into a remarkable diversity of responses. The best understood, or so-called canonical mechanism of auxin perception ultimately results in variable adjustments of the cellular transcriptome, via a short, nuclear signal transduction pathway. Additional findings that accumulated over decades implied that an additional, presumably, cell surface-based auxin perception mechanism mediates very rapid cellular responses and decisively contributes to the cell's overall hormonal response. Recent investigations into both, nuclear and cell surface auxin signalling challenged this assumed partition of roles for different auxin signalling pathways and revealed an unexpected complexity in transcriptional and non-transcriptional cellular responses mediated by auxin.

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.

Evolutionary Conserved Cysteines Function as cis-Acting Regulators of Arabidopsis PIN-FORMED 2 Distribution.

Coordination of plant development requires modulation of growth responses that are under control of the phytohormone auxin. PIN-FORMED plasma membrane proteins, involved in intercellular transport of the growth regulator, are key to the transmission of such auxin signals and subject to multilevel surveillance mechanisms, including reversible post-translational modifications. Apart from well-studied PIN protein modifications, namely phosphorylation and ubiquitylation, no further post-translational modifications have been described so far. Here, we focused on root-specific Arabidopsis PIN2 and explored functional implications of two evolutionary conserved cysteines, by a combination of in silico and molecular approaches. PIN2 sequence alignments and modeling predictions indicated that both cysteines are facing the cytoplasm and therefore would be accessible to redox status-controlled modifications. Notably, mutant pin2C-A alleles retained functionality, demonstrated by their ability to almost completely rescue defects of a pin2 null allele, whereas high resolution analysis of pin2C-A localization revealed increased intracellular accumulation, and altered protein distribution within plasma membrane micro-domains. The observed effects of cysteine replacements on root growth and PIN2 localization are consistent with a model in which redox status-dependent cysteine modifications participate in the regulation of PIN2 mobility, thereby fine-tuning polar auxin transport.

PPP1, a plant-specific regulator of transcription controls Arabidopsis development and PIN expression.

Directional transport of auxin is essential for plant development, with PIN auxin transport proteins representing an integral part of the machinery that controls hormone distribution. However, unlike the rapidly emerging framework of molecular determinants regulating PIN protein abundance and subcellular localization, insights into mechanisms controlling PIN transcription are still limited. Here we describe PIN2 PROMOTER BINDING PROTEIN 1 (PPP1), an evolutionary conserved plant-specific DNA binding protein that acts on transcription of PIN genes. Consistent with PPP1 DNA-binding activity, PPP1 reporter proteins are nuclear localized and analysis of PPP1 null alleles and knockdown lines indicated a function as a positive regulator of PIN expression. Furthermore, we show that ppp1 pleiotropic mutant phenotypes are partially reverted by PIN overexpression, and results are presented that underline a role of PPP1-PIN promoter interaction in PIN expression control. Collectively, our findings identify an elementary, thus far unknown, plant-specific DNA-binding protein required for post-embryonic plant development, in general, and correct expression of PIN genes, in particular.

Tricho- and atrichoblast cell files show distinct PIN2 auxin efflux carrier exploitations and are jointly required for defined auxin-dependent root organ growth.

The phytohormone auxin is a vital growth regulator in plants. In the root epidermis auxin steers root organ growth. However, the mechanisms that allow adjacent tissues to integrate growth are largely unknown. Here, the focus is on neighbouring epidermal root tissues to assess the integration of auxin-related growth responses. The pharmacologic, genetic, and live-cell imaging approaches reveal that PIN2 auxin efflux carriers are differentially controlled in tricho- and atrichoblast cells. PIN2 proteins show lower abundance at the plasma membrane of trichoblast cells, despite showing higher rates of intracellular trafficking in these cells. The data suggest that PIN2 proteins display distinct cell-type-dependent trafficking rates to the lytic vacuole for degradation. Based on this insight, it is hypothesized that auxin-dependent processes are distinct in tricho- and atrichoblast cells. Moreover, genetic interference with epidermal patterning supports this assumption and suggests that tricho- and atrichoblasts have distinct importance for auxin-sensitive root growth and gravitropic responses.

DIAGEOTROPICA: news from the auxin swamp.

Recently established links between the tomato cyclophilin A-type protein DIAGEOTROPICA and the regulation of polar auxin transport provide first mechanistic insights into the function of this enigmatic locus.

Meta-regulation of Arabidopsis auxin responses depends on tRNA maturation.

Polar transport of the phytohormone auxin throughout plants shapes morphogenesis and is subject to stringent and specific control. Here, we identify basic cellular activities connected to translational control of gene expression as sufficient to specify auxin-mediated development. Mutants in subunits of Arabidopsis Elongator, a protein complex modulating translational efficiency via maturation of tRNAs, exhibit defects in auxin-controlled developmental processes, associated with reduced abundance of PIN-formed (PIN) auxin transport proteins. Similar anomalies are observed upon interference with tRNA splicing by downregulation of RNA ligase (AtRNL), pointing to a general role of tRNA maturation in auxin signaling. Elongator Protein 6 (ELP6) and AtRNL expression patterns underline an involvement in adjusting PIN protein levels, whereas rescue of mutant defects by auxin indicates rate-limiting activities in auxin-controlled organogenesis. This emphasizes mechanisms in which auxin serves as a bottleneck for plant morphogenesis, translating common cellular activities into defined developmental readouts.

Ubiquitylation-mediated control of polar auxin transport: analysis of Arabidopsis PIN2 auxin transport protein.

Reversible, covalent modification by the small protein ubiquitin acts in a variety of pathways controlling protein fate in virtually all aspects of cellular function. For example, ubiquitylation of plasma membrane proteins modulates their intracellular sorting and turnover, thereby decisively influencing crosstalk between cells and their environment. In recent years, experimental work performed with the model plant Arabidopsis thaliana demonstrated ubiquitylation of a number of plasma membrane proteins, including the auxin efflux carrier protein PIN2. By using solubilized membrane protein immunoprecipitation assays, we established quantitative approaches, suitable for analysis of PIN2 ubiquitylation and variations therein. Applicability of this robust approach is not restricted to PIN auxin carriers, but could be extended to analysis of further plant membrane proteins that are controlled by variations in their ubiquitylation status.

The dynamics of plant plasma membrane proteins: PINs and beyond.

Plants are permanently situated in a fixed location and thus are well adapted to sense and respond to environmental stimuli and developmental cues. At the cellular level, several of these responses require delicate adjustments that affect the activity and steady-state levels of plasma membrane proteins. These adjustments involve both vesicular transport to the plasma membrane and protein internalization via endocytic sorting. A substantial part of our current knowledge of plant plasma membrane protein sorting is based on studies of PIN-FORMED (PIN) auxin transport proteins, which are found at distinct plasma membrane domains and have been implicated in directional efflux of the plant hormone auxin. Here, we discuss the mechanisms involved in establishing such polar protein distributions, focusing on PINs and other key plant plasma membrane proteins, and we highlight the pathways that allow for dynamic adjustments in protein distribution and turnover, which together constitute a versatile framework that underlies the remarkable capabilities of plants to adjust growth and development in their ever-changing environment.