SORTING NEXIN1 is required for modulating the trafficking and stability of the Arabidopsis IRON-REGULATED TRANSPORTER1

Quantitative tools for analyzing rhizosphere pH dynamics: localized and integrated approaches

The rhizosphere, the region surrounding plant roots, plays a critical role in nutrient acquisition, root development, and plant-soil interactions. Spatial variations in rhizosphere pH along the root axis are shaped by environmental cues, nutrient availability, microbial activity, and root growth patterns. Precise detection and quantification of these pH changes are essential for understanding plant plasticity and nutrient efficiency. Here, we present a refined methodology integrating pH indicator bromocresol purple with a rapid, non-destructive electrode-based system to visualize and quantify pH variations along the root axis, enabling high-resolution and scalable monitoring of root-induced pH changes in the rhizosphere. Using this approach, we investigated the impact of iron (Fe) availability on rhizosphere pH dynamics in wild-type (WT) and bHLH39-overexpressing (39Ox) seedlings. bHLH39, a key basic helix-loop-helix transcription factor in Fe uptake, enhances Fe acquisition when overexpressed, often leading to Fe toxicity and reduced root growth under Fe-sufficient conditions. However, its role in root-mediated acidification remains unclear. Our findings reveal that 39Ox plants exhibit enhanced rhizosphere acidification, whereas WT roots display zone-specific pH responses depending on Fe availability. To refine pH measurements, we developed two complementary electrode-based methodologies: localized rhizosphere pH change for region-specific assessment and integrated rhizosphere pH change for net root system variation. These techniques improve resolution, accuracy, and efficiency in large-scale experiments, providing robust tools for investigating natural and genetic variations in rhizosphere pH regulation and their role in nutrient mobilization and ecological adaptation.

The plant retromer components SNXs bind to ATG8 and CLASP to mediate autophagosome movement along microtubules

In eukaryotic cells, autophagosomes are double-membrane vesicles that are highly mobile and traffic along cytoskeletal tracks. While core autophagy-related proteins (ATGs) and other regulators involved in autophagosome biogenesis in plants have been extensively studied, the specific components regulating plant autophagosome motility remain elusive. In this study, using TurboID-based proximity labeling, we identify the retromer subcomplex comprising sorting nexin 1 (SNX1), SNX2a, and SNX2b as interacting partners of ATG8. Remarkably, SNX proteins decorate ATG8-labeled autophagosomes and facilitate their coordinated movement along microtubules. Depletion of SNX proteins restricts the motility of autophagosomes in the cytoplasm, resulting in decreased autophagic flux. Furthermore, we show that the microtubule-associated protein CLASP is a bridge, connecting the SNX-ATG8-decorated autophagosomes to the microtubules. Genetically, the clasp-1 mutant phenotype resembles that of plants with disrupted SNXs or microtubule networks, displaying diminished autophagosome motility and reduced autophagic flux. Collectively, our study unveils a hitherto unanticipated role of the SNXs subcomplex in connecting autophagosomes with microtubules to promote autophagosome mobility in Arabidopsis.

Function, structure, and regulation of Iron Regulated Transporter 1

Iron (Fe) is an essential mineral for the growth and development of plants, as it serves as a vital co-factor for a multitude of enzymes that participate in a variety of physiological processes. Plants obtain Fe from the soil through their Fe uptake systems. Non-graminaceous plants utilize a reduction-based system for Fe uptake, which involves the conversion of Fe(III) to Fe(II) and subsequent absorption of Fe(II). Iron-Regulated Transporter 1 (IRT1), a predominant transporter of Fe(II), is a central element of the Fe uptake mechanism in plants. In Arabidopsis thaliana, IRT1 exhibits a broad-spectrum of substrate specificity and functions as a transceptor, capable of sensing the levels of its non-Fe metal substrates. Over the past two decades, significant advancements have been achieved in understanding the functions and regulatory mechanisms of IRT1 and its orthologs across various plant species. This review provides a systematic overview of the functional attributes of IRT1, with a particular focus on the intricate regulatory mechanisms at the transcriptional, post-transcriptional, and post-translational levels that are pivotal in modulating the expression and activity of IRT1. Moreover, we offer insights and directions for future research on this important transporter.

SORTING NEXIN1 facilitates SALT OVERLY SENSITIVE1 protein accumulation to enhance salt tolerance in Arabidopsis

The plasma membrane (PM)-localized Na+/H+ antiporter Salt Overly Sensitive1 (SOS1) is essential for plant salt tolerance through facilitating Na+ efflux; however, how SOS1 localization and protein accumulation is regulated in plants remains elusive. Here, we report that Sorting Nexin 1 (SNX1) is required for plant salt-stress tolerance through affecting endosomal trafficking of SOS1 in Arabidopsis (Arabidopsis thaliana). Disruption of SNX1 caused salt hypersensitivity with increased Na+ accumulation and decreased Na+ efflux in Arabidopsis when challenged with high salinity stress. SNX1 co-localized and interacted with SOS1 in endosomes, promoting its PM localization and protein stability in plants under saline conditions. SOS1 overexpression promoted salt tolerance in the wild-type, whereas such effect was greatly compromised in the snx1-2 mutant. Pharmaceutical results showed that SOS1 recycling from the cytosol to the PM was largely blocked while its vacuolar degradation was accelerated in the snx1-2 mutant. Furthermore, salt-induced SOS1 phosphorylation enhanced its interaction and co-localization with SNX1, which is required for SOS1 PM localization in plants. Our study elucidates that SNX1 facilitates SOS1 PM localization and protein accumulation through endosomal trafficking, thereby enhancing salt tolerance in plants.

The small ARF-like 2 GTPase TITAN5 is linked with the dynamic regulation of IRON-REGULATED TRANSPORTER 1

Iron acquisition is crucial for plants. The abundance of IRON-REGULATED TRANSPORTER 1 (IRT1) is controlled through endomembrane trafficking, a process that requires small ARF-like GTPases. Only few components that are involved in the vesicular trafficking of specific cargo are known. Here, we report that the ARF-like GTPase TITAN5 (TTN5) interacts with the large cytoplasmic variable region and protein-regulatory platform of IRT1. Heterozygous ttn5-1 plants can display reduced root iron reductase activity. This activity is needed for iron uptake via IRT1. Fluorescent fusion proteins of TTN5 and IRT1 colocalize at locations where IRT1 sorting and cycling between the plasma membrane and the vacuole are coordinated. TTN5 can also interact with peripheral membrane proteins that are components of the IRT1 regulation machinery, like the trafficking factor SNX1, the C2 domain protein EHB1 and the SEC14-GOLD protein PATL2. Hence, the link between iron acquisition and vesicular trafficking involving a small GTPase of the ARF family opens up the possibility to study the involvement of TTN5 in nutritional cell biology and the endomembrane system.

Endocytic recycling in plants: pathways and regulation

Endocytic recycling is an intracellular trafficking pathway that returns endocytosed molecules to the plasma membrane via the recycling endosome. This pathway plays a crucial role in remodelling plasma membrane composition and is thus essential for cellular homeostasis. In plants, endocytic recycling regulates the localization and abundance of receptors, transporters, and channels at the plasma membrane that are involved in many aspects of plant growth and development. Despite its importance, the recycling endosome and the underlying sorting mechanisms for cargo recycling in plants remain understudied in comparison to the endocytic recycling pathways in animals. In this review, we focus on the cumulative evidence suggesting the existence of endosomes decorated by regulators that contribute to recycling in plant cells. We summarize the chemical inhibitors used for analysing cargo recycling and discuss recent advances in our understanding of how endocytic recycling participates in various plant cellular and physiological events.

Characterization of the small Arabidopsis thaliana GTPase and ADP-ribosylation factor-like 2 protein TITAN 5

Small GTPases switch between GDP- and GTP-bound states during cell signaling. The ADP-ribosylation factor (ARF) family of small GTPases is involved in vesicle trafficking. Although evolutionarily well conserved, little is known about ARF and ARF-like GTPases in plants. We characterized biochemical properties and cellular localization of the essential small ARF-like GTPase TITAN 5 (TTN5; also known as HALLIMASCH, ARL2 and ARLC1) from Arabidopsis thaliana, and two TTN5 proteins with point mutants in conserved residues, TTN5T30N and TTN5Q70L, that were expected to be unable to perform nucleotide exchange and GTP hydrolysis, respectively. TTN5 exhibited very rapid intrinsic nucleotide exchange and remarkably low GTP hydrolysis activity, functioning as a non-classical small GTPase being likely present in a GTP-loaded active form. We analyzed signals from YFP-TTN5 and HA3-TTN5 by in situ immunolocalization in Arabidopsis seedlings and through use of a transient expression system. Colocalization with endomembrane markers and pharmacological treatments suggests that TTN5 can be present at the plasma membrane and that it dynamically associates with membranes of vesicles, Golgi stacks and multivesicular bodies. Although TTN5Q70L mirrored wild-type TTN5 behavior, the TTN5T30N mutant differed in some aspects. Hence, the unusual rapid nucleotide exchange activity of TTN5 is linked with its membrane dynamics, and TTN5 likely has a role in vesicle transport within the endomembrane system.

Characterization of the small Arabidopsis thaliana GTPase and ADP-ribosylation factor-like 2 protein TITAN 5

Small GTPases function by conformational switching ability between GDP- and GTP-bound states in rapid cell signaling events. The ADP-ribosylation factor (ARF) family is involved in vesicle trafficking. Though evolutionarily well conserved, little is known about ARF and ARF-like GTPases in plants. Here, we characterized biochemical properties and cellular localization of the essential small ARF-like GTPase TITAN 5/HALLIMASCH/ARL2/ARLC1 (hereafter termed TTN5) from Arabidopsis thaliana. Two TTN5 variants were included in the study with point mutations at conserved residues, suspected to be functional for nucleotide exchange and GTP hydrolysis, TTN5T30N and TTN5Q70L. We found that TTN5 had a very rapid intrinsic nucleotide exchange capacity with a conserved nucleotide switching mechanism. TTN5 acted as a non-classical small GTPase with a remarkably low GTP hydrolysis activity, suggesting it is likely present in GTP-loaded active form in the cell. We analyzed signals from yellow fluorescent protein (YFP)-tagged TTN5 and from in situ immunolocalization of hemagglutine-tagged HA3-TTN5 in Arabidopsis seedlings and in a transient expression system. Together with colocalization using endomembrane markers and pharmacological treatments the microscopic analysis suggests that TTN5 can be present at the plasma membrane and dynamically associated with membranes of vesicles, Golgi stacks and multivesicular bodies. While the TTN5Q70L variant showed similar GTPase activities and localization behavior as wild-type TTN5, the TTN5T30N mutant differed in some aspects. Hence, the unusual capacity of rapid nucleotide exchange activity of TTN5 is linked with cell membrane dynamics, likely associated with vesicle transport pathways in the endomembrane system.

The plant-unique protein DRIF1 coordinates with sorting nexin 1 to regulate membrane protein homeostasis

Membrane protein homeostasis is fine-tuned by the cellular pathways for vacuolar degradation and recycling, which ultimately facilitate plant growth and cell-environment interactions. The endosomal sorting complex required for transport (ESCRT) machinery plays important roles in regulating intraluminal vesicle (ILV) formation and membrane protein sorting to vacuoles. We previously showed that the plant-specific ESCRT component FYVE DOMAIN PROTEIN REQUIRED FOR ENDOSOMAL SORTING1 (FREE1) performs multiple functions in plants, although the underlying mechanisms remain elusive. In this study, we performed a suppressor screen of the FREE1-RNAi mutant and identified and characterized 2 suppressor of free1 (sof) mutants in Arabidopsis (Arabidopsis thaliana). These mutants, sof10 and sof641, result in a premature stop codon or a missense mutation in AT5G10370, respectively. This gene was named DEAH and RING domain-containing protein as FREE1 suppressor 1 (DRIF1). DRIF1 has a homologous gene, DRIF2, in the Arabidopsis genome with 95% identity to DRIF1. The embryos of drif1 drif2 mutants arrested at the globular stage and formed enlarged multivesicular bodies (MVBs) with an increased number of ILVs. DRIF1 is a membrane-associated protein that coordinates with retromer component sorting nexin 1 to regulate PIN-FORMED2 recycling to the plasma membrane. Altogether, our data demonstrate that DRIF1 is a unique retromer interactor that orchestrates FREE1-mediated ILV formation of MVBs and vacuolar sorting of membrane proteins for degradation in plants.

Calcium-dependent protein kinases CPK21 and CPK23 phosphorylate and activate the iron-regulated transporter IRT1 to regulate iron deficiency in Arabidopsis

Iron (Fe) is an essential micronutrient for all organisms. Fe availability in the soil is usually much lower than that required for plant growth, and Fe deficiencies seriously restrict crop growth and yield. Calcium (Ca2+) is a second messenger in all eukaryotes; however, it remains largely unknown how Ca2+ regulates Fe deficiency. In this study, mutations in CPK21 and CPK23, which are two highly homologous calcium-dependent protein kinases, conferredimpaired growth and rootdevelopment under Fe-deficient conditions, whereas constitutively active CPK21 and CPK23 enhanced plant tolerance to Fe-deficient conditions. Furthermore, we found that CPK21 and CPK23 interacted with and phosphorylated the Fe transporter IRON-REGULATED TRANSPORTER1 (IRT1) at the Ser149 residue. Biochemical analyses and complementation of Fe transport in yeast and plants indicated that IRT1 Ser149 is critical for IRT1 transport activity. Taken together, these findings suggest that the CPK21/23-IRT1 signaling pathway is critical for Fe homeostasis in plants and provides targets for improving Fe-deficient environments and breeding crops resistant to Fe-deficient conditions.

Multilayered regulation of iron homeostasis in Arabidopsis

Iron (Fe) is an essential micronutrient for plant growth and development due to its role in crucial processes such as photosynthesis and modulation of the redox state as an electron donor. While Fe is one of the five most abundant metals in the Earth's crust, it is poorly accessible to plants in alkaline soils due to the formation of insoluble complexes. To limit Fe deficiency symptoms, plant have developed a highly sophisticated regulation network including Fe sensing, transcriptional regulation of Fe-deficiency responsive genes, and post-translational modifications of Fe transporters. In this mini-review, we detail how plants perceive intracellular Fe status and how they regulate transporters involved in Fe uptake through a complex cascade of transcription factors. We also describe the current knowledge about intracellular trafficking, including secretion to the plasma membrane, endocytosis, recycling, and degradation of the two main Fe transporters, IRON-REGULATED TRANSPORTER 1 (IRT1) and NATURAL RESISTANCE ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1). Regulation of these transporters by their non-Fe substrates is discussed in relation to their functional role to avoid accumulation of these toxic metals during Fe limitation.

Nutrient carriers at the heart of plant nutrition and sensing

Plants require water and several essential nutrients for their development. The radial transport of nutrients from the soil to the root vasculature is achieved through a combination of three different pathways: apoplastic, symplastic, and transcellular. A common feature for these pathways is the requirement of carriers to transport nutrients across the plasma membrane. An efficient transport of nutrients across the root cell layers relies on a large number of carriers, each of them having their own substrate specificity, tissular and subcellular localization. Polarity is also emerging as a major feature allowing their function. Recent advances on radial transport of nutrients, especially carrier mediated nutrient transport will be discussed in this review, as well as the role of transporters as nutrient sensors.

Diversity of retromer-mediated vesicular trafficking pathways in plants

The plant endomembrane system is organized and regulated by large gene families that encode proteins responsible for the spatiotemporal delivery and retrieval of cargo throughout the cell and to and from the plasma membrane. Many of these regulatory molecules form functional complexes like the SNAREs, exocyst, and retromer, which are required for the delivery, recycling, and degradation pathways of cellular components. The functions of these complexes are well conserved in eukaryotes, but the extreme expansion of the protein subunit families in plants suggests that plant cells require more regulatory specialization when compared with other eukaryotes. The retromer is associated with retrograde sorting and trafficking of protein cargo back towards the TGN and vacuole in plants, while in animals, there is new evidence that the VPS26C ortholog is associated with recycling or 'retrieving' proteins back to the PM from the endosomes. The human VPS26C was shown to rescue vps26c mutant phenotypes in Arabidopsis thaliana, suggesting that the retriever function could be conserved in plants. This switch from retromer to retriever function may be associated with core complexes that include the VPS26C subunit in plants, similar to what has been suggested in other eukaryotic systems. We review what is known about retromer function in light of recent findings on functional diversity and specialization of the retromer complex in plants.

SEC14-GOLD protein PATELLIN2 binds IRON-REGULATED TRANSPORTER1 linking root iron uptake to vitamin E

Organisms require micronutrients, and Arabidopsis (Arabidopsis thaliana) IRON-REGULATED TRANSPORTER1 (IRT1) is essential for iron (Fe2+) acquisition into root cells. Uptake of reactive Fe2+ exposes cells to the risk of membrane lipid peroxidation. Surprisingly little is known about how this is avoided. IRT1 activity is controlled by an intracellular variable region (IRT1vr) that acts as a regulatory protein interaction platform. Here, we describe that IRT1vr interacted with peripheral plasma membrane SEC14-Golgi dynamics (SEC14-GOLD) protein PATELLIN2 (PATL2). SEC14 proteins bind lipophilic substrates and transport or present them at the membrane. To date, no direct roles have been attributed to SEC14 proteins in Fe import. PATL2 affected root Fe acquisition responses, interacted with ROS response proteins in roots, and alleviated root lipid peroxidation. PATL2 had high affinity in vitro for the major lipophilic antioxidant vitamin E compound α-tocopherol. Molecular dynamics simulations provided insight into energetic constraints and the orientation and stability of the PATL2-ligand interaction in atomic detail. Hence, this work highlights a compelling mechanism connecting vitamin E with root metal ion transport at the plasma membrane with the participation of an IRT1-interacting and α-tocopherol-binding SEC14 protein.

Regulatory module WRKY33-ATL31-IRT1 mediates cadmium tolerance in Arabidopsis

Cadmium (Cd) is one of the most dangerous environmental pollutants among heavy metals, and threatens food safety and human health by accumulating in plant sink tissues. Here, we report a novel regulatory cascade that profoundly influences Cd tolerance in Arabidopsis. Phenotypic analysis showed that an insertional knockdown mutation at the Arabidopsis Tóxicos en Levadura 31 (ATL31) locus resulted in hypersensitivity to Cd stress, most likely due to a significant increase in Cd accumulation. Consistently, ATL31-overexpressing lines exhibited enhanced Cd stress tolerance and reduced Cd accumulation. Further, IRON-REGULATED TRANSPORTER 1 (IRT1) was identified, and yeast two-hybrid, co-immunoprecipitation and bimolecular fluorescence complementation assays demonstrated its interaction with ATL31. Biochemical, molecular, and genetic analyses showed that IRT1 is targeted by ATL31 for ubiquitin-conjugated degradation in response to Cd stress. Intriguingly, transcription of ATL31 was strongly induced by Cd stress. In addition, transgenic and molecular analyses showed that WRKY33 directly activated the transcription of ATL31 in response to Cd stress and positively regulated Cd tolerance. Genetic analysis indicated that ATL31 acts upstream of IRT1 and downstream of WRKY33 to regulate Cd tolerance. Our study revealed that the WRKY33-ATL31-IRT1 module plays a crucial role in timely blocking Cd absorption to prevent metal toxicity in Arabidopsis.

Multiple gene co-options underlie the rapid evolution of sexually deceptive flowers in Gorteria diffusa

Gene co-option, the redeployment of an existing gene in an unrelated developmental context, is an important mechanism underlying the evolution of morphological novelty. In most cases described to date, novel traits emerged by co-option of a single gene or genetic network. Here, we show that the integration of multiple co-opted genetic elements facilitated the rapid evolution of complex petal spots that mimic female bee-fly pollinators in the sexually deceptive South African daisy Gorteria diffusa. First, co-option of iron homeostasis genes altered petal spot pigmentation, producing a color similar to that of female pollinators. Second, co-option of the root hair gene GdEXPA7 enabled the formation of enlarged papillate petal epidermal cells, eliciting copulation responses from male flies. Third, co-option of the miR156-GdSPL1 transcription factor module altered petal spot placement, resulting in better mimicry of female flies resting on the flower. The three genetic elements were likely co-opted sequentially, and strength of sexual deception in different G. diffusa floral forms strongly correlates with the presence of the three corresponding morphological alterations. Our findings suggest that gene co-options can combine in a modular fashion, enabling rapid evolution of novel complex traits.

Retrograde transport in plants: Circular economy in the endomembrane system

The study of endomembrane trafficking is crucial for understanding how cells and whole organisms function. Moreover, there is a special interest in investigating endomembrane trafficking in plants, given its role in transport and accumulation of seed storage proteins and in secretion of cell wall material, arguably the two most essential commodities obtained from crops. The mechanisms of anterograde transport in the biosynthetic and endocytic pathways of plants have been thoroughly discussed in recent reviews, but, comparatively, retrograde trafficking pathways have received less attention. Retrograde trafficking is essential to recover membranes, retrieve proteins that have escaped from their intended localization, maintain homeostasis in maturing compartments, and recycle trafficking machinery for its reuse in anterograde transport reactions. Here, we review the current understanding on retrograde trafficking pathways in the endomembrane system of plants, discussing their integration with anterograde transport routes, describing conserved and plant-specific retrieval mechanisms at play, highlighting contentious issues and identifying open questions for future research.

Endophytic yeast protect plants against metal toxicity by inhibiting plant metal uptake through an ethylene-dependent mechanism

Toxic metal pollution requires significant adjustments in plant metabolism. Here, we show that the plant microbiota plays an important role in this process. The endophytic Sporobolomyces ruberrimus isolated from a serpentine population of Arabidopsis arenosa protected plants against excess metals. Coculture with its native host and Arabidopsis thaliana inhibited Fe and Ni uptake. It had no effect on host Zn and Cd uptake. Fe uptake inhibition was confirmed in wheat and rape. Our investigations show that, for the metal inhibitory effect, the interference of microorganisms in plant ethylene homeostasis is necessary. Application of an ethylene synthesis inhibitor, as well as loss-of-function mutations in canonical ethylene signalling genes, prevented metal uptake inhibition by the fungus. Coculture with S. ruberrimus significantly changed the expression of Fe homeostasis genes: IRT1, OPT3, OPT6, bHLH38 and bHLH39 in wild-type (WT) A. thaliana. The expression pattern of these genes in WT plants and in the ethylene signalling defective mutants significantly differed and coincided with the plant accumulation phenotype. Most notably, down-regulation of the expression of IRT1 solely in WT was necessary for the inhibition of metal uptake in plants. This study shows that microorganisms optimize plant Fe and Ni uptake by fine-tuning plant metal homeostasis.

Monoubiquitinated MxIRT1 acts as an iron receptor to determine MxIRT1 vacuole degradation or plasma membrane recycling via endocytosis

IRON-REGULATED TRANSPORTER 1 (IRT1) is critical for iron uptake in roots, and its exocytosis to the plasma membrane (PM) is regulated by the iron status sensed by the histidine-rich domain (HRM). However, studies on the fate of IRT1 after fusion with PM in response to iron conditions are still limited. In this study, we found that K165 and K196 regulate the monoubiquitination of MxIRT1 (mUb-MxIRT1), which acts as a receptor delivering signals from HRM to downstream effectors such as clathrin to determine the fate of MxIRT1. Iron supply led MxIRT1 in the PM to monoubiquitin-dependent endocytosis which could be inhibited by endocytosis inhibitor TyrA23 or in the double site-directed mutant K165/K196R. Subsequently, the endocytosis pathway to the vacuole was inhibited by vacuolar protease inhibitor Leupeptin in excessive iron conditions and the inability of being able to respond to iron change, indicated by the protein accumulating in the PM, contributed to iron toxicity in K165/K196R transgenic Arabidopsis. With iron availability decreasing again, MxIRT1 could dock close to the PM waiting for to be recycled. Another monoubiquitination site, K26, was necessary for MxIRT1 Endoplasmic Reticulum (ER) export as site-directed mutant K26R lost the ability of PM targeting, and co-localized with the COPII subunit of the coat protein OsSec24. Therefore, after K26-directed ER export and iron-induced PM fusion, mUb-MxIRT1 determines subsequent vacuolar degradation or recycling to the PM via endocytosis for maintaining iron homeostasis.

HRM and CRAC in MxIRT1 act as iron sensors to determine MxIRT1 vesicle-PM fusion and metal transport

The IRON-REGULATED TRANSPORTER1 (IRT1) is critical for iron uptake in roots, and its exocytosis to the plasma membrane (PM) is regulated by detergent-resistant membranes. However, studies on IRT1 exocytosis and function in response to iron status are limited. Presently, we found that the histidine-rich motif (HRM) of MxIRT1 could bind to iron directly and HRM determined the delivery of MxIRT1 to the PM, after which the cholesterol recognition amino acid consensus (CRAC) motif-regulated MxIRT1 mediated metal transport. IMAC assay revealed that H192 was the vital site for HRM binding to Fe²⁺, and metal-binding activity was stopped after the deletion of HRM (MxIRT1∆HM) or in H192 site-directed mutants (H₁₉₂A). MxIRT1∆HM or H₁₉₂A in transgenic yeast and Arabidopsis failed to localize in the PM and displayed impaired iron absorption. In the PM, Y266 in CRAC was required for metal transport; Y266A transgenic Arabidopsis displayed the same root length, Cd²⁺ flux, and Fe concentration as Arabidopsis mutant irt1 under iron-deficient conditions. Therefore, H192 in HRM may be an iron sensor to regulate delivery of MxIRT1 vesicles to the PM after binding with iron; Y266 in CRAC acts as an iron sensor for active metal transport under iron-deficient conditions.

Birth, life and death of the Arabidopsis IRT1 iron transporter: the role of close friends and foes

IRT1 intracellular dynamics and function are finely controlled through protein-protein interactions. In plants, iron uptake from the soil is tightly regulated to allow optimal growth and development. Iron acquisition in Arabidopsis root epidermal cells requires the IRT1 transporter, which also mediates the entry of non-iron metals. In this mini-review, we describe how protein-protein interactions regulate IRT1 intracellular dynamics and IRT1-mediated metal uptake to maintain iron homeostasis. Recent interactomic data provided interesting clues on IRT1 secretion and the putative involvement of COPI- and COPII-mediated pathways. Once delivered to the plasma membrane, IRT1 can interact with other components of the iron uptake machinery to form an iron acquisition complex that likely optimizes iron entrance in root epidermal cells. Then, IRT1 may be internalized from the plasma membrane. In the past decade, IRT1 endocytosis emerged as an essential mechanism to control IRT1 subcellular localization and thus to tune iron uptake. Interestingly, IRT1 endocytosis and degradation are regulated by its non-iron metal substrates in an ubiquitin-dependent manner, which requires a set of interacting-proteins including kinases, E3 ubiquitin ligases and ESCRT complex subunits. This mechanism is essential to avoid non-iron metal overload in Arabidopsis when the iron is scarce.

Emerging roles of protein phosphorylation in plant iron homeostasis

Remarkable progress has been made in dissecting the molecular mechanisms involved in iron (Fe) homeostasis in plants, especially the identification of key transporter and transcriptional regulatory networks. But how the protein activity of these master players is regulated by Fe status remains underexplored. Recent studies show that major players toggle switch their properties by protein phosphorylation under different Fe conditions and consequently control the signaling cascade and metabolic adjustment. Moreover, Fe deficiency causes changes of multiple kinases and phosphatases. Here, we discuss how these findings highlight the emergence of the protein phosphorylation-dependent regulation for rapid and precise responses to Fe status to attain Fe homeostasis. Further studies will be needed to fully understand the regulation of these intricate networks.

Subcellular trafficking and post-translational modification regulate PIN polarity in plants

Auxin regulates plant growth and tropism responses. As a phytohormone, auxin is transported between its synthesis sites and action sites. Most natural auxin moves between cells via a polar transport system that is mediated by PIN-FORMED (PIN) auxin exporters. The asymmetrically localized PINs usually determine the directionality of intercellular auxin flow. Different internal cues and external stimuli modulate PIN polar distribution and activity at multiple levels, including transcription, protein stability, subcellular trafficking, and post-translational modification, and thereby regulate auxin-distribution-dependent development. Thus, the different regulation levels of PIN polarity constitute a complex network. For example, the post-translational modification of PINs can affect the subcellular trafficking of PINs. In this review, we focus on subcellular trafficking and post-translational modification of PINs to summarize recent progress in understanding PIN polarity.

Sorting Nexin1 negatively modulates phosphate uptake by facilitating Phosphate Transporter1;1 degradation in Arabidopsis

High-affinity phosphate (Pi) transporters (PHTs) PHT1;1 and PHT1;4 are necessary for plant root Pi uptake especially under Pi-deficient conditions, but how their protein stability is modulated remains elusive. Here, we identified a Ttransfer DNA insertion mutant of Sorting Nexin1 (SNX1), which had more Pi content and less anthocyanin accumulation than the wild type under deficient Pi. By contrast, the snx1-2 mutant displayed higher sensitivity to exogenous arsenate in terms of seed germination and root elongation, revealing higher Pi uptake rates. Further study showed that SNX1 could co-localize and interact with PHT1;1 and PHT1;4 in vesicles and at the plasma membrane. Genetic analysis showed that increased Pi content in the snx1-2 mutant under low Pi conditions could be extensively compromised by mutating PHT1;1 in the double mutant snx1-2 pht1;1, revealing that SNX1 is epistatic to PHT1;1. In addition, SNX1 negatively controls PHT1;1 protein stability; therefore, PHT1;1 protein abundance in the plasma membrane was increased in the snx1-2 mutant compared with the wild type under either sufficient or deficient Pi. Together, our study (i) identifies SNX1 as a key modulator of the plant response to low Pi and (ii) unravels its role in the modulation of PHT1;1 protein stability, PHT1;1 accumulation at the plasma membrane, and root Pi uptake.

The Iron Deficiency-Regulated Small Protein Effector FEP3/IRON MAN1 Modulates Interaction of BRUTUS-LIKE1 With bHLH Subgroup IVc and POPEYE Transcription Factors

In light of climate change and human population growth one of the most challenging tasks is to generate plants that are Fe-efficient, resilient to low Fe supply and Fe-biofortified. For such endeavors, it is crucial to understand the regulation of Fe acquisition and allocation in plants. One open question is how identified Fe-regulatory proteins comprising positive and negative regulators act together to steer Fe homeostasis. bHLH transcription factors (TFs) belonging to the subgroups IVb and IVc can initiate a bHLH cascade controlling the -Fe response in roots. In Arabidopsis thaliana, the -Fe-induced genes are sub-divided into several gene co-expression clusters controlled by different sets of TFs. Some of the co-expressed genes encode regulatory E3 ligase proteins BRUTUS (BTS)/BTS-LIKE (BTSL) and small proteins belonging to the group of FE UPTAKE-INDUCING PEPTIDE/IRON MAN (FEP/IMA). Recently, it was described that FEP1/IMA3 and FEP3/IMA1 proteins inhibit the repression of bHLH factors by BTS. We had postulated that -Fe-regulated co-expression clusters provide new information about regulatory protein interaction complexes. Here, we report a targeted yeast two-hybrid screen among 23 proteins of the -Fe response. This identified a novel protein interactome involving another E3 ligase, namely BTSL1, basic helix-loop-helix (bHLH) protein POPEYE (PYE) and transcription factors of the subgroup IVc as well as FEP3/IMA1. Because of the difficulty in stable BTSL1 protein expression in plant cells, we used a yeast two hybrid-based deletion mapping, homology modeling and molecular docking, to pinpoint interaction sites in BTSL1 and FEP3/IMA1. bHLH IVc TFs have similar residues at their C-terminus as FEP3/IMA1 interacting sites. FEP3/IMA1 attenuated interaction of BTSL1 and bHLH proteins in a yeast three-hybrid assay, in line with physiological data pointing to enhanced Fe acquisition and allocation in FEP3/IMA1 overexpression and btsl1 btsl2 mutant plants. Hence, exploiting -Fe-induced gene co-expression networks identified FEP3/IMA1 as a small effector protein that binds and inhibits the BTSL1 complex with PYE and bHLH subgroup IVc proteins. Structural analysis resolved interaction sites. This information helps improving models of Fe regulation and identifying novel targets for breeding of Fe-efficient crops.

SORTING NEXIN2 proteins mediate stomatal movement and the response to drought stress by modulating trafficking and protein levels of the ABA exporter ABCG25

The phytohormone abscisic acid (ABA) regulates ion channel activity and stomatal movement in response to drought stress. Cellular ABA levels change depending on cellular and environmental conditions via modulation of its biosynthesis, catabolism and transport. Although factors involved in ABA biosynthesis and degradation have been studied extensively, how ABA transporters are modulated to fine-tune ABA levels, especially under drought stress, remains elusive. Here, we show that Arabidopsis thaliana SORTING NEXIN 2 (SNX2) proteins play a critical role in endosomal trafficking of the ABA exporter ATP BINDING CASETTE G25 (ABCG25) via direct interaction at endosomes, leading to its degradation in the vacuole. In agreement, snx2a and snx2b mutant plants showed enhanced recycling of GFP-ABCG25 from early endosomes to the plasma membrane and higher accumulation of GFP-ABCG25. Phenotypically, snx2a and snx2b plants were highly sensitive to exogenous ABA and displayed enhanced ABA-mediated inhibition of inward K⁺ currents and ABA-mediated activation of slow anion currents in guard cells, resulting in an increased tolerance to drought stress. Based on these results, we propose that SNX2 proteins play a crucial role in stomatal movement and tolerance to drought stress by modulating the endosomal trafficking of ABCG25 and thus cellular ABA levels.

Primary nutrient sensors in plants

Nutrients are scarce and valuable resources, so plants developed sophisticated mechanisms to optimize nutrient use efficiency. A crucial part of this is monitoring external and internal nutrient levels to adjust processes such as uptake, redistribution, and cellular compartmentation. Measurement of nutrient levels is carried out by primary sensors that typically involve either transceptors or transcription factors. Primary sensors are only now starting to be identified in plants for some nutrients. In particular, for nitrate, there is detailed insight concerning how the external nitrate status is sensed by members of the nitrate transporter 1 (NRT1) family. Potential sensors for other macronutrients such as potassium and sodium have also been identified recently, whereas for micronutrients such as zinc and iron, transcription factor type sensors have been reported. This review provides an overview that interprets and evaluates our current understanding of how plants sense macro and micronutrients in the rhizosphere and root symplast.

Plant iron nutrition: the long road from soil to seeds

Iron (Fe) is an essential plant micronutrient since many cellular processes including photosynthesis, respiration, and the scavenging of reactive oxygen species depend on adequate Fe levels; however, non-complexed Fe ions can be dangerous for cells, as they can act as pro-oxidants. Hence, plants possess a complex homeostatic control system for safely taking up Fe from the soil and transporting it to its various cellular destinations, and for its subcellular compartmentalization. At the end of the plant's life cycle, maturing seeds are loaded with the required amount of Fe needed for germination and early seedling establishment. In this review, we discuss recent findings on how the microbiota in the rhizosphere influence and interact with the strategies adopted by plants to take up iron from the soil. We also focus on the process of seed-loading with Fe, and for crop species we also consider its associated metabolism in wild relatives. These two aspects of plant Fe nutrition may provide promising avenues for a better comprehension of the long pathway of Fe from soil to seeds.

An Update on Coat Protein Complexes for Vesicle Formation in Plant Post-Golgi Trafficking

Endomembrane trafficking is an evolutionarily conserved process for all eukaryotic organisms. It is a fundamental and essential process for the transportation of proteins, lipids, or cellular metabolites. The aforementioned cellular components are sorted across multiple membrane-bounded organelles. In plant cells, the endomembrane mainly consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network or early endosome (TGN/EE), prevacuolar compartments or multivesicular bodies (PVCs/MVBs), and vacuole. Among them, Golgi apparatus and TGN represent two central sorting intermediates for cargo secretion and recycling from other compartments by anterograde or retrograde trafficking. Several protein sorting machineries have been identified to function in these pathways for cargo recognition and vesicle assembly. Exciting progress has been made in recent years to provide novel insights into the sorting complexes and also the underlying sorting mechanisms in plants. Here, we will highlight the recent findings for the adaptor protein (AP) complexes, retromer, and retriever complexes, and also their functions in the related coated vesicle formation in post-Golgi trafficking.

BrpNAC895 and BrpABI449 coregulate the transcription of the afflux-type Cd transporter BrpHMA2 in Brassica parachinensis

Brassica parachinensis is a popular leafy vegetable. It is able to accumulate high concentration of Cd, however, the molecular mechanism of Cd accumulation is unknown. This study investigated the function and regulatory mechanism of the Cd-responsive metal ion transporter gene BrpHMA2. BrpHMA2 was induced by Cd stress and specifically expressed in vascular tissues, and the protein was localized in the plasma membrane. Heterologous expression of BrpHMA2 enhanced Cd accumulation and Cd sensitivity in transgenic Arabidopsis and yeast. After Cd stress, the transcriptional factors BrpNAC895 and BrpABI449, which may recognize the ABREs in the BrpHMA2 promoter, were also differentially expressed. The transcriptional regulation of BrpHMA2 was further investigated using ChIP-qPCR, EMSA and LUC reporter activity analysis employing the transient expression system of Brassica parachinensis protoplasts and tobacco leaves and the E. coli expression system. By binding to the promoter, BrpNAC895 induced the transcription of BrpHMA2. BrpABI449 might bind to the BrpHMA2 promoter or interact with BrpNAC895 to interfere with the action of BrpNAC895. The findings suggest that BrpHMA2 is a membrane-based afflux-type Cd transporter involved in the Cd2+ uptake and long-distance transport in plants. BrpNAC895 and BrpABI449, which function as the transcription activator and repressor respectively, coregulate BrpHMA2 expression.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Specific expression of AtIRT1 in phloem companion cells suggests its role in iron translocation in aboveground plant organs

IRON-REGULATED TRANSPORTER 1 (IRT1) is a central iron transporter responsible for the uptake of iron from the rhizosphere to root epidermal cells. This study uses immunohistochemistry, histochemistry, and fluorometry to show that this gene's promoter is also active in the aboveground parts, specifically in phloem companied cells. Promoter activity here was regulated by iron as it was in the roots. The promoter of the close IRT2 homolog was root-specific and only weakly active in the stem pits. RT-PCR showed the presence of a long splicing form exclusively in iron-deficient roots. The short splicing form was present in all organs regardless of the presence of iron. Immunohistology exhibited labeling on the periphery of the epidermal cells in matured root zone and intracellular patches in the meristematic cells. In the aboveground organs, the protein was seen in the whole volume of companion cells and in neighboring sieve elements as bodies. The fluorescent protein technique revealed the short IRT1 form to be present in the patches accumulated mainly around the nucleus and the long form as a continuous layer along the cells periphery. These results suggest that IRT1 has a role also in the aboveground organs.

Organic nitrogen nutrition: LHT1.2 protein from hybrid aspen (Populus tremula L. x tremuloides Michx) is a functional amino acid transporter and a homolog of Arabidopsis LHT1

The contribution of amino acids (AAs) to soil nitrogen (N) fluxes is higher than previously thought. The fact that AA uptake is pivotal for N nutrition in boreal ecosystems highlights plant AA transporters as key components of the N cycle. At the same time, very little is known about AA transport and respective transporters in trees. Tree genomes may contain 13 or more genes encoding the lysine histidine transporter (LHT) family proteins, and this complicates the study of their significance for tree N-use efficiency. With the strategy of obtaining a tool to study N-use efficiency, our aim was to identify and characterize a relevant AA transporter in hybrid aspen (Populus tremula L. x tremuloides Michx.). We identified PtrLHT1.2, the closest homolog of Arabidopsis thaliana (L.) Heynh AtLHT1, which is expressed in leaves, stems and roots. Complementation of a yeast AA uptake mutant verified the function of PtrLHT1.2 as an AA transporter. Furthermore, PtrLHT1.2 was able to fully complement the phenotypes of the Arabidopsis AA uptake mutant lht1 aap5, including early leaf senescence-like phenotype, reduced growth, decreased plant N levels and reduced root AA uptake. Amino acid uptake studies finally showed that PtrLHT1.2 is a high affinity transporter for neutral and acidic AAs. Thus, we identified a functional AtLHT1 homolog in hybrid aspen, which harbors the potential to enhance overall plant N levels and hence increase biomass production. This finding provides a valuable tool for N nutrition studies in trees and opens new avenues to optimizing tree N-use efficiency.

Attracted to membranes: lipid-binding domains in plants

Membranes are essential for cells and organelles to function. As membranes are impermeable to most polar and charged molecules, they provide electrochemical energy to transport molecules across and create compartmentalized microenvironments for specific enzymatic and cellular processes. Membranes are also responsible for guided transport of cargoes between organelles and during endo- and exocytosis. In addition, membranes play key roles in cell signaling by hosting receptors and signal transducers and as substrates and products of lipid second messengers. Anionic lipids and their specific interaction with target proteins play an essential role in these processes, which are facilitated by specific lipid-binding domains. Protein crystallography, lipid-binding studies, subcellular localization analyses, and computer modeling have greatly advanced our knowledge over the years of how these domains achieve precision binding and what their function is in signaling and membrane trafficking, as well as in plant development and stress acclimation.

The many facets of protein ubiquitination and degradation in plant root iron-deficiency responses

Organisms need to deal with the absolute requirement for metals and also their possible toxicity. This is achieved through an intricate network of signaling pathways that are integrated to ultimately fine-tune iron uptake and metabolism. The mechanisms by which plants cope with iron limitation and the associated genomic responses are well characterized. On top of this transcriptional cascade is another level of regulation involving the post-translational protein modification and degradation. The ubiquitination and/or degradation of several transcription factors in the iron-deficiency signaling pathways and metal transporters has recently come to light. In this review we discuss the mechanisms and possible roles of protein modification and turnover in the regulation of root iron-deficiency responses. We also highlight the tight coupling between metal sensing by E3 ubiquitin ligases or bifunctional transporters and protein degradation.

Reactive oxygen species coordinate the transcriptional responses to iron availability in Arabidopsis

Reactive oxygen species play a central role in the regulation of plant responses to environmental stress. Under prolonged iron (Fe) deficiency, increased levels of hydrogen peroxide (H2O2) initiate signaling events, resulting in the attenuation of Fe acquisition through the inhibition of FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT). As this H2O2 increase occurs in a FIT-dependent manner, our aim was to understand the processes involved in maintaining H2O2 levels under prolonged Fe deficiency and the role of FIT. We identified the CAT2 gene, encoding one of the three Arabidopsis catalase isoforms, as regulated by FIT. CAT2 loss-of-function plants displayed severe susceptibility to Fe deficiency and greatly increased H2O2 levels in roots. Analysis of the Fe homeostasis transcription cascade revealed that H2O2 influences the gene expression of downstream regulators FIT, BHLH genes of group Ib, and POPEYE (PYE); however, H2O2 did not affect their upstream regulators, such as BHLH104 and ILR3. Our data shows that FIT and CAT2 participate in a regulatory loop between H2O2 and prolonged Fe deficiency.

Interaction Between Sulfur and Iron in Plants

It is well known that S interacts with some macronutrients, such as N, P, and K, as well as with some micronutrients, such as Fe, Mo, Cu, Zn, and B. From our current understanding, such interactions could be related to the fact that: (i) S shares similar chemical properties with other elements (e.g., Mo and Se) determining competition for the acquisition/transport process (SULTR transporter family proteins); (ii) S-requiring metabolic processes need the presence of other nutrients or regulate plant responses to other nutritional deficiencies (S-containing metabolites are the precursor for the synthesis of ethylene and phytosiderophores); (iii) S directly interacts with other elements (e.g., Fe) by forming complexes and chemical bonds, such as Fe-S clusters; and (iv) S is a constituent of organic molecules, which play crucial roles in plants (glutathione, transporters, etc.). This review summarizes the current state of knowledge of the interplay between Fe and S in plants. It has been demonstrated that plant capability to take up and accumulate Fe strongly depends on S availability in the growth medium in both monocots and dicot plants. Moreover, providing S above the average nutritional need enhances the Fe content in wheat grains, this beneficial effect being particularly pronounced under severe Fe limitation. On the other hand, Fe shortage induces a significant increase in the demand for S, resulting in enhanced S uptake and assimilation rate, similar to what happens under S deficiency. The critical evaluation of the recent studies on the modulation of Fe/S interaction by integrating old and new insights gained on this topic will help to identify the main knowledge gaps. Indeed, it remains a challenge to determine how the interplay between S and Fe is regulated and how plants are able to sense environmental nutrient fluctuations and then to adapt their uptake, translocation, assimilation, and signaling. A better knowledge of the mechanisms of Fe/S interaction might considerably help in improving crop performance within a context of limited nutrient resources and a more sustainable agriculture.

Endocytosis in plants: Peculiarities and roles in the regulated trafficking of plant metal transporters

The removal of transmembrane proteins from the plasma membrane via endocytosis has emerged as powerful strategy in the regulation of receptor signalling and molecule transport. In the last decade, IRON-REGULATED TRANSPORTER1 (IRT1) has been established as one of the key plant model proteins for studying endomembrane trafficking. The use of IRT1 and additional other metal transporters has uncovered novel factors involved in plant endocytosis and facilitated a better understanding of the role of endocytosis in the fine balancing of plant metal homoeostasis. In this review, we outline the specifics of plant endocytosis compared to what is known in yeast and mammals, and based on several examples, we demonstrate how studying metal transport has contributed to extending our knowledge of endocytic trafficking by shedding light on novel regulatory mechanisms and factors.

EMAC, Retromer, and VSRs: do they connect?

Eukaryotic organisms share many common features in terms of endomembrane trafficking. This fact has helped plant scientists to propose testable hypotheses on how plant intracellular membrane trafficking is achieved and regulated based on knowledge from yeast and mammals. However, when a new compartment has been identified in a plant cell that has a vesicle tethering complex located at a position which is completely different to its counterpart in yeast and mammalian cells, caution is demanded when interpreting possible interactions with other trafficking elements. This is exemplified by the recently discovered EMAC (ER and microtubule-associated compartment). It has been postulated that this compartment is the recipient of vacuolar sorting receptors (VSRs) transported retrogradely via "retromer vesicles" from a post-Golgi location. Unfortunately, this suggestion was based entirely on our knowledge of retromer from yeast and mammalian cells, and did not take into account the available literature on the composition, localization, and function of the plant retromer. It also lacked reference to recent contradictory findings on VSR trafficking. In this short article, we have tried to rectify this situation, pointing out that plant retromer may not function as a pentameric complex of two subunits: the retromer core and the sorting nexins.

Dynamic Control of the High-Affinity Iron Uptake Complex in Root Epidermal Cells

In plants, iron uptake from the soil is tightly regulated to ensure optimal growth and development. Iron absorption in Arabidopsis root epidermal cells requires the IRT1 transporter that also allows the entry of certain non-iron metals, such as Zn, Mn, and Co. Recent work demonstrated that IRT1 endocytosis and degradation are controlled by IRT1 non-iron metal substrates in a ubiquitin-dependent manner. To better understand how metal uptake is regulated, we identified IRT1-interacting proteins in Arabidopsis roots by mass spectrometry and established an interactome of IRT1. Interestingly, the AHA2 proton pump and the FRO2 reductase, both of which work in concert with IRT1 in the acidification-reduction-transport strategy of iron uptake, were part of this interactome. We confirmed that IRT1, FRO2, and AHA2 associate through co-immunopurification and split-ubiquitin analyses, and uncovered that they form tripartite direct interactions. We characterized the dynamics of the iron uptake complex and showed that FRO2 and AHA2 ubiquitination is independent of the non-iron metal substrates transported by IRT1. In addition, FRO2 and AHA2 are not largely endocytosed in response to non-iron metal excess, unlike IRT1. Indeed, we provide evidence that the phosphorylation of IRT1 in response to high levels of non-iron metals likely triggers dissociation of the complex. Overall, we propose that a dedicated iron-acquisition protein complex exists at the cell surface of Arabidopsis root epidermal cells to optimize iron uptake.

Alteration of iron responsive gene expression in Arabidopsis glutaredoxin S17 loss of function plants with or without iron stress

Iron (Fe) is a mineral nutrient and a metal cofactor essential for plants. Iron limitation can have detrimental effects on plant growth and development, while excess iron inside plant cells leads to oxidative damage. As a result, plants have evolved complex regulatory networks to respond to fluctuations in cellular iron concentrations. The mechanisms that regulate these responses however, are not fully understood. Heterologous expression of an Arabidopsis thaliana monothiol glutaredoxin S17 (GRXS17) suppresses the over-accumulation of iron in the Saccharomyces cerevisiae Grx3/Grx4 mutant and disruption of GRXS17 causes plant sensitivity to exogenous oxidants and iron deficiency stress. GRXS17 may act as an important regulator in the plant's ability to respond to iron deficiency stress and maintain redox homeostasis. Here, we extend this investigation by analyzing iron-responsive gene expression of the Fer-like iron deficiency-induced transcription factor (FIT) network (FIT, IRT1, FRO1, and FRO2) and the bHLH transcription factor POPEYE (PYE) network (PYE, ZIF1, FRO3, NAS4, and BTS) in GRXS17 KO plants and wildtype controls grown under iron sufficiency and deficiency conditions. Our findings suggest that GRXS17 is required for tolerance to iron deficiency, and plays a negative regulatory role under conditions of iron sufficiency.

Phosphorylation at Ser28 stabilizes the Arabidopsis nitrate transporter NRT2.1 in response to nitrate limitation

Nitrate is one of the main inorganic nitrogen sources for plants. Nitrate absorption from soils is achieved through the combined activities of specific nitrate transporters. Nitrate transporter 2.1 (NRT2.1) is the major component of the root high-affinity nitrate transport system in Arabidopsis thaliana. Studies to date have mainly focused on transcriptional control of NRT2.1. Here, we show that NRT2.1 protein stability is also regulated in response to nitrogen nutrition availability. When seedlings were transferred to nitrate-limited conditions, the apparent half-life of NRT2.1 in roots increased from 3 to 9 h. This stabilization of NRT2.1 protein occurred rapidly, even prior to the transcriptional stimulation of NRT2.1. Furthermore, we revealed that phosphorylation at serine 28 (Ser28) of NRT2.1 is involved in regulating the stability of this protein. Substitution of Ser28 by alanine resulted in unstable NRT2.1, and this loss-of-phosphorylation mutant (NRT2.1^S28A ) failed to complement the growth-restricted phenotype of the nrt2.1 mutant when a low concentration of nitrate was the sole nitrogen source. These results demonstrate that phosphorylation at Ser28 is crucial for NRT2.1 protein stabilization and accumulation in response to nitrate limitation.

The Role of Selective Protein Degradation in the Regulation of Iron and Sulfur Homeostasis in Plants

Plants are able to synthesize all essential metabolites from minerals, water, and light to complete their life cycle. This plasticity comes at a high energy cost, and therefore, plants need to tightly allocate resources in order to control their economy. Being sessile, plants can only adapt to fluctuating environmental conditions, relying on quality control mechanisms. The remodeling of cellular components plays a crucial role, not only in response to stress, but also in normal plant development. Dynamic protein turnover is ensured through regulated protein synthesis and degradation processes. To effectively target a wide range of proteins for degradation, plants utilize two mechanistically-distinct, but largely complementary systems: the 26S proteasome and the autophagy. As both proteasomal- and autophagy-mediated protein degradation use ubiquitin as an essential signal of substrate recognition, they share ubiquitin conjugation machinery and downstream ubiquitin recognition modules. Recent progress has been made in understanding the cellular homeostasis of iron and sulfur metabolisms individually, and growing evidence indicates that complex crosstalk exists between iron and sulfur networks. In this review, we highlight the latest publications elucidating the role of selective protein degradation in the control of iron and sulfur metabolism during plant development, as well as environmental stresses.

Phospholipase D-derived phosphatidic acid promotes root hair development under phosphorus deficiency by suppressing vacuolar degradation of PIN-FORMED2

Root hair development is crucial for phosphate absorption, but how phosphorus deficiency affects root hair initiation and elongation remains unclear. We demonstrated the roles of auxin efflux carrier PIN-FORMED2 (PIN2) and phospholipase D (PLD)-derived phosphatidic acid (PA), a key signaling molecule, in promoting root hair development in Arabidopsis thaliana under a low phosphate (LP) condition. Root hair elongation under LP conditions was greatly suppressed in pin2 mutant or under treatment with a PLDζ2-specific inhibitor, revealing that PIN2 and polar auxin transport and PLDζ2-PA are crucial in LP responses. PIN2 was accumulated and degraded in the vacuole under a normal phosphate (NP) condition, whereas its vacuolar accumulation was suppressed under the LP or NP plus PA conditions. Vacuolar accumulation of PIN2 was increased in pldζ2 mutants under LP conditions. Increased or decreased PIN2 vacuolar accumulation is not observed in sorting nexin1 (snx1) mutant, indicating that vacuolar accumulation of PIN2 is mediated by SNX1 and the relevant trafficking process. PA binds to SNX1 and promotes its accumulation at the plasma membrane, especially under LP conditions, and hence promotes root hair development by suppressing the vacuolar degradation of PIN2. We uncovered a link between PLD-derived PA and SNX1-dependent vacuolar degradation of PIN2 in regulating root hair development under phosphorus deficiency.

Evolutionary insights into FYVE and PHOX effector proteins from the moss Physcomitrella patens

Genome-wide identification, together with gene expression patterns and promoter region analysis of FYVE and PHOX proteins in Physcomitrella patens, emphasized their importance in regulating mainly developmental processes in P. patens. Phosphatidylinositol 3-phosphate (PtdIns3P) is a signaling phospholipid, which regulates several aspects of plant growth and development, as well as responses to biotic and abiotic stresses. The mechanistic insights underlying PtdIns3P mode of action, specifically through effector proteins have been partially explored in plants, with main focus on Arabidopsis thaliana. In this study, we searched for genes coding for PtdIns3P-binding proteins such as FYVE and PHOX domain-containing sequences from different photosynthetic organisms to gather evolutionary insights on these phosphoinositide binding domains, followed by an in silico characterization of the FYVE and PHOX gene families in the moss Physcomitrella patens. Phylogenetic analysis showed that PpFYVE proteins can be grouped in 7 subclasses, with an additional subclass whose FYVE domain was lost during evolution to higher plants. On the other hand, PpPHOX proteins are classified into 5 subclasses. Expression analyses based on RNAseq data together with the analysis of cis-acting regulatory elements and transcription factor (TF) binding sites in promoter regions suggest the importance of these proteins in regulating stress responses but mainly developmental processes in P. patens. The results provide valuable information and robust candidate genes for future functional analysis aiming to further explore the role of this signaling pathway mainly during growth and development of tip growing cells and during the transition from 2 to 3D growth. These studies would identify ancestral regulatory players undertaken during plant evolution.

The roles of endomembrane trafficking in plant abiotic stress responses

Endomembrane trafficking is a fundamental cellular process in all eukaryotic cells and its regulatory mechanisms have been extensively studied. In plants, the endomembrane trafficking system needs to be constantly adjusted to adapt to the ever-changing environment. Evidence has accumulated supporting the idea that endomembrane trafficking is tightly linked to stress signaling pathways to meet the demands of rapid changes in cellular processes and to ensure the correct delivery of stress-related cargo molecules. However, the underlying mechanisms remain unknown. In this review, we summarize the recent findings on the functional roles of both secretory trafficking and endocytic trafficking in different types of abiotic stresses. We also highlight and discuss the unique properties of specific regulatory molecules beyond their conventional functions in endosomal trafficking during plant growth under stress conditions.

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.

Mobility and localization of the iron deficiency-induced transcription factor bHLH039 change in the presence of FIT

Regulation of iron (Fe) acquisition and homeostasis is critical for plant survival. In Arabidopsis, Fe deficiency-induced bHLH039 forms a complex with the master regulator FIT and activates it to upregulate Fe acquisition genes. FIT is partitioned between cytoplasm and nucleus, whereby active FIT accumulates more in the nucleus than inactive FIT. At the same time, there is so far no information on the subcellular localization of bHLH039 protein and how it is controlled. We report here that the bHLH039 localization pattern changes depending on the presence of FIT in the cell. When expressed in cells lacking FIT, bHLH039 localizes predominantly in the cytoplasm, including cytoplasmic foci in close proximity to the plasma membrane. The presence of FIT enhances the mobility of bHLH039 and redirects the protein toward primarily nuclear localization, abolishing its accumulation in cytoplasmic foci. This FIT-dependent change in localization of bHLH039 found in transient fluorescent protein expression experiments was confirmed in both leaves and roots of Arabidopsis transgenic plants, stably expressing hemagglutinin-tagged bHLH039 in wild-type or fit mutant background. This posttranslational mechanism for intracellular partitioning of Fe-responsive transcription factors suggests a signaling cascade that translates Fe sensing at the plasma membrane to nuclear accumulation of the transcriptional regulators.

Keep talking: crosstalk between iron and sulfur networks fine-tunes growth and development to promote survival under iron limitation

Plants are capable of synthesizing all the molecules necessary to complete their life cycle from minerals, water, and light. This plasticity, however, comes at a high energetic cost and therefore plants need to regulate their economy and allocate resources accordingly. Iron-sulfur (Fe-S) clusters are at the center of photosynthesis, respiration, amino acid, and DNA metabolism. Fe-S clusters are extraordinary catalysts, but their main components (Fe2+ and S2-) are highly reactive and potentially toxic. To prevent toxicity, plants have evolved mechanisms to regulate the uptake, storage, and assimilation of Fe and S. Recent advances have been made in understanding the cellular economy of Fe and S metabolism individually, and growing evidence suggests that there is dynamic crosstalk between Fe and S networks. In this review, we summarize and discuss recent literature on Fe sensing, allocation, use efficiency, and, when pertinent, its relationship to S metabolism. Our future perspectives include a discussion about the open questions and challenges ahead and how the plant nutrition field can come together to approach these questions in a cohesive and more efficient way.