Subcellular trafficking of the Arabidopsis auxin influx carrier AUX1 uses a novel pathway distinct from PIN1

Investigation into the Synthesis, Bioactivity, and Mechanism of Action of the Novel 6-Pyrazolyl-2-picolinic Acid as a Herbicide

A series of new 4-amino-3,5-dicholo-6-(5-aryl-substituted-1H-pyrazol-1-yl)-2-picolinic acid compounds were designed and prepared to discover herbicidal molecules. The inhibitory activities of all new compounds against the root growth ofArabidopsis thaliana were assayed. On the whole, the new synthesized compounds displayed good inhibition effects and had excellent herbicidal activities on root growth of weed at 500 μM. Importantly, a selection of compounds demonstrated comparable herbicidal properties to picloram. At the dosage of 250 g/ha, most of the compounds showed a 100% postemergence herbicidal activity to control Chenopodium album and Amaranthus retroflexus. Using compound V-2, the mechanism of action was investigated based on a phenotype study using AFB5-deficient Arabidopsis thaliana. It was found that the novel 6-pyrazolyl-2-picolinic acids were auxinic compounds. In addition, it was proposed that V-2 may be an immune activator due to its upregulation of defense genes and the increased content of jasmonic acid.

Distinct ADP-ribosylation factor-GTP exchange factors govern the opposite polarity of 2 receptor kinases

Polarity of plasma membrane proteins is essential for cell morphogenesis and control of cell division and, thus, influences organ and whole plant development. In Arabidopsis (Arabidopsis thaliana) root endodermal cells, 2 transmembrane kinases, INFLORESCENCE AND ROOT APICES RECEPTOR KINASE (IRK) and KINASE ON THE INSIDE (KOIN), accumulate at opposite lateral domains. Their polarization is tightly linked to their activities regulating cell division and ground tissue patterning. The polarization of IRK and KOIN relies solely on the secretion of newly synthesized protein. However, the secretion machinery by which their opposite, lateral polarity is achieved remains largely unknown. Here, we show that different sets of ADP-ribosylation factor (ARF)-guanine-nucleotide exchange factors (ARF-GEFs) mediate their secretion. ARF-GEF GNOM-like-1 (GNL1) regulates KOIN secretion to the inner polar domain, thereby directing KOIN sorting early in the secretion pathway. For IRK, combined chemical and genetic analyses showed that the ARG-GEF GNL1, GNOM, and the BREFELDIN A-INHIBITED-GUANINE NUCLEOTIDE-EXCHANGE FACTORs 1 to 4 (BIG1-BIG4) collectively regulate its polar secretion. The ARF-GEF-dependent mechanisms guiding IRK or KOIN lateral polarity were active across different root cell types and functioned regardless of the protein's inner/outer polarity in those cells. Therefore, we propose that specific polar trafficking of IRK and KOIN occurs via distinct mechanisms that are not constrained by cell identity or polar axis and likely rely on individual protein recognition.

Regulation of PIN polarity in response to abiotic stress

Plants have evolved robust adaptive mechanisms to withstand the ever-changing environment. Tightly regulated distribution of the hormone auxin throughout the plant body controls an impressive variety of developmental processes that tailor plant growth and morphology to environmental conditions. The proper flow and directionality of auxin between cells is mainly governed by asymmetrically localized efflux carriers - PINs - ensuring proper coordination of developmental processes in plants. Discerning the molecular players and cellular dynamics involved in the establishment and maintenance of PINs in specific membrane domains, as well as their ability to readjust in response to abiotic stressors is essential for understanding how plants balance adaptability and stability. While much is known about how PINs get polarized, there is still limited knowledge about how abiotic stresses alter PIN polarity by acting on these systems. In this review, we focus on the current understanding of mechanisms involved in (re)establishing and maintaining PIN polarity under abiotic stresses.

A methane-cGMP module positively influences adventitious rooting

KEY MESSAGE

Endogenous cGMP operates downstream of CH₄ control of adventitious rooting, following by the regulation in the expression of cell cycle regulatory and auxin signaling-related genes. Methane (CH₄) is a natural product from plants and microorganisms. Although exogenously applied CH₄ and cyclic guanosine monophosphate (cGMP) are separately confirmed to be involved in the control of adventitious root (AR) formation, the possible interaction still remains elusive. Here, we observed that exogenous CH₄ not only rapidly promoted cGMP synthesis through increasing the activity of guanosine cyclase (GC), but also induced cucumber AR development. These responses were obviously impaired by the removal of endogenous cGMP with two GC inhibitors. Anatomical evidence showed that the emerged stage (V) among AR primordia development might be the main target of CH₄-cGMP module. Genetic evidence revealed that the transgenic Arabidopsis that overexpressed the methyl-coenzyme M reductase gene (MtMCR) from Methanobacterium thermoautotrophicum not only increased-cGMP production, but also resulted in a pronounced AR development compared to wild-type (WT), especially with the addition of CH₄ or the cell-permeable cGMP derivative 8-Br-cGMP. qPCR analysis confirmed that some marker genes associated with cell cycle regulatory and auxin signaling were closely related to the brand-new CH₄-cGMP module in AR development. Overall, our results clearly revealed an important function of cGMP in CH₄ governing AR formation by modulating auxin-dependent pathway and cell cycle regulation.

Genome-wide identification and expression analysis of AUX/LAX family genes in Chinese hickory (Carya cathayensis Sarg.) Under various abiotic stresses and grafting

Auxin is essential for regulating plant growth and development as well as the response of plants to abiotic stresses. AUX/LAX proteins are auxin influx transporters belonging to the amino acid permease family of proton-driven transporters, and are involved in the transport of indole-3-acetic acid (IAA). However, how AUX/LAX genes respond to abiotic stresses in Chinese hickory is less studied. For the first time identification, structural characteristics as well as gene expression analysis of the AUX/LAX gene family in Chinese hickory were conducted by using techniques of gene cloning and real-time fluorescent quantitative PCR. Eight CcAUX/LAXs were identified in Chinese hickory, all of which had the conserved structural characteristics of AUX/LAXs. CcAUX/LAXs were most closely related to their homologous proteins in Populus trichocarpa , which was in consistence with their common taxonomic character of woody trees. CcAUX/LAXs exhibited different expression profiles in different tissues, indicating their varying roles during growth and development. A number of light-, hormone-, and abiotic stress responsive cis-acting regulatory elements were detected on the promoters of CcAUX/LAX genes. CcAUX/LAX genes responded differently to drought and salt stress treatments to varying degrees. Furthermore, CcAUX/LAX genes exhibited complex expression changes during Chinese hickory grafting. These findings not only provide a valuable resource for further functional validation of CcAUX/LAXs, but also contribute to a better understanding of their potential regulatory functions during grafting and abiotic stress treatments in Chinese hickory.

Pleiotropic and nonredundant effects of an auxin importer in Setaria and maize

Directional transport of auxin is critical for inflorescence and floral development in flowering plants, but the role of auxin influx carriers (AUX1 proteins) has been largely overlooked. Taking advantage of available AUX1 mutants in green millet (Setaria viridis) and maize (Zea mays), we uncover previously unreported aspects of plant development that are affected by auxin influx, including higher order branches in the inflorescence, stigma branch number, glume (floral bract) development, and plant fertility. However, disruption of auxin flux does not affect all parts of the plant, with little obvious effect on inflorescence meristem size, time to flowering, and anther morphology. In double mutant studies in maize, disruptions of ZmAUX1 also affect vegetative development. A green fluorescent protein (GFP)-tagged construct of the Setaria AUX1 protein Sparse Panicle1 (SPP1) under its native promoter showed that SPP1 localizes to the plasma membrane of outer tissue layers in both roots and inflorescences, and accumulates specifically in inflorescence branch meristems, consistent with the mutant phenotype and expected auxin maxima. RNA-seq analysis indicated that most gene expression modules are conserved between mutant and wild-type plants, with only a few hundred genes differentially expressed in spp1 inflorescences. Using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology, we disrupted SPP1 and the other four AUX1 homologs in S. viridis. SPP1 has a larger effect on inflorescence development than the others, although all contribute to plant height, tiller formation, and leaf and root development. The AUX1 importers are thus not fully redundant in S. viridis. Our detailed phenotypic characterization plus a stable GFP-tagged line offer tools for future dissection of the function of auxin influx proteins.

Pleiotropic and nonredundant effects of an auxin importer in Setaria and maize

Directional transport of auxin is critical for inflorescence and floral development in flowering plants, but the role of auxin influx carriers (AUX1 proteins) has been largely overlooked. Taking advantage of available AUX1 mutants in green millet (Setaria viridis) and maize (Zea mays), we uncover previously unreported aspects of plant development that are affected by auxin influx, including higher order branches in the inflorescence, stigma branch number, glume (floral bract) development, and plant fertility. However, disruption of auxin flux does not affect all parts of the plant, with little obvious effect on inflorescence meristem size, time to flowering, and anther morphology. In double mutant studies in maize, disruptions of ZmAUX1 also affect vegetative development. A green fluorescent protein (GFP)-tagged construct of the Setaria AUX1 protein Sparse Panicle1 (SPP1) under its native promoter showed that SPP1 localizes to the plasma membrane of outer tissue layers in both roots and inflorescences, and accumulates specifically in inflorescence branch meristems, consistent with the mutant phenotype and expected auxin maxima. RNA-seq analysis indicated that most gene expression modules are conserved between mutant and wild-type plants, with only a few hundred genes differentially expressed in spp1 inflorescences. Using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology, we disrupted SPP1 and the other four AUX1 homologs in S. viridis. SPP1 has a larger effect on inflorescence development than the others, although all contribute to plant height, tiller formation, and leaf and root development. The AUX1 importers are thus not fully redundant in S. viridis. Our detailed phenotypic characterization plus a stable GFP-tagged line offer tools for future dissection of the function of auxin influx proteins.

Optimized synthesis of layered double hydroxide lactate nanosheets and their biological effects on Arabidopsis seedlings

BACKGROUND

Layered double hydroxide lactate nanosheets (LDH-lactate-NS) are powerful carriers for delivering macro-molecules into intact plant cells. In the past few years, some studies have been carried out on DNA/RNA transformation and plant disease resistance, but little attention has been paid to these factors during LDH-lactate-NS synthesis and delamination, nor has their relationship to the DNA adsorption capacity or transformation efficiency of plant cells been considered.

RESULTS

Since the temperature during delamination alters particle sizes and zeta potentials of LDH-lactate-NS products, we compared the LDH-lactate-NS stability, DNA adsorption rate and delivery efficiency of fluorescein isothiocyanate isomer I (FITC) of them, found that the LDH-lactate-NS obtained at 25 °C has the best characters for delivering biomolecules into plant cell. To understand the potential side effects and cytotoxicity of LDH-lactate-NS to plants, we compared the root growth rate between the Arabidopsis thaliana seedlings grown in the culture medium with 1-300 μg/mL LDH-lactate-NS and equivalent raw material, Mg(lactate)₂ and Al (lactate)₃. Phenotypic analysis showed LDH in a range of 1-300 μg/mL can enhance the root elongation, whereas the same concentration of raw materials dramatically inhibited root elongation, suggesting the nanocrystallization has a dramatical de-toxic effect to Mg(lactate)₂ and Al (lactate)_3. Since enhancing of root elongation by LDH is an unexpected phenomenon, we further designed experiments to investigate influence of LDH to Arabidopsis seedlings. We further used the gravitropic bending test, qRT-PCR analysis of auxin transport proteins, non-invasive micro-test technology and liquid chromatography-mass spectrometry to investigate the auxin transport and distribution in Arabidopsis root. Results indicated that LDH-lactate-NS affect root growth by increasing the polar auxin transport.

CONCLUSIONS

Optimal synthesized LDH-lactate-NS can delivery biomolecules into intact plant cells with high efficiency and low cytotoxity. The working solution of LDH-lactate-NS can promote root elongation via increase the polar auxin transport in Arabidopsis roots.

Transcriptional landscape of highly lignified poplar stems at single-cell resolution

BACKGROUND

Plant secondary growth depends on the activity of the vascular cambium, which produces xylem and phloem. Wood derived from xylem is the most abundant form of biomass globally and has played key socio-economic and subsistence roles throughout human history. However, despite intensive study of vascular development, the full diversity of cell types and the gene networks engaged are still poorly understood.

RESULTS

Here, we have applied an optimized protoplast isolation protocol and RNA sequencing to characterize the high-resolution single-cell transcriptional landscape of highly lignified poplar stems. We identify 20 putative cell clusters with a series of novel cluster-specific marker genes and find that these cells are highly heterogeneous based on the transcriptome. Analysis of these marker genes' expression dynamics enables reconstruction of the cell differentiation trajectories involved in phloem and xylem development. We find that different cell clusters exhibit distinct patterns of phytohormone responses and emphasize the use of our data to predict potential gene redundancy and identify candidate genes related to vascular development in trees.

CONCLUSIONS

These findings establish the transcriptional landscape of major cell types of poplar stems at single-cell resolution and provide a valuable resource for investigating basic principles of vascular cell specification and differentiation in trees.

SNARE proteins VAMP721 and VAMP722 mediate the post-Golgi trafficking required for auxin-mediated development in Arabidopsis

The plant hormone auxin controls many aspects of plant development. Membrane trafficking processes, such as secretion, endocytosis and recycling, regulate the polar localization of auxin transporters in order to establish an auxin concentration gradient. Here, we investigate the function of the Arabidopsis thaliana R-SNAREs VESICLE-ASSOCIATED MEMBRANE PROTEIN 721 (VAMP721) and VAMP722 in the post-Golgi trafficking required for proper auxin distribution and seedling growth. We show that multiple growth phenotypes, such as cotyledon development, vein patterning and lateral root growth, were defective in the double homozygous vamp721 vamp722 mutant. Abnormal auxin distribution and root patterning were also observed in the mutant seedlings. Fluorescence imaging revealed that three auxin transporters, PIN-FORMED 1 (PIN1), PIN2 and AUXIN RESISTANT 1 (AUX1), aberrantly accumulate within the cytoplasm of the double mutant, impairing the polar localization at the plasma membrane (PM). Analysis of intracellular trafficking demonstrated the involvement of VAMP721 and VAMP722 in the endocytosis of FM4-64 and the secretion and recycling of the PIN2 transporter protein to the PM, but not its trafficking to the vacuole. Furthermore, vamp721 vamp722 mutant roots display enlarged trans-Golgi network (TGN) structures, as indicated by the subcellular localization of a variety of marker proteins and the ultrastructure observed using transmission electron microscopy. Thus, our results suggest that the R-SNAREs VAMP721 and VAMP722 mediate the post-Golgi trafficking of auxin transporters to the PM from the TGN subdomains, substantially contributing to plant growth.

The Coordinated KNR6-AGAP-ARF1 Complex Modulates Vegetative and Reproductive Traits by Participating in Vesicle Trafficking in Maize

The KERNEL NUMBER PER ROW6 (KNR6)-mediated phosphorylation of an adenosine diphosphate ribosylation factor (Arf) GTPase-activating protein (AGAP) forms a key regulatory module for the numbers of spikelets and kernels in the ear inflorescences of maize (Zea mays L.). However, the action mechanism of the KNR6–AGAP module remains poorly understood. Here, we characterized the AGAP-recruited complex and its roles in maize cellular physiology and agronomically important traits. AGAP and its two interacting Arf GTPase1 (ARF1) members preferentially localized to the Golgi apparatus. The loss-of-function AGAP mutant produced by CRISPR/Cas9 resulted in defective Golgi apparatus with thin and compact cisternae, together with delayed internalization and repressed vesicle agglomeration, leading to defective inflorescences and roots, and dwarfed plants with small leaves. The weak agap mutant was phenotypically similar to knr6, showing short ears with fewer kernels. AGAP interacted with KNR6, and a double mutant produced shorter inflorescence meristems and mature ears than the single agap and knr6 mutants. We hypothesized that the coordinated KNR6–AGAP–ARF1 complex modulates vegetative and reproductive traits by participating in vesicle trafficking in maize. Our findings provide a novel mechanistic insight into the regulation of inflorescence development, and ear length and kernel number, in maize.

Actin Isovariant ACT7 Modulates Root Thermomorphogenesis by Altering Intracellular Auxin Homeostasis

High temperature stress is one of the most threatening abiotic stresses for plants limiting the crop productivity world-wide. Altered developmental responses of plants to moderate-high temperature has been shown to be linked to the intracellular auxin homeostasis regulated by both auxin biosynthesis and transport. Trafficking of the auxin carrier proteins plays a major role in maintaining the cellular auxin homeostasis. The intracellular trafficking largely relies on the cytoskeletal component, actin, which provides track for vesicle movement. Different classes of actin and the isovariants function in regulating various stages of plant development. Although high temperature alters the intracellular trafficking, the role of actin in this process remains obscure. Using isovariant specific vegetative class actin mutants, here we demonstrate that ACTIN 7 (ACT7) isovariant plays an important role in regulating the moderate-high temperature response in Arabidopsis root. Loss of ACT7, but not ACT8 resulted in increased inhibition of root elongation under prolonged moderate-high temperature. Consistently, kinematic analysis revealed a drastic reduction in cell production rate and cell elongation in act7-4 mutant under high temperature. Quantification of actin dynamicity reveals that prolonged moderate-high temperature modulates bundling along with orientation and parallelness of filamentous actin in act7-4 mutant. The hypersensitive response of act7-4 mutant was found to be linked to the altered intracellular auxin distribution, resulted from the reduced abundance of PIN-FORMED PIN1 and PIN2 efflux carriers. Collectively, these results suggest that vegetative class actin isovariant, ACT7 modulates the long-term moderate-high temperature response in Arabidopsis root.

The microtubule-associated protein WDL4 modulates auxin distribution to promote apical hook opening in Arabidopsis

The unique apical hook in dicotyledonous plants protects the shoot apical meristem and cotyledons when seedlings emerge through the soil. Its formation involves differential cell growth under the coordinated control of plant hormones, especially ethylene and auxin. Microtubules are essential players in plant cell growth that are regulated by multiple microtubule-associated proteins (MAPs). However, the role and underlying mechanisms of MAP-microtubule modules in differential cell growth are poorly understood. In this study, we found that the previously uncharacterized Arabidopsis MAP WAVE-DAMPENED2-LIKE4 (WDL4) protein plays a positive role in apical hook opening. WDL4 exhibits a temporal expression pattern during hook development in dark-grown seedlings that is directly regulated by ethylene signaling. WDL4 mutants showed a delayed hook opening phenotype while overexpression of WDL4 resulted in enhanced hook opening. In particular, wdl4-1 mutants exhibited stronger auxin accumulation in the concave side of the apical hook. Furthermore, the regulation of the auxin maxima and trafficking of the auxin efflux carriers PIN-FORMED1 (PIN1) and PIN7 in the hook region is critical for WDL4-mediated hook opening. Together, our study demonstrates that WDL4 positively regulates apical hook opening by modulating auxin distribution, thus unraveling a mechanism for MAP-mediated differential plant cell growth.

Periodic root branching is influenced by light through an HY1-HY5-auxin pathway

The spacing of lateral roots (LRs) along the main root in plants is driven by an oscillatory signal, often referred to as the "root clock" that represents a pre-patterning mechanism that can be influenced by environmental signals. Light is an important environmental factor that has been previously reported to be capable of modulating the root clock, although the effect of light signaling on the LR pre-patterning has not yet been fully investigated. In this study, we reveal that light can activate the transcription of a photomorphogenic gene HY1 to maintain high frequency and amplitude of the oscillation signal, leading to the repetitive formation of pre-branch sites. By grafting and tissue-specific complementation experiments, we demonstrated that HY1 generated in the shoot or locally in xylem pole pericycle cells was sufficient to regulate LR branching. We further found that HY1 can induce the expression of HY5 and its homolog HYH, and act as a signalosome to modulate the intracellular localization and expression of auxin transporters, in turn promoting auxin accumulation in the oscillation zone to stimulate LR branching. These fundamental mechanistic insights improve our understanding of the molecular basis of light-controlled LR formation and provide a genetic interconnection between shoot- and root-derived signals in regulating periodic LR branching.

Global Identification of ANTH Genes Involved in Rice Pollen Germination and Functional Characterization of a Key Member, OsANTH3

Pollen in angiosperms plays a critical role in double fertilization by germinating and elongating pollen tubes rapidly in one direction to deliver sperm. In this process, the secretory vesicles deliver cell wall and plasma membrane materials, and excessive materials are sequestered via endocytosis. However, endocytosis in plants is poorly understood. AP180 N-terminal homology (ANTH) domain-containing proteins function as adaptive regulators for clathrin-mediated endocytosis in eukaryotic systems. Here, we identified 17 ANTH domain-containing proteins from rice based on a genome-wide investigation. Motif and phylogenomic analyses revealed seven asparagine-proline-phenylalanine (NPF)-rich and 10 NPF-less subgroups of these proteins, as well as various clathrin-mediated endocytosis-related motifs in their C-terminals. To investigate their roles in pollen germination, we performed meta-expression analysis of all genes encoding ANTH domain-containing proteins in Oryza sativa (OsANTH genes) in anatomical samples, including pollen, and identified five mature pollen-preferred OsANTH genes. The subcellular localization of four OsANTH proteins that were preferentially expressed in mature pollen can be consistent with their role in endocytosis in the plasma membrane. Of them, OsANTH3 represented the highest expression in mature pollen. Functional characterization of OsANTH3 using T-DNA insertional knockout and gene-edited mutants revealed that a mutation in OsANTH3 decreased seed fertility by reducing the pollen germination percentage in rice. Thus, our study suggests OsANTH3-mediated endocytosis is important for rice pollen germination.

A fundamental developmental transition in Physcomitrium patens is regulated by evolutionarily conserved mechanisms

One of the most defining moments in history was the colonization of land by plants approximately 470 million years ago. The transition from water to land was accompanied by significant changes in the plant body plan, from those than resembled filamentous representatives of the charophytes, the sister group to land plants, to those that were morphologically complex and capable of colonizing harsher habitats. The moss Physcomitrium patens (also known as Physcomitrella patens) is an extant representative of the bryophytes, the earliest land plant lineage. The protonema of P. patens emerges from spores from a chloronemal initial cell, which can divide to self-renew to produce filaments of chloronemal cells. A chloronemal initial cell can differentiate into a caulonemal initial cell, which can divide and self-renew to produce filaments of caulonemal cells, which branch extensively and give rise to three-dimensional shoots. The process by which a chloronemal initial cell differentiates into a caulonemal initial cell is tightly regulated by auxin-induced remodeling of the actin cytoskeleton. Studies have revealed that the genetic mechanisms underpinning this transition also regulate tip growth and differentiation in diverse plant taxa. This review summarizes the known cellular and molecular mechanisms underpinning the chloronema to caulonema transition in P. patens.

The TOR-Auxin Connection Upstream of Root Hair Growth

Plant growth and productivity are orchestrated by a network of signaling cascades involved in balancing responses to perceived environmental changes with resource availability. Vascular plants are divided into the shoot, an aboveground organ where sugar is synthesized, and the underground located root. Continuous growth requires the generation of energy in the form of carbohydrates in the leaves upon photosynthesis and uptake of nutrients and water through root hairs. Root hair outgrowth depends on the overall condition of the plant and its energy level must be high enough to maintain root growth. TARGET OF RAPAMYCIN (TOR)-mediated signaling cascades serve as a hub to evaluate which resources are needed to respond to external stimuli and which are available to maintain proper plant adaptation. Root hair growth further requires appropriate distribution of the phytohormone auxin, which primes root hair cell fate and triggers root hair elongation. Auxin is transported in an active, directed manner by a plasma membrane located carrier. The auxin efflux carrier PIN-FORMED 2 is necessary to transport auxin to root hair cells, followed by subcellular rearrangements involved in root hair outgrowth. This review presents an overview of events upstream and downstream of PIN2 action, which are involved in root hair growth control.

The Arabidopsis embryo as a quantifiable model for studying pattern formation

Phenotypic diversity of flowering plants stems from common basic features of the plant body pattern with well-defined body axes, organs and tissue organisation. Cell division and cell specification are the two processes that underlie the formation of a body pattern. As plant cells are encased into their cellulosic walls, directional cell division through precise positioning of division plane is crucial for shaping plant morphology. Since many plant cells are pluripotent, their fate establishment is influenced by their cellular environment through cell-to-cell signaling. Recent studies show that apart from biochemical regulation, these two processes are also influenced by cell and tissue morphology and operate under mechanical control. Finding a proper model system that allows dissecting the relationship between these aspects is the key to our understanding of pattern establishment. In this review, we present the Arabidopsis embryo as a simple, yet comprehensive model of pattern formation compatible with high-throughput quantitative assays.

A Conditional Mutation in SCD1 Reveals Linkage Between PIN Protein Trafficking, Auxin Transport, Gravitropism, and Lateral Root Initiation

Auxin is transported in plants with distinct polarity, defined by transport proteins of the PIN-formed (PIN) family. Components of the complex trafficking machinery responsible for polar PIN protein localization have been identified by genetic approaches, but severe developmental phenotypes of trafficking mutants complicate dissection of this pathway. We utilized a temperature sensitive allele of Arabidopsis thaliana SCD1 (stomatal cytokinesis defective1) that encodes a RAB-guanine nucleotide exchange factor. Auxin transport, lateral root initiation, asymmetric auxin-induced gene expression after gravitropic reorientation, and differential gravitropic growth were reduced in the roots of the scd1-1 mutant relative to wild type at the restrictive temperature of 25°C, but not at the permissive temperature of 18°C. In scd1-1 at 25°C, PIN1- and PIN2-GFP accumulated in endomembrane bodies. Transition of seedlings from 18 to 25°C for as little as 20 min resulted in the accumulation of PIN2-GFP in endomembranes, while gravitropism and root developmental defects were not detected until hours after transition to the non-permissive temperature. The endomembrane compartments that accumulated PIN2-GFP in scd1-1 exhibited FM4-64 signal colocalized with ARA7 and ARA6 fluorescent marker proteins, consistent with PIN2 accumulation in the late or multivesicular endosome. These experiments illustrate the power of using a temperature sensitive mutation in the gene encoding SCD1 to study the trafficking of PIN2 between the endosome and the plasma membrane. Using the conditional feature of this mutation, we show that altered trafficking of PIN2 precedes altered auxin transport and defects in gravitropism and lateral root development in this mutant upon transition to the restrictive temperature.

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

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

Plant Cell Polarity: Creating Diversity from Inside the Box

Cell polarity in plants operates across a broad range of spatial and temporal scales to control processes from acute cell growth to systemic hormone distribution. Similar to other eukaryotes, plants generate polarity at both the subcellular and tissue levels, often through polarization of membrane-associated protein complexes. However, likely due to the constraints imposed by the cell wall and their extremely plastic development, plants possess novel polarity molecules and mechanisms highly tuned to environmental inputs. Considerable progress has been made in identifying key plant polarity regulators, but detailed molecular understanding of polarity mechanisms remains incomplete in plants. Here, we emphasize the striking similarities in the conceptual frameworks that generate polarity in both animals and plants. To this end, we highlight how novel, plant-specific proteins engage in common themes of positive feedback, dynamic intracellular trafficking, and posttranslational regulation to establish polarity axes in development. We end with a discussion of how environmental signals control intrinsic polarity to impact postembryonic organogenesis and growth.

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

An essential role for Arabidopsis Trs33 in cell growth and organization in plant apical meristems

Trafficking protein particle (TRAPP) complexes subunit gene AtTrs33 plays an important role in keeping apical meristematic activity and dominance in Arabidopsis. TRAPP complexes, composed of multimeric subunits, are guanine-nucleotide exchange factors for certain Rab GTPases and are believed to be involved in the regulation of membrane trafficking, but the cases in Arabidopsis are largely unknown. Trs33, recently proposed to be a component of TRAPP IV, is non-essential in yeast cells. A single copy of Trs33 gene, AtTrs33, was identified in Arabidopsis. GUS activity assay indicated that AtTrs33 was ubiquitously expressed. Based on a T-DNA insertion line, we found that loss-of-function of AtTrs33 is lethal for apical growth. Knock-down or knock-in of AtTrs33 affects apical meristematic growth and fertility, which indicates that AtTrs33 plays an important role in keeping apical meristematic activity and dominance in Arabidopsis. Analysis of auxin responses and PIN1/2 localization indicate that impaired apical meristematic activity and dominance were caused by altered auxin responses through non-polarized PIN1 localization. The present study reported that AtTrs33 plays an essential role in Arabidopsis cell growth and organization, which is different with its homologue in yeast. These findings provide new insights into the functional divergence of TRAPP subunits.

CesA6 and PGIP2 Endocytosis Involves Different Subpopulations of TGN-Related Endosomes

Endocytosis is an essential process for the internalization of plasma membrane proteins, lipids and extracellular molecules into the cells. The mechanisms underlying endocytosis in plant cells involve several endosomal organelles whose origins and specific role needs still to be clarified. In this study we compare the internalization events of a GFP-tagged polygalacturonase-inhibiting protein of Phaseolus vulgaris (PGIP2-GFP) to that of a GFP-tagged subunit of cellulose synthase complex of Arabidopsis thaliana (secGFP-CesA6). Through the use of endocytic traffic chemical inhibitors (tyrphostin A23, salicylic acid, wortmannin, concanamycin A, Sortin 2, Endosidin 5 and BFA) it was evidenced that the two protein fusions were endocytosed through distinct endosomes with different mechanisms. PGIP2-GFP endocytosis is specifically sensitive to tyrphostin A23, salicylic acid and Sortin 2; furthermore, SYP51, a tSNARE with interfering effect on late steps of vacuolar traffic, affects its arrival in the central vacuole. SecGFP-CesA6, specifically sensitive to Endosidin 5, likely reaches the plasma membrane passing through the trans Golgi network (TGN), since the BFA treatment leads to the formation of BFA bodies, compatible with the aggregation of TGNs. BFA treatments determine the accumulation and tethering of the intracellular compartments labeled by both proteins, but PGIP2-GFP aggregated compartments overlap with those labeled by the endocytic dye FM4-64 while secGFP-CesA6 fills different compartments. Furthermore, secGFP-CesA6 co-localization with RFP-NIP1.1, marker of the direct ER-to-Vacuole traffic, in small compartments separated from ER suggests that secGFP-CesA6 is sorted through TGNs in which the direct contribution from the ER plays an important role. All together the data indicate the existence of a heterogeneous population of Golgi-independent TGNs.

Secretion of Phospholipase Dδ Functions as a Regulatory Mechanism in Plant Innate Immunity

Plant phospholipase Ds (PLDs), essential regulators of phospholipid signaling, function in multiple signal transduction cascades; however, the mechanisms regulating PLDs in response to pathogens remain unclear. Here, we found that Arabidopsis (Arabidopsis thaliana) PLDδ accumulated in cells at the entry sites of the barley powdery mildew fungus, Blumeria graminis f. sp hordei Using fluorescence recovery after photobleaching and single-molecule analysis, we observed higher PLDδ density in the plasma membrane after chitin treatment; PLDδ also underwent rapid exocytosis. Fluorescence resonance energy transfer with fluorescence lifetime imaging microscopy showed that the interaction between PLDδ and the microdomain marker AtREMORIN1.3 (AtREM1.3) increased in response to chitin, indicating that exocytosis facilitates rapid, efficient sorting of PLDδ into microdomains upon pathogen stimulus. We further unveiled a trade-off between brefeldin A (BFA)-resistant and -sensitive pathways in secretion of PLDδ under diverse conditions. Upon pathogen attack, PLDδ secretion involved syntaxin-associated VAMP721/722-mediated exocytosis sensitive to BFA. Analysis of phosphatidic acid (PA), hydrogen peroxide, and jasmonic acid (JA) levels and expression of related genes indicated that the relocalization of PLDδ is crucial for its activation to produce PA and initiate reactive oxygen species and JA signaling pathways. Together, our findings revealed that the translocation of PLDδ to papillae is modulated by exocytosis, thus triggering PA-mediated signaling in plant innate immunity.plantcell;31/12/3015/FX1F1fx1.

The Road to Auxin-Dependent Growth Repression and Promotion in Apical Hooks

The phytohormone auxin controls growth rates within plant tissues, but the underlying mechanisms are still largely enigmatic. The apical hook is a superb model to understand differential growth, because it displays both auxin-dependent growth repression and promotion. In this special issue on membranes, we illustrate how the distinct utilization of vesicle trafficking contributes to the spatial control of polar auxin transport, thereby pinpointing the site of growth repression in apical hooks. We moreover highlight that the transition to growth promotion is achieved by balancing inter- and intracellular auxin transport. We emphasize here that the apical hook development is a suitable model to further advance our mechanistic knowledge on plant growth regulation.

An Auxin Transport Inhibitor Targets Villin-Mediated Actin Dynamics to Regulate Polar Auxin Transport

Auxin transport inhibitors are essential tools for understanding auxin-dependent plant development. One mode of inhibition affects actin dynamics; however, the underlying mechanisms remain unclear. In this study, we characterized the action of 2,3,5-triiodobenzoic acid (TIBA) on actin dynamics in greater mechanistic detail. By surveying mutants for candidate actin-binding proteins with reduced TIBA sensitivity, we determined that Arabidopsis (Arabidopsis thaliana) villins contribute to TIBA action. By directly interacting with the C-terminal headpiece domain of villins, TIBA causes villin to oligomerize, driving excessive bundling of actin filaments. The resulting changes in actin dynamics impair auxin transport by disrupting the trafficking of PIN-FORMED auxin efflux carriers and reducing their levels at the plasma membrane. Collectively, our study provides mechanistic insight into the link between the actin cytoskeleton, vesicle trafficking, and auxin transport.

A role for the auxin precursor anthranilic acid in root gravitropism via regulation of PIN-FORMED protein polarity and relocalisation in Arabidopsis

distribution of auxin within plant tissues is of great importance for developmental plasticity, including root gravitropic growth. Auxin flow is directed by the subcellular polar distribution and dynamic relocalisation of auxin transporters such as the PIN-FORMED (PIN) efflux carriers, which can be influenced by the main natural plant auxin indole-3-acetic acid (IAA). Anthranilic acid (AA) is an important early precursor of IAA and previously published studies with AA analogues have suggested that AA may also regulate PIN localisation. Using Arabidopsis thaliana as a model species, we studied an AA-deficient mutant displaying agravitropic root growth, treated seedlings with AA and AA analogues and transformed lines to over-produce AA while inhibiting its conversion to downstream IAA precursors. We showed that AA rescues root gravitropic growth in the AA-deficient mutant at concentrations that do not rescue IAA levels. Overproduction of AA affects root gravitropism without affecting IAA levels. Treatments with, or deficiency in, AA result in defects in PIN polarity and gravistimulus-induced PIN relocalisation in root cells. Our results revealed a previously unknown role for AA in the regulation of PIN subcellular localisation and dynamics involved in root gravitropism, which is independent of its better known role in IAA biosynthesis.

Auxin and Cell Wall Crosstalk as Revealed by the Arabidopsis thaliana Cellulose Synthase Mutant Radially Swollen 1

Plant cells sheath themselves in a complex lattice of polysaccharides, proteins and enzymes forming an integral matrix known as the cell wall. Cellulose microfibrils, the primary component of cell walls, are synthesized at the plasma membrane by CELLULOSE SYNTHASE A (CESA) proteins throughout cellular growth and are responsible for turgor-driven anisotropic expansion. Associations between hormone signaling and cell wall biosynthesis have long been suggested, but recently direct links have been found revealing hormones play key regulatory roles in cellulose biosynthesis. The radially swollen 1 (rsw1) allele of Arabidopsis thaliana CESA1 harbors a single amino acid change that renders the protein unstable at high temperatures. We used the conditional nature of rsw1 to investigate how auxin contributes to isotropic growth. We found that exogenous auxin treatment reduces isotropic swelling in rsw1 roots at the restrictive temperature of 30�C. We also discovered decreases in auxin influx between rsw1 and wild-type roots via confocal imaging of AUX1-YFP, even at the permissive temperature of 19�C. Moreover, rsw1 displayed mis-expression of auxin-responsive and CESA genes. Additionally, we found altered auxin maxima in rsw1 mutant roots at the onset of swelling using DII-VENUS and DR5:vYFP auxin reporters. Overall, we conclude disrupted cell wall biosynthesis perturbs auxin transport leading to altered auxin homeostasis impacting both anisotropic and isotropic growth that affects overall root morphology.

Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants

Plant hormone auxin regulates several aspects of plant growth and development. Auxin is predominantly synthesized in the shoot apex and developing leaf primordia and from there it is transported to the target tissues e.g. roots. Auxin transport is polar in nature and is carrier-mediated. AUXIN1/LIKE-AUX1 (AUX1/LAX) family members are the major auxin influx carriers whereas PIN-FORMED (PIN) family and some members of the P-GLYCOPROTEIN/ATP-BINDING CASSETTE B4 (PGP/ABCB) family are major auxin efflux carriers. AUX1/LAX auxin influx carriers are multi-membrane spanning transmembrane proteins sharing similarity to amino acid permeases. Mutations in AUX1/LAX genes result in auxin related developmental defects and have been implicated in regulating key plant processes including root and lateral root development, root gravitropism, root hair development, vascular patterning, seed germination, apical hook formation, leaf morphogenesis, phyllotactic patterning, female gametophyte development and embryo development. Recently AUX1 has also been implicated in regulating plant responses to abiotic stresses. This review summarizes our current understanding of the developmental roles of AUX1/LAX gene family and will also briefly discuss the modelling approaches that are providing new insight into the role of auxin transport in plant development.

Initial Bud Outgrowth Occurs Independent of Auxin Flow from Out of Buds

Apical dominance is the process whereby the shoot tip inhibits the growth of axillary buds along the stem. It has been proposed that the shoot tip, which is the predominant source of the plant hormone auxin, prevents bud outgrowth by suppressing auxin canalization and export from axillary buds into the main stem. In this theory, auxin flow out of axillary buds is a prerequisite for bud outgrowth, and buds are triggered to grow by an enhanced proportional flow of auxin from the buds. A major challenge of directly testing this model is in being able to create a bud- or stem-specific change in auxin transport. Here we evaluate the relationship between specific changes in auxin efflux from axillary buds and bud outgrowth after shoot tip removal (decapitation) in the pea (Pisum sativum). The auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) and to a lesser extent, the auxin perception inhibitor p-chlorophenoxyisobutyric acid (PCIB), effectively blocked auxin efflux from axillary buds of intact and decapitated plants without affecting auxin flow in the main stem. Gene expression analyses indicate that NPA and PCIB regulate auxin-inducible, and biosynthesis and transport genes, in axillary buds within 3 h after application. These inhibitors had no effect on initial bud outgrowth after decapitation or cytokinin (benzyladenine; BA) treatment. Inhibitory effects of PCIB and NPA on axillary bud outgrowth only became apparent from 48 h after treatment. These findings demonstrate that the initiation of decapitation- and cytokinin-induced axillary bud outgrowth is independent of auxin canalization and export from the bud.

FERONIA regulates auxin-mediated lateral root development and primary root gravitropism

The Arabidopsis FERONIA (FER) receptor kinase is a key hub of cell signaling networks mediating various hormone, stress, and immune responses. Previous studies have shown that FER functions correlate with auxin responses, but the underlying molecular mechanism is unknown. Here, we demonstrate that the primary root of the fer-4 mutant displays increased lateral root branching and a delayed gravitropic response, which are associated with polar auxin transport (PAT). Our data suggest that aberrant PIN2 polarity is responsible for the delayed gravitropic response in fer-4. Furthermore, the diminished F-actin cytoskeleton in fer-4 implies that FER modulates F-actin-mediated PIN2 polar localization. Our findings provide new insights into the function of FER in PAT.

Class XI Myosins Contribute to Auxin Response and Senescence-Induced Cell Death in Arabidopsis

The integrity and dynamics of actin cytoskeleton is necessary not only for plant cell architecture but also for membrane trafficking-mediated processes such as polar auxin transport, senescence, and cell death. In Arabidopsis, the inactivation of actin-based molecular motors, class XI myosins, affects the membrane trafficking and integrity of actin cytoskeleton, and thus causes defective plant growth and morphology, altered lifespan and reduced fertility. To evaluate the potential contribution of class XI myosins to the auxin response, senescence and cell death, we followed the flower and leaf development in the triple gene knockout mutant xi1 xi2 xik (3KO) and in rescued line stably expressing myosin XI-K:YFP (3KOR). Assessing the development of primary inflorescence shoots we found that the 3KO plants produced more axillary branches. Exploiting the auxin-dependent reporters DR5::GUS and IAA2::GUS, a significant reduction in auxin responsiveness was found throughout the development of the 3KO plants. Examination of the flower development of the plants stably expressing the auxin transporter PIN1::PIN1-GFP revealed partial loss of PIN1 polarization in developing 3KO pistils. Surprisingly, the stable expression of PIN1::PIN1-GFP significantly enhanced the semi-sterile phenotype of the 3KO plants. Further we investigated the localization of myosin XI-K:YFP in the 3KOR floral organs and revealed its expression pattern in floral primordia, developing pistils, and anther filaments. Interestingly, the XI-K:YFP and PIN1::PIN1-GFP shared partially overlapping but distinct expression patterns throughout floral development. Assessing the foliar development of the 3KO plants revealed increased rosette leaf production with signs of premature yellowing. Symptoms of the premature senescence correlated with massive loss of chlorophyll, increased cell death, early plasmolysis of epidermal cells, and strong up-regulation of the stress-inducible senescence-associated gene SAG13 in 3KO plants. Simultaneously, the reduced auxin responsiveness and premature leaf senescence were accompanied by significant anthocyanin accumulation in 3KO tissues. Collectively, our results provide genetic evidences that Arabidopsis class XI myosins arrange the flower morphogenesis and leaf longevity via contributing to auxin responses, leaf senescence, and cell death.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Regulatory networks controlling the development of the root system and the formation of lateral roots: a comparative analysis of the roles of pericycle and vascular cambium

Background

The production of a new lateral root from parental root primary tissues has been investigated extensively, and the most important regulatory mechanisms are now well known. A first regulatory mechanism is based on the synthesis of small peptides which interact ectopically with membrane receptors to elicit a modulation of transcription factor target genes. A second mechanism involves a complex cross-talk between plant hormones. It is known that lateral roots are formed even in parental root portions characterized by the presence of secondary tissues, but there is not yet agreement about the putative tissue source providing the cells competent to become founder cells of a new root primordium.

Scope

We suggest models of possible regulatory mechanisms for inducing specific root vascular cambium (VC) stem cells to abandon their activity in the production of xylem and phloem elements and to start instead the construction of a new lateral root primordium. Considering the ontogenic nature of the VC, the models which we suggest are the result of a comparative review of mechanisms known to control the activity of stem cells in the root apical meristem, procambium and VC. Stem cells in the root meristems can inherit various competences to play different roles, and their fate could be decided in response to cross-talk between endogenous and exogenous signals.

Conclusions

We have found a high degree of relatedness among the regulatory mechanisms controlling the various root meristems. This fact suggests that competence to form new lateral roots can be inherited by some stem cells of the VC lineage. This kind of competence could be represented by a sensitivity of specific stem cells to factors such as those presented in our models.

Transcriptome Analysis of Intrusively Growing Flax Fibers Isolated by Laser Microdissection

The intrusive growth, a type of plant cell elongation occurring in the depths of plant tissues, is characterized by the invasion of a growing cell between its neighbours due to a higher rate of elongation. In order to reveal the largely unknown molecular mechanisms of intrusive growth, we isolated primary flax phloem fibers specifically at the stage of intrusive growth by laser microdissection. The comparison of the RNA-Seq data from several flax stem parts enabled the characterization of those processes occurring specifically during the fiber intrusive elongation. The revealed molecular players are summarized as those involved in the supply of assimilates and support of turgor pressure, cell wall enlargement and modification, regulation by transcription factors and hormones, and responses to abiotic stress factors. The data obtained in this study provide a solid basis for developing approaches to manipulate fiber intrusive elongation, which is of importance both for plant biology and the yield of fiber crops.

Arabidopsis FIM4 and FIM5 regulates the growth of root hairs in an auxin-insensitive way

Tip-growing cells provide a useful model system for studying the underlying mechanisms of plant cell growth. The apical growth of root hairs is dependent on the microfilament skeleton, and auxin is an important regulator of root hair development. We functionally characterized actin bundling proteins AtFIM4 and AtFIM5, which were preferentially expressed in tip-growing cells such as pollen tubes and root hairs. The morphology and length of root hairs in atfim4/atfim5 double mutant line had obvious defects. In addition, we found the growth of root hairs of atfim4/atfim5 double mutant was insensitive to exogenous IAA (indole-3-acetic acid) treatment. So we consider that AtFIM4 and AtFIM5 act together to regulate the growth of root hair in an auxin-insensitive way.

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

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

Knowns and unknowns of plasma membrane protein degradation in plants

Plasma membrane (PM) not only creates a physical barrier to enclose the intracellular compartments but also mediates the direct communication between plants and the ever-changing environment. A tight control of PM protein homeostasis by selective degradation is thus crucial for proper plant development and plant-environment interactions. Accumulated evidences have shown that a number of plant PM proteins undergo clathrin-dependent or membrane microdomain-associated endocytic routes to vacuole for degradation in a cargo-ubiquitination dependent or independent manner. Besides, several trans-acting determinants involved in the regulation of endocytosis, recycling and multivesicular body-mediated vacuolar sorting have been identified in plants. More interestingly, recent findings have uncovered the participation of selective autophagy in PM protein turnover in plants. Although great progresses have been made to identify the PM proteins that undergo dynamic changes in subcellular localizations and to explore the factors that control the membrane protein trafficking, several questions remain to be answered regarding the molecular mechanisms of PM protein degradation in plants. In this short review article, we briefly summarize recent progress in our understanding of the internalization, sorting and degradation of plant PM proteins. More specifically, we focus on discussing the elusive aspects underlying the pathways of PM protein degradation in plants.

Dose-dependent interactions between two loci trigger altered shoot growth in BG-5 × Krotzenburg-0 (Kro-0) hybrids of Arabidopsis thaliana

Hybrids occasionally exhibit genetic interactions resulting in reduced fitness in comparison to their parents. Studies of Arabidopsis thaliana have highlighted the role of immune conflicts, but less is known about the role of other factors in hybrid incompatibility in plants. Here, we present a new hybrid incompatibility phenomenon in this species. We have characterized a new case of F₁ hybrid incompatibility from a cross between the A. thaliana accessions Krotzenburg-0 (Kro-0) and BG-5, by conducting transcript, metabolite and hormone analyses, and identified the causal loci through genetic mapping. The F₁ hybrids showed arrested growth of the main stem, altered shoot architecture, and altered concentrations of hormones in comparison to parents. The F₁ phenotype could be rescued in a developmental-stage-dependent manner by shifting to a higher growth temperature. These F₁ phenotypes were linked to two loci, one on chromosome 2 and one on chromosome 3. The F₂ generation segregated plants with more severe phenotypes which were linked to the same loci as those in the F₁ . This study provides novel insights into how previously unknown mechanisms controlling shoot branching and stem growth can result in hybrid incompatibility.

Trafficking routes to the plant vacuole: connecting alternative and classical pathways

Due to the numerous roles plant vacuoles play in cell homeostasis, detoxification, and protein storage, the trafficking pathways to this organelle have been extensively studied. Recent evidence, however, suggests that our vision of transport to the vacuole is not as simple as previously imagined. Alternative routes have been identified and are being characterized. Intricate interconnections between routes seem to occur in various cases, complicating the interpretation of data. In this review, we aim to summarize the published evidence and link the emerging data with previous findings. We discuss the current state of information on alternative and classical trafficking routes to the plant vacuole.

The allelochemical farnesene affects Arabidopsis thaliana root meristem altering auxin distribution

Farnesene is a sesquiterpene with semiochemical activity involved in interspecies communication. This molecule, known for its phytotoxic potential and its effects on root morphology and anatomy, caused anisotropic growth, bold roots and a "left-handedness" phenotype. These clues suggested an alteration of auxin distribution, and for this reason, the aim of the present study was to evaluate its effects on: i) PIN-FORMED proteins (PIN) distribution, involved in polar auxin transport; ii) PIN genes expression iii) apical meristem anatomy of primary root, in 7 days old Arabidopsis thaliana seedlings treated with farnesene 250 μM. The following GFP constructs: pSCR::SCR-GFP, pDR5::GFP,pPIN1::PIN1-GFP, pPIN2::PIN2-GFP, pPIN3::PIN3-GFP, pPIN4::PIN4-GFP and pPIN7::PIN7-GFP were used to evaluate auxin distribution. Farnesene caused a reduction in meristematic zone size, an advancement in transition zone, suggesting a premature exit of cells from the meristematic zone, a reduction in cell division and an impairment between epidermal and cortex cells. The auxin-responsive reporter pDR5::GFP highlighted that auxin distribution was impaired in farnesene-treated roots, where auxin distribution appeared maximum in the quiescent center and columella initial cells, without extending to mature columella cells. This finding was further confirmed by the analysis on PIN transport proteins distribution, assessed on individual constructs, which showed an extreme alteration mainly dependent on the PIN 3, 4 and 7, involved in pattern specification during root development and auxin redistribution. Finally, farnesene treatment caused a down-regulation of all the auxin transport genes studied. We propose that farnesene affected auxin transport and distribution causing the alteration of root meristem, and consequently the left-handedness phenotype.

RopGEF1 Plays a Critical Role in Polar Auxin Transport in Early Development

Polar auxin transport, facilitated by the combined activities of auxin influx and efflux carriers to maintain asymmetric auxin distribution, is essential for plant growth and development. Here, we show that Arabidopsis (Arabidopsis thaliana) RopGEF1, a guanine nucleotide exchange factor and activator of Rho GTPases of plants (ROPs), is critically involved in polar distribution of auxin influx carrier AUX1 and differential accumulation of efflux carriers PIN7 and PIN2 and is important for embryo and early seedling development when RopGEF1 is prevalently expressed. Knockdown or knockout of RopGEF1 induces embryo defects, cotyledon vein breaks, and delayed root gravity responses. Altered expression from the auxin response reporter DR5rev:GFP in the root pole of RopGEF1-deficient embryos and loss of asymmetric distribution of DR5rev:GFP in their gravistimulated root tips suggest that auxin distribution is affected in ropgef1 mutants. This is reflected by the polarity of AUX1 being altered in ropgef1 embryos and roots, shifting from the normal apical membrane location to a basal location in embryo central vascular and root protophloem cells and also reduced PIN7 accumulation at embryos and altered PIN2 distribution in gravistimulated roots of mutant seedlings. In establishing that RopGEF1 is critical for AUX1 localization and PIN differential accumulation, our results reveal a role for RopGEF1 in cell polarity and polar auxin transport whereby it imapcts auxin-mediated plant growth and development.

Ethylene Regulates Differential Growth via BIG ARF-GEF-Dependent Post-Golgi Secretory Trafficking in Arabidopsis

During early seedling development, the shoot apical meristem is protected from damage as the seedling emerges from soil by the formation of apical hook. Hook formation requires differential growth across the epidermis below the meristem in the hypocotyl. The plant hormones ethylene and auxin play key roles during apical hook development by controlling differential growth. We provide genetic and cell biological evidence for the role of ADP-ribosylation factor 1 (ARF1)-GTPase and its effector ARF-guanine-exchange factors (GEFs) of the Brefeldin A-inhibited GEF (BIG) family and GNOM in ethylene- and auxin-mediated control of hook development. We show that ARF-GEF GNOM acts early, whereas BIG ARF-GEFs act at a later stage of apical hook development. We show that the localization of ARF1 and BIG4 at the trans-Golgi network (TGN) depends on ECHIDNA (ECH), a plant homolog of yeast Triacylglycerol lipase (TLG2/SYP4) interacting protein Tgl2-Vesicle Protein 23 (TVP23). BIGs together with ECH and ARF1 mediate the secretion of AUX1 influx carrier to the plasma membrane from the TGN during hook development and defects in BIG or ARF1 result in insensitivity to ethylene. Thus, our data indicate a division of labor within the ARF-GEF family in mediating differential growth with GNOM acting during the formation phase whereas BIGs act during the hook maintenance phase downstream of plant hormone ethylene.

Chemical Genetic Dissection of Membrane Trafficking

The plant endomembrane system is an extensively connected functional unit for exchanging material between compartments. Secretory and endocytic pathways allow dynamic trafficking of proteins, lipids, and other molecules, regulating a myriad of biological processes. Chemical genetics-the use of compounds to perturb biological processes in a fast, tunable, and transient manner-provides elegant tools for investigating this system. Here, we review how chemical genetics has helped to elucidate different aspects of membrane trafficking. We discuss different strategies for uncovering the modes of action of such compounds and their use in unraveling membrane trafficking regulators. We also discuss how the bioactive chemicals that are currently used as probes to interrogate endomembrane trafficking were discovered and analyze the results regarding membrane trafficking and pathway crosstalk. The integration of different expertises and the rational implementation of chemical genetic strategies will improve the identification of molecular mechanisms that drive intracellular trafficking and our understanding of how trafficking interfaces with plant physiology and development.

Small acidic protein 1 and SCFTIR1 ubiquitin proteasome pathway act in concert to induce 2,4-dichlorophenoxyacetic acid-mediated alteration of actin in Arabidopsis roots

2,4-Dichlorophenoxyacetic acid (2,4-D), a functional analogue of auxin, is used as an exogenous source of auxin as it evokes physiological responses like the endogenous auxin, indole-3-acetic acid (IAA). Previous molecular analyses of the auxin response pathway revealed that IAA and 2,4-D share a common mode of action to elicit downstream physiological responses. However, recent findings with 2,4-D-specific mutants suggested that 2,4-D and IAA might also use distinct pathways to modulate root growth in Arabidopsis. Using genetic and cellular approaches, we demonstrate that the distinct effects of 2,4-D and IAA on actin filament organization partly dictate the differential responses of roots to these two auxin analogues. 2,4-D but not IAA altered the actin structure in long-term and short-term assays. Analysis of the 2,4-D-specific mutant aar1-1 revealed that small acidic protein 1 (SMAP1) functions positively to facilitate the 2,4-D-induced depolymerization of actin. The ubiquitin proteasome mutants tir1-1 and axr1-12, which show enhanced resistance to 2,4-D compared with IAA for inhibition of root growth, were also found to have less disrupted actin filament networks after 2,4-D exposure. Consistently, a chemical inhibitor of the ubiquitin proteasome pathway mitigated the disrupting effects of 2,4-D on the organization of actin filaments. Roots of the double mutant aar1-1 tir1-1 also showed enhanced resistance to 2,4-D-induced inhibition of root growth and actin degradation compared with their respective parental lines. Collectively, these results suggest that the effects of 2,4-D on actin filament organization and root growth are mediated through synergistic interactions between SMAP1 and SCF^TIR1 ubiquitin proteasome components.

Inhibition of phosphatidylinositol 3,5-bisphosphate production has pleiotropic effects on various membrane trafficking routes in Arabidopsis

Phosphoinositides play an important role in various membrane trafficking events in eukaryotes. One of them, however, phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2], has not been studied widely in plants. Using a combination of fluorescent reporter proteins and the PI(3,5)P2-specific inhibitor YM202636, here we demonstrated that in Arabidopsis thaliana, PI(3,5)P2 affects various membrane trafficking events, mostly in the post-Golgi routes. We found that YM201636 treatment effectively reduced PI(3,5)P2 concentration not only in the wild type but also in FAB1A-overexpressing Arabidopsis plants. In particular, reduced PI(3,5)P2 levels caused abnormal membrane dynamics of plasma membrane proteins, AUX1 and BOR1, with different trafficking patterns. Secretion and morphological characteristics of late endosomes and vacuoles were also affected by the decreased PI(3,5)P2 production. These pleiotropic defects in the post-Golgi trafficking events were caused by the inhibition of PI(3,5)P2 production. This effect is probably mediated by the inhibition of maturation of FAB1-positive late endosomes, thereby impairing late endosome function. In conclusion, our results imply that in Arabidopsis, late endosomes are involved in multiple post-Golgi membrane trafficking routes including not only vacuolar trafficking and endocytosis but also secretion.

In Arabidopsis thaliana Cadmium Impact on the Growth of Primary Root by Altering SCR Expression and Auxin-Cytokinin Cross-Talk

Cadmium is one of the most widespread pollutant in both terrestrial and marine environment, and its inhibitory effect on plant growth has been largely demonstrated. However, the molecular mechanisms underlying Cd toxicity in plant and mainly in root, as the first organ sensing soil heavy metals, need to be better investigated. To this aim, in the present work we analyzed the growth and the organization of Arabidopsis thaliana primary root in seedlings exposed to Cd (25 and 50 μM) for 8 days starting from germination. Root length, root meristem size, and organization were evaluated together with the behavior of some of the major molecular players in root growth and patterning. In particular, by using different GFP transgenic lines, we monitored: (i) the expression pattern of WOX5 and SCR transcription factors involved in the establishment and maintenance of stem cell niche and in the control of meristem size; (ii) the expression pattern of the IAA-inducible pDR5::GFP reporter, PIN 1, 2, 3, 7 auxin carriers and TCSn::GFP cytokinin-sensitive sensor as relevant components of hormone circuit controlling root growth. We report that Cd exposure inhibits primary root growth via affecting RAM stem cell niche and root radial pattern. At the molecular level, an impairment of auxin maximum accumulation at the root tip, related to a down-regulation and mislocalisation of PIN proteins, and an enhancement of TCSn::GFP cytokinin-sensitive sensor signal is also detected under Cd treatment, thus suggesting an alteration in the homeostasis of auxin/cytokinin signaling. Moreover, and for the first time Cd toxicity on root growth and pattern has been related to a misexpression of SCR transcription factors which is known to interplay with auxin/cytokinin cross-talk in the control of RAM maintenance and activity.