The Twisted Dwarf's ABC: How Immunophilins Regulate Auxin Transport

Arabidopsis TWISTED DWARF1 regulates stamen elongation by differential activation of ABCB1,19-mediated auxin transport

Despite clear evidence that a local accumulation of auxin is likewise critical for male fertility, much less is known about the components that regulate auxin-controlled stamen development. In this study, we analyzed physiological and morphological parameters in mutants of key players of ABCB-mediated auxin transport, and spatially and temporally dissected their expression on the protein level as well as auxin fluxes in the Arabidopsis stamens. Our analyses revealed that the FKBP42, TWISTED DWARF1 (TWD1), promotes stamen elongation and, to a lesser extent, anther dehiscence, as well as pollen maturation, and thus is required for seed development. Most of the described developmental defects in twd1 are shared with the abcb1 abcb19 mutant, which can be attributed to the fact that TWD1-as a described ABCB chaperone-is a positive regulator of ABCB1- and ABCB19-mediated auxin transport. However, reduced stamen number was dependent on TWD1 but not on investigated ABCBs, suggesting additional players downstream of TWD1. We predict an overall housekeeping function for ABCB1 during earlier stages, while ABCB19 seems to be responsible for the key event of rapid elongation at later stages of stamen development. Our data indicate that TWD1 controls stamen development by differential activation of ABCB1,19-mediated auxin transport in the stamen.

Role of ATP-binding cassette transporters in maintaining plant homeostasis under abiotic and biotic stresses

The ATP-binding cassette (ABC) transporters belong to a large protein family predominantly present in diverse species. ABC transporters are driven by ATP hydrolysis and can act as exporters as well as importers. These proteins are localized in the membranes of chloroplasts, mitochondria, peroxisomes and vacuoles. ABC proteins are involved in regulating diverse biological processes in plants, such as growth, development, uptake of nutrients, tolerance to biotic and abiotic stresses, tolerance to metal toxicity, stomatal closure, shape and size of grains, protection of pollens, transport of phytohormones, etc. In mitochondria and chloroplast, the iron metabolism and its transport across the membrane are mediated by ABC transporters. Tonoplast-localized ABC transporters are involved in internal detoxification of metal ion; thus protecting against the DNA impairment and maintaining cell growth. ABC transporters are involved in the transport of secondary metabolites inside the cells. Microorganisms also engage a large number of ABC transporters to import and expel substrates decisive for their pathogenesis. ABC transporters also suppress the seed embryonic growth until favorable conditions come. This review aims at giving insights on ABC transporters, their evolution, structure, functions and roles in different biological processes for helping the terrestrial plants to survive under adverse environmental conditions. These specialized plant membrane transporters ensure a sustainable economic yield and high-quality products, especially under unfavorable conditions of growth. These transporters can be suitably manipulated to develop 'Plants for the Future'.

BcHTT4 Inhibits Branching of Non-Heading Chinese Cabbage at the Vegetative Stage

Branching is speculated to contribute to the plant architecture and crop yield. As a quantitative trait, branching is regulated by multiple genes in non-heading Chinese cabbage (NHCC). Several related candidate genes have been discovered in previous studies on the branching of NHCC, but their specific functions and regulatory mechanisms still need to be verified and explored. In this study, we found that the expression of BcHTT4, the ortholog to HEAT-INDUCED TAS1 TARGET4 (HTT4) in Arabidopsis, was significantly different between ‘Suzhouqing’ (common type) and ‘Maertou’ (multiple shoot branching type) in NHCC, which was consistent with the previous transcriptome sequencing results. The silencing of BcHTT4 expression in non-heading Chinese cabbage promotes axillary bud growth at the vegetative stage. When BcHTT4 is overexpressed in Arabidopsis, branching will decrease. In further study, we found that BcHTT4 interacts with immunophilin BcFKBP13 in vivo and in vitro through yeast two-hybrid analysis and bimolecular fluorescence complementation (BiFC) assays. Moreover, quantitative real-time PCR analysis showed that when the expression of BcHTT4 was silenced in ‘Suzhouqing’, the expression of BcFKBP13 also decreased significantly. Our findings reveal that BcHTT4 is involved in the branching mechanism and interacts with immunophilin BcFKBP13 in NHCC.

A twist in the ABC: regulation of ABC transporter trafficking and transport by FK506-binding proteins

Post-transcriptional regulation of ATP-binding cassette (ABC) proteins has been so far shown to encompass protein phosphorylation, maturation, and ubiquitination. Yet, recent accumulating evidence implicates FK506-binding proteins (FKBPs), a type of peptidylprolyl cis-trans isomerase (PPIase) proteins, in ABC transporter regulation. In this perspective article, we summarize current knowledge on ABC transporter regulation by FKBPs, which seems to be conserved over kingdoms and ABC subfamilies. We uncover striking functional similarities but also differences between regulatory FKBP-ABC modules in plants and mammals. We dissect a PPIase- and HSP90-dependent and independent impact of FKBPs on ABC biogenesis and transport activity. We propose and discuss a putative new mode of transient ABC transporter regulation by cis-trans isomerization of X-prolyl bonds.

Auxin-transporting ABC transporters are defined by a conserved D/E-P motif regulated by a prolylisomerase

The plant hormone auxin must be transported throughout plants in a cell-to-cell manner to affect its various physiological functions. ABCB transporters are critical for this polar auxin distribution, but the regulatory mechanisms controlling their function is not fully understood. The auxin transport activity of ABCB1 was suggested to be regulated by a physical interaction with FKBP42/Twisted Dwarf1 (TWD1), a peptidylprolyl cis-trans isomerase (PPIase), but all attempts to demonstrate such a PPIase activity by TWD1 have failed so far. By using a structure-based approach, we identified several surface-exposed proline residues in the nucleotide binding domain and linker of Arabidopsis ABCB1, mutations of which do not alter ABCB1 protein stability or location but do affect its transport activity. P1008 is part of a conserved signature D/E-P motif that seems to be specific for auxin-transporting ABCBs, which we now refer to as ATAs. Mutation of the acidic residue also abolishes auxin transport activity by ABCB1. All higher plant ABCBs for which auxin transport has been conclusively proven carry this conserved motif, underlining its predictive potential. Introduction of this D/E-P motif into malate importer, ABCB14, increases both its malate and its background auxin transport activity, suggesting that this motif has an impact on transport capacity. The D/E-P1008 motif is also important for ABCB1-TWD1 interactions and activation of ABCB1-mediated auxin transport by TWD1. In summary, our data imply a new function for TWD1 acting as a putative activator of ABCB-mediated auxin transport by cis-trans isomerization of peptidyl-prolyl bonds.

Cell death at the cross roads of host-pathogen interaction in Mycobacterium tuberculosis infection

Tuberculosis (TB) continues to be the leading cause of death by any single infectious agent, accounting for around 1.7 million annual deaths globally, despite several interventions and support programs by national and international agencies. With the development of drug resistance in Mycobacterium tuberculosis (M. tb), there has been a paradigm shift in TB research towards host-directed therapy. The potential targets include the interactions between host and bacterial proteins that are crucial for pathogenesis. Hence, collective efforts are being made to understand the molecular details of host-pathogen interaction for possible translation into host-directed therapy. The present review focuses on 'host cell death modalities' of host-pathogen interaction, which play a crucial role in determining the outcome of TB disease progression. Several cell death modalities that occur in response to mycobacterial infection have been identified in human macrophages either as host defences for bacterial clearance or as pathogen strategies for multiplication and dissemination. These cell death modalities include apoptosis, necrosis, pyroptosis, necroptosis, pyronecrosis, NETosis, and autophagy. These processes are highly overlapping with several mycobacterial proteins participating in more than one cell death pathway. Until now, reviews in M. tb and host cell death have discussed either focusing on host evasion strategies, apoptosis, autophagy, and necrosis or describing all these forms with limited discussions of their role in host-pathogen interactions. Here, we present a comprehensive review of various mycobacterial factors modulating host cell death pathways and the cross-talk between them. Besides this, we have discussed the networking of host cell death pathways including the interference of host miRNA during M. tb infection with their respective targets. Through this review, we present the host targets that overlap across several cell death modalities and the technical limitations of methodology in cell death research. Given the compelling need to discover alternative drug target(s), this review identifies these overlapping cell death factors as potential targets for host-directed therapy.

Enhanced multiple stress tolerance in Arabidopsis by overexpression of the polar moss peptidyl prolyl isomerase FKBP12 gene

PaFKBP12 overexpression in Arabidopsis resulted in stress tolerance to heat, ABA, drought, and salt stress, in addition to growth promotion under normal conditions. Polytrichastrum alpinum (alpine haircap moss) is one of polar organisms that can withstand the severe conditions of the Antarctic. In this study, we report the isolation of a peptidyl prolyl isomerase FKBP12 gene (PaFKBP12) from P. alpinum collected in the Antarctic and its functional implications in development and stress responses in plants. In P. alpinum, PaFKBP12 expression was induced by heat and ABA. Overexpression of PaFKBP12 in Arabidopsis increased the plant size, which appeared to result from increased rates of cell cycle. Under heat stress conditions, PaFKBP12-overexpressing lines (PaFKBP12-OE) showed better growth and survival than the wild type. PaFKBP12-OE also showed higher root elongation rates, better shoot growth and enhanced survival at higher concentrations of ABA in comparison to the wild type. In addition, PaFKBP12-OE were more tolerant to drought and salt stress than the wild type. All these phenotypes were accompanied with higher induction of the stress responsive genes in PaFKBP12-OE than in the wild type. Taken together, our findings revealed important functions of PaFKBP12 in plant development and abiotic stress responses.

Plant hormone transporters: what we know and what we would like to know

Hormone transporters are crucial for plant hormone action, which is underlined by severe developmental and physiological impacts caused by their loss-of-function mutations. Here, we summarize recent knowledge on the individual roles of plant hormone transporters in local and long-distance transport. Our inventory reveals that many hormones are transported by members of distinct transporter classes, with an apparent dominance of the ATP-binding cassette (ABC) family and of the Nitrate transport1/Peptide transporter family (NPF). The current need to explore further hormone transporter regulation, their functional interaction, transport directionalities, and substrate specificities is briefly reviewed.

A Critical View on ABC Transporters and Their Interacting Partners in Auxin Transport

Different subclasses of ATP-binding cassette (ABC) transporters have been implicated in the transport of native variants of the phytohormone auxin. Here, the putative, individual roles of key members belonging to the ABCB, ABCD and ABCG families, respectively, are highlighted and the knowledge of their assumed expression and transport routes is reviewed and compared with their mutant phenotypes. Protein-protein interactions between ABC transporters and regulatory components during auxin transport are summarized and their importance is critically discussed. There is a focus on the functional interaction between members of the ABCB family and the FKBP42, TWISTED DWARF1, acting as a chaperone during plasma membrane trafficking of ABCBs. Further, the mode and relevance of functional ABCB-PIN interactions is diagnostically re-evaluated. A new nomenclature describing precisely the most likely ABCB-PIN interaction scenarios is suggested. Finally, available tools for the detection and prediction of ABC transporter interactomes are summarized and the potential of future ABC transporter interactome maps is highlighted.

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

Plant development and architecture are greatly influenced by the polar distribution of the essential hormone auxin. The directional influx and efflux of auxin from plant cells depends primarily on AUX1/LAX, PIN, and ABCB/PGP/MDR families of auxin transport proteins. The functional analysis of these proteins has progressed rapidly within the last decade thanks to the establishment of heterologous auxin transport systems. Heterologous co-expression allowed also for the testing of protein-protein interactions involved in the regulation of transporters and identified relationships with members of the FK506-Binding Protein (FKBP) and cyclophilin protein families, which are best known in non-plant systems as cellular receptors for the immunosuppressant drugs, FK506 and cyclosporin A, respectively. Current evidence that such interactions affect membrane trafficking, and potentially the activity of auxin transporters is reviewed. We also propose that FKBPs andcyclophilins might integrate the action of auxin transport inhibitors, such as NPA, on members of the ABCB and PIN family, respectively. Finally, we outline open questions that might be useful for further elucidation of the role of immunophilins as regulators (servants) of auxin transporters (masters).

Mechanisms of thaxtomin A-induced root toxicity revealed by a thaxtomin A sensitive Arabidopsis mutant (ucu2-2/gi-2)

The Arabidopsis mutant ( ucu2 - 2/gi - 2 ) is thaxtomin A, isoxaben and NPA-sensitive indicated by root growth and ion flux responses providing new insights into these compounds mode of action and interactions. Thaxtomin A (TA) is a cellulose biosynthetic inhibitor (CBI) that promotes plant cell hypertrophy and cell death. Electrophysiological analysis of steady-state K(+) and Ca(2+) fluxes in Arabidopsis thaliana roots pretreated with TA for 24 h indicated a disturbance in the regulation of ion movement across the plant cell membrane. The observed inability to control solute movement, recorded in rapidly growing meristematic and elongation root zones, may partly explain typical root toxicity responses to TA treatment. Of note, the TA-sensitive mutant (ucu2-2/gi-2) was more susceptible with K(+) and Ca(2+) fluxes altered between 1.3 and eightfold compared to the wild-type control where fluxes altered between 1.2 and threefold. Root growth inhibition assays showed that the ucu2-2/gi-2 mutant had an increased sensitivity to the auxin 2,4-D, but not IAA or NAA; it also had increased sensitivity to the auxin efflux transport inhibitor, 1-naphthylphthalamic acid (NPA), but not 2,3,5- Triiodobenzoic acid (TIBA), when compared to the WT. The NPA sensitivity data were supported by electrophysiological analysis of H(+) fluxes in the mature (but not elongation) root zone. Increased sensitivity to the CBI, isoxaben (IXB), but not dichlobenil was recorded. Increased sensitivity to both TA and IXB corresponded with higher levels of accumulation of these toxins in the root tissue, compared to the WT. Further root growth inhibition assays showed no altered sensitivity of ucu2-2/gi-2 to two other plant pathogen toxins, alternariol and fusaric acid. Identification of a TA-sensitive Arabidopsis mutant provides further insight into how this CBI toxin interacts with plant cells.

Polar auxin transport is essential for medial versus lateral tissue specification and vascular-mediated valve outgrowth in Arabidopsis gynoecia

Although it is generally accepted that auxin is important for the patterning of the female reproductive organ, the gynoecium, the flow as well as the temporal and spatial actions of auxin have been difficult to show during early gynoecial development. The primordium of the Arabidopsis (Arabidopsis thaliana) gynoecium is composed of two congenitally fused, laterally positioned carpel primordia bisected by two medially positioned meristematic regions that give rise to apical and internal tissues, including the ovules. This organization makes the gynoecium one of the most complex plant structures, and as such, the regulation of its development has remained largely elusive. By determining the spatiotemporal expression of auxin response reporters and localization of PINFORMED (PIN) auxin efflux carriers, we have been able to create a map of the auxin flow during the earliest stages of gynoecial primordium initiation and outgrowth. We show that transient disruption of polar auxin transport (PAT) results in ectopic auxin responses, broadened expression domains of medial tissue markers, and disturbed lateral preprocambium initiation. Based on these results, we propose a new model of auxin-mediated gynoecial patterning, suggesting that valve outgrowth depends on PIN1-mediated lateral auxin maxima as well as subsequent internal auxin drainage and provascular formation, whereas the growth of the medial domains is less dependent on correct PAT. In addition, PAT is required to prevent the lateral domains, at least in the apical portion of the gynoecial primordium, from obtaining medial fates.

Regulation of ABCB1/PGP1-catalysed auxin transport by linker phosphorylation

Polar transport of the plant hormone auxin is controlled by PIN- and ABCB/PGP-efflux catalysts. PIN polarity is regulated by the AGC protein kinase, PINOID (PID), while ABCB activity was shown to be dependent on interaction with the FKBP42, TWISTED DWARF1 (TWD1). Using co-immunoprecipitation (co-IP) and shotgun LC-MS/MS analysis, we identified PID as a valid partner in the interaction with TWD1. In-vitro and yeast expression analyses indicated that PID specifically modulates ABCB1-mediated auxin efflux in an action that is dependent on its kinase activity and that is reverted by quercetin binding and thus inhibition of PID autophosphorylation. Triple ABCB1/PID/TWD1 co-transfection in tobacco revealed that PID enhances ABCB1-mediated auxin efflux but blocks ABCB1 in the presence of TWD1. Phospho-proteomic analyses identified S634 as a key residue of the regulatory ABCB1 linker and a very likely target of PID phosphorylation that determines both transporter drug binding and activity. In summary, we provide evidence that PID phosphorylation has a dual, counter-active impact on ABCB1 activity that is coordinated by TWD1-PID interaction.

Tail-anchored membrane proteins: exploring the complex diversity of tail-anchored-protein targeting in plant cells

Tail-anchored (TA) proteins are special class of integral membrane proteins that in recent years have received a considerable amount of attention due to their diverse cellular functions and unique targeting and insertion mechanisms. Defined by the presence of a single, hydrophobic membrane-spanning domain at or near their C terminus, TA proteins must be inserted into membranes post-translationally and are orientated such that their larger N-terminal domain (most often the functional domain) faces the cytosol, while their shorter C-terminal domain faces the interior of the organelle. The C-terminal domain of TA proteins also usually contains the information responsible for their selective targeting to the proper subcellular membrane, a process that, based primarily on studies with yeasts and mammals, appears to be highly complex due to the presence of multiple pathways. Within this context, we discuss here the biogenesis of plant TA proteins and the potential for hundreds of new TA proteins identified via bioinformatics screens to contribute to the already remarkable number of roles that this class of membrane proteins participates in throughout plant growth and development.

Modulation of P-glycoproteins by auxin transport inhibitors is mediated by interaction with immunophilins

The immunophilin-like FKBP42 TWISTED DWARF1 (TWD1) has been shown to control plant development via the positive modulation of ABCB/P-glycoprotein (PGP)-mediated transport of the plant hormone auxin. TWD1 functionally interacts with two closely related proteins, ABCB1/PGP1 and ABCB19/PGP19/MDR1, both of which exhibit the ability to bind to and be inhibited by the synthetic auxin transport inhibitor N-1-naphylphtalamic acid (NPA). They are also inhibited by flavonoid compounds, which are suspected modulators of auxin transport. The mechanisms by which flavonoids and NPA interfere with auxin efflux components are unclear. We report here the specific disruption of PGP1-TWD1 interaction by NPA and flavonoids using bioluminescence resonance energy transfer with flavonoids functioning as a classical established inhibitor of mammalian and plant PGPs. Accordingly, TWD1 was shown to mediate modulation of PGP1 efflux activity by these auxin transport inhibitors. NPA bound to both PGP1 and TWD1 but was excluded from the PGP1-TWD1 complex expressed in yeast, suggesting a transient mode of action in planta. As a consequence, auxin fluxes and gravitropism in twd1 roots are less affected by NPA treatment, whereas TWD1 gain-of-function promotes root bending. Our data support a novel model for the mode of drug-mediated P-glycoprotein regulation mediated via protein-protein interaction with immunophilin-like TWD1.

Tête-à-tête: the function of FKBPs in plant development

Compared with that of other eukaryotes, the nuclear genome of the model plant Arabidopsis thaliana encodes an expanded family of FK506-binding proteins (FKBPs). Whereas approximately half of the FKBPs are implicated in the regulation of photosynthetic processes, a subcluster appears to be stress responsive. Recent reports indicate that a discrete group of Arabidopsis multidomain FKBPs regulate plant hormone pathways by recruiting or modulating client proteins via direct protein-protein interactions (tête-à-tête). This suggests that multidomain FKBPs function as central elements in plant development by linking hormone responses with other signal transduction pathways. Here, we present a summary of current research demonstrating that, in addition to their role in protein folding, subsets of plant FKBPs exhibit diverse functionality.