FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Genome-wide identification and characterization of ABC transporter superfamily in the legume Cajanus cajan

Plant ATP-binding cassette (ABC) protein family is the largest multifunctional highly conserved protein superfamily that transports diverse substrates across biological membranes by the hydrolysis of ATP and is also the part of the several other biological processes like cellular detoxification, growth and development, stress biology, and signaling processes. In the agriculturally important legume crop Cajanus cajan, a genome-wide identification and characterization of the ABC gene family was carried out. A total of 159 ABC genes were identified that belong to eight canonical classes CcABCA to CcABCG and CcABCI based on the phylogenetic analysis. The number of genes was highest in CcABCG followed by CcABCC and CcABCB class. A total of 85 CcABC genes were found on 11 chromosomes and 74 were found on scaffold. Tandem duplication was the major driver of CcABC gene family expansion. The dN/dS ratio revealed the purifying selection. The phylogenetic analysis revealed class-specific eight superclades which reflect their functional importance. The largest clade was found to be CcABCG which reflects their functional significance. CcABC proteins were mainly basic in nature and found to be localized in the plasma membrane. The secondary structure prediction revealed the dominance of α-helix. The canonical transmembrane and nucleotide binding domain, signature motif LSSGQ, Walker A, Walker B region, and Q loop were also identified. A class-specific exon-intron pattern was also observed. In addition to core elements, different cis-acting regulatory elements like stress, hormone, and cellular responsive were also identified. Expression profiling of CcABC genes at various developmental stages of different anatomical tissues was performed and it was noticed that CcABCF3, CcABCF4, CcABCF5, CcABCG66, and CcABCI3 had the highest expression. The results of the current study endow us with the further functional analysis of Cajanus ABC in the future.

Phytochelatins: Sulfur-Containing Metal(loid)-Chelating Ligands in Plants

Phytochelatins (PCs) are small cysteine-rich peptides capable of binding metal(loid)s via SH-groups. Although the biosynthesis of PCs can be induced in vivo by various metal(loid)s, PCs are mainly involved in the detoxification of cadmium and arsenic (III), as well as mercury, zinc, lead, and copper ions, which have high affinities for S-containing ligands. The present review provides a comprehensive account of the recent data on PC biosynthesis, structure, and role in metal(loid) transport and sequestration in the vacuoles of plant cells. A comparative analysis of PC accumulation in hyperaccumulator plants, which accumulate metal(loid)s in their shoots, and in the excluders, which accumulate metal(loid)s in their roots, investigates the question of whether the endogenous PC concentration determines a plant's tolerance to metal(loid)s. Summarizing the available data, it can be concluded that PCs are not involved in metal(loid) hyperaccumulation machinery, though they play a key role in metal(loid) homeostasis. Unraveling the physiological role of metal(loid)-binding ligands is a fundamental problem of modern molecular biology, plant physiology, ionomics, and toxicology, and is important for the development of technologies used in phytoremediation, biofortification, and phytomining.

Transport of acylated anthocyanins by the Arabidopsis ATP-binding cassette transporters AtABCC1, AtABCC2, and AtABCC14

Anthocyanins are a group of pigments that have various roles in plants including attracting pollinators and seed dispersers and protecting against various types of stress. In vegetative tissue, these anthocyanins are sequestered in the vacuole following biosynthesis in the cytoplasm, though there remain questions as to the events leading to the vacuolar sequestration. In this study, we were able to show that the uptake of acylated anthocyanins by vacuolar membrane-enriched vesicles isolated from Arabidopsis was stimulated by the addition of MgATP and was inhibited by both vanadate and glybenclamide, but not by gramicidin D or bafilomycin A₁ , suggesting that uptake involves an ATP-binding cassette (ABC) transporter and not an H⁺ -antiporter. Membrane vesicles isolated from yeast expressing the ABC transporters designated AtABCC1, AtABCC2, and AtABCC14 are capable of MgATP-dependent uptake of acylated anthocyanins. This uptake was not dependent on glutathione as seen previously for anthocyanidin 3-O-monoglucosides. Compared to the wild-type, the transport of acylated anthocyanins was lower in vacuolar membrane-enriched vesicles isolated from atabcc1 cell cultures providing evidence that AtABCC1 may be the predominant transporter of these compounds in vivo. In addition, the pattern of anthocyanin accumulation differed between the atabcc1, atabcc2, and atabcc14 mutants and the wild-type seedlings under anthocyanin inductive conditions. We suggest that AtABCC1, AtABCC2, and AtABCC14 are involved in the vacuolar transport of acylated anthocyanins produced in the vegetative tissue of Arabidopsis and that the pattern of anthocyanin accumulation can be altered depending on the presence or absence of a specific vacuolar ABC transporter.

Salicylate and glutamate mediate different Cd accumulation and tolerance between Brassica napus and B. juncea

Most hyperaccumulator plants have little economic values, and therefore have not been widely used in Cd-contaminated soils. Rape species are Cd hyperaccumulators with high economic values. Black mustard seed (Brassica juncea) has a higher accumulation ability and a higher tolerance for Cd than oilseed rape (Brassica napus), but its biomass is relatively low and its geographical distribution is limited. However, it is unknown why B. juncea (Bj) is more tolerant to and accumulates more Cd than B. napus (Bn). Here, we found that the differences in Cd accumulation and tolerance between the two species is mainly because Bj plants have higher levels of salicylic acid and glutamic acid than Bn plants. Exogenous salicylate and glutamate treatments enhanced Cd accumulation (salicylate + glutamate co-treatment doubled Cd accumulation level in Bn seedlings) but reduced oxidative stresses by increasing glutathione biosynthesis and activating phytochelatin-based sequestration of Cd into vacuoles. Our results provide a new idea to simultaneously improve Cd accumulation and Cd tolerance in B. napus.

A tau class glutathione-S-transferase (OsGSTU5) confers tolerance against arsenic toxicity in rice by accumulating more arsenic in root

Arsenic (As) considered as one of the hazardous metalloid that hampers various physiological activities in rice. To study the mechanism of As tolerance in rice, one differentially expressed tau class glutathione-S-transferase (OsGSTU5) has been selected and transgenic rice plants with knockdown (KD) and overexpressing (OE) OsGSTU5 were generated. Our results suggested that KD lines became less tolerant to As stress than WT plants, while OE lines showed enhanced tolerance to As. Under As toxicity, OE and KD lines showed enhanced and reduced antioxidant activities such as, SOD, PRX and catalase, respectively indicating its role in ROS homeostasis. In addition, higher malondialdehyde content, poor photosynthetic parameters and higher reactive oxygen species (ROS) in KD plant, suggests that knockdown of OsGSTU5 renders KD plants more susceptible to oxidative damage. Also, the relative expression profile of various transporters such as OsABCC1 (As sequestration), Lsi2 and Lsi6 (As translocaters) and GSH dependent activity of GSTU5 suggests that GSTU5 might help in chelation of As with GSH and sequester it into the root vacuole using OsABCC1 transporter and thus limits the upward translocation of As towards shoot. This study suggests the importance of GSTU5 as a good target to improve the As tolerance in rice.

Differential response of physiology and metabolic response to drought stress in different sweetpotato cultivars

Sweetpotato (Ipomoea batatas [L.] Lam) is a widely cultivated food crop with generally good adaptability. However, drought stress can cause a significant decline in yield. To reveal the response mechanism of sweetpotato to drought stress, an integrated physiological, proteomic and metabolomic investigation was conducted in leaves of two sweetpotato varieties with differing responses to drought stress, drought-resistant Wanzishu56 (WZ56) and a more sensitive variety, Ningzishu2(NZ2). Physiological analysis showed that the variety with better drought tolerance had superior performance in water retention capacity and photosynthetic efficiency under drought stress. A total of 1140 proteins were identified within the two varieties. Among them, 192 differentially expressed proteins were detected under drought conditions, including 97 that were up-regulated. Functional analysis showed that these up-regulated proteins were primarily involved in photosynthesis, reactive oxygen species metabolism, organonitrogen compound metabolism, and precursor metabolite catabolism and energy generation. All differentially expressed proteins in WZ56 that were involved in photosynthetic and glutathione metabolic processes were up-regulated. Enzyme activity assays were carried out to validate the proteomics data. Moreover, 75 metabolites were found to have a higher expression level in WZ56 than NZ2 under drought stress. The higher concentration of carbohydrates, amino acids, flavonoids and organic acids found in drought-stressed leaves of WZ56 suggested that these metabolites may improve the drought resistance of sweetpotato. This study uncovered specific-proteins and metabolites associated with drought resistance, providing new insights into the molecular mechanisms of drought tolerance in sweetpotato.

The E3 Ubiquitin Ligase Gene Sl1 Is Critical for Cadmium Tolerance in Solanum lycopersicum L

Heavy metal cadmium (Cd) at high concentrations severely disturbs plant growth and development. The E3 ubiquitin ligase involved in protein degradation is critical for plant tolerance to abiotic stress, but the role of E3 ubiquitin ligases in Cd tolerance is largely unknown in tomato. Here, we characterized an E3 ubiquitin ligase gene Sl1, which was highly expressed in roots under Cd stress in our previous study. The subcellular localization of Sl1 revealed that it was located in plasma membranes. In vitro ubiquitination assays confirmed that Sl1 had E3 ubiquitin ligase activity. Knockout of the Sl1 gene by CRISPR/Cas9 genome editing technology reduced while its overexpression increased Cd tolerance as reflected by the changes in the actual quantum efficiency of PSII photochemistry (Φ_PSII) and hydrogen peroxide (H₂O₂) accumulation. Cd-induced increased activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were compromised in sl1 mutants but were enhanced in Sl1 overexpressing lines. Furthermore, the content of Cd in both shoots and roots increased in sl1 mutants while reduced in Sl1 overexpressing plants. Gene expression assays revealed that Sl1 regulated the transcript levels of heavy metal transport-related genes to inhibit Cd accumulation. These findings demonstrate that Sl1 plays a critical role in regulating Cd tolerance by relieving oxidative stress and resisting heavy metal transportation in tomato. The study provides a new understanding of the mechanism of plant tolerance to heavy metal stress.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

Responses to Cadmium in Early-Diverging Streptophytes (Charophytes and Bryophytes): Current Views and Potential Applications

Several transition metals are essential for plant growth and development, as they are involved in various fundamental metabolic functions. By contrast, cadmium (Cd) is a metal that can prove extremely toxic for plants and other organisms in a dose-dependent manner. Charophytes and bryophytes are early-diverging streptophytes widely employed for biomonitoring purposes, as they are able to cope with high concentrations of toxic metal(loid)s without showing any apparent heavy damage. In this review, we will deal with different mechanisms that charophytes and bryophytes have evolved to respond to Cd at a cellular level. Particular attention will be addressed to strategies involving Cd vacuolar sequestration and cell wall immobilization, focusing on specific mechanisms that help achieve detoxification. Understanding the effects of metal(loid) pollution and accumulation on the morpho-physiological traits of charophytes and bryophytes can be in fact fundamental for optimizing their use as phytomonitors and/or phytoremediators.

Non-target-Site Resistance in Lolium spp. Globally: A Review

The Lolium genus encompasses many species that colonize a variety of disturbed and non-disturbed environments. Lolium perenne L. spp. perenne, L. perenne L. spp. multiflorum, and L. rigidum are of particular interest to weed scientists because of their ability to thrive in agricultural and non-agricultural areas. Herbicides are the main tool to control these weeds; however, Lolium spp. populations have evolved multiple- and cross-resistance to at least 14 herbicide mechanisms of action in more than 21 countries, with reports of multiple herbicide resistance to at least seven mechanisms of action in a single population. In this review, we summarize what is currently known about non-target-site resistance in Lolium spp. to acetyl CoA carboxylase, acetohydroxyacid synthase, microtubule assembly, photosystem II, 5-enolpyruvylshikimate-3-phosphate synthase, glutamine synthetase, very-long chain fatty acids, and photosystem I inhibitors. We suggest research topics that need to be addressed, as well as strategies to further our knowledge and uncover the mechanisms of non-target-site resistance in Lolium spp.

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.

iTRAQ protein profile analysis of sugar beet under salt stress: different coping mechanisms in leaves and roots

BACKGROUND

Salinity is one of the most serious threats to world agriculture. An important sugar-yielding crop sugar beet, which shows some tolerance to salt via a mechanism that is poorly understood. Proteomics data can provide important clues that can contribute to finally understand this mechanism.

RESULTS

Differentially abundant proteins (DAPs) in sugar beet under salt stress treatment were identified in leaves (70 DAPs) and roots (76 DAPs). Functions of these DAPs were predicted, and included metabolism and cellular, environmental information and genetic information processing. We hypothesize that these processes work in concert to maintain cellular homeostasis. Some DAPs are closely related to salt resistance, such as choline monooxygenase, betaine aldehyde dehydrogenase, glutathione S-transferase (GST) and F-type H+-transporting ATPase. The expression pattern of ten DAPs encoding genes was consistent with the iTRAQ data.

CONCLUSIONS

During sugar beet adaptation to salt stress, leaves and roots cope using distinct mechanisms of molecular metabolism regulation. This study provides significant insights into the molecular mechanism underlying the response of higher plants to salt stress, and identified some candidate proteins involved in salt stress countermeasures.

The Moss Leptodictyum riparium Counteracts Severe Cadmium Stress by Activation of Glutathione Transferase and Phytochelatin Synthase, but Slightly by Phytochelatins

In the present work, we investigated the response to Cd in Leptodictyum riparium, a cosmopolitan moss (Bryophyta) that can accumulate higher amounts of metals than other plants, even angiosperms, with absence or slight apparent damage. High-performance liquid chromatography followed by electrospray ionization tandem mass spectrometry of extracts from L. riparium gametophytes, exposed to 0, 36 and 360 µM Cd for 7 days, revealed the presence of γ-glutamylcysteine (γ-EC), reduced glutathione (GSH), and traces of phytochelatins. The increase in Cd concentrations progressively augmented reactive oxygen species levels, with activation of both antioxidant (catalase and superoxide dismutase) and detoxifying (glutathione-S-transferase) enzymes. After Cd treatment, cytosolic and vacuolar localization of thiol peptides was performed by means of the fluorescent dye monochlorobimane and subsequent observation with confocal laser scanning microscopy. The cytosolic fluorescence observed with the highest Cd concentrations was also consistent with the formation of γ-EC-bimane in the cytosol, possibly catalyzed by the peptidase activity of the L. riparium phytochelatin synthase. On the whole, activation of phytochelatin synthase and glutathione-S-transferase, but minimally phytochelatin synthesis, play a role to counteract Cd toxicity in L. riparium, in this manner minimizing the cellular damage caused by the metal. This study strengthens previous investigations on the L. riparium ability to efficiently hinder metal pollution, hinting at a potential use for biomonitoring and phytoremediation purposes.

Adaptation mechanisms of Rhodococcus sp. CNS16 under different temperature gradients: Physiological and transcriptome

Rhodococcus exhibits strong adaptability to environmental stressors and plays a crucial role in environmental bioremediation. However, seasonal changes in ambient temperature, especially rapid temperature drops exert an adverse effect on in situ bioremediation. In this paper, we studied the cell morphology and fatty acid composition of an aniline-degrading strain Rhodococcus sp. CNS16 at temperatures of 30 °C, 20 °C, and 10 °C. At suboptimal temperatures, cell morphology of CNS16 changed from short rod-shaped to long rod or irregular shaped, and the proportion of unsaturated fatty acids was upregulated. Transcriptomic technologies were then utilized to gain detailed insights into the adaptive mechanisms of CNS16 subjected to suboptimal temperatures. The results showed that the number of gene responses was significantly higher at 10 °C than that at 20 °C. The inhibition of peptidoglycan synthase expression and up-regulation of Filamentous Temperature Sensitive as well as unsaturated fatty acid synthesis genes at suboptimal temperatures might be closely related to corresponding changes in cell morphology and fatty acids composition. Strain CNS16 showed loss of catalase and superoxide dismutase activity, and utilized thioredoxin-dependent thiol peroxidase to resist oxidative stress. The up-regulation of carotenoid and Vitamin B2 synthesis at 10 °C might also be involved in the resistance to oxidative stress. Amino acid metabolism, coenzyme and vitamin metabolism, ABC transport, and energy metabolism are essential for peptidoglycan synthesis and regulation of cellular metabolism; therefore, synergistically resisting environmental stress. This study provides a mechanistic basis for the regulation of aniline degradation in Rhodococcus sp. CNS16 at low temperatures.

Functional characterisation of two phytochelatin synthases in rice (Oryza sativa cv. Milyang 117) that respond to cadmium stress

Cadmium (Cd) is one of the most toxic heavy metals and a non-essential element to all organisms, including plants; however, the genes involved in Cd resistance in plants remain poorly characterised. To identify Cd resistance genes in rice, we screened a rice cDNA expression library treated with CdCl₂ using a yeast (Saccharomyces cerevisiae) mutant ycf1 strain (DTY167) and isolated two rice phytochelatin synthases (OsPCS5 and OsPCS15). The genes were strongly induced by Cd treatment and conferred increased resistance to Cd when expressed in the ycf1 mutant strain. In addition, the Cd concentration was twofold higher in yeast expressing OsPCS5 and OsPCS15 than in vector-transformed yeast, and OsPCS5 and OsPCS15 localised in the cytoplasm. Arabidopsis thaliana plants overexpressing OsPCS5/-15 paradoxically exhibited increased sensitivity to Cd, suggesting that overexpression of OsPCS5/-15 resulted in toxicity due to excess phytochelatin production in A. thaliana. These data indicate that OsPCS5 and OsPCS15 are involved in Cd tolerance, which may be related to the relative abundances of phytochelatins synthesised by these phytochelatin synthases.

Transport of Anthocyanins and other Flavonoids by the Arabidopsis ATP-Binding Cassette Transporter AtABCC2

Flavonoids have important developmental, physiological, and ecological roles in plants and are primarily stored in the large central vacuole. Here we show that both an ATP-binding cassette (ABC) transporter(s) and an H⁺-antiporter(s) are involved in the uptake of cyanidin 3-O-glucoside (C3G) by Arabidopsis vacuolar membrane-enriched vesicles. We also demonstrate that vesicles isolated from yeast expressing the ABC protein AtABCC2 are capable of MgATP-dependent uptake of C3G and other anthocyanins. The uptake of C3G by AtABCC2 depended on the co-transport of glutathione (GSH). C3G was not altered during transport and a GSH conjugate was not formed. Vesicles from yeast expressing AtABCC2 also transported flavone and flavonol glucosides. We performed ligand docking studies to a homology model of AtABCC2 and probed the putative binding sites of C3G and GSH through site-directed mutagenesis and functional studies. These studies identified residues important for substrate recognition and transport activity in AtABCC2, and suggest that C3G and GSH bind closely, mutually enhancing each other’s binding. In conclusion, we suggest that AtABCC2 along with possibly other ABCC proteins are involved in the vacuolar transport of anthocyanins and other flavonoids in the vegetative tissue of Arabidopsis.

Involvement of LeMRP, an ATP-binding cassette transporter, in shikonin transport and biosynthesis in Lithospermum erythrorhizon

Shikonin and its derivatives are important medicinal secondary metabolites accumulating in roots of Lithospermum erythrorhizon. Although some membrane proteins have been identified as transporters of secondary metabolites, the mechanisms underlying shikonin transport and accumulation in L. erythrorhizon cells still remain largely unknown. In this study, we isolated a cDNA encoding LeMRP, an ATP-binding cassette transporter from L. erythrorhizon, and further investigated its functions in the transport and biosynthesis of shikonin using the yeast transformation and transgenic hairy root methods, respectively. Real-time PCR was applied for expression analyses of LeMRP and shikonin biosynthetic enzyme genes. Functional analysis of LeMRP using the heterologous yeast cell expression system showed that LeMRP could be involved in shikonin transport. Transgenic hairy roots of L. erythrorhizon demonstrated that LeMRP overexpressing hairy roots produced more shikonin than the empty vector (EV) control. Real-time PCR results revealed that the enhanced shikonin biosynthesis in the overexpression lines was mainly caused by highly up-regulated expression of genes coding key enzymes (LePAL, HMGR, Le4CL and LePGT) involved in shikonin biosynthesis. Conversely, LeMRP RNAi decreased the accumulation of shikonin and effectively down-regulated expression level of the above genes. Typical inhibitors of ABC proteins, such as azide and buthionine sulphoximine, dramatically inhibited accumulation of shikonin in hairy roots. Our findings provide evidence for the important direct or indirect role of LeMRP in transmembrane transport and biosynthesis of shikonin.

Tracing the role of plant proteins in the response to metal toxicity: a comprehensive review

Plants are sessile in nature, but are capable to evade from high level concentration of heavy metals like Cd, Hg, Cu, through various metabolic pathways. Some of the pathways regulate normal metabolism in plants, whereas others are required for for their survival under metal toxicity. Different plant proteins act as transporters to transfer metal from one organelle to the other and further eliminate it out from the plants. Initially, exposure of heavy metals/metalloids to plants lead to over expression of proteins which in turn stimulate other stress-related genes. Further, they activate signalling mechanism like MAPK cascade, Cd-Calmodulin signalling pathway, and oxidation signalling pathway that lead to generation of ROS (reactive oxygen species). Once these ROS (highly unstable) are formed, they generate free radicals which react with macromolecules like proteins and DNA. This has negative impact on plant growth and leads to ageing and, eventually, cell death. The uncontrolled, destructive processes damage plants physiologically and ultimately lead to oxidative stress. Activation of antioxidant enzymes like SOD (superoxide dismutase) and CAT (catalase) allows plants to cope under oxidative stress conditions. Among plant proteins, some of the antioxidant enzymes like glutathione, and APX (ascorbate peroxidase) play defensive roles against abiotic stress in plants. Chaperones help in protein folding to maintain protein stability under stress conditions. With this background, the present review gives a brief account of the functions, localization and expression pattern of plant proteins against metal/metalloid toxicity. Moreover, the aim of this review is also to summarize the cutting edge research of plant protein and metal interfaces and their future prospects.

Sedum alfredii SaNramp6 Metal Transporter Contributes to Cadmium Accumulation in Transgenic Arabidopsis thaliana

The plant natural resistance-associated macrophage protein (Nramp) family plays an important role in tolerance to heavy metal stress. However, few Nramps have been functionally characterized in the heavy metal-accumulating plant Sedum alfredii. Here, Nramp6 was cloned and identified from S. alfredii and its function analyzed in transgenic Arabidopsis thaliana. SaNramp6 cDNA contains an open reading frame of 1, 638 bp encoding 545 amino acids. SaNramp6's expression can be induced by cadmium (Cd) stress, and, after treatment, it peaked at one week and 12 h in the roots and leaves, respectively. SaNramp6 localized to the plasma membrane in protoplasts isolated from A. thaliana, Nicotiana benthamiana lower leaf and onion (Allium cepa) epidermal cells. The heterologous expression of SaNramp6 in the Δycf1 yeast mutant increased the Cd content in yeast cells. SaNramp6 also rescued the low Cd accumulation of the A. thaliana nramp1 mutant. Transgenic A. thaliana expressing SaNramp6 exhibited high Cd accumulation levels, as determined by a statistical analysis of the Cd concentration, translocation factors and net Cd²⁺ fluxes under Cd stress. Thus, SaNramp6 may play a significant role in improving Cd accumulation, and the gene may be useful for the biotechnological development of transgenic plants for phytoremediation.

Arabidopsis thaliana MRP1 (AtABCC1) nucleotide binding domain contributes to arsenic stress tolerance with serine triad phosphorylation

Multidrug resistance protein AtMRPs belong to the ATP binding cassette (ABC) transporter super family. ABC proteins are membrane proteins involved in the transport of a broad range of amphipathic organic anions across membranes. MRPs (ABCCs) are one of the highly represented subfamilies of ABC transporters. Plant MRPs also transport various glutathione conjugates across membranes. Arabidopsis thaliana MRP1 is already known to be involved in vacuolar storage of folates. Using heterologously expressed AtMRP1 in yeast and its C-terminal nucleotide binding domain (NBD2) in Escherichia coli, it has been shown that Casein kinase II (CKII) mediated phosphorylation is a potential regulator of AtMRP1 function. AtMRP1 showed enhanced tolerance towards arsenite As(III) in yeast. CKIIII/CKII mediated phosphorylation of AtMRP1 was found to be involved in As(III) mediated signaling. AtMRP1-NBD2 and its serine mutants showed distinct change in secondary structure in the presence of arsenite and methotrexate (MTX) controlled by serine triad phosphorylation. Results showed that AtMRP1 is important for vacuolar accumulation of antifolates as well as tolerance against arsenic, both of which involved phosphorylation in the serine triads at the C terminal NBD of AtMRP1. The experiments provide an important insight into the role of AtMRP1 serine triad phosphorylation under AsIII stress conditions.

Engineering an ABC Transporter for Enhancing Resistance to Caffeine in Saccharomyces cerevisiae

In addressing caffeine toxicity to the producing cells, engineering a transporter that can move caffeine from cytoplasm across the cell membrane to the extracellular space, thus enhancing caffeine resistance and potentially increasing the yield in yeast, is important. An ABC-transporter bfr1 from Schizosaccharomyces pombe was cloned and transformed into S. cerevisiae, resulting in enhancing caffeine resistance. Afterward, a library of randomly mutagenized bfr1 mutants through error-prone PCR was generated. One mutant was identified with drastically increased caffeine resistance (15 mg/mL). Sequencing and structural analysis illustrated that many of the mutations occurred at the cytosolic domain. Site-directed mutagenesis of these mutations confirmed at least one amino acid that conferred enhancing caffeine resistance in the mutated bfr1. These data demonstrated engineering ABC-transporters can be an efficient way to reduce product toxicity in heterologous systems.

Expression of Echmr gene from Eichhornia offers multiple stress tolerance to Cd sensitive Escherichia coli Δgsh mutants

The detoxification of heavy metals frequently involves conjugation to glutathione prior to compartmentalization and eflux in higher plants. We have expressed a heavy metal stress responsive (Echmr) gene from water hyacinth, which conferred tolerance to Cd sensitive Escherichia coli Δgsh mutants against heavy metals and abiotic stresses. The recombinant E. coli Δgsh mutant cells showed better growth recovery and survival than control cells under Cd (200 μM), Pb(200 μM), heat shock (50 °C), cold stress at 4 °C for 4 h, and UV-B (20 min) exposure. The enhanced expression of Echmr gene revealed by northern analysis during above stresses further advocates its role in multi-stress tolerance. Heterologous expression of EcHMR from Eichhornia rescued Cd(2+) sensitive E. coli mutants from Cd(2+) toxicity and induced better recovery post abiotic stresses. This may suggests a possible role of Echmr in Cd(II) and desiccation tolerance in plants for enhanced stress response.

ABC transporter and metallothionein expression affected by NI and Epichloe endophyte infection in tall fescue

Epichloe endophytes are symbiotic fungi which unlike mycorrhiza grow within aerial parts of host plants. The fungi may increase host tolerance to both biotic and abiotic stresses. In this study, the effect of endophyte infection on growth and tolerance, carbohydrate contents and ABC (ABC transporter) and MET (metallothionein) expression in the leaves of tall fescue (Festuca arundinacea) plants cultivated in Ni polluted soil were evaluated. The endophyte infected (E+) and non-infected (E-) fescue plants were cultivated in soil under different Ni concentrations (30, 90 and 180mgkg(-1)). Growth parameters including root, shoot, total biomass, tiller number and total chlorophyll content of plants and H2O2 content of shoots were measured at the end of experiment. Ni translocation to the shoots, carbohydrate contents in roots and expression of ABC and MET of the leaves were also measured after 10 weeks of growth. Results demonstrated the beneficial effect of endophyte association on growth and Ni tolerance of tall fescue under Ni stress through an avoidance mechanism (reduction of Ni accumulation and translocation to the shoots). Endophyte infected plants showed less ABC and MET expression compared to the endophyte free plants. In endophyte free plants, H2O2 production had a significant positive correlation with genes expression, indicating that an increase in H2O2 might be involved in the up-regulation of ABC and MET under Ni stress.

The Sugar Kinase That Is Necessary for the Catabolism of Rhamnose in Rhizobium leguminosarum Directly Interacts with the ABC Transporter Necessary for Rhamnose Transport

Rhamnose catabolism in Rhizobium leguminosarum was found to be necessary for the ability of the organism to compete for nodule occupancy. Characterization of the locus necessary for the catabolism of rhamnose showed that the transport of rhamnose was dependent upon a carbohydrate uptake transporter 2 (CUT2) ABC transporter encoded by rhaSTPQ and on the presence of RhaK, a protein known to have sugar kinase activity. A linker-scanning mutagenesis analysis of rhaK showed that the kinase and transport activities of RhaK could be separated genetically. More specifically, two pentapeptide insertions defined by the alleles rhaK72 and rhaK73 were able to uncouple the transport and kinase activities of RhaK, such that the kinase activity was retained, but cells carrying these alleles did not have measurable rhamnose transport rates. These linker-scanning alleles were localized to the C terminus and N terminus of RhaK, respectively. Taken together, the data led to the hypothesis that RhaK might interact either directly or indirectly with the ABC transporter defined by rhaSTPQ. In this work, we show that both N- and C-terminal fragments of RhaK are capable of interacting with the N-terminal fragment of the ABC protein RhaT using a 2-hybrid system. Moreover, if RhaK fragments carrying either the rhaK72 or rhaK73 allele were used, this interaction was abolished. Phylogenetic and bioinformatic analysis of the RhaK fragments suggested that a conserved region in the N terminus of RhaK may represent a putative binding domain. Alanine-scanning mutagenesis of this region followed by 2-hybrid analysis revealed that a substitution of any of the conserved residues greatly affected the interaction between RhaT and RhaK fragments, suggesting that the sugar kinase RhaK and the ABC protein RhaT interact directly.

IMPORTANCE

ABC transporters involved in the transport of carbohydrates help define the overall physiological fitness of bacteria. The two largest groups of transporters are the carbohydrate uptake transporter classes 1 and 2 (CUT1 and CUT2, respectively). This work provides the first evidence that a kinase that is necessary for the catabolism of a sugar can directly interact with a domain from the ABC protein that is necessary for its transport.

Abiotic stressors and stress responses: What commonalities appear between species across biological organization levels?

Organisms are regularly subjected to abiotic stressors related to increasing anthropogenic activities, including chemicals and climatic changes that induce major stresses. Based on various key taxa involved in ecosystem functioning (photosynthetic microorganisms, plants, invertebrates), we review how organisms respond and adapt to chemical- and temperature-induced stresses from molecular to population level. Using field-realistic studies, our integrative analysis aims to compare i) how molecular and physiological mechanisms related to protection, repair and energy allocation can impact life history traits of stressed organisms, and ii) to what extent trait responses influence individual and population responses. Common response mechanisms are evident at molecular and cellular scales but become rather difficult to define at higher levels due to evolutionary distance and environmental complexity. We provide new insights into the understanding of the impact of molecular and cellular responses on individual and population dynamics and assess the potential related effects on communities and ecosystem functioning.

Milestones in plant sulfur research on sulfur-induced-resistance (SIR) in Europe

Until the 1970's of the last century sulfur (S) was mainly regarded as a pollutant being the main contributor of acid rain, causing forest dieback in central Europe. When Clean Air Acts came into force at the start of the 1980's SO2 contaminations in the air were consequently reduced within the next years. S changed from an unwanted pollutant into a lacking plant nutrient in agriculture since agricultural fields were no longer "fertilized" indirectly by industrial pollution. S deficiency was first noticed in Brassica crops that display an especially high S demand because of its content of S-containing secondary metabolites, the glucosinolates. In Scotland, where S depositions decreased even faster than in continental Europe, an increasing disease incidence with Pyrenopeziza brassicae was observed in oilseed rape in the beginning 1990's and the concept of sulfur-induced-resistance (SIR) was developed after a relationship between the S status and the disease incidence was uncovered. Since then a lot of research was carried out to unravel the background of SIR in the metabolism of agricultural crops and to identify metabolites, enzymes and reactions, which are potentially activated by the S metabolism to combat fungal pathogens. The S status of the crop is affecting many different plant features such as color and scent of flowers, pigments in leaves, metabolite concentrations and the release of gaseous S compounds which are directly influencing the desirability of a crop for a variety of different organisms from microorganisms, over insects and slugs to the point of grazing animals. The present paper is an attempt to sum up the knowledge about the effect of the S nutritional status of agricultural crops on parameters that are directly related to their health status and by this to SIR. Milestones in SIR research are compiled, open questions are addressed and future projections were developed.

Proteomic analysis of eucalyptus leaves unveils putative mechanisms involved in the plant response to a real condition of soil contamination by multiple heavy metals in the presence or absence of mycorrhizal/rhizobacterial additives

Here we report on the growth, accumulation performances of, and leaf proteomic changes in Eucalyptus camaldulensis plants harvested for different periods of time in an industrial, heavy metals (HMs)-contaminated site in the presence or absence of soil microorganism (AMs/PGPRs) additives. Data were compared to those of control counterparts grown in a neighboring nonpolluted district. Plants harvested in the contaminated areas grew well and accumulated HMs in their leaves. The addition of AMs/PGPRs to the polluted soil determined plant growth and metal accumulation performances that surpassed those observed in the control. Comparative proteomics suggested molecular mechanisms underlying plant adaptation to the HMs challenge. Similarly to what was observed in laboratory-scale investigations on other metal hyperaccumulators but not on HMs-sensitive plants, eucalyptus grown in the contaminated areas showed an over-representation of enzymes involved in photosynthesis and the Calvin cycle. AMs/PGPRs addition to the soil increased the activation of these energetic pathways, suggesting the existence of signaling mechanisms that address the energy/reductive power requirement associated with augmented growth performances. HMs-exposed plants presented an over-representation of antioxidant enzymes, chaperones, and proteins involved in glutathione metabolism. While some antioxidant enzymes/chaperones returned to almost normal expression values in the presence of AMs/PGPRs or in plants exposed to HMs for prolonged periods, proteins guaranteeing elevated glutathione levels were constantly over-represented. These data suggest that glutathione (and related phytochelatins) could act as key molecules for ensuring the effective formation of HMs-chelating complexes that are possibly responsible for the observed plant tolerance to metal stresses. Overall, these results suggest potential genetic traits for further selection of phytoremediating plants based on dedicated cloning or breeding programs.

Mechanisms of metal resistance and homeostasis in haloarchaea

Haloarchaea are the predominant microflora of hypersaline econiches such as solar salterns, soda lakes, and estuaries where the salinity ranges from 35 to 400 ppt. Econiches like estuaries and solar crystallizer ponds may contain high concentrations of metals since they serve as ecological sinks for metal pollution and also as effective traps for river borne metals. The availability of metals in these econiches is determined by the type of metal complexes formed and the solubility of the metal species at such high salinity. Haloarchaea have developed specialized mechanisms for the uptake of metals required for various key physiological processes and are not readily available at high salinity, beside evolving resistance mechanisms for metals with high solubility. The present paper seeks to give an overview of the main molecular mechanisms involved in metal tolerance in haloarchaea and focuses on factors such as salinity and metal speciation that affect the bioavailability of metals to haloarchaea. Global transcriptomic analysis during metal stress in these organisms will help in determining the various factors differentially regulated and essential for metal physiology.

Transcriptome response to nitrogen starvation in rice

Nitrogen is an essential mineral nutrient required for plant growth and development. Insufficient nitrogen (N) supply triggers extensive physiological and biochemical changes in plants. In this study, we used Affymetrix GeneChip rice genome arrays to analyse the dynamics of rice transcriptome under N starvation. N starvation induced or suppressed transcription of 3518 genes, representing 10.88 percent of the genome. These changes, mostly transient, affected various cellular metabolic pathways, including stress response, primary and secondary metabolism, molecular transport, regulatory process and organismal development. 462 or 13.1 percent transcripts for N starvation expressed similarly in root and shoot. Comparative analysis between rice and Arabidopsis identified 73 orthologous groups that responded to N starvation, demonstrated the existence of conserved N stress coupling mechanism among plants. Additional analysis of transcription profiles of microRNAs revealed differential expression of miR399 and miR530 under N starvation, suggesting their potential roles in plant nutrient homeostasis.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

New insights into the transport mechanisms in plant vacuoles

The vacuole is the largest compartment in plant cells, often occupying more than 80% of the total cell volume. This organelle accumulates a large variety of endogenous ions, metabolites, and xenobiotics. The compartmentation of divergent substances is relevant for a wide range of biological processes, such as the regulation of stomata movement, defense mechanisms against herbivores, flower coloration, etc. Progress in molecular and cellular biology has revealed that a large number of transporters and channels exist at the tonoplast. In recent years, various biochemical and physiological functions of these proteins have been characterized in detail. Some are involved in maintaining the homeostasis of ions and metabolites, whereas others are related to defense mechanisms against biotic and abiotic stresses. In this review, we provide an updated inventory of vacuolar transport mechanisms and a comprehensive summary of their physiological functions.

Phytoremediation potential of Arabidopsis thaliana, expressing ectopically a vacuolar proton pump, for the industrial waste phosphogypsum

Phosphogypsum (PG) is a by-product of the phosphorus-fertiliser industry and represents an environmental concern since it contains pollutants such as cadmium (Cd). We have recently shown that the overexpression of a proton pump gene (TaVP1) in transgenic tobacco (Nicotiana tabacum) led to an enhanced Cd tolerance and accumulation. The aim of this study was to evaluate the potential of transgenic Arabidopsis thaliana plants harbouring the TaVP1 gene to phytoremediate phosphogypsum. A pot experiment was carried out under greenhouse conditions. Transgenic A. thaliana plants harbouring the TaVP1 gene were grown on various substrates containing phosphogypsum (0, 25, 50 and 100 %) for 40 days. At the end of the growth period, we examined the growth (germination, root length, fresh weight) and physiological parameters (chlorophyll and protein contents, catalase activity and proteolysis) as well as the cadmium, Mg, Ca, and P contents of the A. thaliana plants. In order to evaluate Cd tolerance of the A. thaliana lines harbouring the TaVP1 gene, an in vitro experiment was also carried out. One week-old seedlings were transferred to Murashige and Skoog agar plates containing various concentrations of cadmium; the germination, total leaf area and root length were determined. The growth and physiological parameters of all A. thaliana plants were significantly altered by PG. The germination capacity, root growth and biomass production of wild-type (WT) plants were more severely inhibited by PG compared with the TaVP1 transgenic A. thaliana lines. In addition, TaVP1 transgenic A. thaliana plants maintained a higher antioxidant capacity than the WT. Interestingly, elemental analysis of leaf material derived from plants grown on PG revealed that the transgenic A. thaliana line accumulated up to ten times more Cd than WT. Despite its higher Cd content, the transgenic A. thaliana line performed better than the WT counterpart. In vitro evaluation of Cd tolerance showed that TaVP1 transgenic A. thaliana lines were more Cd-tolerant than the WT plants. These results suggested that ectopic expression of a vacuolar proton pump in A. thaliana plants can lead to various biotechnological applications including the phytoremediation of industrial wastes.

Differentially expressed transcripts from leaf and root tissue of Chlorophytum borivilianum: a plant with high medicinal value

Chlorophytum borivilianum is one of the important medicinal plants used for treating different health problems such as diabetes, arthritis, physical weakness, etc. Saponins present in C. borivilianum are the primary source of its significant medicinal properties and are synthesized by mevalonate and non-mevalonate pathways in plants. However, the biosynthesis of these compounds at molecular level is not studied in C. borivilianum. Cloning and sequencing of genes involved in metabolic processes are prerequisite to study the gene expression, their regulation and genetic engineering experiments. Expressed sequence tags (ESTs) provide a quick insight into various genes and their tissue specific expression. Suppression subtractive hybridization (SSH) libraries were constructed using mRNA from leaf and root tissues of C. borivilianum. High quality non-redundant 506 and 303 ESTs were generated from leaf and root specific libraries respectively. These sequences were analyzed using bioinformatics tools and grouped into different categories based on their similarity and cellular functions such as photosynthesis, metabolism, transcription factors, cell signaling, defense, stress response etc. ESTs also showed similarity with genes involved in saponins biosynthesis such as squalene synthase, squalene epoxidase, cytochrome p450, glycosyltransferase, etc. Semi-quantitative analysis of some of the ESTs involved in saponins biosynthesis confirmed their differential regulation in leaves and roots. These ESTs will provide an efficient resource to accelerate gene discovery in C. borivilianum and will help in determining promising targets for genetic engineering of saponins pathway.

Phytochelatin synthase: of a protease a peptide polymerase made

Of the mechanisms known to protect vascular plants and some algae, fungi and invertebrates from the toxic effects of non-essential heavy metals such as As, Cd or Hg, one of the most sophisticated is the enzyme-catalyzed synthesis of phytochelatins (PCs). PCs, (γ-Glu-Cys)(n) Gly polymers, which serve as high-affinity, thiol-rich cellular chelators and contribute to the detoxification of heavy metal ions, are derived from glutathione (GSH; γ-Glu-Cys-Gly) and related thiols in a reaction catalyzed by phytochelatin synthases (PC synthases, EC 2.3.2.15). Using the enzyme from Arabidopsis thaliana (AtPCS1) as a model, the reasoning and experiments behind the conclusion that PC synthases are novel papain-like Cys protease superfamily members are presented. The status of S-substituted GSH derivatives as generic PC synthase substrates and the sufficiency of the N-terminal domain of the enzyme from eukaryotic and its half-size equivalents from prokaryotic sources, for net PC synthesis and deglycylation of GSH and its derivatives, respectively, are emphasized. The question of the common need or needs met by PC synthases and their homologs is discussed. Of the schemes proposed to account for the combined protease and peptide polymerase capabilities of the eukaryotic enzymes vs the limited protease capabilities of the prokaryotic enzymes, two that will be considered are the storage and homeostasis of essential heavy metals in eukaryotes and the metabolism of S-substituted GSH derivatives in both eukaryotes and prokaryotes.

Vacuolar transporters in their physiological context

Vacuoles in vegetative tissues allow the plant surface to expand by accumulating energetically cheap inorganic osmolytes, and thereby optimize the plant for absorption of sunlight and production of energy by photosynthesis. Some specialized cells, such as guard cells and pulvini motor cells, exhibit rapid volume changes. These changes require the rapid release and uptake of ions and water by the vacuole and are a prerequisite for plant survival. Furthermore, seed vacuoles are important storage units for the nutrients required for early plant development. All of these fundamental processes rely on numerous vacuolar transporters. During the past 15 years, the transporters implicated in most aspects of vacuolar function have been identified and characterized. Vacuolar transporters appear to be integrated into a regulatory network that controls plant metabolism. However, little is known about the mode of action of these fundamental processes, and deciphering the underlying mechanisms remains a challenge for the future.

Cd tolerance and accumulation in the aquatic macrophyte, Chara australis: potential use for charophytes in phytoremediation

We investigated the potential use of the alga Chara australis (R. Br.) forphytore mediation of Cd-contaminated sediments in aquatic systems. Chara tolerated up to 20 mg added Cd (kg soil)⁻¹ in laboratory culture. Chlorophyll a and b levels were not affected even at Cd concentrations that suppressed growth. Levels of glutathione were suppressed at 2-35 mg added Cd (kg soil)⁻¹ to 200-350 nmol GSH (g DW)⁻¹, while control levels were 660 nmol GSH (g DW)⁻¹). Histochemical studies showed Cd occurred throughout cell walls and cytoplasm in plants grown in 5-20 mg Cd (kg soil)⁻¹. Quantification using ICP-MS showed the maximum concentration in shoots was 72 mg Cd (kg DW)⁻¹ at 35 mg added Cd (kg soil)⁻¹, while the maximum in rhizoids was 116 mg Cd (kg DW)⁻¹ at 25 mg added Cd (kg soil)⁻¹. The bioconcentration factor (BCF, concentration in plant/concentration in soil) exceeded 1.0, the critical value for hyperaccumulators, for shoots exposed to 35 mg Cd (kg soil)⁻¹ and rhizoids exposed to ≥25 mg Cd (kg soil)⁻¹. Translocation factors (TF, shoot concentration/rhizoid concentration) did not exceed 1.0 for any treatment. While Chara cannot be considered a hyperaccumulator, it shows promise for use in phytoremediation efforts.

Sortin1-hypersensitive mutants link vacuolar-trafficking defects and flavonoid metabolism in Arabidopsis vegetative tissues

Sortin1 is a chemical genetic-hit molecule that causes specific mislocalization of plant and yeast-soluble and membrane vacuolar markers. To better understand its mode of action, we designed a Sortin1-hypersensitive screen and identified several Sortin1-hypersensitive and flavonoid-defective mutants. Mechanistically, Sortin1 mimics the effect of the glutathione inhibitor buthionine sulfoximine and alters the vacuolar accumulation of flavonoids, likely blocking their transport through vacuole-localized ABC transporters. Structure-activity relationship studies conducted in Arabidopsis revealed the structural requirements for Sortin1 bioactivity and demonstrated that overlapping Sortin1 substructures can be used to discriminate between vacuolar-flavonoid accumulations and vacuolar-biogenesis defects. We conclude that Sortin1 is a valuable probe for dissecting novel links among flavonoid transport, vacuolar integrity, and the trafficking of vacuolar targeted cargoes in Arabidopsis.