Plant ABC Transporters Enable Many Unique Aspects of a Terrestrial Plant's Lifestyle

The Roles of Plant Hormones and Their Interactions with Regulatory Genes in Determining Meristem Activity

Plants, unlike animals, have developed a unique system in which they continue to form organs throughout their entire life cycle, even after embryonic development. This is possible because plants possess a small group of pluripotent stem cells in their meristems. The shoot apical meristem (SAM) plays a key role in forming all of the aerial structures of plants, including floral meristems (FMs). The FMs subsequently give rise to the floral organs containing reproductive structures. Studies in the past few decades have revealed the importance of transcription factors and secreted peptides in meristem activity using the model plant Arabidopsis thaliana. Recent advances in genomic, transcriptomic, imaging, and modeling technologies have allowed us to explore the interplay between transcription factors, secreted peptides, and plant hormones. Two different classes of plant hormones, cytokinins and auxins, and their interaction are particularly important for controlling SAM and FM development. This review focuses on the current issues surrounding the crosstalk between the hormonal and genetic regulatory network during meristem self-renewal and organogenesis.

ECF-type ABC transporters for uptake of vitamins and transition metal ions into prokaryotic cells

Energy-coupling factor (ECF) transporters mediate the uptake of micronutrients in prokaryotes. They consist of two ATP-binding-cassette family ATPases, a transmembrane coupling protein (T component) and a substrate-binding membrane protein (S component). ECF transporters for Co²⁺ and Ni²⁺ ions have one or two additional proteins with extracytoplasmic regions but poorly understood function. Homologs of T components with a predicted localization in plastids are widespread in plants but their physiological role is unclear. S components in eukaryotes are very rare and restricted to biotin-specific variants. Apart from a potential contribution to the export of flavins to serve the assembly of extracytoplasmic electron transfer chains, ECF transporters function as importers.

The C-type ATP-Binding Cassette Transporter OsABCC7 Is Involved in the Root-to-Shoot Translocation of Arsenic in Rice

Rice is a major dietary source of inorganic arsenic (As), a nonthreshold carcinogen. Reducing As accumulation in rice grain is of critical importance for food safety. In the present study, we investigated the role of a member of the rice C-type ATP-binding cassette (ABC) transporter (OsABCC) family, OsABCC7, in arsenite [As(III)] accumulation in rice. Quantitative real-time RT-PCR showed that OsABCC7 was expressed intensively in the roots and the expression was strongly suppressed by As(III) exposure. Transgenic rice plants expressing OsABCC7 Promoter-GUS (β-glucuronidase) suggest that the gene was predominantly expressed in the xylem parenchyma cells in the stele region of the primary and lateral roots. Transient expression of OsABCC7: GFP fusion protein in Nicotiana benthamiana leaf cells showed that the protein was localized at the plasma membrane. When expressed in Xenopus laevis oocytes, OsABCC7 showed an efflux activity for As(III)-phytochelatin and As(III)-glutathione complexes, but not for As(III). Knockout of OsABCC7 in rice significantly decreased As concentration in the xylem sap and As concentration in the shoots, but had little effect on root As concentration. Taken together, our results indicate that OsABCC7 is involved in the root-to-shoot translocation of As(III).

Abscisic acid is a substrate of the ABC transporter encoded by the durable wheat disease resistance gene Lr34

The wheat Lr34res allele, coding for an ATP-binding cassette transporter, confers durable resistance against multiple fungal pathogens. The Lr34sus allele, differing from Lr34res by two critical nucleotide polymorphisms, is found in susceptible wheat cultivars. Lr34res is functionally transferrable as a transgene into all major cereals, including rice, barley, maize, and sorghum. Here, we used transcriptomics, physiology, genetics, and in vitro and in vivo transport assays to study the molecular function of Lr34. We report that Lr34res results in a constitutive induction of transcripts reminiscent of an abscisic acid (ABA)-regulated response in transgenic rice. Lr34-expressing rice was altered in biological processes that are controlled by this phytohormone, including dehydration tolerance, transpiration and seedling growth. In planta seedling and in vitro yeast accumulation assays revealed that both LR34res and LR34sus act as ABA transporters. However, whereas the LR34res protein was detected in planta the LR34sus version was not, suggesting a post-transcriptional regulatory mechanism. Our results identify ABA as a substrate of the LR34 ABC transporter. We conclude that LR34res-mediated ABA redistribution has a major effect on the transcriptional response and physiology of Lr34res-expressing plants and that ABA is a candidate molecule that contributes to Lr34res-mediated disease resistance.

Targeted transcriptional and proteomic studies explicate specific roles of Bacillus subtilis iturin A, fengycin, and surfactin on elicitation of defensive systems in mandarin fruit during stress

Application of Bacillus cyclic lipopeptides (CLPs); fengycin, iturin A and surfactin has shown a great potential in controlling the spread of green mold pathogen invasion (Penicillium digitatum) in wounded mandarin fruit during postharvest period. The limited defensive protein profiles followed specific expression of pivotal genes relating to plant hormone mediating signaling pathways of the CLPs’ action on stimulating host plant resistance have been exhibited. The present study aimed to elucidate the specific effect of individual CLP obtained from Bacillus subtilis ABS-S14 as elicitor role on activation of plant defensive system at transcriptional and proteomic levels with and without P. digitatum co-application in mandarin fruit. Fengycin and iturin A elevated the gene expression of PAL, ACS1, ACO, CHI, and GLU while significantly stimulating plant POD transcription was only detected in the treatments of surfactin both with and without following P. digitatum. An increase of LOX and PR1 gene transcripts was determined in the treatments of individual CLP with fungal pathogen co-application. Fengycin activated production of unique defensive proteins such as protein involved in ubiquinone biosynthetic process in treated flavedo without P. digitatum infection. Proteins involved in the auxin modulating pathway were present in the iturin A and surfactin treatments. CLP-protein binding assay following proteome analysis reveals that iturin A attached to 12-oxophytodienoate reductase 2 involved in the oxylipin biosynthetic process required for jasmonic acid production which is implicated in induced systemic resistance (ISR). This study suggests specific elicitor action of individual CLP, particularly iturin A showed the most powerful in stimulating the ISR system in response to stresses in postharvest mandarins.

Standing genomic variation within coding and regulatory regions contributes to the adaptive capacity to climate in a foundation tree species

Global climate is rapidly changing, and the ability for tree species to adapt is dependent on standing genomic variation; however, the distribution and abundance of functional and adaptive variants are poorly understood in natural systems. We test key hypotheses regarding the genetics of adaptive variation in a foundation tree: genomic variation is associated with climate, and genomic variation is more likely to be associated with temperature than precipitation or aridity. To test these hypotheses, we used 9,593 independent, genomic single-nucleotide polymorphisms (SNPs) from 270 individuals sampled from Corymbia calophylla's entire distribution in south-western Western Australia, spanning orthogonal temperature and precipitation gradients. Environmental association analyses returned 537 unique SNPs putatively adaptive to climate. We identified SNPs associated with climatic variation (i.e., temperature [458], precipitation [75] and aridity [78]) across the landscape. Of these, 78 SNPs were nonsynonymous (NS), while 26 SNPs were found within gene regulatory regions. The NS and regulatory candidate SNPs associated with temperature explained more deviance (27.35%) than precipitation (5.93%) and aridity (4.77%), suggesting that temperature provides stronger adaptive signals than precipitation. Genes associated with adaptive variants include functions important in stress responses to temperature and precipitation. Patterns of allelic turnover of NS and regulatory SNPs show small patterns of change through climate space with the exception of an aldehyde dehydrogenase gene variant with 80% allelic turnover with temperature. Together, these findings provide evidence for the presence of adaptive variation to climate in a foundation species and provide critical information to guide adaptive management practices.

ABC Transporters in Prorocentrum lima and Their Expression Under Different Environmental Conditions Including Okadaic Acid Production

Prorocentrum lima is a typical benthic toxic dinoflagellate, which can produce phycotoxins such as okadaic acid (OA). In this study, we identified three ABC transporter genes (ABCB1, ABCC1 and ABCG2) and characterized their expression patterns, as well as OA production under different environmental conditions in P. lima. We found that the three ABC transporters all showed high identity with related ABC proteins from other species, and contained classical features of ABC transport proteins. Among them, ABCG2 was a half size transporter. The three ABC transporter genes displayed various expression profiles under different conditions. The high concentration of Cu²⁺ could up-regulate ABCB1, ABCC1 and ABCG2 transcripts in P. lima, suggesting the potential defensive role of ABC transporters against metal ions in surrounding waters. Cu²⁺, in some concentration, could induce OA production; meanwhile, tributyltin inhibited OA accumulation. The grazer Artemia salina could induce OA production, and P. lima displayed some toxicity to the grazer, indicating the possibility of OA as an anti-grazing chemical. Collectively, our results revealed intriguing data about OA production and the expression patterns of three ABC transporter genes. However, we could not find any significant correlation between OA production and expression pattern of the three ABC transporters in P. lima. Our results might provide new molecular insights on the defensive responses of P. lima to the surrounding environment.

Genome-wide identification and gene expression pattern of ABC transporter gene family in Capsicum spp

ATP-binding cassette (ABC) transporter genes act as transporters for different molecules across biological membranes and are involved in a diverse range of biological processes. In this study, we performed a genome-wide identification and expression analysis of genes encoding ABC transporter proteins in three Capsicum species, i.e., Capsicum annuum, Capsicum baccatum and Capsicum chinense. Capsicum is a valuable horticultural crop worldwide as an important constituent of many foods while containing several medicinal compounds including capsaicin and dihydrocapsaicin. Our results identified the presence of a total of 200, 185 and 187 ABC transporter genes in C. annuum, C. baccatum and C. chinense genomes, respectively. Capsaicin and dihydrocapsaicin content were determined in green pepper fruits (16 dpa). Additionally, we conducted different bioinformatics analyses including ABC genes classification, gene chromosomal location, Cis elements, conserved motifs identification and gene ontology classification, as well as profile expression of selected genes. Based on phylogenetic analysis and domain organization, the Capsicum ABC gene family was grouped into eight subfamilies. Among them, members within the ABCG, ABCB and ABCC subfamilies were the most abundant, while ABCD and ABCE subfamilies were less abundant throughout all species. ABC members within the same subfamily showed similar motif composition. Furthermore, common cis-elements involved in the transcriptional regulation were also identified in the promoter regions of all Capsicum ABC genes. Gene expression data from RNAseq and reverse transcription-semi-quantitative PCR analysis revealed development-specific stage expression profiles in placenta tissues. It suggests that ABC transporters, specifically the ABCC and ABCG subfamilies, may be playing important roles in the transport of secondary metabolites such as capsaicin and dihydrocapsaicin to the placenta vacuoles, effecting on their content in pepper fruits. Our results provide a more comprehensive understanding of ABC transporter gene family in different Capsicum species while allowing the identification of important candidate genes related to capsaicin content for subsequent functional validation.

The different tolerance to magnesium deficiency of two grapevine rootstocks relies on the ability to cope with oxidative stress

BACKGROUND

Magnesium (Mg) deficiency causes physiological and molecular responses, already dissected in several plant species. The study of these responses among genotypes showing a different tolerance to the Mg shortage can allow identifying the mechanisms underlying the resistance to this nutritional disorder. To this aim, we compared the physiological and molecular responses (e.g. changes in root metabolome and transcriptome) of two grapevine rootstocks exhibiting, in field, different behaviors with respect to Mg shortage (1103P, tolerant and SO4 susceptible).

RESULTS

The two grapevine rootstocks confirmed, in a controlled growing system, their behavior in relation to the tolerance to Mg deficiency. Differences in metabolite and transcriptional profiles between the roots of the two genotypes were mainly linked to antioxidative compounds and the cell wall constituents. In addition, differences in secondary metabolism, in term of both metabolites (e.g. alkaloids, terpenoids and phenylpropanoids) and transcripts, assessed between 1103P and SO4 suggest a different behavior in relation to stress responses particularly at early stages of Mg deficiency.

CONCLUSIONS

Our results suggested that the higher ability of 1103P to tolerate Mg shortage is mainly linked to its capability of coping, faster and more efficiently, with the oxidative stress condition caused by the nutritional disorder.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Comparative transcriptomics sheds light on differential adaptation and species diversification between two Melastoma species and their F1 hybrid

Variation in gene expression has been shown to promote adaptive divergence, and can lead to speciation. The plant genus Melastoma, thought to have diversified through adaptive radiation, provides an excellent model for the study of gene expressional changes during adaptive differentiation and following interspecific hybridization. In this study, we performed RNA-seq on M. candidum, M. sanguineum and their F1 hybrid, to investigate the role of gene expression in species diversification within the genus. Reference transcriptomes were assembled using combined data from both parental species, resulting in 50 519 and 48 120 transcripts for the leaf and flower petal, after removing redundancy. Differential expression analysis uncovered 3793 and 2116 differentially expressed (DE) transcripts, most of which are between M. candidum and M. sanguineum. Differential expression was observed for genes related to light responses, as well as genes that regulate the development of leaf trichomes, a trait that among others is thought to protect plants against sunlight, suggesting the differential adaptation of the species to sunlight intensity. The analysis of positively selected genes between the two species also revealed possible differential adaptation to other abiotic stresses such as drought and temperature. In the hybrid, almost all possible modes of expression were observed at the DE transcripts, although at most transcripts, the expression levels were similar to that of either parent instead of being intermediate. A small number of transgressively expressed transcripts that matched genes known to promote plant growth and adaptation to stresses in new environments were also found, possibly explaining the vigour observed in the hybrid. The findings in this study provided insights into the role of gene expression in the diversification of Melastoma, which we believe is an important example for more cross-taxa comparisons in the future.

Cloning and expression of selected ABC transporters from the Arabidopsis thaliana ABCG family in Pichia pastoris

Phytohormones play a major role in plant growth and development. They are in most cases not synthesized in their target location and hence need to be transported to the site of action, by for instance ATP-binding cassette transporters. Within the ATP-binding cassette transporter family, Pleiotropic Drug Resistance transporters are known to be involved in phytohormone transport. Interestingly, PDRs are only present in plants and fungi. In contrast to fungi, there are few biochemical studies of plant PDRs and one major reason is that suitable overexpression systems have not been identified. In this study, we evaluate the expression system Pichia pastoris for heterologous overexpression of PDR genes of the model plant Arabidopsis thaliana. We successfully cloned and expressed the potential phytohormone transporters PDR2 and PDR8 in P. pastoris. Sucrose gradient centrifugation confirmed that the overexpressed proteins were correctly targeted to the plasma membrane of P. pastoris and initial functional studies demonstrated ATPase activity for WBC1. However, difficulties in cloning and heterologous overexpression might be particular obstacles of the PDR family, since cloning and overexpression of White Brown Complex 1, a half-size transporter of the same ABCG subfamily with comparable domain organization, was more easily achieved. We present strategies and highlight critical factors to successfully clone plant PDR genes and heterologously expressed in P. pastoris.

In Silico Genome-Wide Analysis of the ATP-Binding Cassette Transporter Gene Family in Soybean (Glycine max L.) and Their Expression Profiling

ATP-binding cassette (ABC) transporters constitute one of the largest gene families in all living organisms, most of which mediate transport across biological membranes by hydrolyzing ATP. However, detailed studies of ABC transporter genes in the important oil crop, soybean, are still lacking. In the present study, we carried out genome-wide identification and phylogenetic and transcriptional analyses of the ABC gene family in G. max. A total of 261 G. max ABC (GmABCs) genes were identified and unevenly localized onto 20 chromosomes. Referring to protein-domain orientation and phylogeny, the GmABC family could be classified into eight (ABCA-ABCG and ABCI) subfamilies and ABCG were the most abundantly present. Further, investigation of whole genome duplication (WGD) signifies the role of segmental duplication in the expansion of the ABC transporter gene family in soybean. The Ka/Ks ratio indicates that several duplicated genes are governed by intense purifying selection during evolution. In addition, in silico expression analysis based on RNA-sequence using publicly available database revealed that ABC transporters are differentially expressed in tissues and developmental stages and in dehydration. Overall, we provide an extensive overview of the GmABC transporter gene family and it promises the primary basis for the study in development and response to dehydration tolerance.

Transcriptome-wide mining, characterization, and development of microsatellite markers in Lychnis kiusiana (Caryophyllaceae)

BACKGROUND

Lychnis kiusiana Makino is an endangered perennial herb native to wetland areas in Korea and Japan. Despite its conservational and evolutionary significance, population genetic resources are lacking for this species. Next-generation sequencing has been accepted as a rapid and cost-effective solution for the identification of microsatellite markers in nonmodel plants.

RESULTS

Using Illumina HiSeq 2000 sequencing technology, we assembled 67,498,600 reads into 91,900 contigs and identified 11,403 microsatellite repeat motifs in 9563 contigs. A total of 4510 microsatellite-containing transcripts had Gene Ontology (GO) annotations, and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 124 pathways with significant scores. Many microsatellites in the L. kiusiana leaf transcriptome were linked to genes involved in the plant response to light intensity, salt stress, temperature stimulus, and nutrient and water deprivation. A total of 12,486 single-nucleotide polymorphisms (SNPs) were identified on transcripts harboring microsatellites. The analysis of nucleotide substitution rates for 2389 unigenes indicated that 39 genes were under strong positive selection. The primers of 6911 microsatellites were designed, and 40 of 50 selected primer pairs were consistently and successfully amplified from 51 individuals. Twenty-five of these were polymorphic, and the average number of alleles per SSR locus was 6.96, with a range from 2 to 15. The observed and expected heterozygosities ranged from 0.137 to 0.902 and 0.131 to 0.827, respectively, and locus-specific F_IS estimates ranged from - 0.116 to 0.290. Eleven of the 25 primer pairs were successfully amplified in three additional species of Lychnis: 56% in L. wilfordii, 64% in L. cognata and 80% in L. fulgens.

CONCLUSIONS

The transcriptomic SSR markers of Lychnis kiusiana provide a valuable resource for understanding the population genetics, evolutionary history, and effective conservation management of this species. Furthermore, the identified microsatellite loci linked to the annotated genes should be useful for developing functional markers of L. kiusiana. The developed markers represent a potentially valuable source of transcriptomic SSR markers for population genetic analyses with moderate levels of cross-taxon portability.

A Novel Prokaryote-Type ECF/ABC Transporter Module in Chloroplast Metal Homeostasis

During evolution, chloroplasts, which originated by endosymbiosis of a prokaryotic ancestor of today's cyanobacteria with a eukaryotic host cell, were established as the site for photosynthesis. Therefore, chloroplast organelles are loaded with transition metals including iron, copper, and manganese, which are essential for photosynthetic electron transport due to their redox capacity. Although transport, storage, and cofactor-assembly of metal ions in chloroplasts are tightly controlled and crucial throughout plant growth and development, knowledge on the molecular nature of chloroplast metal-transport proteins is still fragmentary. Here, we characterized the soluble, ATP-binding ABC-transporter subunits ABCI10 and ABCI11 in Arabidopsis thaliana, which show similarities to components of prokaryotic, multisubunit ABC transporters. Both ABCI10 and ABCI11 proteins appear to be strongly attached to chloroplast-intrinsic membranes, most likely inner envelopes for ABCI10 and possibly plastoglobuli for ABCI11. Loss of ABCI10 and ABCI11 gene products in Arabidopsis leads to extremely dwarfed, albino plants showing impaired chloroplast biogenesis and deregulated metal homeostasis. Further, we identified the membrane-intrinsic protein ABCI12 as potential interaction partner for ABCI10 in the inner envelope. Our results suggest that ABCI12 inserts into the chloroplast inner envelope membrane most likely with five predicted α-helical transmembrane domains and represents the membrane-intrinsic subunit of a prokaryotic-type, energy-coupling factor (ECF) ABC-transporter complex. In bacteria, these multisubunit ECF importers are widely distributed for the uptake of nickel and cobalt metal ions as well as for import of vitamins and several other metabolites. Therefore, we propose that ABCI10 (as the ATPase A-subunit) and ABCI12 (as the membrane-intrinsic, energy-coupling T-subunit) are part of a novel, chloroplast envelope-localized, AAT energy-coupling module of a prokaryotic-type ECF transporter, most likely involved in metal ion uptake.

A Single Nucleotide Deletion in an ABC Transporter Gene Leads to a Dwarf Phenotype in Watermelon

Dwarf habit is one of the most important traits in crop plant architecture, as it can increase plant density and improved land utilization, especially for protected cultivation, as well as increasing lodging resistance and economic yield. At least four dwarf genes have been identified in watermelon, but none of them has been cloned. In the current study, the Cldw-1 gene was primary-mapped onto watermelon chromosome 9 by next-generation sequencing-aided bulked-segregant analysis (BSA-seq) of F₂ plants derived from a cross between a normal-height line, WT4, and a dwarf line, WM102, in watermelon. The candidate region identified by BSA-seq was subsequently validated and confirmed by linkage analysis using 30 simple sequence repeat (SSR) markers in an F₂ population of 124 plants. The Cldw-1 gene was further fine-mapped by chromosome walking in a large F₂ population of 1,053 plants and was delimited into a candidate region of 107.00 kb. Six genes were predicted to be in the candidate region, and only one gene, Cla010337, was identified to have two single nucleotide polymorphisms (SNPs) and a single nucleotide deletion in the exons in the dwarf line, WM102. A derived cleaved amplified polymorphic sequence (dCAPS) marker was developed from the single nucleotide deletion, co-segregated with the dwarf trait in both the F₂ population and a germplasm collection of 165 accessions. Cla010337 encoded an ATP-binding cassette transporter (ABC transporter) protein, and the expression levels of Cla010337 were significantly reduced in all the tissues tested in the dwarf line, WM102. The results of this study will be useful in achieving a better understanding of the molecular mechanism of the dwarf plant trait in watermelon and for the development of marker-assisted selection (MAS) for new dwarf cultivars.

Designating ligand specificities to metal uptake ABC transporters in Thermus thermophilus HB8

Acquisition of different metal ions by metal uptake ABC transporters of Thermus thermophilus HB8 for accomplishing its various cellular functions.

A Novel Prokaryote-Type ECF/ABC Transporter Module in Chloroplast Metal Homeostasis

During evolution, chloroplasts, which originated by endosymbiosis of a prokaryotic ancestor of today's cyanobacteria with a eukaryotic host cell, were established as the site for photosynthesis. Therefore, chloroplast organelles are loaded with transition metals including iron, copper, and manganese, which are essential for photosynthetic electron transport due to their redox capacity. Although transport, storage, and cofactor-assembly of metal ions in chloroplasts are tightly controlled and crucial throughout plant growth and development, knowledge on the molecular nature of chloroplast metal-transport proteins is still fragmentary. Here, we characterized the soluble, ATP-binding ABC-transporter subunits ABCI10 and ABCI11 in Arabidopsis thaliana, which show similarities to components of prokaryotic, multisubunit ABC transporters. Both ABCI10 and ABCI11 proteins appear to be strongly attached to chloroplast-intrinsic membranes, most likely inner envelopes for ABCI10 and possibly plastoglobuli for ABCI11. Loss of ABCI10 and ABCI11 gene products in Arabidopsis leads to extremely dwarfed, albino plants showing impaired chloroplast biogenesis and deregulated metal homeostasis. Further, we identified the membrane-intrinsic protein ABCI12 as potential interaction partner for ABCI10 in the inner envelope. Our results suggest that ABCI12 inserts into the chloroplast inner envelope membrane most likely with five predicted α-helical transmembrane domains and represents the membrane-intrinsic subunit of a prokaryotic-type, energy-coupling factor (ECF) ABC-transporter complex. In bacteria, these multisubunit ECF importers are widely distributed for the uptake of nickel and cobalt metal ions as well as for import of vitamins and several other metabolites. Therefore, we propose that ABCI10 (as the ATPase A-subunit) and ABCI12 (as the membrane-intrinsic, energy-coupling T-subunit) are part of a novel, chloroplast envelope-localized, AAT energy-coupling module of a prokaryotic-type ECF transporter, most likely involved in metal ion uptake.

Filling the Gap: Functional Clustering of ABC Proteins for the Investigation of Hormonal Transport in planta

Plant hormones regulate a myriad of plant processes, from seed germination to reproduction, from complex organ development to microelement uptake. Much has been discovered on the factors regulating the activity of phytohormones, yet there are gaps in knowledge about their metabolism, signaling as well as transport. In this review we analyze the potential of the characterized phytohormonal transporters belonging to the ATP-Binding Cassette family (ABC proteins), thus to identify new candidate orthologs in model plants and species important for human health and food production. Previous attempts with phylogenetic analyses on transporters belonging to the ABC family suggested that sequence homology per se is not a powerful tool for functional characterization. However, we show here that sequence homology might indeed support functional conservation of characterized members of different classes of ABC proteins in several plant species, e.g., in the case of ABC class G transporters of strigolactones and ABC class B transporters of auxinic compounds. Also for the low-affinity, vacuolar abscisic acid (ABA) transporters belonging to the ABCC class we show that localization-, rather than functional-clustering occurs, possibly because of sequence conservation for targeting the tonoplast. The ABC proteins involved in pathogen defense are phylogenetically neighboring despite the different substrate identities, suggesting that sequence conservation might play a role in their activation/induction after pathogen attack. Last but not least, in case of the multiple lipid transporters belong to different ABC classes, we focused on ABC class D proteins, reported to transport/affect the synthesis of hormonal precursors. Based on these results, we propose that phylogenetic approaches followed by transport bioassays and in vivo investigations might accelerate the discovery of new hormonal transport routes and allow the designing of transgenic and genome editing approaches, aimed to improve our knowledge on plant development, plant-microbe symbioses, plant nutrient uptake and plant stress resistance.

General control non-repressible 20 (GCN20) functions in root growth by modulating DNA damage repair in Arabidopsis

BACKGROUND

Most ABC transporters are engaged in transport of various compounds, but its subfamily F lacks transmembrane domain essential for chemical transportation. Thus the function of subfamily F remains further elusive.

RESULTS

Here, we identified General Control Non-Repressible 20 (GCN20), a member of subfamily F, as new factor for DNA damage repair in root growth. While gcn20-1 mutant had a short primary root with reduced meristem size and cell number, similar primary root lengths were assayed in both wild-type and GCN20::GCN20 gcn20-1 plants, indicating the involvement of GCN20 in root elongation. Further experiments with EdU incorporation and comet assay demonstrated that gcn20-1 displays increased cell cycle arrest at G2/M checkpoint and accumulates more damaged DNA. This is possible due to impaired ability of DNA repair in gcn20-1 since gcn20-1 seedlings are hypersensitive to DNA damage inducers MMC and MMS compared with the wild type plants. This note was further supported by the observation that gcn20-1 is more sensitive than the wild type when subjected to UV treatment in term of changes of both fresh weight and survival rate.

CONCLUSIONS

Our study indicates that GCN20 functions in primary root growth by modulating DNA damage repair in Arabidopsis. Our study will be useful to understand the functions of non-transporter ABC proteins in plant growth.

Down regulation of a heavy metal transporter gene influences several domestication traits and grain Fe-Zn content in rice

Biofortification of rice (Oryza sativa L.) would alleviate iron and zinc deficiencies in the target populations. We identified two alleles 261 and 284 of a Gramineae-specific heavy metal transporter gene OsHMA7 by analyzing expression patterns and sequences of genes within QTLs for high Fe & Zn, in Madhukar x Swarna recombinant inbred lines (RILs) with high (HL) or low (LL) grain Fe & Zn. Overexpression of 261 allele increased grain Fe and Zn but most of the transgenic plants either did not survive or did not yield enough seeds and could not be further characterized. Knocking down expression of OsHMA7 by RNAi silencing of endogenous gene resulted in plants with altered domestication traits such as plant height, tiller number, panicle size and architecture, grain color, shape, size, grain shattering, heading date and increased sensitivity to Fe and Zn deficiency. However, overexpression of 284 allele resulted in transgenic lines with either high grain Fe & Zn content (HL-ox) and tolerance to Fe and Zn deficiency or low grain Fe & Zn content (LL-ox) and phenotype similar to RNAi-lines. OsHMA7 transcript levels were five-fold higher in the HL-ox plants whereas LL-ox and RNAi plants showed 2-3 fold reduced levels compared to Kitaake control. Spraying LL-ox and RNAi lines with Fe & Zn at grain filling stage resulted in increased grain yield, significant increase in Fe & Zn content and brown pericarp. Altered expression of OsHMA7 influenced transcript levels of iron-responsive genes indicating cellular Fe-Zn homeostasis and also several domestication-related genes in rice. Our study shows that a novel heavy metal transporter gene influences yield and grain Fe & Zn content and has potential to improve rice production and biofortification.

BANFF: ending of bilayer membranes by mphiphilic α-helices is ecessary for orm and unction of organelles 1

By binding to and inserting into the lipid bilayer, amphiphilic α-helices of proteins are involved in the curvature of biological membranes in all organisms. In particular, they are involved in establishing the complex membrane architecture of intracellular organelles like the endoplasmatic reticulum, Golgi apparatus, mitochondria, and chloroplasts. Thus, amphiphilic α-helices are essential for maintenance of cellular metabolism and fitness of organisms. Here we focus on the structure and function of membrane-intrinsic proteins, which are involved in membrane curvature by amphiphilic α-helices, in mitochondria and chloroplasts of the eukaryotic model organisms yeast and Arabidopsis thaliana. Further, we propose a model for transport of fatty acids and lipid compounds across the envelope of chloroplasts in which amphiphilic α-helices might play a role.

The ATP-binding protein FgArb1 is essential for penetration, infectious and normal growth of Fusarium graminearum

ATP-binding cassette (ABC) transporters act mainly to transport compounds across cellular membranes and are important for diverse biological processes. However, their roles in pathogenesis have not been well-characterized in Fusarium graminearum. Sixty F. graminearum ABC protein genes were functionally characterized. Among them, FgArb1 regulates normal growth and importantly is essential for pathogenicity. Thus, the regulatory mechanisms of FgArb1 in pathogenicity were analyzed in this study. FgArb1 interacts with the mitogen-activated protein kinase (MAPK) FgSte7, and partially modulates plant penetration by regulating the phosphorylation of FgGpmk1 (the downstream kinase of FgSte7). The FgArb1 mutant exhibited dramatically reduced infective growth within wounded host tissues, likely resulting from its increased sensitivity to oxidative stresses, defective cell wall integrity and reduced deoxynivalenol (DON) production. FgArb1 also is important for the production of sexual and asexual spores that are important propagules for plant infection. In addition, FgArb1 is involved in the regulation of protein biosynthesis through impeding nuclear export of small ribosomal subunit. Finally, acetylation modification at sites K28, K65, K341 and K525 in FgArb1 is required for its biological functions. Taken together, results of this study provide a novel insight into functions of the ABC transporter in fungal pathogenesis.

OsABCG9 Is an Important ABC Transporter of Cuticular Wax Deposition in Rice

The importance of the cuticular layer in regulating a plant's water status and providing protection from environmental challenges has been recognized for a long time. The cuticular layer in plants restricts non-stomatal water loss and protects plants against damage from pathogen infection and UV radiation. Much genetic and biochemical research has been done about cutin and wax transportation in Arabidopsis thaliana, but little is known about it in rice. Here, we report that a rice ATP-binding cassette (ABC) transporter, OsABCG9, is essential for normal development during vegetative growth and could play a critical role in the transportation of epicuticular wax in rice. Rice phenotypes with mutated OsABCG9 exhibited growth retardation and sensitivity to low humidity. The total amount of cuticular wax on the leaves of the osabcg9-1 mutant diminished by 53% compared with the wild type, and wax crystals disappeared completely in osabcg9-2 mutant leaves. However, OsABCG9 does not seem to be involved in cutin transportation, even though its ortholog in Arabidopsis, AtABCG11, transports both wax and cutin. Furthermore, the osabcg9-1 mutant had increased leaf chlorophyll leaching and more severe drought susceptibility. This study provides new insights about differences between rice and A. thaliana in wax and cutin transportation associated with the ABCG family during evolution.

Identification and Characterization of Compounds that Affect Stomatal Movements

Regulation of stomatal aperture is essential for plant growth and survival in response to environmental stimuli. Opening of stomata induces uptake of CO2 for photosynthesis and transpiration, which enhances uptake of nutrients from roots. Light is the most important stimulus for stomatal opening. Under drought stress, the plant hormone ABA induces stomatal closure to prevent water loss. However, the molecular mechanisms of stomatal movements are not fully understood. In this study, we screened chemical libraries to identify compounds that affect stomatal movements in Commelina benghalensis and characterize the underlying molecular mechanisms. We identified nine stomatal closing compounds (SCL1-SCL9) that suppress light-induced stomatal opening by >50%, and two compounds (temsirolimus and CP-100356) that induce stomatal opening in the dark. Further investigations revealed that SCL1 and SCL2 had no effect on autophosphorylation of phototropin or the activity of the inward-rectifying plasma membrane (PM) K+ channel, KAT1, but suppressed blue light-induced phosphorylation of the penultimate residue, threonine, in PM H+-ATPase, which is a key enzyme for stomatal opening. SCL1 and SCL2 had no effect on ABA-dependent responses, including seed germination and expression of ABA-induced genes. These results suggest that SCL1 and SCL2 suppress light-induced stomatal opening at least in part by inhibiting blue light-induced activation of PM H+-ATPase, but not by the ABA signaling pathway. Interestingly, spraying leaves onto dicot and monocot plants with SCL1 suppressed wilting of leaves, indicating that inhibition of stomatal opening by these compounds confers tolerance to drought stress in plants.

CH. QTL analysis reveals quantitative resistant loci for Phytophthora infestans and Tecia solanivora in tetraploid potato (Solanum tuberosum L.)

Late blight and Guatemalan potato tuber moth caused by Phytophthora infestans and Tecia solanivora, respectively, are major phytosanitary problems on potato crops in Colombia and Ecuador. Hence, the development of resistant cultivars is an alternative for their control. However, breeding initiatives for durable resistance using molecular tools are limited due to the genome complexity and high heterozygosity in autotetraploid potatoes. To contribute to a better understanding of the genetic basis underlying the resistance to P. infestans and T. solanivora in potato, the aim of this study was to identify QTLs for resistance to P. infestans and T. solanivora using a F1 tetraploid potato segregant population for both traits. Ninety-four individuals comprised this population. Parent genotypes and their progeny were genotyped using SOLCAP 12K potato array. Forty-five percent of the markers were polymorphic. A genetic linkage map was built with a length of 968.4 cM and 1,287 SNPs showing good distribution across the genome. Severity and incidence were evaluated in two crop cycles for two years. QTL analysis revealed six QTLs linked to P. infestans, four of these related to previous QTLs reported, and two novel QTLs (qrAUDPC-3 and qrAUDPC-8). Fifteen QTLs were linked to T. solanivora, being qIPC-6 and qOPA-6.1, and qIPC-10 and qIPC-10.1 stable in two different trials. This study is one of the first to identify QTLs for T. solanivora. As the population employed is a breeding population, results will contribute significantly to breeding programs to select resistant plant material, especially in countries where P. infestans and T. solanivora limit potato production.

The Vacuolar Transportome of Plant Specialized Metabolites

The plant vacuole is a cellular compartment that is essential to plant development and growth. Often plant vacuoles accumulate specialized metabolites, also called secondary metabolites, which constitute functionally and chemically diverse compounds that exert in planta many essential functions and improve the plant's fitness. These metabolites provide, for example, chemical defense against herbivorous and pathogens or chemical attractants (color and fragrance) to attract pollinators. The chemical composition of the vacuole is dynamic, and is altered during development and as a response to environmental changes. To some extent these alterations rely on vacuolar transporters, which import and export compounds into and out of the vacuole, respectively. During the past decade, significant progress was made in the identification and functional characterization of the transporters implicated in many aspects of plant specialized metabolism. Still, deciphering the molecular players underlying such processes remains a challenge for the future. In this review, we present a comprehensive summary of the most recent achievements in this field.

Comparison of Strategies to Overcome Drug Resistance: Learning from Various Kingdoms

Drug resistance, especially antibiotic resistance, is a growing threat to human health. To overcome this problem, it is significant to know precisely the mechanisms of drug resistance and/or self-resistance in various kingdoms, from bacteria through plants to animals, once more. This review compares the molecular mechanisms of the resistance against phycotoxins, toxins from marine and terrestrial animals, plants and fungi, and antibiotics. The results reveal that each kingdom possesses the characteristic features. The main mechanisms in each kingdom are transporters/efflux pumps in phycotoxins, mutation and modification of targets and sequestration in marine and terrestrial animal toxins, ABC transporters and sequestration in plant toxins, transporters in fungal toxins, and various or mixed mechanisms in antibiotics. Antibiotic producers in particular make tremendous efforts for avoiding suicide, and are more flexible and adaptable to the changes of environments. With these features in mind, potential alternative strategies to overcome these resistance problems are discussed. This paper will provide clues for solving the issues of drug resistance.

Transcriptome analysis of Petunia axillaris flowers reveals genes involved in morphological differentiation and metabolite transport

The biosynthesis of plant secondary metabolites is associated with morphological and metabolic differentiation. As a consequence, gene expression profiles can change drastically, and primary and secondary metabolites, including intermediate and end-products, move dynamically within and between cells. However, little is known about the molecular mechanisms underlying differentiation and transport mechanisms. In this study, we performed a transcriptome analysis of Petunia axillaris subsp. parodii, which produces various volatiles in its corolla limbs and emits metabolites to attract pollinators. RNA-sequencing from leaves, buds, and limbs identified 53,243 unigenes. Analysis of differentially expressed genes, combined with gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses, showed that many biological processes were highly enriched in limbs. These included catabolic processes and signaling pathways of hormones, such as gibberellins, and metabolic pathways, including phenylpropanoids and fatty acids. Moreover, we identified five transporter genes that showed high expression in limbs, and we performed spatiotemporal expression analyses and homology searches to infer their putative functions. Our systematic analysis provides comprehensive transcriptomic information regarding morphological differentiation and metabolite transport in the Petunia flower and lays the foundation for establishing the specific mechanisms that control secondary metabolite biosynthesis in plants.

Sensitivity of Yeast Mutants Deficient in Mitochondrial or Vacuolar ABC Transporters to Pathogenesis-Related Protein TcPR-10 of Theobroma cacao

Pathogenesis-related proteins (PRs) are induced in plants after infection by pathogens and/or abiotic stress. Among these proteins, the family 10 (PR-10) influences the biosynthesis of secondary metabolites and shows antimicrobial ribonuclease activity. TcPR-10p (Pathogenesis-related Protein 10 of Theobroma cacao) was isolated from resistant and susceptible Moniliophthora perniciosa cacao cultivars. Cell survival with Saccharomyces cerevisiae mutant lines deficient in ATP-binding cassette (ABC) transporter proteins indicated the influence on resistance to TcPR-10p. Proteins of the ABC transport type are considered important in the process of resistance to antimicrobials and toxins. Thus, the objective of this work was to observe the sensitivity of ABC transporter yeast mutants in the presence of the TcPR-10p. Chronic exposure of S. cerevisiae mitochondrial (BYatm1Δ and BYmdl1Δ) and vacuole (BYnft1Δ, BYvmr1Δ, BYybt1Δ, BYycf1Δ and BYbpt1Δ) ABC transporter mutants to TcPR-10p (3 μg/mL, 0, 6, 12 and 24 h) was performed. Two TcPR-10p sensitive strains (BYmdl1Δ and BYnft1Δ) were submitted to a fluorescence test with the fluorogenic dihydroethidium (DHE), to visualize the presence of oxidative stress in the cells. Oxidative stress-increased sensitivity was confirmed by flow cytometry indicating induced cell death either via apoptosis or necrosis. This yeast data combined with previous data of literature (of M. perniciosa sensitivity to TcPR-10p) show that increased sensitivity to TcPR-10p in these mutants could be due to the TcPR10p-generated higher levels of intracellular reactive oxygen species (ROS), leading to increased cell death either via necrosis or apoptosis.

Ectopic RING zinc finger gene from hot pepper induces totally different genes in lettuce and tobacco

Advances in molecular biology have improved crops through transferring genes from one organism to new hosts, and these efforts have raised concerns about potential unexpected outcomes. Here, we provide evidence that a gene with a specific function in one organism can yield completely different effects in a new host. CaRZFP1 is a C3HC4-type RING zinc finger protein gene previously isolated from a cDNA library for heat-stressed hot pepper. In our previous work investigating in vivo CaRZFP1 function, we transferred CaRZFP1 into tobacco; transgenic tobacco exhibited enhanced growth and tolerance to abiotic stresses. As further analysis of CaRZFP1 ectopic expression in a heterologous host plant, here we mobilized and constitutively overexpressed CaRZFP1 in lettuce. In contrast to tobacco, transgenic lettuce exhibited poorer growth and delayed flowering compared with vector-only controls. To identify genes that might be involved in this phenotypic effect, transcriptome analyses on transgenic plants of both species were performed, uncovering dozens of genes that reflect the different outcomes between tobacco and lettuce. These included protein kinase, transcriptional factor, transporter protein, hormone and metabolism-related genes, and some unannotated genes. The opposite effects of CaRZFP1 ectopic expression in lettuce and tobacco address concerns of unexpectedly different outcomes in different host species.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

Genetic analysis of the liverwort Marchantia polymorpha reveals that R2R3MYB activation of flavonoid production in response to abiotic stress is an ancient character in land plants

The flavonoid pathway is hypothesized to have evolved during land colonization by plants c. 450 Myr ago for protection against abiotic stresses. In angiosperms, R2R3MYB transcription factors are key for environmental regulation of flavonoid production. However, angiosperm R2R3MYB gene families are larger than those of basal plants, and it is not known whether the regulatory system is conserved across land plants. We examined whether R2R3MYBs regulate the flavonoid pathway in liverworts, one of the earliest diverging land plant lineages. We characterized MpMyb14 from the liverwort Marchantia polymorpha using genetic mutagenesis, transgenic overexpression, gene promoter analysis, and transcriptomic and chemical analysis. MpMyb14 is phylogenetically basal to characterized angiosperm R2R3MYB flavonoid regulators. Mpmyb14 knockout lines lost all red pigmentation from the flavonoid riccionidin A, whereas overexpression conferred production of large amounts of flavones and riccionidin A, activation of associated biosynthetic genes, and constitutive red pigmentation. MpMyb14 expression and flavonoid pigmentation were induced by light- and nutrient-deprivation stress in M. polymorpha as for anthocyanins in angiosperms. MpMyb14 regulates stress-induced flavonoid production in M. polymorpha, and is essential for red pigmentation. This suggests that R2R3MYB regulated flavonoid production is a conserved character across land plants which arose early during land colonization.

The genetics underlying natural variation of plant-plant interactions, a beloved but forgotten member of the family of biotic interactions

Despite the importance of plant-plant interactions on crop yield and plant community dynamics, our understanding of the genetic and molecular bases underlying natural variation of plant-plant interactions is largely limited in comparison with other types of biotic interactions. By listing 63 quantitative trait loci (QTL) mapping and global gene expression studies based on plants directly challenged by other plants, we explored whether the genetic architecture and the function of the candidate genes underlying natural plant-plant interactions depend on the type of interactions between two plants (competition versus commensalism versus reciprocal helping versus asymmetry). The 16 transcriptomic studies are unevenly distributed between competitive interactions (n = 12) and asymmetric interactions (n = 4, all focusing on response to parasitic plants). By contrast, 17 and 30 QTL studies were identified for competitive interactions and asymmetric interactions (either weed suppressive ability or response to parasitic plants), respectively. Surprisingly, no studies have been carried out on the identification of genetic and molecular bases underlying natural variation in positive interactions. The candidate genes underlying natural plant-plant interactions can be classified into seven categories of plant function that have been identified in artificial environments simulating plant-plant interactions either frequently (photosynthesis, hormones), only recently (cell wall modification and degradation, defense pathways against pathogens) or rarely (ABC transporters, histone modification and meristem identity/life history traits). Finally, we introduce several avenues that need to be explored in the future to obtain a thorough understanding of the genetic and molecular bases underlying plant-plant interactions within the context of realistic community complexity.

Functions of ABC transporters in plant growth and development

ABC transporters are essential for plant development, playing roles in processes such as gametogenesis, seed development, seed germination, organ formation, and secondary growth. ABC transporters are directly energized by ATP and can transport complex organic materials against concentration gradients; thus, they are uniquely suited to provide the complex building blocks required for the development of specialized plant cells. We review recent progress in our understanding of the contribution ABC transporters make to the growth and development of plants, including their roles in protective layer formation and in transporting phytohormones.

No Time to Waste: Transcriptome Study Reveals that Drought Tolerance in Barley May Be Attributed to Stressed-Like Expression Patterns that Exist before the Occurrence of Stress

Plant survival in adverse environmental conditions requires a substantial change in the metabolism, which is reflected by the extensive transcriptome rebuilding upon the occurrence of the stress. Therefore, transcriptomic studies offer an insight into the mechanisms of plant stress responses. Here, we present the results of global gene expression profiling of roots and leaves of two barley genotypes with contrasting ability to cope with drought stress. Our analysis suggests that drought tolerance results from a certain level of transcription of stress-influenced genes that is present even before the onset of drought. Genes that predispose the plant to better drought survival play a role in the regulatory network of gene expression, including several transcription factors, translation regulators and structural components of ribosomes. An important group of genes is involved in signaling mechanisms, with significant contribution of hormone signaling pathways and an interplay between ABA, auxin, ethylene and brassinosteroid homeostasis. Signal transduction in a drought tolerant genotype may be more efficient through the expression of genes required for environmental sensing that are active already during normal water availability and are related to actin filaments and LIM domain proteins, which may function as osmotic biosensors. Better survival of drought may also be attributed to more effective processes of energy generation and more efficient chloroplasts biogenesis. Interestingly, our data suggest that several genes involved in a photosynthesis process are required for the establishment of effective drought response not only in leaves, but also in roots of barley. Thus, we propose a hypothesis that root plastids may turn into the anti-oxidative centers protecting root macromolecules from oxidative damage during drought stress. Specific genes and their potential role in building up a drought-tolerant barley phenotype is extensively discussed with special emphasis on processes that take place in barley roots. When possible, the interconnections between particular factors are emphasized to draw a broader picture of the molecular mechanisms of drought tolerance in barley.

ABCE Proteins: From Molecules to Development

Most members of the large family of ATP-Binding Cassette (ABC) proteins function as membrane transporters. However, the most evolutionarily conserved group, the ABCE protein subfamily, comprises soluble proteins that were initially denoted RNase L inhibitor (RLI) proteins. ABCE proteins are present in all eukaryotes and archaea and are encoded by a single gene in most genomes, or by two genes in a few cases. Functional analysis of ABCE genes, primarily in Saccharomyces cerevisiae, has shown that ABCE proteins have essential functions as part of the translational apparatus. In this review, we summarize the current understanding of ABCE protein function in ribosome biogenesis and recycling, with a particular focus on their known and proposed developmental roles in different species. The ABCE proteins might represent another class of factors contributing to the role of the ribosome in gene expression regulation.

Changes in the allocation of endogenous strigolactone improve plant biomass production on phosphate-poor soils

Strigolactones (SLs) are carotenoid-derived phytohormones shaping plant architecture and inducing the symbiosis with endomycorrhizal fungi. In Petunia hybrida, SL transport within the plant and towards the rhizosphere is driven by the ABCG-class protein PDR1. PDR1 expression is regulated by phytohormones and by the soil phosphate abundance, and thus SL transport integrates plant development with nutrient conditions. We overexpressed PDR1 (PDR1 OE) to investigate whether increased endogenous SL transport is sufficient to improve plant nutrition and productivity. Phosphorus quantification and nondestructive X-ray computed tomography were applied. Morphological and gene expression changes were quantified at cellular and whole tissue levels via time-lapse microscopy and quantitative PCR. PDR1 OE significantly enhanced phosphate uptake and plant biomass production on phosphate-poor soils. PDR1 OE plants showed increased lateral root formation, extended root hair elongation, faster mycorrhization and reduced leaf senescence. PDR1 overexpression allowed considerable SL biosynthesis by releasing SL biosynthetic genes from an SL-dependent negative feedback. The increased endogenous SL transport/biosynthesis in PDR1 OE plants is a powerful tool to improve plant growth on phosphate-poor soils. We propose PDR1 as an as yet unexplored trait to be investigated for crop production. The overexpression of PDR1 is a valuable strategy to investigate SL functions and transport routes.

Feed Your Friends: Do Plant Exudates Shape the Root Microbiome?

Plant health in natural environments depends on interactions with complex and dynamic communities comprising macro- and microorganisms. While many studies have provided insights into the composition of rhizosphere microbiomes (rhizobiomes), little is known about whether plants shape their rhizobiomes. Here, we discuss physiological factors of plants that may govern plant-microbe interactions, focusing on root physiology and the role of root exudates. Given that only a few plant transport proteins are known to be involved in root metabolite export, we suggest novel families putatively involved in this process. Finally, building off of the features discussed in this review, and in analogy to well-known symbioses, we elaborate on a possible sequence of events governing rhizobiome assembly.

Plant membrane-protein mediated intracellular traffic of fatty acids and acyl lipids

In plants, de novo synthesis of fatty acids (FAs) occurs in plastids, whereas assembly and modification of acyl lipids is accomplished in the endoplasmic reticulum (ER) and plastids as well as in mitochondria. Subsequently, lipophilic compounds are distributed within the cell and delivered to their destination site. Thus, constant acyl-exchanges between subcellular compartments exist. These can occur via several modes of transport and plant membrane-intrinsic proteins for FA/lipid transfer have been shown to play an essential role in delivery and distribution. Lately, substantial progress has been made in identification and characterization of transport proteins for lipid compounds in plant organelle membranes. In this review, we focus on our current understanding of protein mediated lipid traffic between organelles of land plants.

Largely additive effects of gibberellin and strigolactone on gene expression in Arabidopsis thaliana seedlings

The phytohormones gibberellin (GA) and strigolactone (SL) are involved in essential processes in plant development. Both GA and SL signal transduction mechanisms employ α/β-hydrolase-derived receptors that confer E3 ubiquitin ligase-mediated protein degradation processes. This suggests a common evolutionary origin of these pathways and possibly a molecular interaction between them. One such indication stems from rice, where the DELLA protein of the GA pathway was reported to interact with the SL receptor. Here, we examine the physiological interaction between both pathways through the analysis of GA (ga1) and SL biosynthesis (max1 and max3) mutants. In ga1 max double mutants, we find indications only for additive interactions when examining several phenotypic readouts. We further identify short-term transcriptional responses to GA and the synthetic SL rac-GR24 through next-generation sequencing of poly-adenylated RNAs (RNA-seq) in ga1 max1. Remarkably, both hormones lead to predominantly additive transcriptional changes of a largely overlapping set of genes. The expression of only a few genes was altered in a synergistic manner but, interestingly, these include the genes encoding the GA catabolic enzyme GA2 OXIDASE2 (GA2ox2) as well as the SL pathway regulators BRANCHED1 (BRC1) and SUPPRESSOR OF max2 1-LIKE8 (SMXL8). We conclude that GA and rac-GR24 signaling in Arabidopsis seedlings converge at the level of transcription of a common gene-set.

Activity of the purified plant ABC transporter NtPDR1 is stimulated by diterpenes and sesquiterpenes involved in constitutive and induced defenses

Within the plant ATP-binding cassette transporter family, pleiotropic drug resistance (PDR) transporters play essential functions, such as in hormone transport or defense against biotic and abiotic stresses. NtPDR1 from Nicotiana tabacum has been shown to be involved in the constitutive defense against pathogens through the secretion of toxic cyclic diterpenes, such as the antimicrobial substrates cembrene and sclareol from the leaf hairs (trichomes). However, direct evidence of an interaction between NtPDR1 and terpenes is lacking. Here, we stably expressed NtPDR1 in N. tabacum BY-2 suspension cells. NtPDR1 was purified as an active monomer glycosylated at a single site in the third external loop. NtPDR1 reconstitution in proteoliposomes stimulated its basal ATPase activity from 21 to 38 nmol of Pi·mg-1·min-1, and ATPase activity was further stimulated by the NtPDR1 substrates cembrene and sclareol, providing direct evidence of an interaction between NtPDR1 and its two substrates. Interestingly, NtPDR1 was also stimulated by capsidiol, a sesquiterpene produced by N. tabacum upon pathogen attack. We also monitored the transcriptional activity from the NtPDR1 promoter in situ with a reporter gene and found that, although NtPDR1 expression was limited to trichomes under normal conditions, addition of methyl jasmonate, a biotic stress hormone, induced expression in all leaf tissues. This finding indicated that NtPDR1 is involved not only in constitutive but also in induced plant defenses. In conclusion, we provide direct evidence of an interaction between the NtPDR1 transporter and its substrates and that NtPDR1 transports compounds involved in both constitutive (diterpenes) and induced (sesquiterpenes) plant defenses.

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.

Bacterial production and reconstitution in proteoliposomes of Solanum lycopersicum CAT2: a transporter of basic amino acids and organic cations

The vacuolar SlCAT2 was cloned, over-produced in E. coli and reconstituted in proteoliposomes. Arg, Ornithine and Lys were identified as substrates. Unexpectedly, also the organic cations Tetraethylammonium and Acetylcholine were transported indicating involvement of SlCAT2 in signaling. In land plants several transporters are involved in ion and metabolite flux across membranes of cells or intracellular organelles. The vacuolar amino acid transporter CAT2 from Solanum lycopersicum was investigated in this work. SlCAT2 was cloned from tomato flower cDNA, over-produced in Escherichia coli and purified by Nichel-chelating chromatography. For functional studies, the transporter was reconstituted in proteoliposomes. Competence of SlCAT2 for Arg transport was demonstrated measuring uptake of [³H]Arg in proteoliposomes which was trans-stimulated by internal Arg or ornithine. Uptake of [³H]Ornithine and [³H]Lys was also detected at lower efficiency with respect to [³H]Arg. Transport was activated by the presence of intraliposomal ATP suggesting regulation by the nucleotide. The prototype for organic cations tetraethylammonium (TEA) was also transported by SlCAT2. However, scarce reciprocal inhibition between TEA and Arg was found, while the biguanide metformin was able to strongly inhibit uptake of both substrates. These findings suggest that amino acids and organic cations may interact with the transporter through different functional groups some of which are common for the two types of substrates. Interestingly, reconstituted SlCAT2 showed competence for acetylcholine transport, which was also inhibited by metformin. Kinetics of Arg and Ach transport were performed from which Km values of 0.29 and 0.79 mM were derived, respectively.

Lipid transfer from plants to arbuscular mycorrhiza fungi

Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi. However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that ¹³C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

Lipid transfer from plants to arbuscular mycorrhiza fungi

Arbuscular mycorrhiza (AM) symbioses contribute to global carbon cycles as plant hosts divert up to 20% of photosynthate to the obligate biotrophic fungi. Previous studies suggested carbohydrates as the only form of carbon transferred to the fungi. However, de novo fatty acid (FA) synthesis has not been observed in AM fungi in absence of the plant. In a forward genetic approach, we identified two Lotus japonicus mutants defective in AM-specific paralogs of lipid biosynthesis genes (KASI and GPAT6). These mutants perturb fungal development and accumulation of emblematic fungal 16:1ω5 FAs. Using isotopolog profiling we demonstrate that 13C patterns of fungal FAs recapitulate those of wild-type hosts, indicating cross-kingdom lipid transfer from plants to fungi. This transfer of labelled FAs was not observed for the AM-specific lipid biosynthesis mutants. Thus, growth and development of beneficial AM fungi is not only fueled by sugars but depends on lipid transfer from plant hosts.

Arabidopsis Transporter ABCG37/PDR9 contributes primarily highly oxygenated Coumarins to Root Exudation

The chemical composition of root exudates strongly impacts the interactions of plants with microorganisms in the rhizosphere and the efficiency of nutrient acquisition. Exudation of metabolites is in part mediated by ATP-binding cassette (ABC) transporters. In order to assess the contribution of individual ABC transporters to root exudation, we performed an LC-MS based non-targeted metabolite profiling of semi-polar metabolites accumulating in root exudates of Arabidopsis thaliana plants and mutants deficient in the expression of ABCG36 (PDR8/PEN3), ABCG37 (PDR9) or both transporters. Comparison of the metabolite profiles indicated distinct roles for each ABC transporter in root exudation. Thymidine exudation could be attributed to ABCG36 function, whereas coumarin exudation was strongly reduced only in ABCG37 deficient plants. However, coumarin exudation was compromised in abcg37 mutants only with respect to certain metabolites of this substance class. The specificity of ABCG37 for individual coumarins was further verified by a targeted LC-MS based coumarin profiling method. The response to iron deficiency, which is known to strongly induce coumarin exudation, was also investigated. In either treatment, the distribution of individual coumarins between roots and exudates in the investigated genotypes suggested the involvement of ABCG37 in the exudation specifically of highly oxygenated rather than monohydroxylated coumarins.

Arbuscular mycorrhiza-specific enzymes FatM and RAM2 fine-tune lipid biosynthesis to promote development of arbuscular mycorrhiza

During arbuscular mycorrhizal symbiosis (AMS), considerable amounts of lipids are generated, modified and moved within the cell to accommodate the fungus in the root, and it has also been suggested that lipids are delivered to the fungus. To determine the mechanisms by which root cells redirect lipid biosynthesis during AMS we analyzed the roles of two lipid biosynthetic enzymes (FatM and RAM2) and an ABC transporter (STR) that are required for symbiosis and conserved uniquely in plants that engage in AMS. Complementation analyses indicated that the biochemical function of FatM overlaps with that of other Fat thioesterases, in particular FatB. The essential role of FatM in AMS was a consequence of timing and magnitude of its expression. Lipid profiles of fatm and ram2 suggested that FatM increases the outflow of 16:0 fatty acids from the plastid, for subsequent use by RAM2 to produce 16:0 β-monoacylglycerol. Thus, during AMS, high-level, specific expression of key lipid biosynthetic enzymes located in the plastid and the endoplasmic reticulum enables the root cell to fine-tune lipid biosynthesis to increase the production of β-monoacylglycerols. We propose a model in which β-monoacylglycerols, or a derivative thereof, are exported out of the root cell across the periarbuscular membrane for ultimate use by the fungus.

Purification and biochemical characterization of NpABCG5/NpPDR5, a plant pleiotropic drug resistance transporter expressed in Nicotiana tabacum BY-2 suspension cells

Pleiotropic drug resistance (PDR) transporters belong to the ABCG subfamily of ATP-binding cassette (ABC) transporters and are involved in the transport of various molecules across plasma membranes. During evolution, PDR genes appeared independently in fungi and in plants from a duplication of a half-size ABC gene. The enzymatic properties of purified PDR transporters from yeast have been characterized. This is not the case for any plant PDR transporter, or, incidentally, for any purified plant ABC transporter. Yet, plant PDR transporters play important roles in plant physiology such as hormone signaling or resistance to pathogens or herbivores. Here, we describe the expression, purification, enzymatic characterization and 2D analysis by electron microscopy of NpABCG5/NpPDR5 from Nicotiana plumbaginifolia, which has been shown to be involved in the plant defense against herbivores. We constitutively expressed NpABCG5/NpPDR5, provided with a His-tag in a homologous system: suspension cells from Nicotiana tabacum (Bright Yellow 2 line). NpABCG5/NpPDR5 was targeted to the plasma membrane and was solubilized by dodecyl maltoside and purified by Ni-affinity chromatography. The ATP-hydrolyzing specific activity (27 nmol min-1 mg-1) was stimulated seven-fold in the presence of 0.1% asolectin. Electron microscopy analysis indicated that NpABCG5/NpPDR5 is monomeric and with dimensions shorter than those of known ABC transporters. Enzymatic data (optimal pH and sensitivity to inhibitors) confirmed that plant and fungal PDR transporters have different properties. These data also show that N. tabacum suspension cells are a convenient host for the purification and biochemical characterization of ABC transporters.