RNA-seq analysis of the effect of kanamycin and the ABC transporter AtWBC19 on Arabidopsis thaliana seedlings reveals changes in metal content

GmABCG5, an ATP-binding cassette G transporter gene, is involved in the iron deficiency response in soybean

Iron deficiency is a major nutritional problem causing iron deficiency chlorosis (IDC) and yield reduction in soybean, one of the most important crops. The ATP-binding cassette G subfamily plays a crucial role in substance transportation in plants. In this study, we cloned the GmABCG5 gene from soybean and verified its role in Fe homeostasis. Analysis showed that GmABCG5 belongs to the ABCG subfamily and is subcellularly localized at the cell membrane. From high to low, GmABCG5 expression was found in the stem, root, and leaf of young soybean seedlings, and the order of expression was flower, pod, seed stem, root, and leaf in mature soybean plants. The GUS assay and qRT-PCR results showed that the GmABCG5 expression was significantly induced by iron deficiency in the leaf. We obtained the GmABCG5 overexpressed and inhibitory expressed soybean hairy root complexes. Overexpression of GmABCG5 promoted, and inhibition of GmABCG5 retarded the growth of soybean hairy roots, independent of nutrient iron conditions, confirming the growth-promotion function of GmABCG5. Iron deficiency has a negative effect on the growth of soybean complexes, which was more obvious in the GmABCG5 inhibition complexes. The chlorophyll content was increased in the GmABCG5 overexpression complexes and decreased in the GmABCG5 inhibition complexes. Iron deficiency treatment widened the gap in the chlorophyll contents. FCR activity was induced by iron deficiency and showed an extraordinary increase in the GmABCG5 overexpression complexes, accompanied by the greatest Fe accumulation. Antioxidant capacity was enhanced when GmABCG5 was overexpressed and reduced when GmABCG5 was inhibited under iron deficiency. These results showed that the response mechanism to iron deficiency is more actively mobilized in GmABCG5 overexpression seedlings. Our results indicated that GmABCG5 could improve the plant's tolerance to iron deficiency, suggesting that GmABCG5 might have the function of Fe mobilization, redistribution, and/or secretion of Fe substances in plants. The findings provide new insights into the ABCG subfamily genes in the regulation of iron homeostasis in plants.

A dynamic compartment model for xylem loading and long-distance transport of iron explains the effect of kanamycin on metal uptake in Arabidopsis

Arabidopsis plants exposed to the antibiotic kanamycin (Kan) display altered metal homeostasis. Further, mutation of the WBC19 gene leads to increased sensitivity to kanamycin and changes in iron (Fe) and zinc (Zn) uptake. Here we propose a model that explain this surprising relationship between metal uptake and exposure to Kan. We first use knowledge about the metal uptake phenomenon to devise a transport and interaction diagram on which we base the construction of a dynamic compartment model. The model has three pathways for loading Fe and its chelators into the xylem. One pathway, involving an unknown transporter, loads Fe as a chelate with citrate (Ci) into the xylem. This transport step can be significantly inhibited by Kan. In parallel, FRD3 transports Ci into the xylem where it can chelate with free Fe. A third critical pathway involves WBC19, which transports metal-nicotianamine (NA), mainly as Fe-NA chelate, and possibly NA itself. To permit quantitative exploration and analysis, we use experimental time series data to parameterize this explanatory and predictive model. Its numerical analysis allows us to predict responses by a double mutant and explain the observed differences between data from wildtype, mutants and Kan inhibition experiments. Importantly, the model provides novel insights into metal homeostasis by permitting the reverse-engineering of mechanistic strategies with which the plant counteracts the effects of mutations and of the inhibition of iron transport by kanamycin.

ATP-Binding Cassette G Transporters and Their Multiple Roles Especially for Male Fertility in Arabidopsis, Rice and Maize

ATP-binding cassette subfamily G (ABCG) transporters are extensive in plants and play essential roles in various processes influencing plant fitness, but the research progress varies greatly among Arabidopsis, rice and maize. In this review, we present a consolidated nomenclature and characterization of the whole 51 ABCG transporters in maize, perform a phylogenetic analysis and classification of the ABCG subfamily members in maize, and summarize the latest research advances in ABCG transporters for these three plant species. ABCG transporters are involved in diverse processes in Arabidopsis and rice, such as anther and pollen development, vegetative and female organ development, abiotic and biotic stress response, and phytohormone transport, which provide useful clues for the functional investigation of ABCG transporters in maize. Finally, we discuss the current challenges and future perspectives for the identification and mechanism analysis of substrates for plant ABCG transporters. This review provides a basic framework for functional research and the potential application of ABCG transporters in multiple plants, including maize.

The ABC transporter G subfamily in Arabidopsis thaliana

ABC transporters are ubiquitously present in all kingdoms and mediate the transport of a large spectrum of structurally different compounds. Plants possess high numbers of ABC transporters in relation to other eukaryotes; the ABCG subfamily in particular is extensive. Earlier studies demonstrated that ABCG transporters are involved in important processes influencing plant fitness. This review summarizes the functions of ABCG transporters present in the model plant Arabidopsis thaliana. These transporters take part in diverse processes such as pathogen response, diffusion barrier formation, or phytohormone transport. Studies involving knockout mutations reported pleiotropic phenotypes of the mutants. In some cases, different physiological roles were assigned to the same protein. The actual transported substrate(s), however, still remain to be determined for the majority of ABCG transporters. Additionally, the proposed substrate spectrum of different ABCG proteins is not always reflected by sequence identities between ABCG members. Applying only reverse genetics is thereby insufficient to clearly identify the substrate(s). We therefore stress the importance of in vitro studies in addition to in vivo studies in order to (i) clarify the substrate identity; (ii) determine the transport characteristics including directionality; and (iii) identify dimerization partners of the half-size proteins, which might in turn affect substrate specificity.

Biotechnological strategies for enhancing heavy metal tolerance in neglected and underutilized legume crops: A comprehensive review

Contamination of agricultural land and water by heavy metals due to rapid industrialization and urbanization including various natural processes have become one of the major constraints to crop growth and productivity. Several studies have reported that to counteract heavy metal stress, plants should be able to maneuver various physiological, biochemical and molecular processes to improve their growth and development under heavy metal stress. With the advent of modern biotechnological tools and techniques it is now possible to tailor legume and other plants overexpressing stress-induced genes, transcription factors, proteins, and metabolites that are directly involved in heavy metal stress tolerance. This review provides an in-depth overview of various biotechnological approaches and/or strategies that can be used for enhancing detoxification of the heavy metals by stimulating phytoremediation processes. Synthetic biology tools involved in the engineering of legume and other crop plants against heavy metal stress tolerance are also discussed herewith some pioneering examples where synthetic biology tools that have been used to modify plants for specific traits. Also, CRISPR based genetic engineering of plants, including their role in modulating the expression of several genes/ transcription factors in the improvement of abiotic stress tolerance and phytoremediation ability using knockdown and knockout strategies has also been critically discussed.

ABCG1 contributes to suberin formation in Arabidopsis thaliana roots

Diffusion barriers enable plant survival under fluctuating environmental conditions. They control internal water potential and protect against biotic or abiotic stress factors. How these protective molecules are deposited to the extracellular environment is poorly understood. We here examined the role of the Arabidopsis ABC half-size transporter AtABCG1 in the formation of the extracellular root suberin layer. Quantitative analysis of extracellular long-chain fatty acids and aliphatic alcohols in the atabcg1 mutants demonstrated altered root suberin composition, specifically a reduction in longer chain dicarboxylic acids, fatty alcohols and acids. Accordingly, the ATP-hydrolyzing activity of heterologous expressed and purified AtABCG1 was strongly stimulated by fatty alcohols (C₂₆-C₃₀) and fatty acids (C₂₄-C₃₀) in a chain length dependent manner. These results are a first indication for the function of AtABCG1 in the transport of longer chain aliphatic monomers from the cytoplasm to the apoplastic space during root suberin formation.

ABCG1 contributes to suberin formation in Arabidopsis thaliana roots

Diffusion barriers enable plant survival under fluctuating environmental conditions. They control internal water potential and protect against biotic or abiotic stress factors. How these protective molecules are deposited to the extracellular environment is poorly understood. We here examined the role of the Arabidopsis ABC half-size transporter AtABCG1 in the formation of the extracellular root suberin layer. Quantitative analysis of extracellular long-chain fatty acids and aliphatic alcohols in the atabcg1 mutants demonstrated altered root suberin composition, specifically a reduction in longer chain dicarboxylic acids, fatty alcohols and acids. Accordingly, the ATP-hydrolyzing activity of heterologous expressed and purified AtABCG1 was strongly stimulated by fatty alcohols (C₂₆–C₃₀) and fatty acids (C₂₄–C₃₀) in a chain length dependent manner. These results are a first indication for the function of AtABCG1 in the transport of longer chain aliphatic monomers from the cytoplasm to the apoplastic space during root suberin formation.

Towards Identification of the Substrates of ATP-Binding Cassette Transporters

Most ATP-binding cassette (ABC) proteins function in transmembrane transport, and plant genomes encode a large number of ABC transporters compared with animal or fungal genomes. These transporters have been classified into eight subfamilies according to their topology and phylogenetic relationships. Transgenic plants and mutants with altered ABC transporter expression or function have contributed to deciphering the physiological roles of these proteins, such as in plant development, responses to biotic and abiotic stress, or detoxification activities within the cell. In agreement with the diversity of these functions, a large range of substrates (e.g. hormones and primary and secondary metabolites) have been identified. We review in detail transporters for which substrates have been unambiguously identified. However, some cases are far from clear, because some ABC transporters have the ability to transport several structurally unrelated substrates or because the identification of their substrates was performed indirectly without any flux measurement. Various heterologous or homologous expression systems have been used to better characterize the transport activity and other biochemical properties of ABC transporters, opening the way to addressing new issues such as the particular structural features of plant ABC transporters, the bidirectionality of transport, or the role of posttranslational modifications.

Physical contact and carbon transfer between a lichen-forming Trebouxia alga and a novel Alphaproteobacterium

Recent progress in molecular techniques has begun to alter traditional recognition of lichens as symbiotic organisms comprised of a fungus and photosynthetic partners (green algae and/or cyanobacteria). Diverse organisms, especially various non-photosynthetic bacteria, are now indicated to be integral components of lichen symbiosis. Although lichen-associated bacteria are inferred to have functions that could support the symbiosis, little is known about their physical and nutritional interaction with fungi and algae. In the present study, we identified specific interaction between a lichen-forming alga and a novel bacterium. Trebouxia alga was isolated from a lichen, Usnea hakonensis, and kept as a strain for 8 years. Although no visible bacterial colonies were observed in this culture, high-throughput sequencing of DNA isolated from the culture revealed that the strain is composed of a Trebouxia alga and an Alphaproteobacterium species. In situ hybridization showed that bacterial cells were localized on the surface of the algal cells. Physiological assays revealed that the bacterium was able to use ribitol, glucose and mannitol, all of which are known to exist abundantly in lichens. It was resistant to three antibiotics. Bacteria closely related to this species were also identified in lichen specimens, indicating that U. hakonensis may commonly associate with this group of bacteria. These features of the novel bacterium suggest that it may be involved in carbon cycling of U. hakonensis as a member of lichen symbiosis and less likely to have become associated with the alga after isolation from a lichen.