OsPSTOL but not TaPSTOL can play a role in nutrient use efficiency and works through conserved pathways in both wheat and rice

There is a large demand to reduce inputs for current crop production, particularly phosphate and nitrogen inputs which are the two most frequently added supplements to agricultural production. Gene characterization is often limited to the native species from which it was identified, but may offer benefits to other species. To understand if the rice gene Phosphate Starvation Tolerance 1 (PSTOL) OsPSTOL, a gene identified from rice which improves tolerance to low P growth conditions, might improve performance and provide the same benefit in wheat, OsPSTOL was transformed into wheat and expressed from a constitutive promoter. The ability of OsPSTOL to improve nutrient acquisition under low phosphate or low nitrogen was evaluated. Here we show that OsPSTOL works through a conserved pathway in wheat and rice to improve yields under both low phosphate and low nitrogen. This increase is yield is mainly driven by improved uptake from the soil driving increased biomass and ultimately increased seed number, but does not change the concentration of N in the straw or grain. Overexpression of OsPSTOL in wheat modifies N regulated genes to aid in this uptake whereas the putative homolog TaPSTOL does not suggesting that expression of OsPSTOL in wheat can help to improve yields under low input agriculture.

Nutritional and genetic variation in a core set of Ethiopian Tef (Eragrostis tef) varieties

BACKGROUND

Tef (Eragrostis tef) is a tropical cereal domesticated and grown in the Ethiopian highlands, where it has been a staple food of Ethiopians for many centuries. Food insecurity and nutrient deficiencies are major problems in the country, so breeding for enhanced nutritional traits, such as Zn content, could help to alleviate problems with malnutrition.

RESULTS

To understand the breeding potential of nutritional traits in tef a core set of 24 varieties were sequenced and their mineral content, levels of phytate and protein, as well as a number of nutritionally valuable phenolic compounds measured in grain. Significant variation in all these traits was found between varieties. Genome wide sequencing of the 24 tef varieties revealed 3,193,582 unique SNPs and 897,272 unique INDELs relative to the tef reference var. Dabbi. Sequence analysis of two key transporter families involved in the uptake and transport of Zn by the plant led to the identification of 32 Zinc Iron Permease (ZIP) transporters and 14 Heavy Metal Associated (HMA) transporters in tef. Further analysis identified numerous variants, of which 14.6% of EtZIP and 12.4% of EtHMA variants were non-synonymous changes. Analysis of a key enzyme in flavanol synthesis, flavonoid 3'-hydroxylase (F3'H), identified a T-G variant in the tef homologue Et_s3159-0.29-1.mrna1 that was associated with the differences observed in kaempferol glycoside and quercetin glycoside levels.

CONCLUSION

Wide genetic and phenotypic variation was found in 24 Ethiopian tef varieties which would allow for breeding gains in many nutritional traits of importance to human health.

Ectopic expression of TaBG1 increases seed size and alters nutritional characteristics of the grain in wheat but does not lead to increased yields

BACKGROUND

Grain size is thought to be a major component of yield in many plant species. Here we set out to understand if knowledge from other cereals such as rice could translate to increased yield gains in wheat and lead to increased nitrogen use efficiency. Previous findings that the overexpression of OsBG1 in rice increased yields while increasing seed size suggest translating gains from rice to other cereals may help to increase yields.

RESULTS

The orthologous genes of OsBG1 were identified in wheat. One homoeologous wheat gene was cloned and overexpressed in wheat to understand its role in controlling seed size. Potential alteration in the nutritional profile of the grains were also analyzed in wheat overexpressing TaBG1. It was found that increased TaBG1-A expression could indeed lead to larger seed size but was linked to a reduction in seed number per plant leading to no significant overall increase in yield. Other important components of yield such as biomass or tillering did not change significantly with increased TaBG1-A expression. The nutritional profile of the grain was altered, with a significant decrease in the Zn levels in the grain associated with increased seed size, but Fe and Mn concentrations were unchanged. Protein content of the wheat grain also fell under moderate N fertilization levels but not under deficient or adequate levels of N.

CONCLUSIONS

TaBG1 does control seed size in wheat but increasing the seed size per se does not increase yield and may come at the cost of lower concentrations of essential elements as well as potentially lower protein content. Nevertheless, TaBG1 could be a useful target for further breeding efforts in combination with other genes for increased biomass.

Genetic control of iron bioavailability is independent from iron concentration in a diverse winter wheat mapping population

BACKGROUND

Anemia is thought to affect up to 1.6 billion people worldwide. One of the major contributors to low iron (Fe) absorption is a higher proportion of cereals compared to meats and pulse crops in people's diets. This has now become a problem in both the developed and developing world, as a result of both modern food choice and food availability. Bread wheat accounts for 20 % of the calories consumed by humans and is an important source of protein, vitamins and minerals meaning it could be a major vehicle for bringing more bioavailable Fe into the diet.

RESULTS

To investigate whether breeding for higher concentrations of Fe in wheat grains could help increase Fe absorption, a multiparent advanced generation intercross (MAGIC) population, encompassing more than 80 % of UK wheat polymorphism, was grown over two seasons in the UK. The population was phenotyped for both Fe concentration and Fe bioavailability using an established Caco-2 cell bioassay. It was found that increasing Fe concentrations in the grains was not correlated with higher Fe bioavailability and that the underlying genetic regions controlling grain Fe concentrations do not co-localise with increased Fe absorption. Furthermore, we show that phytate concentrations do not correlate with Fe bioavailability in our wheat population and thus phytate-binding is insufficient to explain the lack of correlation between Fe bioavailability and Fe concentrations in the wheat grain. Finally, we observed no (Fe bioavailability) or low (Fe concentration) correlation between years for these traits, confirming that both are under strong environmental influence.

CONCLUSIONS

This suggests that breeders will have to select not only for Fe concentrations directly in grains, but also increased bioavailability. However the use of numerous controls and replicated trials limits the practicality of adoption of screening by Caco-2 cells by many breeders.

Overcoming the trade-off between grain weight and number in wheat by the ectopic expression of expansin in developing seeds leads to increased yield potential

Wheat is the most widely grown crop globally, providing 20% of all human calories and protein. Achieving step changes in genetic yield potential is crucial to ensure food security, but efforts are thwarted by an apparent trade-off between grain size and number. Expansins are proteins that play important roles in plant growth by enhancing stress relaxation in the cell wall, which constrains cell expansion. Here, we describe how targeted overexpression of an α-expansin in early developing wheat seeds leads to a significant increase in grain size without a negative effect on grain number, resulting in a yield boost under field conditions. The best-performing transgenic line yielded 12.3% higher average grain weight than the control, and this translated to an increase in grain yield of 11.3% in field experiments using an agronomically appropriate plant density. This targeted transgenic approach provides an opportunity to overcome a common bottleneck to yield improvement across many crops.

Turning Up the Temperature on CRISPR: Increased Temperature Can Improve the Editing Efficiency of Wheat Using CRISPR/Cas9

The application of CRISPR/Cas9 technologies has transformed our ability to target and edit designated regions of a genome. It's broad adaptability to any organism has led to countless advancements in our understanding of many biological processes. Many current tools are designed for simple plant systems such as diploid species, however, efficient deployment in crop species requires a greater efficiency of editing as these often contain polyploid genomes. Here, we examined the role of temperature to understand if CRISPR/Cas9 editing efficiency can be improved in wheat. The recent finding that plant growth under higher temperatures could increase mutation rates was tested with Cas9 expressed from two different promoters in wheat. Increasing the temperature of the tissue culture or of the seed germination and early growth phase increases the frequency of mutation in wheat when the Cas9 enzyme is driven by the ZmUbi promoter but not OsActin. In contrast, Cas9 expression driven by the OsActin promoter did not increase the mutations detected in either transformed lines or during the transformation process itself. These results demonstrate that CRISPR/Cas9 editing efficiency can be significantly increased in a polyploid cereal species with a simple change in growth conditions to facilitate increased mutations for the creation of homozygous or null knock-outs.

Identification of genes involved in male sterility in wheat (Triticum aestivum L.) which could be used in a genic hybrid breeding system

Wheat is grown on more land than any other crop in the world. Current estimates suggest that yields will have to increase sixty percent by 2050 to meet the demand of an ever-increasing human population; however, recent wheat yield gains have lagged behind other major crops such as rice and maize. One of the reasons suggested for the lag in yield potential is the lack of a robust hybrid system to harness the potential yield gains associated with heterosis, also known as hybrid vigor. Here, we set out to identify candidate genes for a genic hybrid system in wheat and characterize their function in wheat using RNASeq on stamens and carpels undergoing meiosis. Twelve genes were identified as potentially playing a role in pollen viability. CalS5- and RPG1-like genes were identified as pre- and post-meiotic genes for further characterization and to determine their role in pollen viability. It appears that all three homoeologues of both CalS5 and RPG1 are functional in wheat as all three homoeologues need to be knocked out in order to cause male sterility. However, one functional homoeologue is sufficient to maintain male fertility in wheat.

Long Non-Coding RNAs as Endogenous Target Mimics and Exploration of Their Role in Low Nutrient Stress Tolerance in Plants

Long non-coding RNA (lncRNA) research in plants has recently gained momentum taking cues from studies in animals systems. The availability of next-generation sequencing has enabled genome-wide identification of lncRNA in several plant species. Some lncRNAs are inhibitors of microRNA expression and have a function known as target mimicry with the sequestered transcript known as an endogenous target mimic (eTM). The lncRNAs identified to date show diverse mechanisms of gene regulation, most of which remain poorly understood. In this review, we discuss the role of identified putative lncRNAs that may act as eTMs for nutrient-responsive microRNAs (miRNAs) in plants. If functionally validated, these putative lncRNAs would enhance current understanding of the role of lncRNAs in nutrient homeostasis in plants.

A PSTOL-like gene, TaPSTOL, controls a number of agronomically important traits in wheat

BACKGROUND

Phosphorus (P) is an essential macronutrient for plant growth, and is required in large quantities by elite varieties of crops to maintain yields. Approximately 70% of global cultivated land suffers from P deficiency, and it has recently been estimated that worldwide P resources will be exhausted by the end of this century, increasing the demand for crops more efficient in their P usage. A greater understanding of how plants are able to maintain yield with lower P inputs is, therefore, highly desirable to both breeders and farmers. Here, we clone the wheat (Triticum aestivum L.) homologue of the rice PSTOL gene (OsPSTOL), and characterize its role in phosphate nutrition plus other agronomically important traits.

RESULTS

TaPSTOL is a single copy gene located on the short arm of chromosome 5A, encoding a putative kinase protein, and shares a high level of sequence similarity to OsPSTOL. We re-sequenced TaPSTOL from 24 different wheat accessions and (3) three T. durum varieties. No sequence differences were detected in 26 of the accessions, whereas two indels were identified in the promoter region of one of the durum wheats. We characterised the expression of TaPSTOL under different P concentrations and demonstrated that the promoter was induced in root tips and hairs under P limiting conditions. Overexpression and RNAi silencing of TaPSTOL in transgenic wheat lines showed that there was a significant effect upon root biomass, flowering time independent of P treatment, tiller number and seed yield, correlating with the expression of TaPSTOL. However this did not increase PUE as elevated P concentration in the grain did not correspond to increased yields.

CONCLUSIONS

Manipulation of TaPSTOL expression in wheat shows it is responsible for many of the previously described phenotypic advantages as OsPSTOL except yield. Furthermore, we show TaPSTOL contributes to additional agronomically important traits including flowering time and grain size. Analysis of TaPSTOL sequences from a broad selection of wheat varieties, encompassing 91% of the genetic diversity in UK bread wheat, showed that there is very little genetic variation in this gene, which would suggest that this locus may have been under high selection pressure.

Evidence for a Double Well in the First Triplet Excited State of 2-Thiouracil

The computationally predicted presence of two structurally distinct minima in the first triplet excited (T₁) state of 2-thiouracil (2TU) is substantiated by sub-picosecond transient vibrational absorption spectroscopy (TVAS) in deuterated acetonitrile solution. Following 300 nm ultraviolet excitation to the second singlet excited state of 2TU, a transient infrared absorption band centered at 1643 cm^-1 is observed within our minimum time resolution of 0.3 ps. It is assigned either to 2TU molecules in the S₁ state or to vibrationally hot T₁-state molecules, with the latter assignment more consistent with recent computational and experimental studies. The 1643 cm^-1 band decays with a time constant of 7.2 ± 0.8 ps, and there is corresponding growth of several further bands centered at 1234, 1410, 1424, 1443, 1511, 1626, and 1660 cm^-1 which show no decline in intensity over the 1 ns time limit of our measurements. These spectral features are assigned to two different conformations of 2TU, corresponding to separate energy minima on the T₁-state potential energy surface, on the basis of their extended lifetimes, computed infrared frequencies, and the observed quenching of the bands by addition of styrene. Corresponding measurements for the 4-thiouracil (4TU) isomer show sub-picosecond population of the T₁ state, which vibrationally cools with a time constant of 5.2 ± 0.6 ps. However, TVAS measurements in the carbonyl stretching region do not distinguish the two computed T₁-state conformers of 4TU because of the similarity of their vibrational frequencies.

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by N,N'-ethylenebis(salicylimine)(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes 3 and (±)-4 and the aluminum analogues 5 and (±)-6. The alkoxygallium complex (±)-2, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl₃/BnOH/NEt₃, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-2 with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann-Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity.

Extra-Large G Proteins Expand the Repertoire of Subunits in Arabidopsis Heterotrimeric G Protein Signaling

Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, are a conserved signal transduction mechanism in eukaryotes. However, G protein subunit numbers in diploid plant genomes are greatly reduced as compared with animals and do not correlate with the diversity of functions and phenotypes in which heterotrimeric G proteins have been implicated. In addition to GPA1, the sole canonical Arabidopsis (Arabidopsis thaliana) Gα subunit, Arabidopsis has three related proteins: the extra-large GTP-binding proteins XLG1, XLG2, and XLG3. We demonstrate that the XLGs can bind Gβγ dimers (AGB1 plus a Gγ subunit: AGG1, AGG2, or AGG3) with differing specificity in yeast (Saccharomyces cerevisiae) three-hybrid assays. Our in silico structural analysis shows that XLG3 aligns closely to the crystal structure of GPA1, and XLG3 also competes with GPA1 for Gβγ binding in yeast. We observed interaction of the XLGs with all three Gβγ dimers at the plasma membrane in planta by bimolecular fluorescence complementation. Bioinformatic and localization studies identified and confirmed nuclear localization signals in XLG2 and XLG3 and a nuclear export signal in XLG3, which may facilitate intracellular shuttling. We found that tunicamycin, salt, and glucose hypersensitivity and increased stomatal density are agb1-specific phenotypes that are not observed in gpa1 mutants but are recapitulated in xlg mutants. Thus, XLG-Gβγ heterotrimers provide additional signaling modalities for tuning plant G protein responses and increase the repertoire of G protein heterotrimer combinations from three to 12. The potential for signal partitioning and competition between the XLGs and GPA1 is a new paradigm for plant-specific cell signaling.

Root and shoot transcriptome analysis of two ecotypes of Noccaea caerulescens uncovers the role of NcNramp1 in Cd hyperaccumulation

The Zn/Cd hyperaccumulator, Noccaea caerulescens, has been studied extensively for its ability to accumulate high levels of Zn and Cd in its leaves. Previous studies have indicated that the Zn and Cd hyperaccumulation trait exhibited by this species involves different transport and tolerance mechanisms. It has also been well documented that certain ecotypes of N. caerulescens are much better Cd hyperaccumulators than others. However, there does not seem to be much ecotypic variation for Zn hyperaccumulation in N. caerulescens. In this study we employed a comparative transcriptomics approach to look at root and shoot gene expression in Ganges and Prayon plants in response to Cd stress to identify transporter genes that were more highly expressed in either the roots or shoots of the superior Cd accumulator, Ganges. Comparison of the transcriptomes from the two ecotypes of Noccaea caerulescens identified a number of genes that encoded metal transporters that were more highly expressed in the Ganges ecotype in response to Cd stress. Characterization of one of these transporters, NcNramp1, showed that it is involved in the influx of Cd across the endodermal plasma membrane and thus may play a key role in Cd flux into the stele and root-to-shoot Cd transport. NcNramp1 may be one of the main transporters involved in Cd hyperaccumulation in N. caerulescens and copy number variation appears to be the main reason for high NcNramp1 gene expression underlying the increased Cd accumulation in the Ganges ecotype.

Identification of a novel pathway involving a GATA transcription factor in yeast and possibly in plant Zn uptake and homeostasis

To gain a better understanding of the regulation of Zn homeostasis in plants and the degree of conservation of Zn homeostasis between plants and yeast, a cDNA library from the Zn/Cd hyperaccumulating plant species, Noccaea caerulescens, was screened for its ability to restore growth under Zn limiting conditions in the yeast mutant zap1Δ. ZAP1 is a transcription factor that activates the Zn dependent transcription of yeast genes involved in Zn uptake, including ZRT1, the yeast high affinity Zn transporter. From this screen two members of the E2F family of transcription factors were found to activate ZRT1 expression in a Zn independent manner. The activation of ZRT1 by the plant E2F proteins involves E2F-mediated activation of a yeast GATA transcription factor which in turn activates ZRT1 expression. A ZRT1 promoter region necessary for activation by E2F and GATA proteins is upstream of two zinc responsive elements previously shown to bind ZAP1 in ZRT1. This activation may not involve direct binding of E2F to the ZRT1 promoter. The expression of E2F genes in yeast does not replace function of ZAP1; instead it appears to activate a novel GATA regulatory pathway involved in Zn uptake and homeostasis that is not Zn responsive.

Transport properties of members of the ZIP family in plants and their role in Zn and Mn homeostasis

A better understanding of the role of the Arabidopsis ZIP family of micronutrient transporters is necessary in order to advance our understanding of plant Zn, Fe, Mn, and Cu homeostasis. In the current study, the 11 Arabidopsis ZIP family members not yet well characterized were first screened for their ability to complement four yeast mutants defective in Zn, Fe, Mn, or Cu uptake. Six of the Arabidopsis ZIP genes complemented a yeast Zn uptake-deficient mutant, one was able partially to complement a yeast Fe uptake-deficient mutant, six ZIP family members complemented an Mn uptake-deficient mutant, and none complemented the Cu uptake-deficient mutant. AtZIP1 and AtZIP2 were then chosen for further study, as the preliminary yeast and in planta analysis suggested they both may be root Zn and Mn transporters. In yeast, AtZIP1 and AtZIP2 both complemented the Zn and Mn uptake mutants, suggesting that they both may transport Zn and/or Mn. Expression of both genes is localized to the root stele, and AtZIP1 expression was also found in the leaf vasculature. It was also found that AtZIP1 is a vacuolar transporter, while AtZIP2 is localized to the plasma membrane. Functional studies with Arabidopsis AtZIP1 and AtZIP2 T-DNA knockout lines suggest that both transporters play a role in Mn (and possibly Zn) translocation from the root to the shoot. AtZIP1 may play a role in remobilizing Mn from the vacuole to the cytoplasm in root stellar cells, and may contribute to radial movement to the xylem parenchyma. AtZIP2, on the other hand, may mediate Mn (and possibly Zn) uptake into root stellar cells, and thus also may contribute to Mn/Zn movement in the stele to the xylem parenchyma, for subsequent xylem loading and transport to the shoot.

Characterization of the high affinity Zn transporter from Noccaea caerulescens, NcZNT1, and dissection of its promoter for its role in Zn uptake and hyperaccumulation

• In this paper, we conducted a detailed analysis of the ZIP family transporter, NcZNT1, in the zinc (Zn)/cadmium (Cd) hyperaccumulating plant species, Noccaea caerulescens, formerly known as Thlaspi caerulescens. NcZNT1 was previously suggested to be the primary root Zn/Cd uptake transporter. Both a characterization of NcZNT1 transport function in planta and in heterologous systems, and an analysis of NcZNT1 gene expression and NcZNT1 protein localization were carried out. • We show that NcZNT1 is not only expressed in the root epidermis, but also is highly expressed in the root and shoot vasculature, suggesting a role in long-distance metal transport. Also, NcZNT1 was found to be a plasma membrane transporter that mediates Zn but not Cd, iron (Fe), manganese (Mn) or copper (Cu) uptake into plant cells. • Two novel regions of the NcZNT1 promoter were identified which may be involved in both the hyperexpression of NcZNT1 and its ability to be regulated by plant Zn status. • In conclusion, we demonstrate here that NcZNT1 plays a role in Zn and not Cd uptake from the soil, and based on its strong expression in the root and shoot vasculature, could be involved in long-distance transport of Zn from the root to the shoot via the xylem.

Elevated expression of TcHMA3 plays a key role in the extreme Cd tolerance in a Cd-hyperaccumulating ecotype of Thlaspi caerulescens

Cadmium (Cd) is a highly toxic heavy metal for plants, but several unique Cd-hyperaccumulating plant species are able to accumulate this metal to extraordinary concentrations in the aboveground tissues without showing any toxic symptoms. However, the molecular mechanisms underlying this hypertolerance to Cd are poorly understood. Here we have isolated and functionally characterized an allelic gene, TcHMA3 (heavy metal ATPase 3) from two ecotypes (Ganges and Prayon) of Thlaspi caerulescens contrasting in Cd accumulation and tolerance. The TcHMA3 alleles from the higher (Ganges) and lower Cd-accumulating ecotype (Prayon) share 97.8% identity, and encode a P(1B)-type ATPase. There were no differences in the expression pattern, cell-specificity of protein localization and transport substrate-specificity of TcHMA3 between the two ecotypes. Both alleles were characterized by constitutive expression in the shoot and root, a tonoplast localization of the protein in all leaf cells and specific transport activity for Cd. The only difference between the two ecotypes was the expression level of TcHMA3: Ganges showed a sevenfold higher expression than Prayon, partly caused by a higher copy number. Furthermore, the expression level and localization of TcHMA3 were different from AtHMA3 expression in Arabidopsis. Overexpression of TcHMA3 in Arabidopsis significantly enhanced tolerance to Cd and slightly increased tolerance to Zn, but did not change Co or Pb tolerance. These results indicate that TcHMA3 is a tonoplast-localized transporter highly specific for Cd, which is responsible for sequestration of Cd into the leaf vacuoles, and that a higher expression of this gene is required for Cd hypertolerance in the Cd-hyperaccumulating ecotype of T. caerulescens.

The final stages of wing development in Drosophila melanogaster

A brief description is given of the behavioural changes in Drosophila melanogaster at the time of eclosion and wing expansion. The developmental changes in the wing during this period are classified into four stages (0-III). Details of changes in the morphology of both the cuticle and the wing epidermal cells are given using both light and electron microscopy. It was found that the breakdown of the wing epidermis is initiated before wing expansion starts. The results obtained for Drosophila are compared with those for other insect species.

Investigation of heavy metal hyperaccumulation at the cellular level: development and characterization of Thlaspi caerulescens suspension cell lines

The ability of Thlaspi caerulescens, a zinc (Zn)/cadmium (Cd) hyperaccumulator, to accumulate extremely high foliar concentrations of toxic heavy metals requires coordination of uptake, transport, and sequestration to avoid damage to the photosynthetic machinery. The study of these metal hyperaccumulation processes at the cellular level in T. caerulescens has been hampered by the lack of a cellular system that mimics the whole plant, is easily transformable, and competent for longer term studies. Therefore, to better understand the contribution of the cellular physiology and molecular biology to Zn/Cd hyperaccumulation in the intact plant, T. caerulescens suspension cell lines were developed. Differences in cellular metal tolerance and accumulation between the cell lines of T. caerulescens and the related nonhyperaccumulator, Arabidopsis (Arabidopsis thaliana), were examined. A number of Zn/Cd transport-related differences between T. caerulescens and Arabidopsis cell lines were identified that also are seen in the whole plant. T. caerulescens suspension cell lines exhibited: (1) higher growth requirements for Zn; (2) much greater Zn and Cd tolerance; (3) enhanced expression of specific metal transport-related genes; and (4) significant differences in metal fluxes compared with Arabidopsis. One interesting feature exhibited by the T. caerulescens cell lines was that they accumulated less Zn and Cd than the Arabidopsis cell lines, most likely due to a greater metal efflux. This finding suggests that the T. caerulescens suspension cells represent cells of the Zn/Cd transport pathway between the root epidermis and leaf. We also show it is possible to stably transform T. caerulescens suspension cells, which will allow us to alter the expression of candidate hyperaccumulation genes and thus dissect the molecular and physiological processes underlying metal hyperaccumulation in T. caerulescens.

Investigating heavy-metal hyperaccumulation using Thlaspi caerulescens as a model system

BACKGROUND

Metal-hyperaccumulating plant species are plants that are endemic to metalliferous soils and are able to tolerate and accumulate metals in their above-ground tissues to very high concentrations. One such hyperaccumulator, Thlaspi caerulescens, has been widely studied for its remarkable properties to tolerate toxic levels of zinc (Zn), cadmium (Cd) and sometimes nickel (Ni) in the soil, and accumulate these metals to very high levels in the shoot. The increased awareness regarding metal-hyperaccumulating plants by the plant biology community has helped spur interest in the possible use of plants to remove heavy metals from contaminated soils, a process known as phytoremediation. Hence, there has been a focus on understanding the mechanisms that metal-hyperaccumulator plant species such as Thlaspi caerulescens employ to absorb, detoxify and store metals in order to use this information to develop plants better suited for the phytoremediation of metal-contaminated soils.

SCOPE

In this review, an overview of the findings from recent research aimed at better understanding the physiological mechanisms of Thlaspi caerulescens heavy-metal hyperaccumulation as well as the underlying molecular and genetic determinants for this trait will be discussed. Progress has been made in understanding some of the fundamental Zn and Cd transport physiology in T. caerulescens. Furthermore, some interesting metal-related genes have been identified and characterized in this plant species, and regulation of the expression of some of these genes may be important for hyperaccumulation.

CONCLUSIONS

Thlaspi caerulescens is a fascinating and useful model system not only for studying metal hyperaccumulation, but also for better understanding micronutrient homeostasis and nutrition. Considerable future research is still needed to elucidate the molecular, genetic and physiological bases for the extreme metal tolerance and hyperaccumulation exhibited by plant species such as T. caerulescens.

Transcriptional profiling of aluminum toxicity and tolerance responses in maize roots

Aluminum (Al) toxicity is a major factor limiting crop yields on acid soils. There is considerable genotypic variation for Al tolerance in most common plant species. In maize (Zea mays), Al tolerance is a complex phenomenon involving multiple genes and physiological mechanisms yet uncharacterized. To begin elucidating the molecular basis of maize Al toxicity and tolerance, a detailed temporal analysis of root gene expression under Al stress was performed using microarrays with Al-tolerant and Al-sensitive genotypes. Al altered the expression of significantly more genes in the Al-sensitive genotype, presumably as a result of more severe Al toxicity. Nevertheless, several Al-regulated genes exhibited higher expression in the Al-tolerant genotype. Cell wall-related genes, as well as low phosphate-responsive genes, were found to be regulated by Al. In addition, the expression patterns of genes related to Al-activated citrate release indicated that in maize this mechanism is probably regulated by the expression level and/or function of the citrate transporter. This study is the first comprehensive survey of global transcriptional regulation under Al stress. The results described here will help to further our understanding of how mechanisms of Al toxicity and tolerance in maize are regulated at the transcriptional level.

Estrogen receptor α as a target for indole-3-carbinol

A wealth of preclinical evidence supports the antitumorigenic properties of indole-3-carbinol (I3C), which is a major bioactive food component in cruciferous vegetables. However, the underlying molecular mechanism(s) accounting for these effects remain unresolved. In the present study, estrogen receptor alpha (ER-alpha) was identified as a potential molecular target for I3C. Treating MCF-7 cells with 100 microM I3C reduced ER-alpha mRNA expression by approximately 60% compared to controls. This reduction in ER-alpha transcript levels was confirmed using real-time polymerase chain reaction. The I3C dimer, 3,3'-diindolylmethane (DIM), was considerably more effective in depressing ER-alpha mRNA in MCF-7 cells than the monomeric unit. The suppressive effects of 5 microM DIM on ER-alpha mRNA was comparable to that caused by 100 microM I3C. DIM is known to accumulate in the nucleus and is a preferred ligand for aryl hydrocarbon receptor (AhR) to I3C. The addition of other AhR ligands, alpha-naphthoflavone (alpha-NF, 10 microM) and luteolin (10 microM), to the culture media resulted in a similar suppression in ER-alpha mRNA levels to that caused by 5 microM DIM. Thus, it is likely that the binding of ligands to AhR inhibits nuclear ER-alpha transcript. The results from these experiments suggest that the antitumorigenic effects of I3C in MCF-7 human breast cancer cells may arise from its ability to reduce ER-alpha expression through the binding of its metabolite, DIM, to the nuclear AhR.

Common phytochemicals are ecdysteroid agonists and antagonists: a possible evolutionary link between vertebrate and invertebrate steroid hormones

Many plant compounds are able to modulate growth and reproduction of herbivores by directly interacting with steroid hormone systems. In insects, several classes of phytochemicals, including the phytoestrogens, interfere with molting and reproduction. We investigated whether the anti-ecdysone activity may be due to interaction with the ecdysone receptor (EcR) using a reporter-gene assay and a cell differentiation assay of an ecdysone-responsive cell line, Cl.8+. We tested rutin (delays molt in insects); four flavones: luteolin and quercetin (metabolites of rutin), and apigenin and chrysin; and three non-flavones, coumestrol and genistein (both estrogenic) and tomatine (alters molt in insects). None of the phytochemicals tested were ecdysone agonists in the reporter-gene assay, but the flavones were able to significantly inhibit EcR-dependent gene transcription. In the Cl.8+ cells, quercetin and coumestrol were mixed agonists/antagonists, while genistein, tomatine and apigenin showed a synergistic effect with ecdysteroid in the reduction of cell growth. We suggest that the rutin effects on molting in insects are most likely due to the metabolites, luteolin or quercetin, while tomatine acts via a non-EcR pathway. Flavones not only interact with EcR and estrogen receptor (ER), but also signal nitrogen-fixing bacteria to form root nodules. The NodD protein which regulates this symbiosis has two ligand-binding domains similar to human ERalpha. The evolutionary significance of these findings are discussed.

Cell-cell and cell-substrate adhesion in cultured Drosophila imaginal disc cells

Drosophila imaginal disc cell lines were used to investigate various aspects of cellular adhesion in vitro. The distribution of PS integrins and their involvement in cell-cell and cell-substrate adhesion were assessed with the monoclonal antibody aBG-1 against the betaPS subunit, and both forms of adhesion were found to be impeded by the presence of the antibody. Adhesion to a number of extracellular matrix components was investigated, and the cells were found to adhere to human fibronectin. This adhesion was inhibited by aBG-1. The adhesion molecule fasciclin III was also found in these cells. Given that the cells are competent to perform cell-cell and cell-substrate adhesion, it was thought that apical basal polarity might be restored when other suitable conditions were provided, i.e., an artificial basement layer with feeder cells to provide nutrients basally to the cells, and some features of apical-basal morphology were seen in cells cultured under these conditions.

Three subfamilies of pheromone and receptor genes generate multiple B mating specificities in the mushroom Coprinus cinereus

The B mating type locus of the basidiomycete Coprinus cinereus encodes a large family of lipopeptide pheromones and their seven transmembrane domain receptors. Here we show that the B42 locus, like the previously described B6 locus, derives its unique specificity from nine multiallelic genes that are organized into three subgroups each comprising a receptor and two pheromone genes. We show that the three genes within each group are kept together as a functional unit by being embedded in an allele-specific DNA sequence. Using a combination of sequence analysis, Southern blotting, and DNA-mediated transformation with cloned genes, we demonstrate that different B loci may share alleles of one or two groups of genes. This is consistent with the prediction that the three subgroups of genes are functionally redundant and that it is the different combinations of their alleles that generate the multiple B mating specificities found in nature. The B42 locus was found to contain an additional gene, mfs1, that encodes a putative multidrug transporter belonging to the major facilitator family. In strains with other B mating specificities, this gene, whose functional significance was not established, lies in a region of shared homology flanking the B locus.

Adhesion of Drosophila imaginal disc cells in vitro

We have used our imaginal disc cell lines to carry out in vitro studies on the cell-cell and cell-substrate adhesion of Drosophila leg and wing disc cells. Single cells were allowed to reaggregate in roller culture, and this process was found to be partially dependent on the presence of magnesium and calcium ions in the suspension medium. Varying rates of reaggregation were observed in cells from different stages of a passage, correlating with the pattern of morphogenesis which occurs during the passage. We have demonstrated that cloned cell lines can be produced showing certain selected characteristics, such as reduced cell adhesiveness.

Interaction of PAHs and PCBs with ecdysone-dependent gene expression and cell proliferation

This study was done to determine whether PAHs and PCBs can interact with the arthropod steroid hormone system. Ecdysteroid molting hormones control growth, molting, and reproduction in arthropods. A spike in 20-OH ecdysone (20 HE) triggers the molt cycle in crustaceans, and earlier studies have shown that PAHs can affect this molt cycle in several crab species. However, the mechanism of this molt cycle interaction is unknown. Both PAHs and PCBs interact with other nuclear receptors; however, nothing is known about their ability to interact with the invertebrate ecdysone receptor (EcR). Four PAHs, benzo[a]pyrene, benzo[b]fluoranthene, pyrene, and chrysene, and the commercial PCB mixture, Aroclor 1254, were used to determine the ability of these classes of compounds to induce ecdysone-dependent reporter gene expression and to modify the proliferation and differentiation response of the ecdysteroid-responsive Cl.8+ cell line. The four PAHs were each able to enhance the ecdysteroid response in both the reporter gene and the cell proliferation assays only when given in conjunction with ecdysteroids. Aroclor 1254 had no effect in either system, either alone or in conjunction with ecdysteroids. These studies show that although the PAHs alone do not activate ecdysteroid-dependent gene expression or cell differentiation, they are able to enhance the effect of ecdysteroids, presumably through a non-receptor-mediated process. This mechanism may explain the effects on molting which have been reported after low-level crude oil exposures in crustaceans.

The effect of juvenile hormone on the response of the Drosophila imaginal disc cell line Cl 8+ to moulting hormone

The Drosophila wing imaginal disc cell line Cl 8+ was used to investigate the interaction between juvenile hormone III (JH) and 20-hydroxyecdysone (20HE). Cell cultures were exposed to either or both hormones at a range of concentrations and cell growth was observed. JH was found to ameliorate the effects of 20HE on cell growth, even when added after the cells had been exposed to 20HE for 4 or 24h.

Effect of age on the growth and response of a Drosophila cell line to moulting hormone

Insect cell lines in culture are used for a variety of studies. In this laboratory imaginal disc cell lines have been established from primary cultures from third instar larvae, and used for a number of experiments. The effect of ageing on the morphology and physiology of Drosophila cell lines has received very little attention, although problems of genotypic or phenotypic changes in cell lines with age are recognized in other areas of animal cell culture. We tested our cell line Cl8+ for any difference in growth, morphology and response to 20-hydroxyecdysone (20HE) at different ages (passage numbers). The cells were found to multiply faster, adhere less firmly to the substrate and to lose the tendency to aggregate at higher passages. The response to 20HE in terms of cell numbers and induction of beta-galactosidase was similar at all passage numbers but morphological changes in hormone-treated cells were less obvious in the higher passages. Cell lines are likely to vary in the extent of ageing effects but workers are advised to be aware of the possibilities. We suggest the effects of age on cell lines should be established, and passage numbers noted in experimental reports.

The effects of several ecdysteroids and ecdysteroid agonists on two Drosophila imaginal disc cell lines

Two Drosophila imaginal disc cell lines, C18+ (sensitive to 20-hydroxyecdysone, 20HE) and C18R (resistant to 20HE) were exposed to the ecdysteroid agonists RH5849 and RH5992 and the ecdysteroids inokosterone, makisterone A and muristerone A. All compounds tested were found to have similar effects on the cells, comparable to the effects of 20HE, although at different concentrations. C18R showed resistance to all compounds, again at varying concentrations. We conclude that it is likely that all the compounds tested use the same receptors as 20HE, but show maximum effectiveness at different concentrations.

Parameters of growth in primary cultures and cell lines established from Drosophila imaginal discs

We have further characterised our tissue culture system for the growth in vitro of Drosophila imaginal disc cells, including the culture medium requirements for optimum growth and we have adjusted the protocol recommended for the initiation of cultures. Many imaginal disc fragments become organised into vesicles, and some of these secrete extracellular material into the lumen. Sensory axons differentiate in primary disc cultures, in the absence of bristle formation. The early stages of cell division to form a cell line are recorded.

Cuticle secretion in Drosophila wing imaginal discs in vitro: parameters of exposure to 20-hydroxy ecdysone

We have cultured Drosophila wing imaginal discs in vitro under a variety of hormonal conditions in order to determine whether cuticle secretion is enhanced by a withdrawal of 20-hydroxy ecdysone at one of two points in development, corresponding to the drop in hormone titer during the prepupal period, and to the fall in hormone levels during the later stages if imaginal differentiation. We found that these treatments did not enhance either pupal or adult cuticle secretion.

The ultrastructure of imaginal disc cells in primary cultures and during cell aggregation in continuous cell lines

We have examined the ultrastructure of cellular vesicles in primary cultures of wing imaginal disc cells of Drosophila melanogaster. These cells maintain the apico-basal polarity characteristic of epithelial cells. The apical surfaces secrete extracellular material into the lumen of the vesicle from plasma membrane plaques at the tip of microvilli. During the course of one passage, cells from the established cell lines grow to confluence and then aggregate into discrete condensations joined by aligned bridges of cells. Cells in these aggregates are tightly packed, and there appears to be a loss of the epithelial polarity characteristic of the vesicle cells. Elongated cell extensions containing numerous microtubules are found in aggregates, and we suggest that these may be epithelial feet involved in the aggregation process. Virus particles are commonly found both within the nucleus and the cytoplasm of cells in the aggregates.

Active ion transport and beta-acdysone induced differentiation of Drosophila melanogaster imaginal discs cultured in vitro

The theory that beta-ecdysone initiates developmental changes during insect metamorphosis by causing an increase in intranuclear levels of potassium, together with a concomitant decrease in sodium levels, has been investigated by two methods. First, imaginal discs from late third instar larvae have been cultured with 0-2 microng/ml of beta-ecdysone together with inhibitors of active ion transport. Non-specific inhibitors, which may have general effects on sulphydryl groups, such as iodoacetic acid, N-ethylmaleimide ethacrinic acid and furosemide, inhibit both eversion and differentiation at concentrations of from 10(-3) M to 2 X 10(-3) M. Ouabain, the only specific inhibitor of the active transport of Na+ and K+ across membranes, had no effect on development even at a concentration of 10(-2) M. Second, a medium containing raised levels of K+, and reduced concentrations of Na+, neither initiated disc development in the absence of beta-ecdysone, nor stimulated development induced by suboptimal levels (0-02 microng/ml) of beta-ecdysone, either in the presence or absence of ouabain. These results suggest that beta-ecdysone induced morphogenesis is not dependent upon Na+ and K+ concentrations, or on the activity of an ouabain-sensitive ion pump.