Transgenic maize lines with cell-type specific expression of fluorescent proteins in plastids

Editing the 19 kDa alpha-zein gene family generates non-opaque2-based quality protein maize

Maize grain is deficient in lysine. While the opaque2 mutation increases grain lysine, o2 is a transcription factor that regulates a wide network of genes beyond zeins, which leads to pleiotropic and often negative effects. Additionally, the drastic reduction in 19 kDa and 22 kDa alpha-zeins causes a floury kernel, unsuitable for agricultural use. Quality protein maize (QPM) overcame the undesirable kernel texture through the introgression of modifying alleles. However, QPM still lacks a functional o2 transcription factor, which has a penalty on non-lysine amino acids due to the o2 mutation. CRISPR/cas9 gives researchers the ability to directly target genes of interest. In this paper, gene editing was used to specifically target the 19 kDa alpha zein gene family. This allows for proteome rebalancing to occur without an o2 mutation and without a total alpha-zein knockout. The results showed that editing some, but not all, of the 19 kDa zeins resulted in up to 30% more lysine. An edited line displayed an increase of 30% over the wild type. While not quite the 55% lysine increase displayed by QPM, the line had little collateral impact on other amino acid levels compared to QPM. Additionally, the edited line containing a partially reduced 19 kDa showed an advantage in kernel texture that had a complete 19 kDa knockout. These results serve as proof of concept that editing the 19 kDa alpha-zein family alone can enhance lysine while retaining vitreous endosperm and a functional O2 transcription factor.

Trehalose 6-phosphate metabolism in C4 species

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signal metabolite in plants, linking growth and development to carbon status. Our current understanding of Tre6P metabolism and signaling pathways in plants is based almost entirely on studies performed with Arabidopsis thaliana, a model plant that performs C₃ photosynthesis. Conversely, our knowledge on the molecular mechanisms involved in Tre6P regulation of carbon partitioning and metabolism in C₄ plants is scarce. This topic is especially relevant due to the agronomic importance of crops performing C₄ photosynthesis, such as maize, sorghum and sugarcane. In this review, we focused our attention on recent developments related to Tre6P metabolism in C₄ species and raised some open questions that should be addressed in the near future to improve the yield of economically important crops.

A vegetative storage protein improves drought tolerance in maize

Vegetative storage proteins (VSPs) are known to serve as nitrogen reserves in many dicot plants but remain undiscovered in grasses, most widely grown group of crops globally. We identified and characterized a VSP in maize and demonstrated that its overexpression improved drought tolerance. Nitrogen supplementation selectively induced a mesophyll lipoxygenase (ZmLOX6), which was targeted to chloroplasts by a novel N-terminal transit peptide of 62 amino acids. When ectopically expressed under the control of various tissue-specific promoters, it accumulated to a fivefold higher level upon expression in the mesophyll cells than the wild-type plants. Constitutive expression or targeted expression specifically to the bundle sheath cells increased its accumulation by less than twofold. The overexpressed ZmLOX6 was remobilized from the leaves like other major proteins during grain development. Evaluated in the field over locations and years, transgenic hybrids overexpressing ZmLOX6 in the mesophyll cells significantly outyielded nontransgenic sibs under managed drought stress imposed at flowering. Additional storage of nitrogen as a VSP in maize leaves ameliorated the effect of drought on grain yield.

Rubisco production in maize mesophyll cells through ectopic expression of subunits and chaperones

C4 plants, such as maize, strictly compartmentalize Rubisco to bundle sheath chloroplasts. The molecular basis for the restriction of Rubisco from the more abundant mesophyll chloroplasts is not fully understood. Mesophyll chloroplasts transcribe the Rubisco large subunit gene and, when normally quiescent transcription of the nuclear Rubisco small subunit gene family is overcome by ectopic expression, mesophyll chloroplasts still do not accumulate measurable Rubisco. Here we show that a combination of five ubiquitin promoter-driven nuclear transgenes expressed in maize leads to mesophyll accumulation of assembled Rubisco. These encode the Rubisco large and small subunits, Rubisco assembly factors 1 and 2, and the assembly factor Bundle sheath defective 2. In these plants, Rubisco large subunit accumulates in mesophyll cells, and appears to be assembled into a holoenzyme capable of binding the substrate analog CABP (carboxyarabinitol bisphosphate). Isotope discrimination assays suggest, however, that mesophyll Rubisco is not participating in carbon assimilation in these plants, most probably due to a lack of the substrate ribulose 1,5-bisphosphate and/or Rubisco activase. Overall, this work defines a minimal set of Rubisco assembly factors in planta and may help lead to methods of regulating the C4 pathway.

Setaria viridis chlorotic and seedling-lethal mutants define critical functions for chloroplast gene expression

Deep insights into chloroplast biogenesis have been obtained by mutant analysis; however, in C₄ plants a relevant mutant collection has only been developed and exploited for maize. Here, we report the initial characterization of an ethyl methyl sulfonate-induced mutant population for the C₄ model Setaria viridis. Approximately 1000 M₂ families were screened for the segregation of pale-green seedlings in the M₃ generation, and a subset of these was identified to be deficient in post-transcriptional steps of chloroplast gene expression. Causative mutations were identified for three lines using deep sequencing-based bulked segregant analysis, and in one case confirmed by transgenic complementation. Using chloroplast RNA-sequencing and other molecular assays, we describe phenotypes of mutants deficient in PSRP7, a plastid-specific ribosomal protein, OTP86, an RNA editing factor, and cpPNP, the chloroplast isozyme of polynucleotide phosphorylase. The psrp mutant is globally defective in chloroplast translation, and has varying deficiencies in the accumulation of chloroplast-encoded proteins. The otp86 mutant, like its Arabidopsis counterpart, is specifically defective in editing of the rps14 mRNA; however, the conditional pale-green mutant phenotype contrasts with the normal growth of the Arabidopsis mutant. The pnp mutant exhibited multiple defects in 3' end maturation as well as other qualitative changes in the chloroplast RNA population. Overall, our collection opens the door to global analysis of photosynthesis and early seedling development in an emerging C₄ model.

Overexpression of Rubisco subunits with RAF1 increases Rubisco content in maize

Rubisco catalyses a rate-limiting step in photosynthesis and has long been a target for improvement due to its slow turnover rate. An alternative to modifying catalytic properties of Rubisco is to increase its abundance within C₄ plant chloroplasts, which might increase activity and confer a higher carbon assimilation rate. Here, we overexpress the Rubisco large (LS) and small (SS) subunits with the Rubisco assembly chaperone RUBISCO ASSEMBLY FACTOR 1 (RAF1). While overexpression of LS and/or SS had no discernable impact on Rubisco content, addition of RAF1 overexpression resulted in a >30% increase in Rubisco content. Gas exchange showed a 15% increase in CO₂ assimilation (A_SAT) in UBI-LSSS-RAF1 transgenic plants, which correlated with increased fresh weight and in vitro V_cmax calculations. The divergence of Rubisco content and assimilation could be accounted for by the Rubisco activation state, which decreased up to 23%, suggesting that Rubisco activase may be limiting V_cmax, and impinging on the realization of photosynthetic potential from increased Rubisco content.

Overexpression of serine acetyltransferase in maize leaves increases seed-specific methionine-rich zeins

Maize kernels do not contain enough of the essential sulphur-amino acid methionine (Met) to serve as a complete diet for animals, even though maize has the genetic capacity to store Met in kernels. Prior studies indicated that the availability of the sulphur (S)-amino acids may limit their incorporation into seed storage proteins. Serine acetyltransferase (SAT) is a key control point for S-assimilation leading to Cys and Met biosynthesis, and SAT overexpression is known to enhance S-assimilation without negative impact on plant growth. Therefore, we overexpressed Arabidopsis thaliana AtSAT1 in maize under control of the leaf bundle sheath cell-specific rbcS1 promoter to determine the impact on seed storage protein expression. The transgenic events exhibited up to 12-fold higher SAT activity without negative impact on growth. S-assimilation was increased in the leaves of SAT overexpressing plants, followed by higher levels of storage protein mRNA and storage proteins, particularly the 10-kDa δ-zein, during endosperm development. This zein is known to impact the level of Met stored in kernels. The elite event with the highest expression of AtSAT1 showed 1.40-fold increase in kernel Met. When fed to chickens, transgenic AtSAT1 kernels significantly increased growth rate compared with the parent maize line. The result demonstrates the efficacy of increasing maize nutritional value by SAT overexpression without apparent yield loss. Maternal overexpression of SAT in vegetative tissues was necessary for high-Met zein accumulation. Moreover, SAT overcomes the shortage of S-amino acids that limits the expression and accumulation of high-Met zeins during kernel development.

A Brief History of Promoter Development for Use in Transgenic Maize Applications

Promoters regulate gene expression, and are essential biotechnology tools. Since its introduction in the mid-1990s, biotechnology has greatly enhanced maize productivity primarily through the development of insect control and herbicide tolerance traits. Additional biotechnology applications include improving seed nutrient composition, industrial protein production, therapeutic production, disease resistance, abiotic stress resistance, and yield enhancement. Biotechnology has also greatly expanded basic research into important mechanisms that govern plant growth and reproduction. Many novel promoters have been developed to facilitate this work, but only a few are widely used. Transgene optimization includes a variety of strategies some of which effect promoter structure. Recent reviews examine the state of the art with respect to transgene design for biotechnology applications. This chapter examines the use of transgene technology in maize, focusing on the way promoters are selected and used. The impact of new developments in genomic technology on promoter structure is also discussed.

Engineering sulfur storage in maize seed proteins without apparent yield loss

Sulfur assimilation may limit the pool of methionine and cysteine available for incorporation into zeins, the major seed storage proteins in maize. This hypothesis was tested by producing transgenic maize with deregulated sulfate reduction capacity achieved through leaf-specific expression of the Escherichia coli enzyme 3'-phosphoadenosine-5'-phosphosulfate reductase (EcPAPR) that resulted in higher methionine accumulation in seeds. The transgenic kernels have higher expression of the methionine-rich 10-kDa δ-zein and total protein sulfur without reduction of other zeins. This overall increase in the expression of the S-rich zeins describes a facet of regulation of these proteins under enhanced sulfur assimilation. Transgenic line PE5 accumulates 57.6% more kernel methionine than the high-methionine inbred line B101. In feeding trials with chicks, PE5 maize promotes significant weight gain compared with nontransgenic kernels. Therefore, increased source strength can improve the nutritional value of maize without apparent yield loss and may significantly reduce the cost of feed supplementation.

Quality Protein Maize Based on Reducing Sulfur in Leaf Cells

Low levels of the essential amino acids lysine (Lys) and methionine (Met) in a maize-based diet are a major cost to feed and food. Lys deficiency is due to the abundance of Lys-poor proteins in maize kernels. Although a maize mutant, opaque-2 (o2), has sufficient levels of Lys, its soft kernel renders it unfit for storage and transportation. Breeders overcame this problem by selecting quantitative trait loci (QTL) restoring kernel hardness in the presence of o2, a variety called Quality Protein Maize (QPM). Although at least one QTL acts by enhancing the expression of the γ-zein proteins, we could surprisingly achieve rebalancing of the Lys content and a vitreous kernel phenotype by targeting suppression of γ-zeins without the o2 mutant. Reduced levels of γ-zeins were achieved with RNA interference (RNAi). Another transgenic event, PE5 expresses the Escherichia coli enzyme 3'-phosphoadenosine-5'-phosphosulfate reductase involved in sulfate assimilation, specifically in leaves. The stacked transgenic events produce a vitreous endosperm, which has higher Lys level than the classical opaque W64Ao2 variant. Moreover, due to the increased sulfate reduction in the leaf, Met level is elevated in the seed. Such a combination of transgenes produces hybrid seeds superior to classical QPMs that would neither require a costly feed mix nor synthetic Met supplementation, potentially creating a novel and cost-effective means for improving maize nutritional quality.

Expression of the Maize Dof1 Transcription Factor in Wheat and Sorghum

Nitrogen is essential for plant growth and development. Improving the ability of plants to acquire and assimilate nitrogen more efficiently is a key agronomic parameter that will augment sustainability in agriculture. A transcription factor approach was pursued to address improvement of nitrogen use efficiency in two major commodity crops. To this end, the Zea mays Dof1 (ZmDof1) transcription factor was expressed in both wheat (Triticum aestivum) and sorghum (Sorghum bicolor) either constitutively, UBI4 promoter from sugarcane, or in a tissue specific fashion via the maize rbcS1 promoter. The primary transcription activation target of ZmDof1, phosphoenolpyruvate carboxylase (PEPC), is observed in transgenic wheat events. Expression ZmDof1 under control of the rbcs1 promoter translates to increase in biomass and yield components in wheat. However, constitutive expression of ZmDof1 led to the down-regulation of genes involved in photosynthesis and the functional apparatus of chloroplasts, and an outcome that negatively impacts photosynthesis, height, and biomass in wheat. Similar patterns were also observed in sorghum transgenic events harboring the constitutive expression cassette of ZmDof1. These results indicate that transcription factor strategies to boost agronomic phenotypic outcomes in crops need to consider expression patterns of the genetic elements to be introduced.

Factors affecting polyhydroxybutyrate accumulation in mesophyll cells of sugarcane and switchgrass

Background

Polyhydroxyalkanoates are linear biodegradable polyesters produced by bacteria as a carbon store and used to produce a range of bioplastics. Widespread polyhydroxyalkanoate production in C₄ crops would decrease petroleum dependency by producing a renewable supply of biodegradable plastics along with residual biomass that could be converted into biofuels or energy. Increasing yields to commercial levels in biomass crops however remains a challenge. Previously, lower accumulation levels of the short side chain polyhydroxyalkanoate, polyhydroxybutyrate (PHB), were observed in the chloroplasts of mesophyll (M) cells compared to bundle sheath (BS) cells in transgenic maize (Zea mays), sugarcane (Saccharum sp.), and switchgrass (Panicum virgatum L.) leading to a significant decrease in the theoretical yield potential. Here we explore various factors which might affect polymer accumulation in mesophyll cells, including targeting of the PHB pathway enzymes to the mesophyll plastid and their access to substrate.

Results

The small subunit of Rubisco from pea effectively targeted the PHB biosynthesis enzymes to both M and BS chloroplasts of sugarcane and switchgrass. PHB enzyme activity was retained following targeting to M plastids and was equivalent to that found in the BS plastids. Leaf total fatty acid content was not affected by PHB production. However, when fatty acid synthesis was chemically inhibited, polymer accumulated in M cells.

Conclusions

In this study, we provide evidence that access to substrate and neither poor targeting nor insufficient activity of the PHB biosynthetic enzymes may be the limiting factor for polymer production in mesophyll chloroplasts of C₄ plants.

Fluorescent labeling and confocal microscopic imaging of chloroplasts and non-green plastids

While chlorophyll has served as an excellent label for plastids in green tissue, the development of fluorescent proteins has allowed their ready visualization in all tissues of the plants, revealing new features of their morphology and motility. Gene regulatory sequences in plastid transgenes can be optimized through the use of fluorescent protein reporters. Fluorescent labeling of plastids simultaneously with other subcellular locations reveals dynamic interactions and mutant phenotypes. Transient expression of fluorescent protein fusions is particularly valuable to determine whether or not a protein of unknown function is targeted to the plastid. Particle bombardment and agroinfiltration methods described here are convenient for imaging fluorescent proteins in plant organelles. With proper selection of fluorophores for labeling the components of the plant cell, confocal microscopy can produce extremely informative images at high resolution at depths not feasible by standard epifluorescence microscopy.

Nonredundant function of zeins and their correct stoichiometric ratio drive protein body formation in maize endosperm

Zeins, the maize (Zea mays) prolamin storage proteins, accumulate at very high levels in developing endosperm in endoplasmic reticulum membrane-bound protein bodies. Products of the multigene α-zein families and the single-gene γ-zein family are arranged in the central hydrophobic core and the cross-linked protein body periphery, respectively, but little is known of the specific roles of family members in protein body formation. Here, we used RNA interference suppression of different zein subclasses to abolish vitreous endosperm formation through a variety of effects on protein body density, size, and morphology. We showed that the 27-kilodalton (kD) γ-zein controls protein body initiation but is not involved in protein body filling. Conversely, other γ-zein family members function more in protein body expansion and not in protein body initiation. Reduction in both 19- and 22-kD α-zein subfamilies severely restricted protein body expansion but did not induce morphological abnormalities, which result from reduction of only the 22-kD α-zein class. Concomitant reduction of all zein classes resulted in severe reduction in protein body number but normal protein body size and morphology.

Differential age-dependent import regulation by signal peptides

Gene-specific, age-dependent regulations are common at the transcriptional and translational levels, while protein transport into organelles is generally thought to be constitutive. Here we report a new level of differential age-dependent regulation and show that chloroplast proteins are divided into three age-selective groups: group I proteins have a higher import efficiency into younger chloroplasts, import of group II proteins is nearly independent of chloroplast age, and group III proteins are preferentially imported into older chloroplasts. The age-selective signal is located within the transit peptide of each protein. A group III protein with its transit peptide replaced by a group I transit peptide failed to complement its own mutation. Two consecutive positive charges define the necessary motif in group III signals for older chloroplast preference. We further show that different members of a gene family often belong to different age-selective groups because of sequence differences in their transit peptides. These results indicate that organelle-targeting signal peptides are part of cells' differential age-dependent regulation networks. The sequence diversity of some organelle-targeting peptides is not a result of the lack of selection pressure but has evolved to mediate regulation.

Ectopic expression of Rubisco subunits in maize mesophyll cells does not overcome barriers to cell type-specific accumulation

In maize (Zea mays), Rubisco accumulates in bundle sheath but not mesophyll chloroplasts, but the mechanisms that underlie cell type-specific expression are poorly understood. To explore the coordinated expression of the chloroplast rbcL gene, which encodes the Rubisco large subunit (LS), and the two nuclear RBCS genes, which encode the small subunit (SS), RNA interference was used to reduce RBCS expression. This resulted in Rubisco deficiency and was correlated with translational repression of rbcL. Thus, as in C3 plants, LS synthesis depends on the presence of its assembly partner SS. To test the hypothesis that the previously documented transcriptional repression of RBCS in mesophyll cells is responsible for repressing LS synthesis in mesophyll chloroplasts, a ubiquitin promoter-driven RBCS gene was expressed in both bundle sheath and mesophyll cells. This did not lead to Rubisco accumulation in the mesophyll, suggesting that LS synthesis is impeded even in the presence of ectopic SS expression. To attempt to bypass this putative mechanism, a ubiquitin promoter-driven nuclear version of the rbcL gene was created, encoding an epitope-tagged LS that was expressed in the presence or absence of the Ubi-RBCS construct. Both transgenes were robustly expressed, and the tagged LS was readily incorporated into Rubisco complexes. However, neither immunolocalization nor biochemical approaches revealed significant accumulation of Rubisco in mesophyll cells, suggesting a continuing cell type-specific impairment of its assembly or stability. We conclude that additional cell type-specific factors limit Rubisco expression to bundle sheath chloroplasts.

Maize histone H2B-mCherry: a new fluorescent chromatin marker for somatic and meiotic chromosome research

Cytological studies of fluorescent proteins are rapidly yielding insights into chromatin structure and dynamics. Here we describe the production and cytological characterization of new transgenic maize lines expressing a fluorescent histone fusion protein, H2B-mCherry. The transgene is expressed under the control of the maize ubiquitin1 promoter, including its first exon and intron. Polymerase chain reaction-based genotyping and root-tip microscopy showed that most of the lines carrying the transgene also expressed it, producing bright uniform staining of nuclei. Further, plants showing expression in root tips at the seedling stage also showed expression during meiosis, late in the life cycle. Detailed high-resolution three-dimensional imaging of cells and nuclei from various somatic and meiotic cell types showed that H2B-mCherry produced remarkably clear images of chromatin and chromosome fiber morphology, as seen in somatic, male meiotic prophase, and early microgametophyte cells. H2B-mCherry also yielded distinct nucleolus staining and was shown to be compatible with fluorescence in situ hybridization. We found several instances where H2B-mCherry was superior to DAPI as a generalized chromatin stain. Our study establishes these histone H2B-mCherry lines as new biological reagents for visualizing chromatin structure, chromosome morphology, and nuclear dynamics in fixed and living cells in a model plant genetic system.

Overproduction of recombinant proteins in plants

Recombinant protein production in microbial hosts and animal cell cultures has revolutionized the pharmaceutical and industrial enzyme industries. Plants as alternative hosts for the production of recombinant proteins are being actively pursued, taking advantage of their unique characteristics. The key to cost-efficient production in any system is the level of protein accumulation, which is inversely proportional to the cost. Levels of up to 5 g/kg biomass have been obtained in plants, making this production system competitive with microbial hosts. Increasing protein accumulation at the cellular level by varying host, germplasm, location of protein accumulation, and transformation procedure is reviewed. At the molecular level increased expression by improving transcription, translation and accumulation of the protein is critically evaluated. The greatest increases in protein accumulation will occur when various optimized parameters are more fully integrated with each other. Because of the complex nature of plants, this will take more time and effort to accomplish than has been the case for the simpler unicellular systems. However the potential for plants to become one of the major avenues for protein production appears very promising.

Genetic engineering of maize (Zea mays) for high-level tolerance to treatment with the herbicide dicamba

Herbicide-tolerant crops have been widely and rapidly adopted by farmers in several countries due to enhanced weed control, lower labor and production costs, increased environmental benefits, and gains in profitability. Soon to be introduced transgenic soybean and cotton varieties tolerant to treatments with the herbicide dicamba offer prospects for excellent broadleaf weed control in these broadleaf crops. Because monocots such as maize (Zea mays) can be treated with dicamba only during a limited window of crop development and because crop injury is sometimes observed when conditions are unfavorable, transgenic maize plants have been produced and tested for higher levels of tolerance to treatment with dicamba. Maize plants expressing the gene encoding dicamba monooxygenase (DMO) linked with an upstream chloroplast transit peptide (CTP) display greatly enhanced tolerance to dicamba applied either pre-emergence or postemergence. Comparisons of DMO coupled to CTPs derived from the Rubisco small subunit from either Arabidopsis thaliana or Z. mays showed that both allowed production of transgenic maize plants tolerant to treatment with levels of dicamba (i.e., 27 kg/ha) greatly exceeding the highest recommended rate of 0.56 kg/ha.

Independent effects of leaf growth and light on the development of the plastid and its DNA content in Zea species

In maize (Zea mays L.), chloroplast development progresses from the basal meristem to the mature leaf tip, and light is required for maturation to photosynthetic competence. During chloroplast greening, it was found that chloroplast DNA (cpDNA) is extensively degraded, falling to undetectable levels in many individual chloroplasts for three maize cultivars, as well as Zea mexicana (the ancestor of cultivated maize) and the perennial species Zea diploperennis. In dark-grown maize seedlings, the proplastid-to-etioplast transition is characterized by plastid enlargement, cpDNA replication, and the retention of high levels of cpDNA. When dark-grown seedlings are transferred to white light, the DNA content per plastid increases slightly during the first 4 h of illumination and then declines rapidly to a minimum at 24 h during the etioplast-to-chloroplast transition. Plastid autofluorescence (from chlorophyll) continues to increase as cpDNA declines, whereas plastid size remains constant. It is concluded that the increase in cpDNA that accompanies plastid enlargement is a consequence of cell and leaf growth, rather than illumination, whereas light stimulates photosynthetic capacity and cpDNA instability. When cpDNA from total tissue was monitored by blot hybridization and real-time quantitative PCR, no decline following transfer from dark to light was observed. The lack of agreement between DNA per plastid and cpDNA per cell may be attributed to nupts (nuclear sequences of plastid origin).

Visualisation of stromules in transgenic wheat expressing a plastid-targeted yellow fluorescent protein

Stromules are stroma-filled tubules that extend from the plastids in all multicellular plants examined to date. To facilitate the visualisation of stromules on different plastid types in various tissues of bread wheat (Triticum aestivum L.), a chimeric gene construct encoding enhanced yellow fluorescent protein (EYFP) targeted to plastids with the transit peptide of wheat granule-bound starch synthase I was introduced by Agrobacterium-mediated transformation. The gene construct was under the control of the rice Actin1 promoter, and EYFP fluorescence was detected in plastids in all cell types throughout the transgenic plants. Stromules were observed on all plastid types, although the stromule length and abundance varied markedly in different tissues. The longest stromules (up to 40 μm) were observed in epidermal cells of leaves, whereas only short beak-like stromules were observed on chloroplasts in mesophyll cells. Epidermal cells in leaves and roots contained the highest proportion of plastids with stromules, and stromules were also abundant on amyloplasts in the endosperm tissue of developing seeds. The general features of stromule morphology and distribution were similar to those shown previously for tobacco (Nicotiana tabacum L.) and arabidopsis (Arabidopsis thaliana (L.) Heynh.).

Stable chloroplast transformation of immature scutella and inflorescences in wheat (Triticum aestivum L.)

Chloroplast transformation in wheat was achieved by bombardment of scutella from immature embryos and immature inflorescences, respectively. A wheat chloroplast site-specific expression vector, pBAGNRK, was constructed by placing an expression cassette containing neomycin phosphotransferase II (nptII) and green fluorescent protein (gfp) as selection and reporter genes, respectively, in the intergenic spacer between atpB and rbcL of wheat chloroplast genome. Integration of gfp gene in the plastome was identified by polymerase chain reaction (PCR) analysis and Southern blotting using gfp gene as a probe. Expression of GFP protein was examined by western blot. Three positive transformants were obtained and the Southern blot of partial fragment of atpB and rbcL (targeting site) probes verified that one of them was homoplasmic. Stable expression of GFP fluorescence was confirmed by confocal microscopy in the leaf tissues from T(1) progeny seedlings. PCR analysis of gfp gene also confirmed the inheritance of transgene in the T(1) progeny. These results strengthen the feasibility of wheat chloroplast transformation and also give a novel method for the introduction of important agronomic traits in wheat through chloroplast transformation.

Setaria viridis: a model for C4 photosynthesis

C(4) photosynthesis drives productivity in several major food crops and bioenergy grasses, including maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), Miscanthus x giganteus, and switchgrass (Panicum virgatum). Gains in productivity associated with C(4) photosynthesis include improved water and nitrogen use efficiencies. Thus, engineering C(4) traits into C(3) crops is an attractive target for crop improvement. However, the lack of a small, rapid cycling genetic model system to study C(4) photosynthesis has limited progress in dissecting the regulatory networks underlying the C(4) syndrome. Setaria viridis is a member of the Panicoideae clade and is a close relative of several major feed, fuel, and bioenergy grasses. It is a true diploid with a relatively small genome of ~510 Mb. Its short stature, simple growth requirements, and rapid life cycle will greatly facilitate genetic studies of the C(4) grasses. Importantly, S. viridis uses an NADP-malic enzyme subtype C(4) photosynthetic system to fix carbon and therefore is a potentially powerful model system for dissecting C(4) photosynthesis. Here, we summarize some of the recent advances that promise greatly to accelerate the use of S. viridis as a genetic system. These include our recent successful efforts at regenerating plants from seed callus, establishing a transient transformation system, and developing stable transformation.