Assessment of ptxD gene as an alternative selectable marker for Agrobacterium-mediated maize transformation

Predicting the allergenic risk of Phosphite-NAD+-Oxidoreductase and purple acid phosphatase 17 proteins in genetically modified canola using bioinformatic approaches

Recent advancements in the generation of high-throughput multi-omics data have provided a vast array of candidate genes for the genetic engineering of plants. However, as part of their safety assessment, newly expressed proteins in genetically modified crops must be evaluated for potential cross-reactivity with known allergens. In this study, we developed transgenic canola plants expressing the Arabidopsis thaliana PAP17 gene and a novel selectable marker composed of the ptxD gene from Pseudomonas stutzeri. To evaluate the potential allergenic cross-reactivity of the AtPAP17 and PTXD proteins expressed in transgenic canola, we applied a comprehensive approach utilizing sequence-based, motif-based, and 3D structure-based analyses. Our results demonstrate that the risk of conferring cross-reactivity with known allergens is negligible, indicating that the expression of these proteins in transgenic canola poses a low allergenic risk.

Prospects of genetics and breeding for low-phosphate tolerance: an integrated approach from soil to cell

Improving phosphorus (P) crop nutrition has emerged as a key factor toward achieving a more resilient and sustainable agriculture. P is an essential nutrient for plant development and reproduction, and phosphate (Pi)-based fertilizers represent one of the pillars that sustain food production systems. To meet the global food demand, the challenge for modern agriculture is to increase food production and improve food quality in a sustainable way by significantly optimizing Pi fertilizer use efficiency. The development of genetically improved crops with higher Pi uptake and Pi-use efficiency and higher adaptability to environments with low-Pi availability will play a crucial role toward this end. In this review, we summarize the current understanding of Pi nutrition and the regulation of Pi-starvation responses in plants, and provide new perspectives on how to harness the ample repertoire of genetic mechanisms behind these adaptive responses for crop improvement. We discuss on the potential of implementing more integrative, versatile, and effective strategies by incorporating systems biology approaches and tools such as genome editing and synthetic biology. These strategies will be invaluable for producing high-yielding crops that require reduced Pi fertilizer inputs and to develop a more sustainable global agriculture.

Improved the Activity of Phosphite Dehydrogenase and its Application in Plant Biotechnology

Phosphorus (P) is a nonrenewable resource, which is one of the major challenges for sustainable agriculture. Although phosphite (Phi) can be absorbed by the plant cells through the Pi transporters, it cannot be metabolized by plant and unable to use as P fertilizers for crops. However, transgenic plants that overexpressed phosphite dehydrogenase (PtxD) from bacteria can utilize phosphite as the sole P source. In this study, we aimed to improve the catalytic efficiency of PtxD from Ralstonia sp.4506 (PtxDR4506), by directed evolution. Five mutations were generated by saturation mutagenesis at the 139th site of PtxD R4506 and showed higher catalytic efficiency than native PtxDR4506. The PtxDQ showed the highest catalytic efficiency (5.83-fold as compared to PtxDR4506) contributed by the 41.1% decrease in the K m and 2.5-fold increase in the k cat values. Overexpression of PtxDQ in Arabidopsis and rice showed increased efficiency of phosphite utilization and excellent development when phosphite was used as the primary source of P. High-efficiency PtxD transgenic plant is an essential prerequisite for future agricultural production using phosphite as P fertilizers.

Leaching Potential of Phosphite Fertilizer in Sandy Soils of the Southern Coastal Plain, USA

Novel biotechnology on transgenic plants capable of metabolizing phosphite (Phi), a reduced form of P, could improve the effectiveness of P fertilizers and reduce the P footprint in agriculture with the benefit of suppressing weed growth. However, potassium Phi (K-Phi) salts used as fertilizer are highly soluble in water. At the same time, sandy soils of the Southern Coastal Plain are vulnerable to leaching losses resulting from long-term Pi fertilizer application. We performed a replicated leaching trial using five soil materials that included three surface and two subsurface layers from cultivated topsoil (Ap horizon) with contrasting Phi and Pi sorption capacities. Each soil received three treatments K-Phi at rates 0 (control), 24, and 49 kg P ha−1 and leached twice with de-ionized water. All K-Phi-treated soils leached Phi except for the controls. A phosphorus saturation ratio (PSR) calculated from P, Al, and Fe in acid extracts indicated increasing environmental risk of Phi leaching in soils with lower Phi and Pi sorption capacities at rising rates of applied K-Phi. Because plants rapidly absorb Phi, further studies on the environmental impact of K-Phi fertilizer use should include the interaction of plants with soil properties and soil microbial activity at optimal Phi application rates for growing transgenic plants able to use Phi as a nutrient source.

A novel dominant selection system for plant transgenics based on phosphite metabolism catalyzed by bacterial alkaline phosphatase

Selective markers are generally indispensable in plant genetic transformation, of which the frequently used are of antibiotic or herbicide resistance. However, the increasing concerns on transgenic biosafety have encouraged many new and safe selective markers emerging, with an eminent representative as phosphite (Phi) in combination to its dehydrogenase (PTDH, e.g. PtxD). As bacterial alkaline phosphatase (BAP) can resemble PtxD to oxidatively convert toxic Phi into metabolizable phosphate (Pi), herein we harnessed it as the substitute of PtxD to develop an alternative Phi-based selection system. We first validated the Escherichia coli BAP (EcBAP) did own an extra enzymatic activity of oxidizing Phi to Pi. We further revealed EcBAP could be used as a dominant selective marker for Agrobacterium-mediated tobacco transformation. Although the involved Phi selection for transformed tobacco cells surprisingly required the presence of Pi, it showed a considerable transformation efficiency and dramatically accelerated transformation procedure, as compared to the routine kanamycin selection and the well-known PtxD/Phi system. Moreover, the EcBAP transgenic tobaccos could metabolize toxic Phi as a phosphorus (P) fertilizer thus underlying Phi-resistance, and competitively possess a dominant growth over wild-type tobacco and weeds under Phi stress. Therefore, this novel BAP/Phi-coupled system, integrating multiple advantages covering biosafe dominant selective marker, plant P utilization and weed management, can provide a PTDH-bypass technological choice to engineer transgenic plant species, especially those of great importance for sustainable agriculture.

ptxD/Phi as alternative selectable marker system for genetic transformation for bio-safety concerns: a review

Antibiotic and herbicide resistance genes are the most common marker genes for plant transformation to improve crop yield and food quality. However, there is public concern about the use of resistance marker genes in food crops due to the risk of potential gene flow from transgenic plants to compatible weedy relatives, leading to the possible development of “superweeds” and antibiotic resistance. Several selectable marker genes such as aph, nptII, aaC3, aadA, pat, bar, epsp and gat, which have been synthesized to generate transgenic plants by genetic transformation, have shown some limitations. These marker genes, which confer antibiotic or herbicide resistance and are introduced into crops along with economically valuable genes, have three main problems: selective agents have negative effects on plant cell proliferation and differentiation, uncertainty about the environmental effects of many selectable marker genes, and difficulty in performing recurrent transformations with the same selectable marker to pyramid desired genes. Recently, a simple, novel, and affordable method was presented for plant cells to convert non-metabolizable phosphite (Phi) to an important phosphate (Pi) for developing cells by gene expression encoding a phosphite oxidoreductase (PTXD) enzyme. The ptxD gene, in combination with a selection medium containing Phi as the sole phosphorus (P) source, can serve as an effective and efficient system for selecting transformed cells. The selection system adds nutrients to transgenic plants without potential risks to the environment. The ptxD/Phi system has been shown to be a promising transgenic selection system with several advantages in cost and safety compared to other antibiotic-based selection systems. In this review, we have summarized the development of selection markers for genetic transformation and the potential use of the ptxD/Phi scheme as an alternative selection marker system to minimize the future use of antibiotic and herbicide marker genes.

Phosphonic acid: a long-standing and versatile crop protectant

Phosphonic acid-based fungicides, also referred to as phosphonates, have been used extensively as crop protectants in horticulture since the late 1970s, and more recently in native ecosystems and forestry. Discovering that phosphonates are effective against foliar and soilborne oomycete diseases, such as those caused by species of Phytophthora, Pythium and Plasmopara, was a significant breakthrough, especially for soilborne pathogens that are notoriously difficult to manage. Phosphonates have played an important role in protection of forests and sensitive natural ecosystems, under threat from these pathogens. Since introduction, their increased application in management of non-oomycete diseases, along with other functionalities, demonstrates their versatility in agriculture and more broadly. Continued use of phosphonic acid crop protectants will be underpinned by demonstrated efficacy and safety, and a better understanding of specific interactions within the plant, pathogen and environment. © 2020 Society of Chemical Industry.

Insights Into Genetic and Molecular Elements for Transgenic Crop Development

Climate change and the exploration of new areas of cultivation have impacted the yields of several economically important crops worldwide. Both conventional plant breeding based on planned crosses between parents with specific traits and genetic engineering to develop new biotechnological tools (NBTs) have allowed the development of elite cultivars with new features of agronomic interest. The use of these NBTs in the search for agricultural solutions has gained prominence in recent years due to their rapid generation of elite cultivars that meet the needs of crop producers, and the efficiency of these NBTs is closely related to the optimization or best use of their elements. Currently, several genetic engineering techniques are used in synthetic biotechnology to successfully improve desirable traits or remove undesirable traits in crops. However, the features, drawbacks, and advantages of each technique are still not well understood, and thus, these methods have not been fully exploited. Here, we provide a brief overview of the plant genetic engineering platforms that have been used for proof of concept and agronomic trait improvement, review the major elements and processes of synthetic biotechnology, and, finally, present the major NBTs used to improve agronomic traits in socioeconomically important crops.

Metabolic engineering of phosphite metabolism in Synechococcus elongatus PCC 7942 as an effective measure to control biological contaminants in outdoor raceway ponds

BACKGROUND

The use of cyanobacteria and microalgae as cell factories to produce biofuels and added-value bioproducts has received great attention during the last two decades. Important investments have been made by public and private sectors to develop this field. However, it has been a challenge to develop a viable and cost-effective platform for cultivation of cyanobacteria and microalgae under outdoor conditions. Dealing with contamination caused by bacteria, weedy algae/cyanobacteria and other organisms is a major constraint to establish effective cultivation processes.

RESULTS

Here, we describe the implementation in the cyanobacterium Synechococcus elongatus PCC 7942 of a phosphorus selective nutrition system to control biological contamination during cultivation. The system is based on metabolic engineering of S. elongatus to metabolize phosphite, a phosphorus source not normally metabolized by most organisms, by expressing a bacterial phosphite oxidoreductase (PtxD). Engineered S. elongatus strains expressing PtxD grow at a similar rate on media supplemented with phosphite as the non-transformed control supplemented with phosphate. We show that when grown in media containing phosphite as the sole phosphorus source in glass flasks, the engineered strain was able to grow and outcompete biological contaminants even when the system was intentionally inoculated with natural competitors isolated from an irrigation canal. The PtxD/phosphite system was successfully used for outdoor cultivation of engineered S. elongatus in 100-L cylindrical reactors and 1000-L raceway ponds, under non-axenic conditions and without the need of sterilizing containers and media. Finally, we also show that the PtxD/phosphite system can be used as selectable marker for S. elongatus PCC 7942 transgenic strains selection, eliminating the need of antibiotic resistance genes.

CONCLUSIONS

Our results suggest that the PtxD/phosphite system is a stable and sufficiently robust strategy to control biological contaminants without the need of sterilization or other complex aseptic procedures. Our data show that the PtxD/phosphite system can be used as selectable marker and allows production of the cyanobacterium S. elongatus PCC 7942 in non-axenic outdoor reactors at lower cost, which in principle should be applicable to other cyanobacteria and microalgae engineered to metabolize phosphite.

The phosphite oxidoreductase gene, ptxD as a bio-contained chloroplast marker and crop-protection tool for algal biotechnology using Chlamydomonas

Edible microalgae have potential as low-cost cell factories for the production and oral delivery of recombinant proteins such as vaccines, anti-bacterials and gut-active enzymes that are beneficial to farmed animals including livestock, poultry and fish. However, a major economic and technical problem associated with large-scale cultivation of microalgae, even in closed photobioreactors, is invasion by contaminating microorganisms. Avoiding this requires costly media sterilisation, aseptic techniques during set-up and implementation of 'crop-protection' strategies during cultivation. Here, we report a strain improvement approach in which the chloroplast of Chlamydomonas reinhardtii is engineered to allow oxidation of phosphite to its bio-available form: phosphate. We have designed a synthetic version of the bacterial gene (ptxD)-encoding phosphite oxidoreductase such that it is highly expressed in the chloroplast but has a Trp→Opal codon reassignment for bio-containment of the transgene. Under mixotrophic conditions, the growth rate of the engineered alga is unaffected when phosphate is replaced with phosphite in the medium. Furthermore, under non-sterile conditions, growth of contaminating microorganisms is severely impeded in phosphite medium. This, therefore, offers the possibility of producing algal biomass under non-sterile conditions. The ptxD gene can also serve as a dominant marker for genetic engineering of any C. reinhardtii strain, thereby avoiding the use of antibiotic resistance genes as markers and allowing the 'retro-fitting' of existing engineered strains. As a proof of concept, we demonstrate the application of our ptxD technology to a strain expressing a subunit vaccine targeting a major viral pathogen of farmed fish.

A Review of the Latest in Phosphorus Fertilizer Technology: Possibilities and Pragmatism

The development of highly concentrated phosphorus (P) fertilizers, such as triple superphosphate, by the Tennessee Valley Authority helped mark the beginning of a revolution in the way we manage food crop nutrition. Since then, scientists, with the help of farmers, have made great advancements in the understanding of P fate and transport across many environments but largely have failed to produce a new generation of products and/or application techniques that are widely accepted and that vastly improve plant acquisition efficiency. Under certain conditions, important advancements have been made. For example, applying liquid formulations of phosphates in lieu of dry granules in some highly calcareous soils has dramatically reduced precipitation as sparingly soluble calcium phosphate minerals, but other attempts, such as the co-application of humic substances, sorption to layered double hydroxides, or use of nanoparticles, have not generated the kind of results necessary to continue economically increasing crop yields without further environmental cost. New sources of fertility will need to be affordable to produce, transport, and furnish P to soil solution in a manner well synchronized with crop demand. This paper provides a review of recent literature on cutting-edge phosphorus fertilizer technology. The goal is that this synthesis will be used as a starting point from which a larger discussion on responsible nutrient management and increased P use efficiency research can be built.

Use of the ptxD gene as a portable selectable marker for chloroplast transformation in Chlamydomonas reinhardtii

Synthetic biology and genetic engineering in algae offer an unprecedented opportunity to develop species with traits that can help solve the problems associated with food and energy supply in the 21st century. In the green alga Chlamydomonas reinhardtii, foreign genes can be expressed from the chloroplast genome for molecular farming and metabolic engineering to obtain commodities and high-value molecules. To introduce these genes, selectable markers, which rely mostly on the use of antibiotics, are needed. This has risen social concern associated with the potential risk of horizontal gene transfer across life kingdoms, which has led to a quest for antibiotic-free selectable markers. Phosphorus (P) is a scarce nutrient element that most organisms can only assimilate in its most oxidized form as phosphate (Pi); however, some organisms are able to oxidize phosphite (Phi) to Pi prior to incorporation into the central metabolism of P. As an alternative to the use of the two positive selectable makers already available for chloroplast transformation in C. reinhardtii, the aadA and the aphA-6 genes, that require the use of antibiotics, we investigated if a phosphite-based selection method could be used for the direct recovery of chloroplast transformed lines in this alga. Here we show that following bombardment with a vector carrying the ptxD gene from Pseudomonas stutzeri WM88, only cells that integrate and express the gene proliferate and form colonies using Phi as the sole P source. Our results demonstrate that a selectable marker based on the assimilation of Phi can be used for chloroplasts transformation in a biotechnologically relevant organism. The portable selectable marker we have developed is, in more than 18 years, the latest addition to the markers available for selection of chloroplast transformed cells in C. reinhardtii. The ptxD gene will contribute to the repertoire of tools available for synthetic biology and genetic engineering in the chloroplast of C. reinhardtii.

Growth and selection of the cyanobacterium Synechococcus sp. PCC 7002 using alternative nitrogen and phosphorus sources

Cyanobacteria, such as Synechococcus sp. PCC 7002 (Syn7002), are promising chassis strains for "green" biotechnological applications as they can be grown in seawater using oxygenic photosynthesis to fix carbon dioxide into biomass. Their other major nutritional requirements for efficient growth are sources of nitrogen (N) and phosphorus (P). As these organisms are more economically cultivated in outdoor open systems, there is a need to develop cost-effective approaches to prevent the growth of contaminating organisms, especially as the use of antibiotic selection markers is neither economically feasible nor ecologically desirable due to the risk of horizontal gene transfer. Here we have introduced a synthetic melamine degradation pathway into Syn7002 and evolved the resulting strain to efficiently use the nitrogen-rich xenobiotic compound melamine as the sole N source. We also show that expression of phosphite dehydrogenase in the absence of its cognate phosphite transporter permits growth of Syn7002 on phosphite and can be used as a selectable marker in Syn7002. We combined these two strategies to generate a strain that can grow on melamine and phosphite as sole N and P sources, respectively. This strain is able to resist deliberate contamination in large excess and should be a useful chassis for metabolic engineering and biotechnological applications using cyanobacteria.

Co-culturing on dry filter paper significantly increased the efficiency of Agrobacterium-mediated transformations of maize immature embryos

In the Agrobacterium tumefaciens-mediated transformations of maize immature embryos (IEs), the common co-culturing media used are MS or N6-based (MC). Here, we used a novel co-culturing method in which maize 'Qi319' IEs inoculated with Agrobacterium-harboring target vector were placed on dry filter paper (DC) in a petri dish. To compare the effects of the DC and MC co-culturing methods on transformation efficiency, we designed three experiments: (1) A. tumefaciens strain AGL1 independently carrying two plasmids, pXQD12 and pXQD70; (2) two A. tumefaciens strains, AGL1 and EHA105, carrying pXQD12; and (3) strains AGL1 and EHA105 each independently inoculated with pXQD12 and pXQD70 for different infiltration periods, 5, 10, 15, 20 and 25 min. We used A. tumefaciens to inoculate IEs derived from maize ears 9-15 d after pollination, and then IEs were placed in petri dishes for co-culturing. The DC treatment significantly increased the percentage of IEs expressing green fluorescence protein (%GFP), indicating positive transformants. DC-treated IEs had ~ 3 to 4 times the %GFP compared with MC-treated IEs at 8 d after inoculation (3 d co-culture and 5 d restoration). The average regeneration frequency (%GFP positive regenerated calli of infected IEs) and stable transformation frequency (%GFP positive T₀ plants of infected IEs) significantly increased with the DC treatment. Thus, the DC method may be used to develop a more efficient Agrobacterium-mediated transformation method for maize IEs.

Developing a flexible, high-efficiency Agrobacterium-mediated sorghum transformation system with broad application

Sorghum is the fifth most widely planted cereal crop in the world and is commonly cultivated in arid and semi-arid regions such as Africa. Despite its importance as a food source, sorghum genetic improvement through transgenic approaches has been limited because of an inefficient transformation system. Here, we report a ternary vector (also known as cohabitating vector) system using a recently described pVIR accessory plasmid that facilitates efficient Agrobacterium-mediated transformation of sorghum. We report regeneration frequencies ranging from 6% to 29% in Tx430 using different selectable markers and single copy, backbone free 'quality events' ranging from 45% to 66% of the total events produced. Furthermore, we successfully applied this ternary system to develop transformation protocols for popular but recalcitrant African varieties including Macia, Malisor 84-7 and Tegemeo. In addition, we report the use of this technology to develop the first stable CRISPR/Cas9-mediated gene knockouts in Tx430.

Selective fertilization with phosphite allows unhindered growth of cotton plants expressing the ptxD gene while suppressing weeds

An increasing number of herbicide-resistant weeds are being reported in the United States, Argentina, and Brazil. This is becoming a global challenge for the production of several major crops, such as cotton, maize, and soybean. New strategies for weed control are required to sustain agricultural production while reducing our dependence on herbicides. Here, we report that selective fertilization of transgenic cotton, expressing a bacterial phosphite dehydrogenase (PTXD), with phosphite provides an effective way to suppress weed growth. Importantly, we show that the ptxD-transgenic cotton plants successfully outcompete a highly aggressive glyphosate-resistant weed. The ptxD/phosphite system represents one of the most promising technologies of recent times with potential to solve many of the agricultural and environmental problems that we encounter currently.

Weeds, which have been the bane of agriculture since the beginning of civilization, are managed manually, mechanically, and, more recently, by chemicals. However, chemical control options are rapidly shrinking due to the recent rise in the number of herbicide-resistant weeds in crop fields, with few alternatives on the horizon. Therefore, there is an urgent need for alternative weed suppression systems to sustain crop productivity while reducing our dependence on herbicides and tillage. Such a development will also allay some of the negative perceptions associated with the use of herbicide-resistance genes and heavy dependence on herbicides. Transgenic plants expressing the bacterial phosphite dehydrogenase (ptxD) gene gain an ability to convert phosphite (Phi) into orthophosphate [Pi, the metabolizable form of phosphorus (P)]. Such plants allow for a selective fertilization scheme, based on Phi as the sole source of P for the crop, while offering an effective alternative for suppressing weed growth. Here, we show that, when P is supplied in the form of Phi, ptxD-expressing cotton (Gossypium hirsutum L.) plants outcompete, in both artificial substrates and natural soils from agricultural fields, three different monocot and dicot weed species intentionally introduced in the experiments, as well as weeds naturally present in the tested soils. Importantly, the ptxD/Phi system proved highly efficacious in inhibiting the growth of glyphosate-resistant Palmer amaranth. With over 250 weed species resistant to currently available herbicides, ptxD-transgenic plants fertilized with Phi could provide an effective alternative to suppressing the growth of these weeds while providing adequate nutrition to the crop.

Growth rate rather than growth duration drives growth heterosis in maize B104 hybrids

Research in maize is often performed using inbred lines that can be readily transformed, such as B104. However, because the B104 line flowers late, the kernels do not always mature before the end of the growing season, hampering routine seed yield evaluations of biotech traits introduced in B104 at many geographical locations. Therefore, we generated five hybrids by crossing B104 with the early-flowering inbred lines CML91, F7, H99, Mo17, and W153R and showed in three consecutive years that the hybrid lines proved to be suitable to evaluate seed yield under field conditions in a temperate climate. By assessing the two main processes driving maize leaf growth, being rate of growth (leaf elongation rate or LER) and the duration of growth (leaf elongation duration or LED) in this panel of hybrids, we showed that leaf growth heterosis was mainly the result of increased LER and not or to a lesser extent of LED. Ectopic expression of the transgenes GA20-oxidase (GA20-OX) and PLASTOCHRON1 (PLA1), known to stimulate the LER and LED, respectively, in the hybrids showed that leaf length heterosis can be stimulated by increased LER, but not by LED, indicating that LER rather than LED is the target for enhancing leaf growth heterosis.

Phosphite: a novel P fertilizer for weed management and pathogen control

The availability of orthophosphate (Pi) is a key determinant of crop productivity because its accessibility to plants is poor due to its conversion to unavailable forms. Weed's competition for this essential macronutrient further reduces its bio-availability. To compensate for the low Pi use efficiency and address the weed hazard, excess Pi fertilizers and herbicides are routinely applied, resulting in increased production costs, soil degradation and eutrophication. These outcomes necessitate the identification of a suitable alternate technology that can address the problems associated with the overuse of Pi-based fertilizers and herbicides in agriculture. The present review focuses on phosphite (Phi) as a novel molecule for its utility as a fertilizer, herbicide, biostimulant and biocide in modern agriculture. The use of Phi-based fertilization will help to reduce the consumption of Pi fertilizers and facilitate weed and pathogen control using the same molecule, thereby providing significant advantages over current orthophosphate-based fertilization.

ptxD gene in combination with phosphite serves as a highly effective selection system to generate transgenic cotton (Gossypium hirsutum L.)

This report demonstrates the usefulness of ptxD/phosphite as a selection system that not only provides a highly efficient and simple means to generate transgenic cotton plants, but also helps address many of the concerns related to the use of antibiotic and herbicide resistance genes in the production of transgenic crops. Two of the most popular dominant selectable marker systems for plant transformation are based on either antibiotic or herbicide resistance genes. Due to concerns regarding their safety and in order to stack multiple traits in a single plant, there is a need for alternative selectable marker genes. The ptxD gene, derived from Pseudomonas stutzeri WM88, that confers to cells the ability to convert phosphite (Phi) into orthophosphate (Pi) offers an alternative selectable marker gene as demonstrated for tobacco and maize. Here, we show that the ptxD gene in combination with a protocol based on selection medium containing Phi, as the sole source of phosphorus (P), can serve as an effective and efficient system to select for transformed cells and generate transgenic cotton plants. Fluorescence microscopy examination of the cultures under selection and molecular analyses on the regenerated plants demonstrate the efficacy of the system in recovering cotton transformants following Agrobacterium-mediated transformation. Under the ptxD/Phi selection, an average of 3.43 transgenic events per 100 infected explants were recovered as opposed to only 0.41% recovery when bar/phosphinothricin (PPT) selection was used. The event recovery rates for nptII/kanamycin and hpt/hygromycin systems were 2.88 and 2.47%, respectively. Molecular analysis on regenerated events showed a selection efficiency of ~ 97% under the ptxD/Phi system. Thus, ptxD/Phi has proven to be a very efficient, positive selection system for the generation of transgenic cotton plants with equal or higher transformation efficiencies compared to the commonly used, negative selection systems.

Risk associated with off-target plant genome editing and methods for its limitation

Assessment for potential adverse effects of plant genome editing logically focuses on the specific characteristics of the derived phenotype and its release environment. Genome-edited crops, depending on the editing objective, can be classified as either indistinguishable from crops developed through conventional plant breeding or as crops which are transgenic. Therefore, existing regulatory regimes and risk assessment procedures accommodate genome-edited crops. The ability for regulators and the public to accept a product focus in the evaluation of genome-edited crops will depend on research which clarifies the precision of the genome-editing process and evaluates unanticipated off-target edits from the process. Interpretation of genome-wide effects of genome editing should adhere to existing frameworks for comparative risk assessment where the nature and degree of effects are considered relative to a baseline of genome-wide mutations as found in crop varieties developed through conventional breeding methods. Research addressing current uncertainties regarding unintended changes from plant genome editing, and adopting procedures that clearly avoid the potential for gene drive initiation, will help to clarify anticipated public and regulatory questions regarding risk of crops derived through genome editing.

Improving phosphorus use efficiency: a complex trait with emerging opportunities

Phosphorus (P) is one of the essential nutrients for plants, and is indispensable for plant growth and development. P deficiency severely limits crop yield, and regular fertilizer applications are required to obtain high yields and to prevent soil degradation. To access P from the soil, plants have evolved high- and low-affinity Pi transporters and the ability to induce root architectural changes to forage P. Also, adjustments of numerous cellular processes are triggered by the P starvation response, a tightly regulated process in plants. With the increasing demand for food as a result of a growing population, the demand for P fertilizer is steadily increasing. Given the high costs of fertilizers and in light of the fact that phosphate rock, the source of P fertilizer, is a finite natural resource, there is a need to enhance P fertilizer use efficiency in agricultural systems and to develop plants with enhanced Pi uptake and internal P-use efficiency (PUE). In this review we will provide an overview of continuing relevant research and highlight different approaches towards developing crops with enhanced PUE. In this context, we will summarize our current understanding of root responses to low phosphorus conditions and will emphasize the importance of combining PUE with tolerance of other stresses, such as aluminum toxicity. Of the many genes associated with Pi deficiency, this review will focus on those that hold promise or are already at an advanced stage of testing (OsPSTOL1, AVP1, PHO1 and OsPHT1;6). Finally, an update is provided on the progress made exploring alternative technologies, such as phosphite fertilizer.

Phosphonate Biochemistry

Organophosphonic acids are unique as natural products in terms of stability and mimicry. The C-P bond that defines these compounds resists hydrolytic cleavage, while the phosphonyl group is a versatile mimic of transition-states, intermediates, and primary metabolites. This versatility may explain why a variety of organisms have extensively explored the use organophosphonic acids as bioactive secondary metabolites. Several of these compounds, such as fosfomycin and bialaphos, figure prominently in human health and agriculture. The enzyme reactions that create these molecules are an interesting mix of chemistry that has been adopted from primary metabolism as well as those with no chemical precedent. Additionally, the phosphonate moiety represents a source of inorganic phosphate to microorganisms that live in environments that lack this nutrient; thus, unusual enzyme reactions have also evolved to cleave the C-P bond. This review is a comprehensive summary of the occurrence and function of organophosphonic acids natural products along with the mechanisms of the enzymes that synthesize and catabolize these molecules.