The Shoot Apical Meristem: An Evolutionary Molding of Higher Plants.

The shoot apical meristem (SAM) gives rise to the aerial structure of plants by producing lateral organs and other meristems. The SAM is responsible for plant developmental patterns, thus determining plant morphology and, consequently, many agronomic traits such as the number and size of fruits and flowers and kernel yield. Our current understanding of SAM morphology and regulation is based on studies conducted mainly on some angiosperms, including economically important crops such as maize (Zea mays) and rice (Oryza sativa), and the model species Arabidopsis (Arabidopsis thaliana). However, studies in other plant species from the gymnosperms are scant, making difficult comparative analyses that help us understand SAM regulation in diverse plant species. This limitation prevents deciphering the mechanisms by which evolution gave rise to the multiple plant structures within the plant kingdom and determines the conserved mechanisms involved in SAM maintenance and operation. This review aims to integrate and analyze the current knowledge of SAM evolution by combining the morphological and molecular information recently reported from the plant kingdom.

Bioactive Compounds with Pesticide Activities Derived from Aged Cultures of Green Microalgae.

The excessive use of synthetic pesticides has caused environmental problems and human health risks and increased the development of resistance in several organisms. Allelochemicals, secondary metabolites produced as part of the defense mechanisms in plants and microorganisms, are an attractive alternative to replace synthetic pesticides to remediate these problems. Microalgae are natural producers of a wide range of allelochemicals. Thus, they provide new opportunities to identify secondary metabolites with pesticide activities and an alternative approach to discover new modes of action and circumvent resistance. We screened 10 green microalgae strains belonging to the Chlorophyta phylum for their potential to inhibit the growth of photosynthetic and nonphotosynthetic organisms. Bioassays were established to assess microalgae extracts' effectiveness in controlling the growth of Chlorella sorokiniana, Arabidopsis thaliana, Amaranthus palmeri, and the model nematode Caenorhabditis elegans. All tested strains exhibited herbicidal, nematocidal, or algicidal activities. Importantly, methanol extracts of a Chlamydomonas strain effectively controlled the germination and growth of a glyphosate-resistant A. palmeri biotype. Likewise, some microalgae extracts effectively killed C. elegans L1 larvae. Comprehensive metabolic profiling using LC-MS of extracts with pesticide activities showed that the metabolite composition of Chlamydomonas, Chlorella, and Chloroidium extracts is diverse. Molecules such as fatty acids, isoquinoline alkaloids, aldehydes, and cinnamic acids were more abundant, suggesting their participation in the pesticide activities.

Multi-omic analyses reveal the unique properties of chia (Salvia hispanica) seed metabolism.

Chia (Salvia hispanica) is an emerging crop considered a functional food containing important substances with multiple potential applications. However, the molecular basis of some relevant chia traits, such as seed mucilage and polyphenol content, remains to be discovered. This study generates an improved chromosome-level reference of the chia genome, resolving some highly repetitive regions, describing methylation patterns, and refining genome annotation. Transcriptomic analysis shows that seeds exhibit a unique expression pattern compared to other organs and tissues. Thus, a metabolic and proteomic approach is implemented to study seed composition and seed-produced mucilage. The chia genome exhibits a significant expansion in mucilage synthesis genes (compared to Arabidopsis), and gene network analysis reveals potential regulators controlling seed mucilage production. Rosmarinic acid, a compound with enormous therapeutic potential, was classified as the most abundant polyphenol in seeds, and candidate genes for its complex pathway are described. Overall, this study provides important insights into the molecular basis for the unique characteristics of chia seeds.

Back to primary endosymbiosis: from plastids to artificial photosynthetic life-forms.

Throughout evolution, only two known primary photosynthetic endosymbiosis occurred, which originated the Archaeplastida and the Paulinella spp. Fundamental questions regarding primary endosymbiosis remain unsolved, but may now be addressed with the recent development of chimeric photosynthetic life-form. Cournoyer et al. could establish artificial photosynthetic endosymbiosis between yeast and cyanobacteria.

Engineering microalgae for water phosphorus recovery to close the phosphorus cycle.

As a finite and non-renewable resource, phosphorus (P) is essential to all life and crucial for crop growth and food production. The boosted agricultural use and associated loss of P to the aquatic environment are increasing environmental pollution, harming ecosystems, and threatening future global food security. Thus, recovering and reusing P from water bodies is urgently needed to close the P cycle. As a natural, eco-friendly, and sustainable reclamation strategy, microalgae-based biological P recovery is considered a promising solution. However, the low P-accumulation capacity and P-removal efficiency of algal bioreactors restrict its application. Herein, it is demonstrated that manipulating genes involved in cellular P accumulation and signalling could triple the Chlamydomonas P-storage capacity to ~7% of dry biomass, which is the highest P concentration in plants to date. Furthermore, the engineered algae could recover P from wastewater almost three times faster than the unengineered one, which could be directly used as a P fertilizer. Thus, engineering genes involved in cellular P accumulation and signalling in microalgae could be a promising strategy to enhance P uptake and accumulation, which have the potential to accelerate the application of algae for P recovery from the water body and closing the P cycle.

Assembly and annotation of the Gossypium barbadense L. 'Pima-S6' genome raise questions about the chromosome structure and gene content of Gossypium barbadense genomes.

BACKGROUND: Gossypium barbadense L. Pima cotton is known for its resistance to Fusarium wilt and for producing fibers of superior quality highly prized in the textile market. We report a high-quality genome assembly and annotation of Pima-S6 cotton and its comparison at the chromosome and protein level to other ten Gossypium published genome assemblies. RESULTS: Synteny and orthogroup analyses revealed important differences on chromosome structure and annotated proteins content between our Pima-S6 and other publicly available G. barbadense assemblies, and across Gossypium assemblies in general. Detailed synteny analyses revealed chromosomal rearrangements between Pima-S6 and other Pima genomes on several chromosomes, with three major inversions in chromosomes A09, A13 and D05, raising questions about the true chromosome structure of Gossypium barbadense genomes. CONCLUSION: Analyses of the re-assembled and re-annotated genome of the close relative G. barbadense Pima 3-79 using our Pima-S6 assembly suggest that contig placement of some recent G. barbadense assemblies might have been unduly influenced by the use of the G. hirsutum TM-1 genome as the anchoring reference. The Pima-S6 reference genome provides a valuable genomic resource and offers new insights on genomic structure, and can serve as G. barbadense genome reference for future assemblies and further support FOV4-related studies and breeding efforts.

Enhanced phenylpropanoid metabolism underlies resistance to Fusarium oxysporum f. sp. vasinfectum race 4 infection in the cotton cultivar Pima-S6 (Gossypium barbadense L.).

Introduction: Fusarium oxysporum f. sp. vasinfectum (FOV) race 4 (FOV4) is a highly pathogenic soil-borne fungus responsible for Fusarium wilt in cotton (Gossypium spp.) and represents a continuing threat to cotton production in the southwest states of the United States, including California, New Mexico, and Texas. Pima (G. barbadense L.) cotton, which is highly valued for its fiber quality, has been shown to be more susceptible to this pathogen than Upland (G. hirsutum L.) cotton. Still, some Pima cultivars present resistance to FOV4 infection. Methods: To gain insights into the FOV4-resistance mechanism, we performed comparative transcriptional and metabolomic analyses between FOV4-susceptible and FOV4-resistant Pima cotton entries. FOV4-resistant Pima-S6 and FOV4-susceptible Pima S-7 and Pima 3-79 cotton plants were infected with FOV4 in the greenhouse, and the roots harvested 11 days post-infection for further analysis. Results: We found that an enhanced root phenylpropanoid metabolism in the resistant Pima-S6 cultivar determines FOV4-resistance. Gene-ontology enrichment of phenylpropanoid biosynthesis and metabolism categories correlated with the accumulation of secondary metabolites in Pima-S6 roots. Specifically, we found esculetin, a coumarin, an inhibitor of Fusarium's growth, accumulated in the roots of Pima-S6 even under non-infected conditions. Genes related to the phenylpropanoid biosynthesis and metabolism, including phenylalanine ammonia-lyase 2 (PAL2) and pleiotropic drug resistance 12 (PDR12) transporter, were found to be upregulated in Pima-S6 roots. Discussion: Our results highlight an essential role for the phenylpropanoid synthesis pathway in FOV4 resistance in Pima-S6 cotton. These genes represent attractive research prospects for FOV4-disease resistance and breeding approaches of other cotton cultivars of economic relevance.

Titanium nanoparticles activate a transcriptional response in Arabidopsis that enhances tolerance to low phosphate, osmotic stress and pathogen infection.

Titanium is a ubiquitous element with a wide variety of beneficial effects in plants, including enhanced nutrient uptake and resistance to pathogens and abiotic stresses. While there is numerous evidence supporting the beneficial effects that Ti fertilization give to plants, there is little information on which genetic signaling pathways the Ti application activate in plant tissues. In this study, we utilize RNA-seq and ionomics technologies to unravel the molecular signals that Arabidopsis plants unleash when treated with Ti. RNA-seq analysis showed that Ti activates abscisic acid and salicylic acid signaling pathways and the expression of NUCLEOTIDE BINDING SITE-LEUCINE RICH REPEAT receptors likely by acting as a chemical priming molecule. This activation results in enhanced resistance to drought, high salinity, and infection with Botrytis cinerea in Arabidopsis. Ti also grants an enhanced nutritional state, even at suboptimal phosphate concentrations by upregulating the expression of multiple nutrient and membrane transporters and by modifying or increasing the production root exudates. Our results suggest that Ti might act similarly to the beneficial element Silicon in other plant species.

Genome-wide chromatin accessibility analysis unveils open chromatin convergent evolution during polyploidization in cotton.

Allopolyploidization, resulting in divergent genomes in the same cell, is believed to trigger a "genome shock", leading to broad genetic and epigenetic changes. However, little is understood about chromatin and gene-expression dynamics as underlying driving forces during allopolyploidization. Here, we examined the genome-wide DNase I-hypersensitive site (DHS) and its variations in domesticated allotetraploid cotton (Gossypium hirsutum and Gossypium barbadense, AADD) and its extant AA (Gossypium arboreum) and DD (Gossypium raimondii) progenitors. We observed distinct DHS distributions between G. arboreum and G. raimondii. In contrast, the DHSs of the two subgenomes of G. hirsutum and G. barbadense showed a convergent distribution. This convergent distribution of DHS was also present in the wild allotetraploids Gossypium darwinii and G. hirsutum var. yucatanense, but absent from a resynthesized hybrid of G. arboreum and G. raimondii, suggesting that it may be a common feature in polyploids, and not a consequence of domestication after polyploidization. We revealed that putative cis-regulatory elements (CREs) derived from polyploidization-related DHSs were dominated by several families, including Dof, ERF48, and BPC1. Strikingly, 56.6% of polyploidization-related DHSs were derived from transposable elements (TEs). Moreover, we observed positive correlations between DHS accessibility and the histone marks H3K4me3, H3K27me3, H3K36me3, H3K27ac, and H3K9ac, indicating that coordinated interplay among histone modifications, TEs, and CREs drives the DHS landscape dynamics under polyploidization. Collectively, these findings advance our understanding of the regulatory architecture in plants and underscore the complexity of regulome evolution during polyploidization.

Conquering compacted soils: uncovering the molecular components of root soil penetration.

Global agriculture and food security face paramount challenges due to climate change and land degradation. Human-induced soil compaction severely affects soil fertility, impairing root system development and crop yield. There is a need to design compaction-resilient crops that can thrive in degraded soils and maintain high yields. To address plausible solutions to this challenging scenario, we discuss current knowledge on plant root penetration ability and delineate potential approaches based on root-targeted genetic engineering (RGE) and genomics-assisted breeding (GAB) for developing crops with enhanced root system penetrability (RSP) into compacted soils. Such approaches could lead to crops with improved resilience to climate change and marginal soils, which can help to boost CO2 sequestration and storage in deeper soil strata.

Root-Knot Nematode Resistance in Gossypium hirsutum Determined by a Constitutive Defense-Response Transcriptional Program Avoiding a Fitness Penalty.

Cotton (Gossypium spp.) is the most important renewable source of natural textile fiber and one of the most cultivated crops around the world. Plant-parasitic nematode infestations, such as the southern Root-Knot Nematode (RKN) Meloidogyne incognita, represent a threat to cotton production worldwide. Host-plant resistance is a highly effective strategy to manage RKN; however, the underlying molecular mechanisms of RKN-resistance remain largely unknown. In this study, we harness the differences in RKN-resistance between a susceptible (Acala SJ-2, SJ2), a moderately resistant (Upland Wild Mexico Jack Jones, WMJJ), and a resistant (Acala NemX) cotton entries, to perform genome-wide comparative analysis of the root transcriptional response to M. incognita infection. RNA-seq data suggest that RKN-resistance is determined by a constitutive state of defense transcriptional behavior that prevails in the roots of the NemX cultivar. Gene ontology and protein homology analyses indicate that the root transcriptional landscape in response to RKN-infection is enriched for responses related to jasmonic and salicylic acid, two key phytohormones in plant defense responses. These responses are constitutively activated in NemX and correlate with elevated levels of these two hormones while avoiding a fitness penalty. We show that the expression of cotton genes coding for disease resistance and receptor proteins linked to RKN-resistance and perception in plants, is enhanced in the roots of RKN-resistant NemX. Members of the later gene families, located in the confidence interval of a previously identified QTL associated with RKN resistance, represent promising candidates that might facilitate introduction of RKN-resistance into valuable commercial varieties of cotton. Our study provides novel insights into the molecular mechanisms that underlie RKN resistance in cotton.

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.

Whole-genome characterization and comparative genomics of a novel freshwater cyanobacteria species: Pseudanabaena punensis.

Cyanobacteria are emerging as a potential source of novel, beneficial bioactive compounds. However, some cyanobacteria species can harm water quality and public health through the production of toxins. Therefore, surveying the occurrence and generating genomic resources of cyanobacteria producing harmful compounds could help develop the control methods necessary to manage their growth and limit the release contaminants into the water bodies. Here, we describe a novel strain, Pseudanabaena punensis isolated from the open ends of pipelines supplying freshwater. This isolate was characterized morphologically, biochemically and by whole-genome sequence analysis. We also provide genomic information for P. punensis to help understand and highlight the features unique to this isolate. Morphological and genetic (analysis using 16S rRNA and rbcL genes) data were used to assign this novel strain to phylogenetic and taxonomic groups. The isolate was identified as a filamentous and non-heterocystous cyanobacteria. Based on morphological and 16S rRNA phylogeny, this isolate shares characteristics with the Pseudanabaenaceae family, but remains distinct from well-characterized species suggesting its polyphyletic assemblage. The whole-genome sequence analysis suggests greater genomic and phenotypic plasticity. Genome-wide sequence and comparative genomic analyses, comparing against several closely related species, revealed diverse and important genes associated with synthesizing bioactive compounds, multi-drug resistance pathway, heavy metal resistance, and virulence factors. This isolate also produces several important fatty acids with potential industrial applications. The observations described in this study emphasize both industrial applications and risks associated with the freshwater contamination, and therefore genomic resources provided in this study offer an opportunity for further investigations.

A microalgal-based preparation with synergistic cellulolytic and detoxifying action towards chemical-treated lignocellulose.

High-temperature bioconversion of lignocellulose into fermentable sugars has drawn attention for efficient production of renewable chemicals and biofuels, because competing microbial activities are inhibited at elevated temperatures and thermostable cell wall degrading enzymes are superior to mesophilic enzymes. Here, we report on the development of a platform to produce four different thermostable cell wall degrading enzymes in the chloroplast of Chlamydomonas reinhardtii. The enzyme blend was composed of the cellobiohydrolase CBM3GH5 from C. saccharolyticus, the ß-glucosidase celB from P. furiosus, the endoglucanase B and the endoxylanase XynA from T. neapolitana. In addition, transplastomic microalgae were engineered for the expression of phosphite dehydrogenase D from Pseudomonas stutzeri, allowing for growth in non-axenic media by selective phosphite nutrition. The cellulolytic blend composed of the glycoside hydrolase (GH) domain GH12/GH5/GH1 allowed the conversion of alkaline-treated lignocellulose into glucose with efficiencies ranging from 14% to 17% upon 48h of reaction and an enzyme loading of 0.05% (w/w). Hydrolysates from treated cellulosic materials with extracts of transgenic microalgae boosted both the biogas production by methanogenic bacteria and the mixotrophic growth of the oleaginous microalga Chlorella vulgaris. Notably, microalgal treatment suppressed the detrimental effect of inhibitory by-products released from the alkaline treatment of biomass, thus allowing for efficient assimilation of lignocellulose-derived sugars by C. vulgaris under mixotrophic growth.

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.

Functional analysis of the OsNPF4.5 nitrate transporter reveals a conserved mycorrhizal pathway of nitrogen acquisition in plants.

Low availability of nitrogen (N) is often a major limiting factor to crop yield in most nutrient-poor soils. Arbuscular mycorrhizal (AM) fungi are beneficial symbionts of most land plants that enhance plant nutrient uptake, particularly of phosphate. A growing number of reports point to the substantially increased N accumulation in many mycorrhizal plants; however, the contribution of AM symbiosis to plant N nutrition and the mechanisms underlying the AM-mediated N acquisition are still in the early stages of being understood. Here, we report that inoculation with AM fungus Rhizophagus irregularis remarkably promoted rice (Oryza sativa) growth and N acquisition, and about 42% of the overall N acquired by rice roots could be delivered via the symbiotic route under N-NO3- supply condition. Mycorrhizal colonization strongly induced expression of the putative nitrate transporter gene OsNPF4.5 in rice roots, and its orthologs ZmNPF4.5 in Zea mays and SbNPF4.5 in Sorghum bicolor OsNPF4.5 is exclusively expressed in the cells containing arbuscules and displayed a low-affinity NO3- transport activity when expressed in Xenopus laevis oocytes. Moreover, knockout of OsNPF4.5 resulted in a 45% decrease in symbiotic N uptake and a significant reduction in arbuscule incidence when NO3- was supplied as an N source. Based on our results, we propose that the NPF4.5 plays a key role in mycorrhizal NO3- acquisition, a symbiotic N uptake route that might be highly conserved in gramineous species.

Assessment of the ptxD gene as a growth and selective marker in Trichoderma atroviride using Pccg6, a novel constitutive promoter.

BACKGROUND: Trichoderma species are among the most effective cell factories to produce recombinant proteins, whose productivity relies on the molecular toolkit and promoters available for the expression of the target protein. Although inducible promoter systems have been developed for producing recombinant proteins in Trichoderma, constitutive promoters are often a desirable alternative. Constitutive promoters are simple to use, do not require external stimuli or chemical inducers to be activated, and lead to purer enzyme preparations. Moreover, most of the promoters for homologous and heterologous expression reported in Trichoderma have been commonly evaluated by directly assessing production of industrial enzymes, requiring optimization of laborious protocols. RESULTS: Here we report the identification of Pccg6, a novel Trichoderma atroviride constitutive promoter, that has similar transcriptional strength as that of the commonly used pki1 promoter. Pccg6 displayed conserved arrangements of transcription factor binding sites between promoter sequences of Trichoderma ccg6 orthologues genes, potentially involved in their regulatory properties. The predicted ccg6-encoded protein potentially belongs to the SPE1/SPI1 protein family and shares high identity with CCG6 orthologue sequences from other fungal species including Trichoderma reesei, Trichoderma virens, Trichoderma asperellum, and to a lesser extent to that of Neurospora crassa. We also report the use of the Pccg6 promoter to drive the expression of PTXD, a phosphite oxidoreductase of bacterial origin, which allowed T. atroviride to utilize phosphite as a sole source of phosphorus. We propose ptxD as a growth reporter gene that allows real-time comparison of the functionality of different promoters by monitoring growth of Trichoderma transgenic lines and enzymatic activity of PTXD. Finally, we show that constitutive expression of ptxD provided T. atroviride a competitive advantage to outgrow bacterial contaminants when supplied with phosphite as a sole source of phosphorus. CONCLUSIONS: A new constitutive promoter, ccg6, for expression of homologous and heterologous proteins has been identified and tested in T. atroviride to express PTXD, which resulted in an effective and visible phenotype to evaluate transcriptional activity of sequence promoters. Use of PTXD as a growth marker holds great potential for assessing activity of other promoters and for biotechnological applications as a contamination control system.

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

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.

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

KEY MESSAGE: 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.

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.