A dual-targeted purple acid phosphatase in Arabidopsis thaliana moderates carbon metabolism and its overexpression leads to faster plant growth and higher seed yield

Acid phosphatase involved in phosphate homeostasis in Brassica napus and the functional analysis of BnaPAP10s

Purple acid phosphatases (PAPs) are involved in activating the rhizosphere's organic phosphorus (P) and promoting P recycling during plant development, especially under the long-term P deficiency conditions in acid soil. However, the function of BnaPAPs in response to P deficiency stress in Brassica napus has rarely been explored. In this study, we found that the acid phosphatase activities (APA) of rapeseed shoot and root increased under P deficienct conditions. Genome-wide identification found that 82 PAP genes were unevenly distributed on 19 chromosomes in B. napus, which could be divided into eight subfamilies. The segmental duplication events were the main driving force for expansion during evolution, and the gene structures and conserved motifs of most members within the same subfamily were highly conservative. Moreover, the expression levels of 37 and 23 different expressed genes were induced by low P in leaf and root, respectively. BnaA09.PAP10a and BnaC09.PAP10a were identified as candidate genes via interaction networks. Significantly, both BnaPAP10a overexpression lines significantly increased root-related APA and total phosphate concentration under P deficiency and ATP supply conditions, thereby improving plant growth and root length. In summary, our results provided a valuable foundation for further study of BnaPAP functions.

Probing the physiological role of the plastid outer-envelope membrane using the oemiR plasmid collection

Plastids are the site of complex biochemical pathways, most prominently photosynthesis. The organelle evolved through endosymbiosis with a cyanobacterium, which is exemplified by the outer envelope membrane that harbors more than 40 proteins in Arabidopsis. Their evolutionary conservation indicates high significance for plant cell function. While a few proteins are well-studied as part of the protein translocon complex the majority of outer envelope protein functions is unclear. Gaining a deeper functional understanding has been complicated by the lack of observable loss-of-function mutant phenotypes, which is often rooted in functional genetic redundancy. Therefore, we designed outer envelope-specific artificial micro RNAs (oemiRs) capable of downregulating transcripts from several loci simultaneously. We successfully tested oemiR function by performing a proof-of-concept screen for pale and cold-sensitive mutants. An in-depth analysis of pale mutant alleles deficient in the translocon component TOC75 using proteomics provided new insights into putative compensatory import pathways. The cold stress screen not only recapitulated 3 previously known phenotypes of cold-sensitive mutants but also identified 4 mutants of additional oemiR outer envelope loci. Altogether our study revealed a role of the outer envelope to tolerate cold conditions and showcasts the power of the oemiR collection to research the significance of outer envelope proteins.

The Role of TCA Cycle Enzymes in Plants

As one of the iconic pathways in plant metabolism, the tricarboxylic acid (TCA) cycle is commonly thought to not only be responsible for the oxidization of respiratory substrate to drive ATP synthesis but also provide carbon skeletons to anabolic processes and contribute to carbon-nitrogen interaction and biotic stress responses. The functions of the TCA cycle enzymes are characterized by a saturation transgenesis approach, whereby the constituent expression of proteins is knocked out or reduced in order to investigate their function in vivo. The alteration of TCA cycle enzyme expression results in changed plant growth and photosynthesis under controlled conditions. Moreover, improvements in plant performance and postharvest properties are reported by overexpression of either endogenous forms or heterologous genes of a number of the enzymes. Given the importance of the TCA cycle in plant metabolism regulation, here, the function of each enzyme and its roles in different tissues are discussed. This article additionally highlights the recent finding that the plant TCA cycle, like that of mammals and microbes, dynamically assembles functional substrate channels or metabolons and discusses the implications of this finding to the current understanding of the metabolic regulation of the plant TCA cycle.

Functional assessment of AtPAP17; encoding a purple acid phosphatase involved in phosphate metabolism in Arabidopsis thaliana

PURPOSE

Purple acid phosphatases (PAPs) includ the largest classes of non-specific plant acid phosphatases. Most characterized PAPs were found to play physiological functions in phosphorus metabolism. In this study, we investigated the function of AtPAP17 gene encoding an important purple acid phosphatase in Arabidopsis thaliana.

METHODS

The full-length cDNA sequence of AtPAP17 gene under the control of CaMV-35S promoter was transferred to the A. thaliana WT plant. The generated homozygote AtPAP17-overexpressed plants were compared by the types of analyses with corresponding homozygote atpap17-mutant plant and WT in both + P (1.2 mM) and - P (0 mM) conditions.

RESULTS

In the + P condition, the highest and the lowest amount of Pi was observed in AtPAP17-overexpressed plants and atpap17-mutant plants by 111% increase and 38% decrease compared with the WT plants, respectively. Furthermore, under the same condition, APase activity of AtPAP17-overexpressed plants increased by 24% compared to the WT. Inversely, atpap17-mutant plant represented a 71% fall compared to WT plants. The comparison of fresh weight and dry weight in the studied plants showed that the highest and the lowest amount of absorbed water belonged to OE plants (with 38 and 12 mg plant^-1) and Mu plants (with 22 and 7 mg plant^-1) in + P and - P conditions, respectively.

CONCLUSION

The lack of AtPAP17 gene in the A. thaliana genome led to a remarkable reduction in the development of root biomass. Thus, AtPAP17 could have an important role in the root but not shoot developmental and structural programming. Consequently, this function enables them to absorb more water and eventually associated with more phosphate absorption.

Understanding source-sink interactions: Progress in model plants and translational research to crops

Agriculture is facing a massive increase in demand per hectare as a result of an ever-expanding population and environmental deterioration. While we have learned much about how environmental conditions and diseases impact crop yield, until recently considerably less was known concerning endogenous factors, including within-plant nutrient allocation. In this review, we discuss studies of source-sink interactions covering both fundamental research in model systems under controlled growth conditions and how the findings are being translated to crop plants in the field. In this respect we detail efforts aimed at improving and/or combining C₃, C₄, and CAM modes of photosynthesis, altering the chloroplastic electron transport chain, modulating photorespiration, adopting bacterial/algal carbon-concentrating mechanisms, and enhancing nitrogen- and water-use efficiencies. Moreover, we discuss how modulating TCA cycle activities and primary metabolism can result in increased rates of photosynthesis and outline the opportunities that evaluating natural variation in photosynthesis may afford. Although source, transport, and sink functions are all covered in this review, we focus on discussing source functions because the majority of research has been conducted in this field. Nevertheless, considerable recent evidence, alongside the evidence from classical studies, demonstrates that both transport and sink functions are also incredibly important determinants of yield. We thus describe recent evidence supporting this notion and suggest that future strategies for yield improvement should focus on combining improvements in each of these steps to approach yield optimization.

Purple acid phosphatase2 stimulates a futile cycle of lipid synthesis and degradation, and mitigates the negative growth effects of triacylglycerol accumulation in vegetative tissues

Storage lipids (mostly triacylglycerols, TAGs) serve as an important energy and carbon reserve in plants, and hyperaccumulation of TAG in vegetative tissues can have negative effects on plant growth. Purple acid phosphatase2 (PAP2) was previously shown to affect carbon metabolism and boost plant growth. However, the effects of PAP2 on lipid metabolism remain unknown. Here, we demonstrated that PAP2 can stimulate a futile cycle of fatty acid (FA) synthesis and degradation, and mitigate negative growth effects associated with high accumulation of TAG in vegetative tissues. Constitutive expression of PAP2 in Arabidopsis thaliana enhanced both lipid synthesis and degradation in leaves and led to a substantial increase in seed oil yield. Suppressing lipid degradation in a PAP2-overexpressing line by disrupting sugar-dependent1 (SDP1), a predominant TAG lipase, significantly elevated vegetative TAG content and improved plant growth. Diverting FAs from membrane lipids to TAGs in PAP2-overexpressing plants by constitutively expressing phospholipid:diacylglycerol acyltransferase1 (PDAT1) greatly increased TAG content in vegetative tissues without compromising biomass yield. These results highlight the potential of combining PAP2 with TAG-promoting factors to enhance carbon assimilation, FA synthesis and allocation to TAGs for optimized plant growth and storage lipid accumulation in vegetative tissues.

AAQSP increases mapping resolution of stable QTLs through applying NGS-BSA in multiple genetic backgrounds

In this study, we present AAQSP as an extension of existing NGS-BSA applications for identifying stable QTLs at high resolution. GhPAP16 and GhIQD14 fine mapped on chromosome D09 of upland cotton are identified as important candidate genes for lint percentage (LP). Bulked segregant analysis combined with next generation sequencing (NGS-BSA) allows rapid identification of genome sequence differences responsible for phenotypic variation. The NGS-BSA approach applied to crops mainly depends on comparing two bulked DNA samples of individuals from an F₂ population. Since some F₂ individuals still maintain high heterozygosity, heterosis will exert complications in pursuing NGS-BSA in such populations. In addition, the genetic background influences the stability of gene expression in crops, so some QTLs mapped in one segregating population may not be widely applied in crop improvement. The AAQSP (Association Analysis of QTL-seq on Semi-homologous Populations) reported in our study combines the optimized scheme of constructing BSA bulks with NGS-BSA analysis in two (or more) different parental genetic backgrounds for isolating the stable QTLs. With application of AAQSP strategy and construction of a high-density linkage map, we have successfully identified a QTL significantly related to lint percentage (LP) in cultivated upland cotton, followed by map-based cloning to dissect two candidate genes, GhPAP16 and GhIQD14. This study demonstrated that AAQSP can efficiently identify stable QTLs for complex traits of interest, and thus accelerate the genetic improvement of upland cotton and other crop plants.

High-throughput measurement of plant fitness traits with an object detection method using Faster R-CNN

Revealing the contributions of genes to plant phenotype is frequently challenging because loss-of-function effects may be subtle or masked by varying degrees of genetic redundancy. Such effects can potentially be detected by measuring plant fitness, which reflects the cumulative effects of genetic changes over the lifetime of a plant. However, fitness is challenging to measure accurately, particularly in species with high fecundity and relatively small propagule sizes such as Arabidopsis thaliana. An image segmentation-based method using the software ImageJ and an object detection-based method using the Faster Region-based Convolutional Neural Network (R-CNN) algorithm were used for measuring two Arabidopsis fitness traits: seed and fruit counts. The segmentation-based method was error-prone (correlation between true and predicted seed counts, r²  = 0.849) because seeds touching each other were undercounted. By contrast, the object detection-based algorithm yielded near perfect seed counts (r²  = 0.9996) and highly accurate fruit counts (r²  = 0.980). Comparing seed counts for wild-type and 12 mutant lines revealed fitness effects for three genes; fruit counts revealed the same effects for two genes. Our study provides analysis pipelines and models to facilitate the investigation of Arabidopsis fitness traits and demonstrates the importance of examining fitness traits when studying gene functions.

Quantitative trait loci mapping and candidate gene analysis of stoma-related traits in wheat (Triticum aestivum L.) glumes

The photosynthesis of wheat glumes makes important contributions to the yield. Stomata play a crucial role in regulating photosynthesis and transpiration in plants. However, the genetic base of wheat glume stomata is not fully understood. In this study, stomatal length (SL), stomatal width (SW), stomatal density (SD), potential conductance index (PCI) of stomata, stomatal area (SA), and stomatal relative area (SRA) were measured in different parts of wheat glumes from a doubled haploid (DH) population and their parents. Quantitative trait loci (QTLs) of these traits were anchored on a high-density genetic linkage map of the DH population. A total of 61 QTLs for stoma-related traits were mapped onto 16 chromosomes, and each one accounted for 3.63 to 19.02% of the phenotypic variations. Two QTL hotspots were detected in two marker intervals, AX-109400932∼AX-110985652 and AX-108972184∼AX-108752564, on chromosome 6A. Five possibly candidate genes (TraesCS6A02G105400, TraesCS6A02G106400, TraesCS6A02G115100, TraesCS6A02G115400, and TraesCS6A02G116200) for stoma-related traits of wheat glumes were screened out , according to their predicted expression levels in wheat glumes or spikes. The expression of these genes may be induced by a variety of abiotic stresses. These findings provide insights for cloning and functional characterization of stoma-related candidate genes in wheat glumes.

Fine-tuning the transcriptional regulatory model of adaptation response to phosphate stress in maize (Zea mays L.)

The post green revolution agriculture is based on generous application of fertilizers and high-yielding genotypes that are suited for such high input regimes. Cereals, like maize (Zea mays L.) are capable of utilizing less than 20% of the applied inorganic phosphate (Pi) - a non-renewable fertilizer resource. A greater understanding of the molecular mechanisms underlying the acquisition, transportation and utilization of Pi may lead to engineering genotypes with high phosphorus use efficiency. In this study, we carried out functional domain similarity analysis, promoter analysis and comparative transcriptional expression profiling of 12 selected Pi responsive genes in the Pi stress tolerant maize inbred line HKI-163 under sufficient and deficient Pi conditions. Pi starvation led to significant increase in root length; marked proliferation of root hairs and lesser number of crown roots. Eleven genes were significantly up or down regulated in Pi deficient condition. The putative acid phosphatase, ZmACP5 expression was up regulated by 162.81 and 74.40 fold in root and leaf tissues, respectively. The RNase, ZmRNS1 showed 115 fold up regulation in roots under Pi deprivation. Among the two putative high affinity Pi transporters ZmPht1;4 was found specific to root, whereas ZmPht2 was found to be up regulated in both root and leaf tissues. The genes involved in Pi homeostasis pathway (ZmSIZ1, SPX1 and Pho2) were up regulated in root and leaf. In light of the expression profiling of selected regulatory genes, an updated model of transcriptional regulation under Pi starvation in maize has been presented.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01155-x.

Transcript Profiling Analysis and ncRNAs' Identification of Male-Sterile Systems of Brassica campestris Reveal New Insights Into the Mechanism Underlying Anther and Pollen Development

Male-sterile mutants are useful materials to study the anther and pollen development. Here, whole transcriptome sequencing was performed for inflorescences in three sterile lines of Chinese cabbage (Brassica campestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis), the genic male-sterile line (A line), the Polima cytoplasmic male-sterile (CMS) line (P line), and the Ogura CMS line (O line) along with their maintainer line (B line). In total, 7,136 differentially expressed genes (DEGs), 361 differentially expressed long non-coding RNAs (lncRNAs) (DELs), 56 differentially expressed microRNAs (miRNAs) (DEMs) were selected out. Specific regulatory networks related to anther cell differentiation, meiosis cytokinesis, pollen wall formation, and tapetum development were constructed based on the abortion characteristics of male-sterile lines. Candidate genes and lncRNAs related to cell differentiation were identified in sporocyteless P line, sixteen of which were common to the DEGs in Arabidopsis spl/nzz mutant. Genes and lncRNAs concerning cell plate formation were selected in A line that is defected in meiosis cytokinesis. Also, the orthologs of pollen wall formation and tapetum development genes in Arabidopsis showed distinct expression patterns in the three different sterile lines. Among 361 DELs, 35 were predicted to interact with miRNAs, including 28 targets, 47 endogenous target mimics, and five precursors for miRNAs. Two lncRNAs were further proved to be functional precursors for bra-miR156 and bra-miR5718, respectively. Overexpression of bra-miR5718HG in B. campestris slowed down the growth of pollen tubes, caused shorter pollen tubes, and ultimately affected the seed set. Our study provides new insights into molecular regulation especially the ncRNA interaction during pollen development in Brassica crops.

Purple acid phosphatases: roles in phosphate utilization and new emerging functions

Plants strive for phosphorus (P), which is an essential mineral for their life. Since P availability is limiting in most of the world's soils, plants have evolved with a complex network of genes and their regulatory mechanisms to cope with soil P deficiency. Among them, purple acid phosphatases (PAPs) are predominantly associated with P remobilization within the plant and acquisition from the soil by hydrolyzing organic P compounds. P in such compounds remains otherwise unavailable to plants for assimilation. PAPs are ubiquitous in plants, and similar enzymes exist in bacteria, fungi, mammals, and unicellular eukaryotes, but having some differences in their catalytic center. In the recent past, PAPs' roles have been extended to multiple plant processes like flowering, seed development, senescence, carbon metabolism, response to biotic and abiotic stresses, signaling, and root development. While new functions have been assigned to PAPs, the underlying mechanisms remained understood poorly. Here, we review the known functions of PAPs, the regulatory mechanisms, and their relevance in crop improvement for P-use-efficiency. We then discuss the mechanisms behind their functions and propose areas worthy of future research. Finally, we argue that PAPs could be a potential target for improving P utilization in crops. In turn, this is essential for sustainable agriculture.

Overlapping Functions of the Paralogous Proteins AtPAP2 and AtPAP9 in Arabidopsis thaliana

Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2), which is anchored to the outer membranes of chloroplasts and mitochondria, affects carbon metabolism by modulating the import of some preproteins into chloroplasts and mitochondria. AtPAP9 bears a 72% amino acid sequence identity with AtPAP2, and both proteins carry a hydrophobic motif at their C-termini. Here, we show that AtPAP9 is a tail-anchored protein targeted to the outer membrane of chloroplasts. Yeast two-hybrid and bimolecular fluorescence complementation experiments demonstrated that both AtPAP9 and AtPAP2 bind to a small subunit of rubisco 1B (AtSSU1B) and a number of chloroplast proteins. Chloroplast import assays using [35S]-labeled AtSSU1B showed that like AtPAP2, AtPAP9 also plays a role in AtSSU1B import into chloroplasts. Based on these data, we propose that AtPAP9 and AtPAP2 perform overlapping roles in modulating the import of specific proteins into chloroplasts. Most plant genomes contain only one PAP-like sequence encoding a protein with a hydrophobic motif at the C-terminus. The presence of both AtPAP2 and AtPAP9 in the Arabidopsis genome may have arisen from genome duplication in Brassicaceae. Unlike AtPAP2 overexpression lines, the AtPAP9 overexpression lines did not exhibit early-bolting or high-seed-yield phenotypes. Their differential growth phenotypes could be due to the inability of AtPAP9 to be targeted to mitochondria, as the overexpression of AtPAP2 on mitochondria enhances the capacity of mitochondria to consume reducing equivalents.

A Balance between the Activities of Chloroplasts and Mitochondria Is Crucial for Optimal Plant Growth

Energy metabolism in plant cells requires a balance between the activities of chloroplasts and mitochondria, as they are the producers and consumers of carbohydrates and reducing equivalents, respectively. Recently, we showed that the overexpression of Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2), a phosphatase dually anchored on the outer membranes of chloroplasts and mitochondria, can boost the plant growth and seed yield of Arabidopsis thaliana by coordinating the activities of both organelles. However, when AtPAP2 is solely overexpressed in chloroplasts, the growth-promoting effects are less optimal, indicating that active mitochondria are required for dissipating excess reducing equivalents from chloroplasts to maintain the optimal growth of plants. It is even more detrimental to plant productivity when AtPAP2 is solely overexpressed in mitochondria. Although these lines contain high level of adenosine triphosphate (ATP), they exhibit low leaf sucrose, low seed yield, and early senescence. These transgenic lines can be useful tools for studying how hyperactive chloroplasts or mitochondria affect the physiology of their counterparts and how they modify cellular metabolism and plant physiology.

Hydrolysis of organophosphorus by diatom purple acid phosphatase and sequential regulation of cell metabolism

Phosphorus (P) limitation affects phytoplankton growth and population size in aquatic systems, and consequently limits aquatic primary productivity. Plants have evolved a range of metabolic responses to cope with P limitation, such as accumulation of purple acid phosphatases (PAPs) to enhance acquisition of phosphates. However, it remains unknown whether algae have evolved a similar mechanism. In this study, we examined the role of PAPs in the model microalga Phaeodactylum tricornutum. Expression of PAP1 was enhanced in P. tricornutum cells grown on organophosphorus compared to inorganic phosphate. PAP1 overexpression improved cellular growth and biochemical composition in a growth-phase dependent manner. PAP1 promoted growth and photosynthesis during growth phases and reallocated carbon flux towards lipogenesis during the stationary phase. PAP1 was found to be localized in the endoplasmic reticulum and it orchestrated the expression of genes involved in key metabolic pathways and translocation of inorganic P (Pi), thereby improving energy use, reducing equivalents and antioxidant potential. RNAi of PAP1 induced expression of its homolog PAP2, thereby compensating for the Pi scavenging activity of PAP1. Our results demonstrate that PAP1 brings about sequential regulation of metabolism, and provide novel insights into algal phosphorus metabolism and aquatic primary productivity.

Modulating the activities of chloroplasts and mitochondria promotes adenosine triphosphate production and plant growth

Efficient photosynthesis requires a balance of ATP and NADPH production/consumption in chloroplasts, and the exportation of reducing equivalents from chloroplasts is important for balancing stromal ATP/NADPH ratio. Here, we showed that the overexpression of purple acid phosphatase 2 on the outer membranes of chloroplasts and mitochondria can streamline the production and consumption of reducing equivalents in these two organelles, respectively. A higher capacity of consumption of reducing equivalents in mitochondria can indirectly help chloroplasts to balance the ATP/NADPH ratio in stroma and recycle NADP⁺, the electron acceptors of the linear electron flow (LEF). A higher rate of ATP and NADPH production from the LEF, a higher capacity of carbon fixation by the Calvin-Benson-Bassham (CBB) cycle and a greater consumption of NADH in mitochondria enhance photosynthesis in the chloroplasts, ATP production in the mitochondria and sucrose synthesis in the cytosol and eventually boost plant growth and seed yields in the overexpression lines.

Differential RNA Editing and Intron Splicing in Soybean Mitochondria during Nodulation

Nitrogen fixation in soybean consumes a tremendous amount of energy, leading to substantial differences in energy metabolism and mitochondrial activities between nodules and uninoculated roots. While C-to-U RNA editing and intron splicing of mitochondrial transcripts are common in plant species, their roles in relation to nodule functions are still elusive. In this study, we performed RNA-seq to compare transcript profiles and RNA editing of mitochondrial genes in soybean nodules and roots. A total of 631 RNA editing sites were identified on mitochondrial transcripts, with 12% or 74 sites differentially edited among the transcripts isolated from nodules, stripped roots, and uninoculated roots. Eight out of these 74 differentially edited sites are located on the matR transcript, of which the degrees of RNA editing were the highest in the nodule sample. The degree of mitochondrial intron splicing was also examined. The splicing efficiencies of several introns in nodules and stripped roots were higher than in uninoculated roots. These include nad1 introns 2/3/4, nad4 intron 3, nad5 introns 2/3, cox2 intron 1, and ccmFc intron 1. A greater splicing efficiency of nad4 intron 1, a higher NAD4 protein abundance, and a reduction in supercomplex I + III2 were also observed in nodules, although the causal relationship between these observations requires further investigation.

Adenine Nucleotide and Nicotinamide Adenine Dinucleotide Measurements in Plants

As the principal co-factors of many metabolic pathways, the measurement of both adenine nucleotides and nicotinamide adenine dinucleotide provides important information about cellular energy metabolism. However, given their rapid and reversible conversion as well as their relatively low concentration ranges, it is difficult to measure these compounds. Here, we describe a highly sensitive and selective ion-pairing HPLC method with fluorescence detection to quantify adenine nucleotides in plants. In addition, nicotinamide adenine dinucleotide is a crucially important redox-active substrate for multiple catabolic and anabolic reactions with the ratios of NAD⁺ /NADH and NADP⁺ /NADPH being suggested as indicators of the general intracellular redox potential and hence metabolic state. Here, we describe highly sensitive enzyme cycling-based colorimetric assays (with a detection limit in the pmol range) performed subsequent to a simple extraction procedure involving acid or base extraction to allow the measurement of the cellular levels of these metabolites. © 2020 The Authors. Basic Protocol 1: Preparation of plant material for the measurement Basic Protocol 2: Measurement of ATP, ADP, and AMP via HPLC Basic Protocol 3: NAD⁺ /NADP⁺ measurements Basic Protocol 4: NADH/NADPH measurements Basic Protocol 5: Data analysis and quality control approaches.

Target genes for plant productivity improvement

The use of chemical fertilizers and pesticides, as well as the development of high-yielding varieties enabled substantial increase in crop productivity during the 20th century. However, the increase in yield over the last two decades has been slower. It is thought that further improvement in productivity of the major crop species using traditional cultivation methods is limited. Therefore, the use of genetic engineering seems to be a promising approach. There is ongoing research concerning genes that have an impact on plant growth, development and yield. The proteins and miRNAs encoded by these genes participate in a variety of processes, such as growth regulation, assimilate transport and partitioning as well as macronutrient uptake and metabolism. This paper presents the major directions in research concerning genes that may be targets of genetic engineering aimed to improve plant productivity.

Improving photosynthesis, plant productivity and abiotic stress tolerance - current trends and future perspectives

With unfavourable climate changes and an increasing global population, there is a great need for more productive and stress-tolerant crops. As traditional methods of crop improvement have probably reached their limits, a further increase in the productivity of crops is expected to be possible using genetic engineering. The number of potential genes and metabolic pathways, which when genetically modified could result in improved photosynthesis and biomass production, is multiple. Photosynthesis, as the only source of carbon required for the growth and development of plants, attracts much attention is this respect, especially the question concerning how to improve CO₂ fixation and limit photorespiration. The most promising direction for increasing CO₂ assimilation is implementating carbon concentrating mechanisms found in cyanobacteria and algae into crop plants, while hitherto performed experiments on improving the CO₂ fixation versus oxygenation reaction catalyzed by Rubisco are less encouraging. On the other hand, introducing the C₄ pathway into C₃ plants is a very difficult challenge. Among other points of interest for increased biomass production is engineering of metabolic regulation, certain proteins, nucleic acids or phytohormones. In this respect, enhanced sucrose synthesis, assimilate translocation to sink organs and starch synthesis is crucial, as is genetic engineering of the phytohormone metabolism. As abiotic stress tolerance is one of the key factors determining crop productivity, extensive studies are being undertaken to develop transgenic plants characterized by elevated stress resistance. This can be accomplished due to elevated synthesis of antioxidants, osmoprotectants and protective proteins. Among other promising targets for the genetic engineering of plants with elevated stress resistance are transcription factors that play a key role in abiotic stress responses of plants. In this review, most of the approaches to improving the productivity of plants that are potentially promising and have already been undertaken are described. In addition to this, the limitations faced, potential challenges and possibilities regarding future research are discussed.

A root-associated purple acid phosphatase, SgPAP23, mediates extracellular phytate-P utilization in Stylosanthes guianensis

As a major component of soil organic phosphorus (P), phytate-P is unavailable to plants unless hydrolysed by phytase to release inorganic phosphate. However, knowledge on natural variation in root-associated phytase activity and its underlying molecular mechanisms in plants remains fragmentary. In this study, variations in root internal and associated phytase activity were observed among 39 genotypes of Stylosanthes guianensis (Stylo), which is well adapted to acid soils. Furthermore, TPRC2001-1, the genotype with the highest root-associated phytase activity, was more capable of utilizing extracellular phytate-P than Fine-stem, the genotype with the lowest root-associated phytase activity. After protein liquid chromatography-tandem mass spectrometry analysis, a purple acid phosphatase (PAP), SgPAP23, was identified and cloned from TPRC2001-1. SgPAP23 exhibited high activity against phytate-P and was mainly localized on the plasma membrane. Furthermore, SgPAP23 overexpression resulted in significant increases of root-associated phytase activity and thus facilitated extracellular phytate-P utilization in both bean (Phaseolus vulgaris) hairy roots and Arabidopsis thaliana. The results herein support the conclusion that SgPAP23 is a primary contributor to the superior extracellular phytate-P utilization in stylo and thus is used to develop cultivars with efficient extracellular phytate-P utilization.

AtPAP2, a Unique Member of the PAP Family, Functions in the Plasma Membrane

Purple acid phosphatases (PAPs) play various physiological roles in plants. AtPAP2 was previously shown to localize to both chloroplasts and mitochondria and to modulate carbon metabolism in Arabidopsis. Over-expression of AtPAP2 resulted in faster growth and increased biomass in several plant species, indicating its great potential for crop improvement of phosphate use and yield. Here, we studied the localization of AtPAP2 by transient expression in tobacco leaves. The results showed AtPAP2 was localized to the plasma membrane through the secretory pathway, which is different from previous studies. We also found that AtPAP2 had a close relationship with fungal PAP2-like proteins based on phylogenetic analysis. In addition, the C-terminal transmembrane domain conserved in land plants is unique among other AtPAPs except AtPAP9, which is a close homolog of AtPAP2. Taken together, our results provide information for further study of AtPAP2 in understanding its special function in crop improvement.

Association of extracellular dNTP utilization with a GmPAP1-like protein identified in cell wall proteomic analysis of soybean roots

Plant root cell walls are dynamic systems that serve as the first plant compartment responsive to soil conditions, such as phosphorus (P) deficiency. To date, evidence for the regulation of root cell wall proteins (CWPs) by P deficiency remains sparse. In order to gain a better understanding of the roles played by CWPs in the roots of soybean (Glycine max) in adaptation to P deficiency, we conducted an iTRAQ (isobaric tag for relative and absolute quantitation) proteomic analysis. A total of 53 CWPs with differential accumulation in response to P deficiency were identified. Subsequent qRT-PCR analysis correlated the accumulation of 21 of the 27 up-regulated proteins, and eight of the 26 down-regulated proteins with corresponding gene expression patterns in response to P deficiency. One up-regulated CWP, purple acid phosphatase 1-like (GmPAP1-like), was functionally characterized. Phaseolus vulgaris transgenic hairy roots overexpressing GmPAP1-like displayed an increase in root-associated acid phosphatase activity. In addition, relative growth and P content were significantly enhanced in GmPAP1-like overexpressing lines compared to control lines when deoxy-ribonucleotide triphosphate (dNTP) was applied as the sole external P source. Taken together, the results suggest that the modulation of CWPs may regulate complex changes in the root system in response to P deficiency, and that the cell wall-localized GmPAP1-like protein is involved in extracellular dNTP utilization in soybean.

RNA editing of cytochrome c maturation transcripts is responsive to the energy status of leaf cells in Arabidopsis thaliana

Overexpression of AtPAP2, a phosphatase located on the outer membranes of chloroplasts and mitochondria, leads to higher energy outputs from these organelles. AtPAP2 interacts with seven MORF proteins of the editosome complex. RNA-sequencing analysis showed that the editing degrees of most sites did not differ significantly between OE and WT, except some sites on the transcripts of several cytochrome c maturation (Ccm) genes. Western blotting of 2D BN-PAGE showed that the patterns of CcmF_N1 polypeptides were different between the lines. We proposed that AtPAP2 may influence cytochrome c biogenesis by modulating RNA editing through its interaction with MORF proteins.

Transgenic Arabidopsis thaliana containing increased levels of ATP and sucrose is more susceptible to Pseudomonas syringae

Disease resistance exerts a fitness cost on plants, presumably due to the extra consumption of energy and carbon. In this study, we examined whether transgenic Arabidopsis thaliana with increased levels of ATP and sucrose is more resistant or susceptible to pathogen infection. Lines of A. thaliana over-expressing purple acid phosphatase 2 (AtPAP2) (OE lines) contain increased levels of ATP and sucrose, with improved growth rate and seed production. Compared to wild type (WT) and pap2 lines, the OE lines were more susceptible to several Pseudomonas syringae pv. tomato (Pst) strains carrying AvrRpm1, AvrRpt2 AvrRps4, AvrPtoB, HrcC and WT strain DC3000. The increased susceptibility of the OE lines to Pst strains cannot solely be attributed to the suppressed expression of R-genes but must also be attributed to the suppression of downstream signaling components, such as MOS2, EDS1 and EDS5. Before infection, the levels of salicylic acid (SA) and jasmonic acid (JA) precursor OPDA were similar in the leaves of OE, pap2 and WT plants, whereas the levels of JA and its derivative JA-Ile were significantly lower in the leaves of OE lines and higher in the pap2 line. The expression of JA marker defense gene PDF1.2 was up-regulated in the OE lines compared to the WT prior to Pst DC3000 infection, but its expression was lower in the OE lines after infection. In summary, high fitness Arabidopsis thaliana exhibited altered JA metabolism and broad suppression of R-genes and downstream genes as well as a higher susceptibility to Pst infections.

AtPAP2 modulates the import of the small subunit of Rubisco into chloroplasts

Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) is the only phosphatase that is dual-targeted to both chloroplasts and mitochondria. Like Toc33/34 of the TOC and Tom 20 of the TOM, AtPAP2 is anchored to the outer membranes of chloroplasts and mitochondria via a hydrophobic C-terminal motif. AtPAP2 on the mitochondria was previously shown to recognize the presequences of several nuclear-encoded mitochondrial proteins and modulate the import of pMORF3 into the mitochondria. Here we show that AtPAP2 binds to the small subunit of Rubisco (pSSU) and that chloroplast import experiments demonstrated that pSSU was imported less efficiently into pap2 chloroplasts than into wild-type chloroplasts. We propose that AtPAP2 is an outer membrane-bound phosphatase receptor that facilitates the import of selected proteins into chloroplasts.

Characterization of purple acid phosphatases involved in extracellular dNTP utilization in Stylosanthes

Stylo (Stylosanthes spp.) is a pasture legume predominant in tropical and subtropical areas, where low phosphorus (P) availability is a major constraint for plant growth. Therefore, stylo might exhibit superior utilization of the P pool on acid soils, particularly organic P. However, little is known about mechanisms of inorganic phosphate (Pi) acquisition employed by stylo. In this study, the utilization of extracellular deoxy-ribonucleotide triphosphate (dNTP) and the underlying physiological and molecular mechanisms were examined for two stylo genotypes with contrasting P efficiency. Results showed that the P-efficient genotype, TPRC2001-1, was superior to the P-inefficient genotype, Fine-stem, when using dNTP as the sole P source. This was reflected by a higher dry weight and total P content for TPRC2001-1 than for Fine-stem, which was correlated with higher root-associated acid phosphatase (APase) activities in TPRC2001-1 under low P conditions. Subsequently, three PAP members were cloned from TPRC2001-1: SgPAP7, SgPAP10, and SgPAP26 Expression levels of these three SgPAPs were up-regulated by Pi starvation in stylo roots. Furthermore, there was a higher abundance of transcripts of SgPAP7 and SgPAP10 in TPRC2001-1 than in Fine-stem. Subcellular localization analysis demonstrated that these three SgPAPs were localized on the plasma membrane. Overexpression of these three SgPAPs could result in significantly increased root-associated APase activities, and thus extracellular dNTP utilization in bean hairy roots. Taken together, the results herein suggest that SgPAP7, SgPAP10, and SgPAP26 may differentially contribute to root-associated APase activities, and thus control extracellular dNTP utilization in stylo.

Comparison of genome-wide gene expression patterns in the seedlings of nascent allohexaploid wheats produced by two combinations of hybrids

Allopolyploidization in plants is an important event that enhances heterosis and environmental adaptation. Common wheat, Triticum aestivum (AABBDD), which is an allohexaploid that evolved from an allopolyploidization event between T. turgidum (AABB) and Aegilops tauschii (DD), shows more growth vigor and wider adaptation than tetraploid wheats. To better understand the molecular basis for the heterosis of hexaploid wheat, we systematically analyzed the genome-wide gene expression patterns of two combinations of newly hybridized triploids (ABD), their chromosome-doubled hexaploids (AABBDD), stable synthetic hexaploids (AABBDD) and natural hexaploids, in addition to their parents, T. turgidum (AABB) and Ae. tauschii (DD), using a microarray to reconstruct the events of allopolyploidization and genome stabilization. Overall comparisons of gene expression profiles showed that the newly generated hexaploids exhibited gene expression patterns similar to those of their maternal tetraploids, irrespective of hybrid combination. With successive generations, the gene expression profiles of nascent hexaploids became less similar to the maternal profiles, and belonged to a separate cluster from the natural hexaploids. Triploids revealed characteristic expression patterns, suggesting endosperm effects. In the newly hybridized triploids (ABD) of two independent synthetic lines, approximately one-fifth of expressed genes displayed non-additive expression; the number of these genes decreased with polyploidization and genome stabilization. Approximately 20% of the non-additively expressed genes were transmitted across generations throughout allopolyploidization and successive self-pollinations, and 43 genes overlapped between the two combinations, indicating that shared gene expression patterns can be seen during allohexaploidization. Furthermore, four of these 43 genes were involved in starch and sucrose metabolism, suggesting that these metabolic events play key roles in the hybrid vigor of hexaploid wheat.

Molecular characterization of a tomato purple acid phosphatase during seed germination and seedling growth under phosphate stress

SlPAP1 is a phosphate starvation responsive purple acid phosphatase during tomato seed germination. Future research on its family members in tomato might improve the phosphate stress tolerance. Phosphate deficiency is a major constraint upon crop growth and yield. In response to phosphate deficiency, plants secrete acid phosphatases (APases) to scavenge organic phosphate from soil. In this study, we investigated the impact of Pi starvation on germination and seedling growth of tomato, and we then cloned and characterized a phosphate starvation responsive purple APase (SlPAP1) that expressed during tomato seedling growth. Our results showed that phosphate deficiency reduced germination and growth rates of tomato, and also increased intracellular and secretory APase activity in a concentration-dependent manner. An in-gel activity assay found that two APases of 50 and 75 kDa were secreted during conditions of phosphate deficiency. SlPAP1 is a single copy gene belonging to a small gene family. It was expressed as a cDNA of approximately 1.5 kbp encoding a secreted glycoprotein of 470 amino acids. Northern blot analysis showed that SlPAP1 was specifically expressed in root tissue during phosphate deficiency. SlPAP1 had high sequence identity (56-89%) with other plant PAPs and contained highly conserved metal-binding residues. SlPAP1 is the first PAP to be cloned and characterized from tomato. This study provides useful information for future research on PAP family members in tomato, leading to better understanding of phosphate deficiency in this important crop plant.

Crop yield: challenges from a metabolic perspective

Considering the dual use of plants, as bio-factories for foods and feedstock for bio-refining, along with a rising world population, the plant biotechnology field is currently facing a dramatic challenge to develop crops with higher yield. Furthermore, convergent studies predict that global changes in climate will influence crop productivity by modifying most yield-associated traits. Here, we review recent advances in the understanding of plant metabolism directly or indirectly impacting on yield and provide an update of the different pathways proposed as targets for metabolic engineering aiming to optimize source-sink relationships.

Heterologous expression of AtPAP2 in transgenic potato influences carbon metabolism and tuber development

Changes in carbon flow and sink/source activities can affect floral, architectural, and reproductive traits of plants. In potato, overexpression (OE) of the purple acid phosphatase 2 of Arabidopsis (AtPAP2) resulted in earlier flowering, faster growth rate, increased tubers and tuber starch content, and higher photosynthesis rate. There was a significant change in sucrose, glucose and fructose levels in leaves, phloem and sink biomass of the OE lines, consistent with an increased expression of sucrose transporter 1 (StSUT1). Furthermore, the expression levels and enzyme activity of sucrose-phosphate synthase (SPS) were also significantly increased in the OE lines. These findings strongly suggest that higher carbon supply from the source and improved sink strength can improve potato tuber yield.

Transgenic tobacco overexpressing Brassica juncea HMG-CoA synthase 1 shows increased plant growth, pod size and seed yield

Seeds are very important not only in the life cycle of the plant but they represent food sources for man and animals. We report herein a mutant of 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS), the second enzyme in the mevalonate (MVA) pathway that can improve seed yield when overexpressed in a phylogenetically distant species. In Brassica juncea, the characterisation of four isogenes encoding HMGS has been previously reported. Enzyme kinetics on recombinant wild-type (wt) and mutant BjHMGS1 had revealed that S359A displayed a 10-fold higher enzyme activity. The overexpression of wt and mutant (S359A) BjHMGS1 in Arabidopsis had up-regulated several genes in sterol biosynthesis, increasing sterol content. To quickly assess the effects of BjHMGS1 overexpression in a phylogenetically more distant species beyond the Brassicaceae, wt and mutant (S359A) BjHMGS1 were expressed in tobacco (Nicotiana tabacum L. cv. Xanthi) of the family Solanaceae. New observations on tobacco OEs not previously reported for Arabidopsis OEs included: (i) phenotypic changes in enhanced plant growth, pod size and seed yield (more significant in OE-S359A than OE-wtBjHMGS1) in comparison to vector-transformed tobacco, (ii) higher NtSQS expression and sterol content in OE-S359A than OE-wtBjHMGS1 corresponding to greater increase in growth and seed yield, and (iii) induction of NtIPPI2 and NtGGPPS2 and downregulation of NtIPPI1, NtGGPPS1, NtGGPPS3 and NtGGPPS4. Resembling Arabidopsis HMGS-OEs, tobacco HMGS-OEs displayed an enhanced expression of NtHMGR1, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Overall, increased growth, pod size and seed yield in tobacco HMGS-OEs were attributed to the up-regulation of native NtHMGR1, NtIPPI2, NtSQS, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Hence, S359A has potential in agriculture not only in improving phytosterol content but also seed yield, which may be desirable in food crops. This work further demonstrates HMGS function in plant reproduction that is reminiscent to reduced fertility of hmgs RNAi lines in let-7 mutants of Caenorhabditis elegans.

Global small RNA analysis in fast-growing Arabidopsis thaliana with elevated concentrations of ATP and sugars

BACKGROUND

In higher eukaryotes, small RNAs play a role in regulating gene expression. Overexpression (OE) lines of Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) were shown to grow faster and exhibit higher ATP and sugar contents. Leaf microarray studies showed that many genes involved in microRNAs (miRNAs) and trans-acting siRNAs (tasiRNAs) biogenesis were significantly changed in the fast-growing lines. In this study, the sRNA profiles of the leaf and the root of 20-day-old plants were sequenced and the impacts of high energy status on sRNA expression were analyzed.

RESULTS

9-13 million reads from each library were mapped to genome. miRNAs, tasiRNAs and natural antisense transcripts-generated small interfering RNAs (natsiRNAs) were identified and compared between libraries. In the leaf of OE lines, 15 known miRNAs increased in abundance and 9 miRNAs decreased in abundance, whereas in the root of OE lines, 2 known miRNAs increased in abundance and 9 miRNAs decreased in abundance. miRNAs with increased abundance in the leaf and root samples of both OE lines (miR158b and miR172a/b) were predicted to target mRNAs coding for Dof zinc finger protein and Apetala 2 (AP2) proteins, respectively. Furthermore, a significant change in the miR173-tasiRNAs-PPR/TPR network was observed in the leaves of both OE lines.

CONCLUSION

In this study, the impact of high energy content on the sRNA profiles of Arabidopsis is reported. While the abundance of many stress-induced miRNAs is unaltered, the abundance of some miRNAs related to plant growth and development (miR172 and miR319) is elevated in the fast-growing lines. An induction of miR173-tasiRNAs-PPR/TPR network was also observed in the OE lines. In contrast, only few cis- and trans-natsiRNAs are altered in the fast-growing lines.

Global transcriptome analysis of AtPAP2--overexpressing Arabidopsis thaliana with elevated ATP

BACKGROUND

AtPAP2 is a purple acid phosphatase that is targeted to both chloroplasts and mitochondria. Over-expression (OE) lines of AtPAP2 grew faster, produced more seeds, and contained higher leaf sucrose and glucose contents. The present study aimed to determine how high energy status affects leaf and root transcriptomes.

RESULTS

ATP and ADP levels in the OE lines are 30-50% and 20-50% higher than in the wild-type (WT) plants. Global transcriptome analyses indicated that transcriptional regulation does play a role in sucrose and starch metabolism, nitrogen, potassium and iron uptake, amino acids and secondary metabolites metabolism when there is an ample supply of energy. While the transcript abundance of genes encoding protein components of photosystem I (PS I), photosystem II (PS II) and light harvesting complex I (LHCI) were unaltered, changes in transcript abundance for genes encoding proteins of LHCII are significant. The gene expressions of most enzymes of the Calvin cycle, glycolysis and the tricarboxylic acid (TCA) cycle were unaltered, as these enzymes are known to be regulated by light/redox status or allosteric modulation by the products (e.g. citrate, ATP/ADP ratio), but not at the level of transcription.

CONCLUSIONS

AtPAP2 overexpression resulted in a widespread reprogramming of the transcriptome in the transgenic plants, which is characterized by changes in the carbon, nitrogen, potassium, and iron metabolism. The fast-growing AtPAP2 OE lines provide an interesting tool for studying the regulation of energy system in plant.

Expression pattern and subcellular localization of Arabidopsis purple acid phosphatase AtPAP9

Purple acid phosphatase (PAP; EC 3.1.3.2) enzymes are metallophosphoesterases that hydrolysis phosphate ester bonds in a wide range of substrates. Twenty-nine PAP-encoding loci have been identified in the Arabidopsis genome, many of which have multiple transcript variants expressed in response to diverse environmental conditions. Having analyzed T-DNA insertion mutants, we have provided strong pieces of evidence that AtPAP9 locus encodes at least two types of transcripts, designated as AtPAP9-1 and AtPAP9-2. These transcript variants expressed distinctly during the course of growth in medium containing sufficient phosphate or none. Further histochemical analysis by the use of AtPAP9-1 promoter fused to β-glucuronidase reporter gene indicated the expression of this gene is regulated in a tissue-specific manner. AtPAP9-1 was highly expressed in stipule and vascular tissue, particularly in response to fungal infection. Subcellular localization of AtPAP9-1:green fluorescent fusion protein showed that it must be involved in plasma membrane and cell wall adhesion.

The outer mitochondrial membrane in higher plants

The acquisition and integration of intracellular organelles, such as mitochondria and plastids, were important steps in the emergence of complex multicellular life. Although the outer membranes of these organelles have lost many of the functions of their free-living bacterial ancestor, others were acquired during organellogenesis. To date, the biological roles of these proteins have not been systematically characterized. In this review, we discuss the evolutionary origins and functions of outer membrane mitochondrial (OMM) proteins in Arabidopsis thaliana. Our analysis, using phylogenetic inference, indicates that several OMM proteins either acquired novel functional roles or were recruited from other subcellular localizations during evolution in Arabidopsis. These observations suggest the existence of novel communication routes and functions between organelles within plant cells.

De novo assembly and characterization of Camelina sativa transcriptome by paired-end sequencing

Background

Biofuels extracted from the seeds of Camelina sativa have recently been used successfully as environmentally friendly jet-fuel to reduce greenhouse gas emissions. Camelina sativa is genetically very close to Arabidopsis thaliana, and both are members of the Brassicaceae. Although public databases are currently available for some members of the Brassicaceae, such as A. thaliana, A. lyrata, Brassica napus, B. juncea and B. rapa, there are no public Expressed Sequence Tags (EST) or genomic data for Camelina sativa. In this study, a high-throughput, large-scale RNA sequencing (RNA-seq) of the Camelina sativa transcriptome was carried out to generate a database that will be useful for further functional analyses.

Results

Approximately 27 million clean “reads” filtered from raw reads by removal of adaptors, ambiguous reads and low-quality reads (2.42 gigabase pairs) were generated by Illumina paired-end RNA-seq technology. All of these clean reads were assembled de novo into 83,493 unigenes and 103,196 transcripts using SOAPdenovo and Trinity, respectively. The average length of the transcripts generated by Trinity was 697 bp (N50 = 976), which was longer than the average length of unigenes (319 bp, N50 = 346 bp). Nonetheless, the assembly generated by SOAPdenovo produced similar number of non-redundant hits (22,435) with that of Trinity (22,433) in BLASTN searches of the Arabidopsis thaliana CDS sequence database (TAIR). Four public databases, the Kyoto Encyclopedia of Genes and Genomes (KEGG), Swiss-prot, NCBI non-redundant protein (NR), and the Cluster of Orthologous Groups (COG), were used for unigene annotation; 67,791 of 83,493 unigenes (81.2%) were finally annotated with gene descriptions or conserved protein domains that were mapped to 25,329 non-redundant protein sequences. We mapped 27,042 of 83,493 unigenes (32.4%) to 119 KEGG metabolic pathways.

Conclusions

This is the first report of a transcriptome database for Camelina sativa, an environmentally important member of the Brassicaceae. We showed that C. savita is closely related to Arabidopsis spp. and more distantly related to Brassica spp. Although the majority of annotated genes had high sequence identity to those of A. thaliana, a substantial proportion of disease-resistance genes (NBS-encoding LRR genes) were instead more closely similar to the genes of other Brassicaceae; these genes included BrCN, BrCNL, BrNL, BrTN, BrTNL in B. rapa. As plant genomes are under long-term selection pressure from environmental stressors, conservation of these disease-resistance genes in C. sativa and B. rapa genomes implies that they are exposed to the threats from closely-related pathogens in their natural habitats.

A somaclonal line SE7 of finger millet (Eleusine coracana) exhibits modified cytokinin homeostasis and increased grain yield

The SE7 somaclonal line of finger millet (Eleusine coracana) achieved increased grain yield in field trials that apparently resulted from a higher number of inflorescences and seeds per plant, compared with the wild type. Levels of endogenous cytokinins, especially those of highly physiologically active iso-pentenyl adenine, were increased during early inflorescence development in SE7 plants. Transcript levels of cytokinin-degrading enzymes but not of a cytokinin-synthesizing enzyme were also decreased in young leaves, seedlings, and initiating inflorescences of SE7. These data suggest that attenuated degradation of cytokinins in SE7 inflorescences leads to higher cytokinin levels that stimulate meristem activity and result in production of more inflorescences. Gene expression was compared between SE7 and wild-type young inflorescences using the barley 12K cDNA array. The largest fraction of up-regulated genes in SE7 was related to transcription, translation, and cell proliferation, cell wall assembly/biosynthesis, and to growth regulation of young and meristematic tissues including floral formation. Other up-regulated genes were associated with protein and lipid degradation and mitochondrial energy production. Down-regulated genes were related to pathogen defence and stress response, primary metabolism, glycolysis, and the C:N balance. The results indicate a prolonged proliferation phase in SE7 young inflorescences characterized by up-regulated protein synthesis, cytokinesis, floral formation, and energy production. In contrast, wild-type inflorescences are similar to a more differentiated status characterized by regulated protein degradation, cell elongation, and defence/stress responses. It is concluded that attenuated degradation of cytokinins in SE7 inflorescences leads to higher cytokinin levels, which stimulate meristem activity, inflorescence formation, and seed set.

AtPAP2 is a tail-anchored protein in the outer membrane of chloroplasts and mitochondria

To date, Arabidopsis purple acid phosphatase 2 (AtPAP2) is the only known plant protein that is dual-targeted to chloroplasts and mitochondria by a C-terminal targeting signal. Using in vitro organelle import and green fluorescence protein (GFP) localization assays, we showed that AtPAP2 is located on, but not imported across the outer membrane (OM) of chloroplasts and mitochondria and exposed its N-terminal enzymatic domain to the cytosol. It was also found that a short stretch of 30 amino acids (a.a.) at the C-terminal region (a.a. 615-644) that contains a stretch of 18 hydrophobic residues, a WYAK motif and 8 hydrophilic residues is sufficient for dual-targeting. Mutation of WYAK to WYAE had no effect on dual-targeting ability suggesting that the charge within this flanking region alone is not an important determinant for dual-targeting.   

A reevaluation of dual-targeting of proteins to mitochondria and chloroplasts

Over 100 proteins are found in both mitochondria and chloroplasts, via a variety of processes known generally as 'dual-targeting'. Dual-targeting has attracted interest from many different research groups because of its profound implications concerning the mechanisms of protein import into these organelles and the evolution of both the protein import machinery and the targeting sequences within the imported proteins. Beyond these aspects, dual-targeting is also interesting for its implications concerning shared functions between mitochondria and chloroplasts, and especially the control of the activities of these two very different energy organelles. We discuss each of these points in the light of the latest relevant research findings and make some suggestions for where research might be most illuminating in the near future. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Over-expression of AtPAP2 in Camelina sativa leads to faster plant growth and higher seed yield

BACKGROUND

Lipids extracted from seeds of Camelina sativa have been successfully used as a reliable source of aviation biofuels. This biofuel is environmentally friendly because the drought resistance, frost tolerance and low fertilizer requirement of Camelina sativa allow it to grow on marginal lands. Improving the species growth and seed yield by genetic engineering is therefore a target for the biofuels industry. In Arabidopsis, overexpression of purple acid phosphatase 2 encoded by Arabidopsis (AtPAP2) promotes plant growth by modulating carbon metabolism. Overexpression lines bolt earlier and produce 50% more seeds per plant than wild type. In this study, we explored the effects of overexpressing AtPAP2 in Camelina sativa.

RESULTS

Under controlled environmental conditions, overexpression of AtPAP2 in Camelina sativa resulted in longer hypocotyls, earlier flowering, faster growth rate, higher photosynthetic rate and stomatal conductance, increased seed yield and seed size in comparison with the wild-type line and null-lines. Similar to transgenic Arabidopsis, activity of sucrose phosphate synthase in leaves of transgenic Camelina was also significantly up-regulated. Sucrose produced in photosynthetic tissues supplies the building blocks for cellulose, starch and lipids for growth and fuel for anabolic metabolism. Changes in carbon flow and sink/source activities in transgenic lines may affect floral, architectural, and reproductive traits of plants.

CONCLUSIONS

Lipids extracted from the seeds of Camelina sativa have been used as a major constituent of aviation biofuels. The improved growth rate and seed yield of transgenic Camelina under controlled environmental conditions have the potential to boost oil yield on an area basis in field conditions and thus make Camelina-based biofuels more environmentally friendly and economically attractive.