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

A converged ubiquitin-proteasome pathway for the degradation of TOC and TOM tail-anchored receptors

In plants, thousands of nucleus-encoded proteins translated in the cytosol are sorted to chloroplasts and mitochondria by binding to specific receptors of the TOC (translocon on the outer chloroplast membrane) and the TOM (translocon on the outer mitochondrial membrane) complexes for import into those organelles. The degradation pathways for these receptors are unclear. Here, we discovered a converged ubiquitin-proteasome pathway for the degradation of Arabidopsis thaliana TOC and TOM tail-anchored receptors. The receptors are ubiquitinated by E3 ligase(s) and pulled from the outer membranes by the AAA+ adenosine triphosphatase CDC48, after which a previously uncharacterized cytosolic protein, transmembrane domain (TMD)-binding protein for tail-anchored outer membrane proteins (TTOP), binds to the exposed TMDs at the C termini of the receptors and CDC48, and delivers these complexes to the 26S proteasome.

Root-Expressed Rice PAP3b Enhances Secreted APase Activity and Helps Utilize Organic Phosphate

Phosphate (Pi) deficiency leads to the induction of purple acid phosphatases (PAPs) in plants, which dephosphorylate organic phosphorus (P) complexes in the rhizosphere and intracellular compartments to release Pi. In this study, we demonstrate that OsPAP3b belongs to group III low-molecular weight PAP and is low Pi-responsive, preferentially in roots. The expression of OsPAP3b is negatively regulated with Pi resupply. Interestingly, OsPAP3b was found to be dual localized to the nucleus and secretome. Furthermore, OsPAP3b is transcriptionally regulated by OsPHR2 as substantiated by DNA-protein binding assay. Through in vitro biochemical assays, we further demonstrate that OsPAP3b is a functional acid phosphatase (APase) with broad substrate specificity. The overexpression (OE) of OsPAP3b in rice led to increased secreted APase activity and improved mineralization of organic P sources, which resulted in better growth of transgenics compared to the wild type when grown on organic P as an exogenous P substrate. Under Pi deprivation, OsPAP3b knock-down and knock-out lines showed no significant changes in total P content and dry biomass. However, the expression of other phosphate starvation-induced genes and the levels of metabolites were found to be altered in the OE and knock-down lines. In addition, in vitro pull-down assay revealed multiple putative interacting proteins of OsPAP3b. Our data collectively suggest that OsPAP3b can aid in organic P utilization in rice. The APase isoform behavior and nuclear localization indicate its additional role, possibly in stress signaling. Considering its important roles, OsPAP3b could be a potential target for improving low Pi adaptation in rice.

Dynamic responses of PA to environmental stimuli imaged by a genetically encoded mobilizable fluorescent sensor

Membrane fluidity, permeability, and surface charges are controlled by phospholipid metabolism and transport. Despite the importance of phosphatidic acid (PA) as a bioactive molecule, the mechanical properties of PA translocation and subcellular accumulation are unknown. Here, we used a mobilizable, highly responsive genetically encoded fluorescent indicator, green fluorescent protein (GFP)-N160_RbohD, to monitor PA dynamics in living cells. The majority of GFP-N160_RbohD accumulated at the plasma membrane and sensitively responded to changes in PA levels. Cellular, pharmacological, and genetic analyses illustrated that both salinity and abscisic acid rapidly enhanced GFP-N160_RbohD fluorescence at the plasma membrane, which mainly depended on hydrolysis of phospholipase D. By contrast, heat stress induced nuclear translocation of PA indicated by GFP-N160_RbohD through a process that required diacylglycerol kinase activity, as well as secretory and endocytic trafficking. Strikingly, we showed that gravity triggers asymmetric PA distribution at the root apex, a response that is suppressed by PLDζ2 knockout. The broad utility of the PA sensor will expand our mechanistic understanding of numerous lipid-associated physiological and cell biological processes and facilitate screening for protein candidates that affect the synthesis, transport, and metabolism of PA.

The PAP Gene Family in Tomato: Comprehensive Comparative Analysis, Phylogenetic Relationships and Expression Profiles

Purple acid phosphatase (PAP) plays a vital role in plant phosphate acquisition and utilization, as well as cell wall synthesis and redox reactions. In this study, comprehensive comparative analyses of PAP genes were carried out using the integration of phylogeny, chromosomal localization, intron/exon structural characteristics, and expression profiling. It was shown that the number of introns of the PAP genes, which were distributed unevenly on 12 chromosomes, ranged from 1 to 12. These findings pointed to the existence of complex structures. Phylogenetic analyses revealed that PAPs from tomato, rice, and Arabidopsis could be divided into three groups (Groups I, II, and III). It was assumed that the diversity of these PAP genes occurred before the monocot–dicot split. RNA-seq analysis revealed that most of the genes were expressed in all of the tissues analyzed, with the exception of SlPAP02, SlPAP11, and SlPAP14, which were not detected. It was also found that expression levels of most of the SlPAP gene family of members were changed under phosphorus stress conditions, suggesting potential functional diversification. The findings of this work will help us to achieve a better insight into the function of SlPAP genes in the future, as well as enhance our understanding of their evolutionary relationships in plants.

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.

Looking for a safe haven: tail-anchored proteins and their membrane insertion pathways

Insertion of membrane proteins into the lipid bilayer is a crucial step during their biosynthesis. Eukaryotic cells face many challenges in directing these proteins to their predestined target membrane. The hydrophobic signal peptide or transmembrane domain (TMD) of the nascent protein must be shielded from the aqueous cytosol and its target membrane identified followed by transport and insertion. Components that evolved to deal with each of these challenging steps range from chaperones to receptors, insertases, and sophisticated translocation complexes. One prominent translocation pathway for most proteins is the signal recognition particle (SRP)-dependent pathway which mediates co-translational translocation of proteins across or into the endoplasmic reticulum (ER) membrane. This textbook example of protein insertion is stretched to its limits when faced with secretory or membrane proteins that lack an amino-terminal signal sequence or TMD. Particularly, a large group of so-called tail-anchored (TA) proteins that harbor a single carboxy-terminal TMD require an alternative, post-translational insertion route into the ER membrane. In this review, we summarize the current research in TA protein insertion with a special focus on plants, address challenges, and highlight future research avenues.

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.

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.

Major Changes in Plastid Protein Import and the Origin of the Chloroplastida

Core components of plastid protein import and the principle of using N-terminal targeting sequences are conserved across the Archaeplastida, but lineage-specific differences exist. Here we compare, in light of plastid protein import, the response to high-light stress from representatives of the three archaeplastidal groups. Similar to land plants, Chlamydomonas reinhardtii displays a broad response to high-light stress, not observed to the same degree in the glaucophyte Cyanophora paradoxa or the rhodophyte Porphyridium purpureum. We find that only the Chloroplastida encode both Toc75 and Oep80 in parallel and suggest that elaborate high-light stress response is supported by changes in plastid protein import. We propose the origin of a phenylalanine-independent import pathway via Toc75 allowed higher import rates to rapidly service high-light stress, but with the cost of reduced specificity. Changes in plastid protein import define the origin of the green lineage, whose greatest evolutionary success was arguably the colonization of land.

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Chloroplastida evolved a dual system, Toc75/Oep80, for high throughput protein import

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Loss of F-based targeting led to dual organelle targeting using a single ambiguous NTS

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Relaxation of functional constraints allowed a wider Toc/Tic modification

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A broad response to high-light stress appears unique to Chloroplastida

The intracellular distribution of the components of the GET system in vascular plants

The guided entry of tail-anchored proteins (GET) pathway facilitates targeting and insertion of tail-anchored proteins into membranes. In plants, such a protein insertion machinery for the endoplasmic reticulum as well as constituents within mitochondrial and chloroplasts were discovered. Previous phylogenetic analysis revealed that Get3 sequences of Embryophyta form two clades representing cytosolic ("a") and organellar ("bc") GET3 homologs, respectively. Cellular fractionation of Arabidopsis thaliana seedlings and usage of the self-assembly GFP system in protoplasts verified the cytosolic (_ATGet3a), plastidic (_ATGet3b) and mitochondrial (_ATGet3c) localization of the different homologs. The identified plant homologs of Get1 and Get4 in A. thaliana are localized in ER and cytosol, respectively, implicating a degree of conservation of the GET pathway in A. thaliana. Transient expression of Get3 homologs of Solanum lycopersicum, Medicago × varia or Physcomitrella patens with the self-assembly GFP technique in homologous and heterologous systems verified that multiple Get3 homologs with differing subcellular localizations are common in plants. Chloroplast localized Get3 homologs were detected in all tested plant systems. In contrast, mitochondrial localized Get3 homologs were not identified in S. lycopersicum, or P. patens, while we confirmed on the example of A. thaliana proteins that mitochondrial localized Get3 proteins are properly targeted in S. lycopersicum as well.

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.

Identification of Purple Acid Phosphatases in Chickpea and Potential Roles of CaPAP7 in Seed Phytate Accumulation

Purple acid phosphatases (PAPs) play important roles in phosphate (Pi) acquisition and utilization. These PAPs hydrolyze organic Phosphorus (P) containing compounds in rhizosphere as well as inside the plant cell. However, roles of PAPs in one of the most widely cultivated legumes, chickpea (Cicer arietnum L.), have not been unraveled so far. In the present study, we identified 25 putative PAPs in chickpea (CaPAPs) which possess functional PAP motifs and domains. Differential regulation of CaPAPs under different nutrient deficiencies revealed their roles under multiple nutrient stresses including Pi deficiency. Interestingly, most of the CaPAPs were prominently expressed in flowers and young pods indicating their roles in flower and seed development. Association mapping of SNPs underlying CaPAPs with seed traits revealed significant association of low Pi inducible CaPAP7 with seed weight and phytate content. Biochemical characterization of recombinant CaPAP7 established it to be a functional acid phosphatase with highest activity on most abundant organic-P substrate, phytate. Exogenous application of recombinant CaPAP7 enhanced biomass and Pi content of Arabidopsis seedlings supplemented with phytate as sole P source. Taken together, our results uncover the PAPs in chickpea and potential roles of CaPAP7 in seed phytate accumulation.

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.

Role of the C-terminal extension peptide of plastid located glutamine synthetase from Medicago truncatula: Crucial for enzyme activity and needless for protein import into the plastids

Glutamine synthetase (GS), a key enzyme in plant nitrogen metabolism, is encoded by a small family of highly homologous nuclear genes that produce cytosolic (GS1) and plastidic (GS2) isoforms. Compared to GS1, GS2 proteins have two extension peptides, one at the N- and the other at the C-terminus, which show a high degree of conservation among plant species. It has long been known that the N-terminal peptide acts as a transit peptide, targeting the protein to the plastids however, the function of the C-terminal extension is still unknown. To investigate whether the C-terminal extension influences the activity of the enzyme, we produced a C-terminal truncated version of Medicago truncatula GS2a in Escherechia coli and studied its catalytic properties. The activity of the truncated protein was found to be lower than that of MtGS2a and with less affinity for glutamate. The importance of the C-terminal extension for the protein import into the chloroplast was also assessed by transient expression of fluorescently-tagged MtGS2a truncated at the C-terminus, which was correctly detected in the chloroplast. The results obtained in this work demonstrate that the C-terminal extension of M. truncatula GS2a is important for the activity of the enzyme and does not contain crucial information for the import process.

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.

Phosphorylation and Dephosphorylation of the Presequence of Precursor MULTIPLE ORGANELLAR RNA EDITING FACTOR3 during Import into Mitochondria from Arabidopsis

The nucleus-encoded mitochondria-targeted proteins, multiple organellar RNA editing factors (MORF3, MORF5, and MORF6), interact with Arabidopsis (Arabidopsis thaliana) PURPLE ACID PHOSPHATASE2 (AtPAP2) located on the chloroplast and mitochondrial outer membranes in a presequence-dependent manner. Phosphorylation of the presequence of the precursor MORF3 (pMORF3) by endogenous kinases in wheat germ translation lysate, leaf extracts, or STY kinases, but not in rabbit reticulocyte translation lysate, resulted in the inhibition of protein import into mitochondria. This inhibition of import could be overcome by altering threonine/serine residues to alanine on the presequence, thus preventing phosphorylation. Phosphorylated pMORF3, but not the phosphorylation-deficient pMORF3, can form a complex with 14-3-3 proteins and HEAT SHOCK PROTEIN70. The phosphorylation-deficient mutant of pMORF3 also displayed faster rates of import when translated in wheat germ lysates. Mitochondria isolated from plants with altered amounts of AtPAP2 displayed altered protein import kinetics. The import rate of pMORF3 synthesized in wheat germ translation lysate into pap2 mitochondria was slower than that into wild-type mitochondria, and this rate disparity was not seen for pMORF3 synthesized in rabbit reticulocyte translation lysate, the latter translation lysate largely deficient in kinase activity. Taken together, these results support a role for the phosphorylation and dephosphorylation of pMORF3 during the import into plant mitochondria. These results suggest that kinases, possibly STY kinases, and AtPAP2 are involved in the import of protein into both mitochondria and chloroplasts and provide a mechanism by which the import of proteins into both organelles may be coordinated.

A purple acid phosphatase plays a role in nodule formation and nitrogen fixation in Astragalus sinicus

The AsPPD1 gene from Astragalus sinicus encodes a purple acid phosphatase. To address the functions of AsPPD1 in legume-rhizobium symbiosis, its expression patterns, enzyme activity, subcellular localization, and phenotypes associated with its over-expression and RNA interference (RNAi) were investigated. The expression of AsPPD1 was up-regulated in roots and nodules after inoculation with rhizobia. Phosphate starvation reduced the levels of AsPPD1 transcripts in roots while increased those levels in nodules. We confirmed the acid phosphatase and phosphodiesterase activities of recombinant AsPPD1 purified from Pichia pastoris, and demonstrated its ability to hydrolyze ADP and ATP in vitro. Subcellular localization showed that AsPPD1 located on the plasma membranes in hairy roots and on the symbiosomes membranes in root nodules. Over-expression of AsPPD1 in hairy roots inhibited nodulation, while its silencing resulted in nodules early senescence and significantly decreased nitrogenase activity. Furthermore, HPLC measurement showed that AsPPD1 overexpression affects the ADP levels in the infected roots and nodules, AsPPD1 silencing affects the ratio of ATP/ADP and the energy charge in nodules, and quantitative observation demonstrated the changes of AsPPD1 transcripts level affected nodule primordia formation. Taken together, it is speculated that AsPPD1 contributes to symbiotic ADP levels and energy charge control, and this is required for effective nodule organogenesis and nitrogen fixation.

The maize (Zea mays ssp. mays var. B73) genome encodes 33 members of the purple acid phosphatase family

Purple acid phosphatases (PAPs) play an important role in plant phosphorus nutrition, both by liberating phosphorus from organic sources in the soil and by modulating distribution within the plant throughout growth and development. Furthermore, members of the PAP protein family have been implicated in a broader role in plant mineral homeostasis, stress responses and development. We have identified 33 candidate PAP encoding gene models in the maize (Zea mays ssp. mays var. B73) reference genome. The maize Pap family includes a clear single-copy ortholog of the Arabidopsis gene AtPAP26, shown previously to encode both major intracellular and secreted acid phosphatase activities. Certain groups of PAPs present in Arabidopsis, however, are absent in maize, while the maize family contains a number of expansions, including a distinct radiation not present in Arabidopsis. Analysis of RNA-sequencing based transcriptome data revealed accumulation of maize Pap transcripts in multiple plant tissues at multiple stages of development, and increased accumulation of specific transcripts under low phosphorus availability. These data suggest the maize PAP family as a whole to have broad significance throughout the plant life cycle, while highlighting potential functional specialization of individual family members.

Impacts of high ATP supply from chloroplasts and mitochondria on the leaf metabolism of Arabidopsis thaliana

Chloroplasts and mitochondria are the major ATP producing organelles in plant leaves. Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) is a phosphatase dually targeted to the outer membranes of both organelles and it plays a role in the import of selected nuclear-encoded proteins into these two organelles. Overexpression (OE) of AtPAP2 in A. thaliana accelerates plant growth and promotes flowering, seed yield, and biomass at maturity. Measurement of ADP/ATP/NADP(+)/NADPH contents in the leaves of 20-day-old OE and wild-type (WT) lines at the end of night and at 1 and 8 h following illumination in a 16/8 h photoperiod revealed that the ATP levels and ATP/NADPH ratios were significantly increased in the OE line at all three time points. The AtPAP2 OE line is therefore a good model to investigate the impact of high energy on the global molecular status of Arabidopsis. In this study, transcriptome, proteome, and metabolome profiles of the high ATP transgenic line were examined and compared with those of WT plants. A comparison of OE and WT at the end of the night provide valuable information on the impact of higher ATP output from mitochondria on plant physiology, as mitochondrial respiration is the major source of ATP in the dark in leaves. Similarly, comparison of OE and WT following illumination will provide information on the impact of higher energy output from chloroplasts on plant physiology. OE of AtPAP2 was found to significantly affect the transcript and protein abundances of genes encoded by the two organellar genomes. For example, the protein abundances of many ribosomal proteins encoded by the chloroplast genome were higher in the AtPAP2 OE line under both light and dark conditions, while the protein abundances of multiple components of the photosynthetic complexes were lower. RNA-seq data also showed that the transcription of the mitochondrial genome is greatly affected by the availability of energy. These data reflect that the transcription and translation of organellar genomes are tightly coupled with the energy status. This study thus provides comprehensive information on the impact of high ATP level on plant physiology, from organellar biology to primary and secondary metabolism.

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.

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.

New insights into the targeting of a subset of tail-anchored proteins to the outer mitochondrial membrane

Tail-anchored (TA) proteins are a unique class of functionally diverse membrane proteins defined by their single C-terminal membrane-spanning domain and their ability to insert post-translationally into specific organelles with an Ncytoplasm-Corganelle interior orientation. The molecular mechanisms by which TA proteins are sorted to the proper organelles are not well-understood. Herein we present results indicating that a dibasic targeting motif (i.e., -R-R/K/H-X({X≠E})) identified previously in the C terminus of the mitochondrial isoform of the TA protein cytochrome b 5, also exists in many other A. thaliana outer mitochondrial membrane (OMM)-TA proteins. This motif is conspicuously absent, however, in all but one of the TA protein subunits of the translocon at the outer membrane of mitochondria (TOM), suggesting that these two groups of proteins utilize distinct biogenetic pathways. Consistent with this premise, we show that the TA sequences of the dibasic-containing proteins are both necessary and sufficient for targeting to mitochondria, and are interchangeable, while the TA regions of TOM proteins lacking a dibasic motif are necessary, but not sufficient for localization, and cannot be functionally exchanged. We also present results from a comprehensive mutational analysis of the dibasic motif and surrounding sequences that not only greatly expands the functional definition and context-dependent properties of this targeting signal, but also led to the identification of other novel putative OMM-TA proteins. Collectively, these results provide important insight to the complexity of the targeting pathways involved in the biogenesis of OMM-TA proteins and help define a consensus targeting motif that is utilized by at least a subset of these proteins.

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.

Elongator protein 3 (Elp3) lysine acetyltransferase is a tail-anchored mitochondrial protein in Toxoplasma gondii

Lysine acetylation has recently emerged as an important, widespread post-translational modification occurring on proteins that reside in multiple cellular compartments, including the mitochondria. However, no lysine acetyltransferase (KAT) has been definitively localized to this organelle to date. Here we describe the identification of an unusual homologue of Elp3 in early-branching protozoa in the phylum Apicomplexa. Elp3 is the catalytic subunit of the well-conserved transcription Elongator complex; however, Apicomplexa lack all other Elongator subunits, suggesting that the Elp3 in these organisms plays a role independent of transcription. Surprisingly, Elp3 in the parasites of this phylum, including Toxoplasma gondii (TgElp3), possesses a unique C-terminal transmembrane domain (TMD) that localizes the protein to the mitochondrion. As TgElp3 is devoid of known mitochondrial targeting signals, we used selective permeabilization studies to reveal that this KAT is oriented with its catalytic components facing the cytosol and its C-terminal TMD inserted into the outer mitochondrial membrane, consistent with a tail-anchored membrane protein. Elp3 trafficking to mitochondria is not exclusive to Toxoplasma as we also present evidence that a form of Elp3 localizes to these organelles in mammalian cells, supporting the idea that Elp3 performs novel functions across eukaryotes that are independent of transcriptional elongation. Importantly, we also present genetic studies that suggest TgElp3 is essential in Toxoplasma and must be positioned at the mitochondrial surface for parasite viability.

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.

Plant mitochondrial retrograde signaling: post-translational modifications enter the stage

Beside their central function in respiration plant mitochondria play important roles in diverse processes such as redox homeostasis, provision of precursor molecules for essential biosynthetic pathways, and programmed cell death. These different functions require the organelle to communicate with the rest of the cell by perceiving, transducing, and emitting signals. As the vast majority of mitochondrial proteins are encoded in the nuclear genome, changes in mitochondrial status must be fed back to the nucleus to coordinate gene expression accordingly, a process termed retrograde signaling. However, the nature of these signaling pathways in plants and their underlying signaling molecules - or indirect metabolite or redox signals - are not completely resolved. We explore the potential of different post-translational modifications (PTMs) to contribute to mitochondrial retrograde signaling. Remarkably, the substrates used for modifying proteins in many major PTMs are either central metabolites or redox-active compounds, as for example ATP, acetyl-CoA, NAD(+), and glutathione. This suggests that the metabolic status of organelles and of the cell in general could be indirectly gaged by the enzymes catalyzing the various PTMs. We examine the evidence supporting this hypothesis with regard to three major PTMs, namely phosphorylation, lysine acetylation, and glutathionylation and assess their potential to regulate not only organellar processes by modifying metabolic enzymes but also to influence nuclear gene expression.