Lipid trafficking between the endoplasmic reticulum and the chloroplast

Two plastidial lysophosphatidic acid acyltransferases differentially mediate the biosynthesis of membrane lipids and triacylglycerols in Phaeodactylum tricornutum

Lysophosphatidic acid acyltransferases (LPAATs) catalyze the formation of phosphatidic acid (PA), a central metabolite in both prokaryotic and eukaryotic organisms for glycerolipid biosynthesis. Phaeodactylum tricornutum contains at least two plastid-localized LPAATs (ptATS2a and ptATS2b), but their roles in lipid synthesis remain unknown. Both ptATS2a and ptATS2b could complement the high temperature sensitivity of the bacterial plsC mutant deficient in LPAAT. In vitro enzyme assays showed that they prefer lysophosphatidic acid over other lysophospholipids. ptATS2a is localized in the plastid inner envelope membrane and CRISPR/Cas9-generated ptATS2a mutants showed compromised cell growth, significantly changed plastid and extra-plastidial membrane lipids at nitrogen-replete condition and reduced triacylglycerols (TAGs) under nitrogen-depleted condition. ptATS2b is localized in thylakoid membranes and its knockout led to reduced growth rate and TAG content but slightly altered molecular composition of membrane lipids. The changes in glycerolipid profiles are consistent with the role of both LPAATs in the sn-2 acylation of sn-1-acyl-glycerol-3-phosphate substrates harboring 20:5 at the sn-1 position. Our findings suggest that both LPAATs are important for membrane lipids and TAG biosynthesis in P. tricornutum and further highlight that 20:5-Lyso-PA is likely involved in the massive import of 20:5 back to the plastid to feed plastid glycerolipid syntheses.

GDSL lipases modulate immunity through lipid homeostasis in rice

Lipids and lipid metabolites play important roles in plant-microbe interactions. Despite the extensive studies of lipases in lipid homeostasis and seed oil biosynthesis, the involvement of lipases in plant immunity remains largely unknown. In particular, GDSL esterases/lipases, characterized by the conserved GDSL motif, are a subfamily of lipolytic enzymes with broad substrate specificity. Here, we functionally identified two GDSL lipases, OsGLIP1 and OsGLIP2, in rice immune responses. Expression of OsGLIP1 and OsGLIP2 was suppressed by pathogen infection and salicylic acid (SA) treatment. OsGLIP1 was mainly expressed in leaf and leaf sheath, while OsGLIP2 showed high expression in elongating internodes. Biochemical assay demonstrated that OsGLIP1 and OsGLIP2 are functional lipases that could hydrolyze lipid substrates. Simultaneous down-regulation of OsGLIP1 and OsGLIP2 increased plant resistance to both bacterial and fungal pathogens, whereas disease resistance in OsGLIP1 and OsGLIP2 overexpression plants was significantly compromised, suggesting that both genes act as negative regulators of disease resistance. OsGLIP1 and OsGLIP2 proteins mainly localize to lipid droplets and the endoplasmic reticulum (ER) membrane. The proper cellular localization of OsGLIP proteins is indispensable for their functions in immunity. Comprehensive lipid profiling analysis indicated that the alteration of OsGLIP gene expression was associated with substantial changes of the levels of lipid species including monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). We show that MGDG and DGDG feeding could attenuate disease resistance. Taken together, our study indicates that OsGLIP1 and OsGLIP2 negatively regulate rice defense by modulating lipid metabolism, thus providing new insights into the function of lipids in plant immunity.

Lipases are a large family of enzymes conferring lipid metabolism. Lipids and their metabolites play diverse roles in plant growth as well as response to environmental stimuli. Accumulating evidence implicates lipids as signaling molecules mediating plant immunity. Therefore, lipases are presumed to be actively involved in plant defense responses. Based on gene expression profiling, we have identified two functional GDSL lipases, encoded by OsGLIP1 and OsGLIP2, whose expression was suppressed by pathogen infection in the model cereal rice. Both OsGLIP1 and OsGLIP2 proteins localize to lipid droplets and the endoplasmic reticulum (ER) membrane, and they likely coordinate lipid metabolism with differential but complementary expression patterns in tissues and developmental stages. Consequently, alteration of OsGLIP gene expression was associated with substantial changes of lipid abundance and plant disease resistance. Our work identifies and characterizes two lipases that function as negative regulators of plant immune responses, strengthening the understanding of lipid metabolism in plant-microbe interactions.

Plant ABC Transporters Enable Many Unique Aspects of a Terrestrial Plant's Lifestyle

Terrestrial plants have two to four times more ATP-binding cassette (ABC) transporter genes than other organisms, including their ancestral microalgae. Recent studies found that plants harboring mutations in these transporters exhibit dramatic phenotypes, many of which are related to developmental processes and functions necessary for life on dry land. These results suggest that ABC transporters multiplied during evolution and assumed novel functions that allowed plants to adapt to terrestrial environmental conditions. Examining the literature on plant ABC transporters from this viewpoint led us to propose that diverse ABC transporters enabled many unique and essential aspects of a terrestrial plant's lifestyle, by transporting various compounds across specific membranes of the plant.

A rice chloroplast transit peptide sequence does not alter the cytoplasmic localization of sheep serotonin N-acetyltransferase expressed in transgenic rice plants

Ectopic overexpression of melatonin biosynthetic genes of animal origin has been used to generate melatonin-rich transgenic plants to examine the functional roles of melatonin in plants. However, the subcellular localization of these proteins expressed in the transgenic plants remains unknown. We studied the localization of sheep (Ovis aries) serotonin N-acetyltransferase (OaSNAT) and a translational fusion of a rice SNAT transit peptide to OaSNAT (TS:OaSNAT) in plants. Laser confocal microscopy analysis revealed that both OaSNAT and TS:OaSNAT proteins were localized to the cytoplasm even with the addition of the transit sequence to OaSNAT. Transgenic rice plants overexpressing the TS:OaSNAT fusion transgene exhibited high SNAT enzyme activity relative to untransformed wild-type plants, but lower activity than transgenic rice plants expressing the wild-type OaSNAT gene. Melatonin levels in both types of transgenic rice plant corresponded well with SNAT enzyme activity levels. The TS:OaSNAT transgenic lines exhibited increased seminal root growth relative to wild-type plants, but less than in the OaSNAT transgenic lines, confirming that melatonin promotes root growth. Seed-specific OaSNAT expression under the control of a rice prolamin promoter did not confer high levels of melatonin production in transgenic rice seeds compared with seeds from transgenic plants expressing OaSNAT under the control of the constitutive maize ubiquitin promoter.

Terpenoid biosynthesis in trichomes--current status and future opportunities

Glandular trichomes are anatomical structures specialized for the synthesis of secreted natural products. In this review we focus on the description of glands that accumulate terpenoid essential oils and oleoresins. We also provide an in-depth account of the current knowledge about the biosynthesis of terpenoids and secretion mechanisms in the highly specialized secretory cells of glandular trichomes, and highlight the implications for metabolic engineering efforts.

The potato mop-top virus TGB2 protein and viral RNA associate with chloroplasts and viral infection induces inclusions in the plastids

The potato mop-top virus (PMTV) triple gene block 2 (TGB2) movement proteins fused to monomeric red fluorescent protein (mRFP-TGB2) was expressed under the control of the PMTV subgenomic promoter from a PMTV vector. The subcellular localizations and interactions of mRFP-TGB2 were investigated using confocal imaging [confocal laser-scanning microscope, (CLSM)] and biochemical analysis. The results revealed associations with membranes of the endoplasmic reticulum (ER), mobile granules, small round structures (1-2 μm in diameter), and chloroplasts. Expression of mRFP-TGB2 in epidermal cells enabled cell-to-cell movement of a TGB2 defective PMTV reporter clone, indicating that the mRFP-TGB2 fusion protein was functional and required for cell-to-cell movement. Protein-lipid interaction assays revealed an association between TGB2 and lipids present in chloroplasts, consistent with microscopical observations where the plastid envelope was labeled later in infection. To further investigate the association of PMTV infection with chloroplasts, ultrastructural studies of thin sections of PMTV-infected potato and Nicotiana benthamiana leaves by electron microscopy revealed abnormal chloroplasts with cytoplasmic inclusions and terminal projections. Viral coat protein (CP), genomic RNA and fluorescently-labeled TGB2 were detected in plastid preparations isolated from the infected leaves, and viral RNA was localized to chloroplasts in infected tissues. The results reveal a novel association of TGB2 and vRNA with chloroplasts, and suggest viral replication is associated with chloroplast membranes, and that TGB2 plays a novel role in targeting the virus to chloroplasts.

Flip-flop of phospholipids in proteoliposomes reconstituted from detergent extract of chloroplast membranes: kinetics and phospholipid specificity

Eukaryotic cells are compartmentalized into distinct sub-cellular organelles by lipid bilayers, which are known to be involved in numerous cellular processes. The wide repertoire of lipids, synthesized in the biogenic membranes like the endoplasmic reticulum and bacterial cytoplasmic membranes are initially localized in the cytosolic leaflet and some of these lipids have to be translocated to the exoplasmic leaflet for membrane biogenesis and uniform growth. It is known that phospholipid (PL) translocation in biogenic membranes is mediated by specific membrane proteins which occur in a rapid, bi-directional fashion without metabolic energy requirement and with no specificity to PL head group. A recent study reported the existence of biogenic membrane flippases in plants and that the mechanism of plant membrane biogenesis was similar to that found in animals. In this study, we demonstrate for the first time ATP independent and ATP dependent flippase activity in chloroplast membranes of plants. For this, we generated proteoliposomes from Triton X-100 extract of intact chloroplast, envelope membrane and thylakoid isolated from spinach leaves and assayed for flippase activity using fluorescent labeled phospholipids. Half-life time of flipping was found to be 6±1 min. We also show that: (a) intact chloroplast and envelope membrane reconstituted proteoliposomes can flip fluorescent labeled analogs of phosphatidylcholine in ATP independent manner, (b) envelope membrane and thylakoid reconstituted proteoliposomes can flip phosphatidylglycerol in ATP dependent manner, (c) Biogenic membrane ATP independent PC flipping activity is protein mediated and (d) the kinetics of PC translocation gets affected differently upon treatment with protease and protein modifying reagents.

Genetic analysis of Arabidopsis mutants impaired in plastid lipid import reveals a role of membrane lipids in chloroplast division

The biogenesis of photosynthetic membranes in plants relies largely on lipid import from the endoplasmic reticulum (ER) and this lipid transport process is mediated by TGD proteins in Arabidopsis. Such a dependency of chloroplast biogenesis on ER-to-plastid lipid transport was recently exemplified by analyzing double mutants between tgd1-1 or tgd4-3 and fad6 mutants. The fad6 mutants are defective in the desaturation of membrane lipids in chloroplasts and therefore dependent on import of polyunsaturated lipid precursors from the ER for constructing a competent thylakoid membrane system. In support of a critical role of TGD proteins in ER-to-plastid lipid trafficking, we showed that the introduction of the tgd mutations into fad6 mutant backgrounds led to drastic reductions in relative amounts of thylakoid lipids. Moreover, the tgd1-1 fad6 and tgd4-3 fad6 double mutants were deficient in polyunsaturated fatty acids in chloroplast membrane lipids, and severely compromised in the biogenesis of photosynthetic membrane systems. Here we report that these double mutants are severely impaired in chloroplast division. The possible role of membrane lipids in chloroplast division is discussed. :

The Arabidopsis multistress regulator TSPO is a heme binding membrane protein and a potential scavenger of porphyrins via an autophagy-dependent degradation mechanism

TSPO, a stress-induced, posttranslationally regulated, early secretory pathway-localized plant cell membrane protein, belongs to the TspO/MBR family of regulatory proteins, which can bind porphyrins. This work finds that boosting tetrapyrrole biosynthesis enhanced TSPO degradation in Arabidopsis thaliana and that TSPO could bind heme in vitro and in vivo. This binding required the His residue at position 91 (H91), but not that at position 115 (H115). The H91A and double H91A/H115A substitutions stabilized TSPO and rendered the protein insensitive to heme-regulated degradation, suggesting that heme binding regulates At-TSPO degradation. TSPO degradation was inhibited in the autophagy-defective atg5 mutant and was sensitive to inhibitors of type III phosphoinositide 3-kinases, which regulate autophagy in eukaryotic cells. Mutation of the two Tyr residues in a putative ubiquitin-like ATG8 interacting motif of At-TSPO did not affect heme binding in vitro but stabilized the protein in vivo, suggesting that downregulation of At-TSPO requires an active autophagy pathway, in addition to heme. Abscisic acid-dependent TSPO induction was accompanied by an increase in unbound heme levels, and downregulation of TSPO coincided with the return to steady state levels of unbound heme, suggesting that a physiological consequence of active TSPO downregulation may be heme scavenging. In addition, overexpression of TSPO attenuated aminolevulinic acid-induced porphyria in plant cells. Taken together, these data support a role for TSPO in porphyrin binding and scavenging during stress in plants.

Cell-type-specific differentiation of chloroplasts in C4 plants

In leaves of C4 grasses such as maize, photosynthetic activities are partitioned between bundle-sheath and mesophyll cells, leading to increased photosynthetic yield, particularly under stress conditions. As we discuss here, recent comparative chloroplast proteome analyses in maize have shown specific adaptation to C4-cell-specific differentiation of the photosynthetic apparatus, primary and secondary metabolism and metabolite transporters, as well as the chloroplast protein homeostasis machinery. Furthermore, a novel bundle-sheath-enriched 1000-kDa NADPH dehydrogenase 'supercomplex' has been identified, and we discuss its possible role in inorganic carbon concentration. These breakthroughs provide new opportunities to further unravel C4 pathways and to increase crop productivity through metabolic engineering of C4 pathways into C3 plants, such as rice.

Host origin of plastid solute transporters in the first photosynthetic eukaryotes

Analysis of plastid transporter proteins in Arabidopsis suggests a host origin and provides new insights into plastid evolution.

Background

It is generally accepted that a single primary endosymbiosis in the Plantae (red, green (including land plants), and glaucophyte algae) common ancestor gave rise to the ancestral photosynthetic organelle (plastid). Plastid establishment necessitated many steps, including the transfer and activation of endosymbiont genes that were relocated to the nuclear genome of the 'host' followed by import of the encoded proteins into the organelle. These innovations are, however, highly complex and could not have driven the initial formation of the endosymbiosis. We postulate that the re-targeting of existing host solute transporters to the plastid fore-runner was critical for the early success of the primary endosymbiosis, allowing the host to harvest endosymbiont primary production.

Results

We tested this model of transporter evolution by conducting a comprehensive analysis of the plastid permeome in Arabidopsis thaliana. Of 137 well-annotated transporter proteins that were initially considered, 83 that are broadly distributed in Plantae were submitted to phylogenetic analysis. Consistent with our hypothesis, we find that 58% of Arabidopsis transporters, including all carbohydrate transporters, are of host origin, whereas only 12% arose from the cyanobacterial endosymbiont. Four transporter genes are derived from a Chlamydia-like source, suggesting that establishment of the primary plastid likely involved contributions from at least two prokaryotic sources.

Conclusion

Our results indicate that the existing plastid solute transport system shared by Plantae is derived primarily from host genes. Important contributions also came from the cyanobacterial endosymbiont and Chlamydia-like bacteria likely co-resident in the first algae.

Allelic mutant series reveal distinct functions for Arabidopsis cycloartenol synthase 1 in cell viability and plastid biogenesis

Sterols have multiple functions in all eukaryotes. In plants, sterol biosynthesis is initiated by the enzymatic conversion of 2,3-oxidosqualene to cycloartenol. This reaction is catalyzed by cycloartenol synthase 1 (CAS1), which belongs to a family of 13 2,3-oxidosqualene cyclases in Arabidopsis thaliana. To understand the full scope of sterol biological functions in plants, we characterized allelic series of cas1 mutations. Plants carrying the weak mutant allele cas1-1 were viable but developed albino inflorescence shoots because of photooxidation of plastids in stems that contained low amounts of carotenoids and chlorophylls. Consistent with the CAS1 catalyzed reaction, mutant tissues accumulated 2,3-oxidosqualene. This triterpenoid precursor did not increase at the expense of the pathway end products. Two strong mutations, cas1-2 and cas1-3, were not transmissible through the male gametes, suggesting a role for CAS1 in male gametophyte function. To validate these findings, we analyzed a conditional CRE/loxP recombination-dependent cas1-2 mutant allele. The albino phenotype of growing leaf tissues was a typical defect observed shortly after the CRE/loxP-induced onset of CAS1 loss of function. In the induced cas1-2 seedlings, terminal phenotypes included arrest of meristematic activity, followed by necrotic death. Mutant tissues accumulated 2,3-oxidosqualene and contained low amounts of sterols. The vital role of sterols in membrane functioning most probably explains the requirement of CAS1 for plant cell viability. The observed impact of cas1 mutations on a chloroplastic function implies a previously unrecognized role of sterols or triterpenoid metabolites in plastid biogenesis.

Making the connections--the crucial role of metabolite transporters at the interface between chloroplast and cytosol

Eukaryotic cells are most fascinating because of their high degree of compartmentation. This is particularly true for plant cells, due to the presence of chloroplasts, photosynthetic organelles of endosymbiotic origin that can be traced back to a single cyanobacterial ancestor. Plastids are major hubs in the metabolic network of plant cells, their metabolism being heavily intertwined with that of the cytosol and of other organelles. Solute transport across the plastid envelope by metabolite transporters is key to integrating plastid metabolism with that of other cellular compartments. Here, we review the advances in understanding metabolite transport across the plastid envelope membrane.