Abstract Chloroplast HSP70 is an essential component of the plastid proteostasis network, supporting protein folding, complex assembly and disassembly, and stress acclimation. Despite extensive genetic evidence for its essentiality, the cellular consequences of reduced chloroplast HSP70 activity remain poorly defined. Here, we investigated the function of the sole chloroplast HSP70 in Chlamydomonas reinhardtii , HSP70B, using an inducible artificial microRNA approach that reduced HSP70B abundance to below 30% of wild-type levels. HSP70B depletion resulted in cell division arrest and extensive proteome remodeling, characterized by strong upregulation of proteins involved in chloroplast protein quality control and membrane remodeling. Notably, this response was accompanied by increased abundance of protein quality control components in the endoplasmic reticulum, cytosol, and mitochondria, indicating pronounced proteostasis cross-talk between cellular compartments. In contrast, chloroplast and cytosolic ribosomes, photosynthetic and respiratory protein complexes, and central metabolic enzymes were broadly depleted, consistent with a collapse of cellular proteostasis. At the ultrastructural level, HSP70B-depleted cells exhibited lesions at thylakoid membrane conversion zones previously described in VIPP1-depleted cells. Accordingly, higher-order oligomeric forms of VIPP1 accumulated, and cells displayed extreme sensitivity to high-light stress. These findings confirm HSP70B as a key regulator of VIPP1 oligomer dynamics and highlight its central role in coordinating chloroplast membrane remodeling with cellular proteostasis in Chlamydomonas . One-sentence summary Depletion of chloroplast HSP70B causes cell division arrest, proteostasis collapse, impaired VIPP1 oligomer dynamics with aberrant thylakoid structures, and increased light sensitivity.

Abstract Botrytis cinerea is a necrotrophic plant pathogen with an extremely wide host range. During invasion, the fungus induces rapid host cell death and proliferates in the necrotic tissue. The mechanisms of host killing are still incompletely understood, they involve secretion of lytic enzymes, phytotoxic metabolites and cell death inducing proteins (CDIPs). We have previously shown that the sequential knockout of up to 12 CDIPs leads to a substantial reduction of virulence of B. cinerea mutants. In this study, we have identified additional CDIPs and generated an extended mutant series culminating in a 29x mutant which is deficient in most currently known CDIPs and two phytotoxic metabolites. These mutants are strongly reduced in infection, but still induced necrosis and grey mould symptoms, demonstrating that additional determinants of host killing remain unidentified. Overexpression of the highly phytotoxic Nep1 in a 22-fold CDIP mutant failed to increase its virulence. Reevaluation of several CDIPs previously described as virulence factors revealed for most tested CDIPs no major contribution to pathogenesis. Together with the observation that none of the CDIPs are specific to B. cinerea , our data question a particular role of CDIPs for necrotrophic pathogenesis. In contrast, generation of a mutant lacking all six predicted endo-polygalacturonases confirmed their major but not exclusive role for tissue degradation and infection.

The chloroplast chaperone HSP70B from Chlamydomonas reinhardtii works with the J-domain co-chaperones CDJ1 to CDJ6. CDJ1 delivers unfolded proteins, while CDJ2 delivers VIPP1 as substrate to HSP70B. CDJ3 to CDJ5 contain 4Fe-4S clusters in addition to the J domain, but their function is unknown. To investigate the function of CDJ5, we performed TurboID - mediated proximity labeling on wild-type CDJ5 (CDJ5-WT) and CDJ5 mutants with an impaired ability to stimulate HSP70B’s ATPase activity (CDJ5-AAA) or bind a 4Fe-4S cluster (CDJ5- SSS). Our results revealed that the proxiomes of all CDJ5 variants contained HSP70B and HSP90C. Furthermore, the proxiomes of CDJ5-WT and CDJ5-AAA overlapped extensively but differed from that of CDJ5-SSS, suggesting that the localization of CDJ5 to a chloroplast microcompartment depends on the presence of a functional 4Fe-4S cluster or its redox state. The CDJ5-WT and CDJ5-AAA proxiomes were enriched with proteins that regulate photosynthetic electron flow or are involved in the biogenesis of thylakoid membrane protein complexes and pigments. These proteins were also present in the proxiome of PGRL1 found in the CDJ5-WT and CDJ5-AAA proxiomes. Overall, our results suggest that CDJ5 acts with HSP70B/HSP90C via its 4Fe-4S cluster to regulate photosynthetic electron flow and thylakoid membrane protein complex biogenesis. Highlight Depending on its 4Fe-4S cluster, HSP70B co-chaperone CDJ5 localizes to a chloroplast microcompartment defined by PGRL1 where it could act in regulating photosynthetic electron flow and thylakoid membrane protein complex biogenesis. ### Competing Interest Statement The authors have declared no competing interest.

Plant proteins of the CURVATURE THYLAKOID 1 (CURT1) family and their prokaryotic CurT homologues are key determinants of the three-dimensional structure of the thylakoid membrane systems in chloroplasts and cyanobacteria. As the evolutionary origin of the CURT1/CurT family appears to coincide with the evolution of thylakoids themselves, shaping the thylakoid system has widely been regarded as their primary role. In this study we present strong evidence that CurT, beyond regulation of thylakoid architecture, is involved in cell division and thylakoid fission/partitioning in both Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942, likely through physical interaction with the key cell division protein FtsZ. Similarly, triple mutants of Chlamydomonas reinhardtii CURT1A, B, and C display an asymmetric chloroplast division phenotype, thus suggesting an evolutionary conserved functionality of CurT/CURT1 in cell/chloroplast division in single-celled oxygenic photosynthesizers.

PSII assembly requires auxiliary factors, including Psb28. Although the absence of Psb28 in cyanobacteria has little effect on PSII assembly, we show here that the Chlamydomonas (Chlamydomonas reinhardtii) psb28 null mutant is severely impaired in PSII assembly, showing drastically reduced PSII supercomplexes, dimers, and monomers, while overaccumulating early PSII assembly intermediates reaction center II (RCII), CP43mod, and D1mod. The mutant had less PSI and more cytochrome b6f complex, its thylakoids were organized mainly as monolayers, and it had a distorted chloroplast morphology. Complexome profiling of the psb28 mutant revealed that THYLAKOID ENRICHED FRACTION 5 (TEF5), the homolog of Arabidopsis (Arabidopsis thaliana) PHOTOSYSTEM B PROTEIN 33/LIGHT HARVESTING-LIKE 8, comigrated particularly with RCII. TEF5 also interacted with PSI. A Chlamydomonas tef5 null mutant was severely impaired in PSII assembly and overaccumulated RCII and CP43mod. RC47 was not detectable in the light-grown tef5 mutant. Our data suggest a possible role for TEF5 in RCII photoprotection or maturation. Both the psb28 and tef5 mutants exhibited decreased synthesis of CP47 and PsbH, suggesting negative feedback regulation possibly exerted by the accumulation of RCII and/or CP43mod in both mutants. The strong effects of missing auxiliary factors on PSII assembly in Chlamydomonas suggest a more effective protein quality control system in this alga than in land plants and cyanobacteria.

While Botrytis cinerea causes gray mold on many plants, its close relative, Botrytis fabae, is host-specifically infecting predominantly faba bean plants. To explore the basis for its narrow host range, a gapless genome sequence of B. fabae strain G12 (BfabG12) was generated. The BfabG12 genome encompasses 45.0 Mb, with 16 chromosomal telomere-to-telomere contigs that show high synteny and sequence similarity to the corresponding B. cinerea B05.10 (BcB0510) chromosomes. Compared to BcB0510, it is 6% larger, due to many AT-rich regions containing remnants of transposable elements, but encodes fewer genes (11,420 vs. 11,707), due to losses of chromosomal segments with up to 20 genes. The coding capacity of BfabG12 is further reduced by nearly 400 genes that had been inactivated by mutations leading to truncations compared to their BcB0510 orthologues. Several species-specific gene clusters for secondary metabolite biosynthesis with stage-specific expression were identified. Comparison of the proteins secreted during infection revealed high similarities, including 17 phytotoxic proteins that were detected in both species. Our data indicate that evolution of the host-specific B. fabae occurred from an ancestral pathogen with wide host range similar to B. cinerea and was accompanied by losses and degeneration of genes, thereby reducing its pathogenic flexibility.

Several auxiliary factors are required for the assembly of photosystem (PS) II, one of which is Psb28. While the absence of Psb28 in cyanobacteria has little effect on PSII assembly, we show here that the Chlamydomonas psb28 -null mutant is severely impaired in PSII assembly, showing drastically reduced PSII supercomplexes, dimers and monomers, while overaccumulating RCII, CP43mod and D1mod. The mutant had less PSI and more Cyt b6f and showed fewer thylakoid stacks and distorted chloroplast morphology. Complexome profiling of the psb28 mutant revealed that TEF5, the homolog of Arabidopsis PSB33/LIL8, co-migrated particularly with RCII. TEF5 also interacted with PSI. A Chlamydomonas tef5 null mutant is also severely impaired in PSII assembly and overaccumulates RCII and CP43mod. RC47 was not detectable in the light-grown tef5 mutant. Our data suggest a possible role for TEF5 in facilitating the assembly of CP47mod into RCII. Both the psb28 and tef5 mutants exhibited decreased synthesis of CP47 and PsbH, suggesting negative feedback regulation possibly exerted by the accumulation of RCII and/or CP43mod in both mutants. The strong effects of missing auxiliary factors on PSII assembly in Chlamydomonas suggest a more effective protein quality control system in this alga than in land plants and cyanobacteria. One-sentence summary The Chlamydomonas psb28 mutant is severely impaired in PSII assembly which via complexome profiling allowed identifying TEF5 as a novel PSII assembly factor that likely facilitates CP47 assembly. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors () is: Michael Schroda (m.schroda{at}rptu.de).

Plants need to adapt to different stresses to optimize growth under unfavorable conditions. The abundance of the chloroplast envelope located Fatty Acid Export Protein1 (FAX1) decreases after the onset of low temperatures. However, it was unclear how FAX1 degradation occurs and whether altered FAX1 abundance contributes to cold tolerance in plants. The rapid cold-induced increase in rhomboid-like protease11 (RBL11) transcript, the physical interaction of RBL11 with FAX1, the specific FAX1 degradation after RBL11 expression, and the absence of cold-induced FAX1 degradation in rbl11 loss-of-function mutants suggest that this enzyme is responsible for FAX1 degradation. Proteomic analyses showed that rbl11 mutants have higher levels of FAX1 and other proteins involved in membrane lipid homeostasis, suggesting that RBL11 is a key element in the remodeling of membrane properties during cold. Consequently, in the cold, rbl11 mutants show a shift in lipid biosynthesis towards the eukaryotic pathway, which coincides with impaired cold tolerance. To demonstrate that cold sensitivity is due to increased FAX1 levels, FAX1 overexpressors were analyzed. rbl11 and FAX1 overexpressor mutants show superimposable phenotypic defects upon exposure to cold temperatures. Our results show that the cold-induced degradation of FAX1 by RBL11 is critical for Arabidopsis to survive cold and freezing periods. One sentence summary Degradation of the inner envelope protein Fatty Acid Export1 via Rhomboid Like Protease11 represents a critical process to achieve cold and frost tolerance in Arabidopsis

Thiol-based redox regulation is a crucial post-translational mechanism to acclimate plants to changing light availability. Here, we conduct a biotin-switch-based redox proteomics study to systematically investigate dynamics of the thiol-redox network in response to temporal changes in light availability and across genotypes lacking parts of the thioredoxin (Trx) or NADPH-Trx-reductase C (NTRC) systems in the chloroplast. Time-resolved dynamics revealed light leading to marked decreases in the oxidation states of many chloroplast proteins with photosynthetic functions during the first 10 min, followed by their partial re-oxidation after 2-6 hours into the photoperiod. This involved f , m and x -type Trx proteins showing similar light-induced reduction-oxidation dynamics, while NTRC, 2-Cys-Prx and Trx y 2 showed an opposing pattern, being more oxidized in the light than the dark. In Arabidopsis trxf1f2 , trxm1m2 or ntrc mutants, in the light most proteins showed increased oxidation states than wild type, suggesting their light-dependent dynamics being related to the NTRC/Trx networks. While NTRC deficiency had a strong influence in all light conditions, deficiencies in f - or m -type Trxs showed differential impacts on the thiol-redox proteome depending on the light environment, being higher in constant or fluctuating light, respectively. Results indicate plant redox proteomes to be subject to dynamic changes in reductive and oxidative pathways to cooperatively fine-tune photosynthetic and metabolic processes in the light. This involves f -type Trxs and NTRC to play a role in constant medium light, while both m -type Trxs and NTRC being important to balance changes in protein redox-pattern during dynamic alterations in fluctuating light intensities. One sentence summary The plant protein redoxome shows light-dependent reduction and reoxidation dynamics linked to Trxs f 1/ f 2, m 1/ m 2 and NTRC, being of different importance depending on the extent of light variability.

Phycobiliproteins (PBPs) play a vital role in light harvesting by cyanobacteria, which enables efficient utilization of photon energy for oxygenic photosynthesis. The PBPs carry phycobilins, open-chain tetrapyrrole chromophores derived from heme. The structure and chromophore composition of PBPs is dependent on the organism’s ecological niche. In cyanobacteria, these holo-proteins typically form large, macromolecular antenna complexes called phycobilisomes (PBSs). The PBS of Synechocystis sp. PCC 6803 (hereafter Synechocystis ) consists of allophycocyanin (APC) and phycocyanin (PC), which exclusively harbor phycocyanobilin (PCB) as a chromophore. Investigations into heterologous PBP biosynthesis in E. coli have proven limiting with respect to PBP assembly and their functional characterization. Consequently, we wanted to engineer a platform for the investigation of heterologously produced PBPs, focusing on unusual, phycoerythrobilin (PEB)-containing light-harvesting proteins called phycoerythrins (PEs) in Synechocystis . As a first step, a gene encoding for the synthesis of the natural cyanobacterial chromophore, PEB, was introduced into Synechocystis . We provide spectroscopic evidence for heterologous PEB formation and show covalent attachment of PEB to the α-subunit of PC, CpcA, by HPLC and LC-MS/MS analyses. Fluorescence microscopy and PBS isolation demonstrate a cellular dispersal of PBPs with modified phycobilin content. However, these modifications have minor effects on physiological responses, as demonstrated by growth rates, oxygen evolution, nutrient accumulation, and PBP content analyses. As a result, Synechocystis demonstrates the capacity to efficiently manage PEB biosynthesis and therefore reflects a promising platform for both biochemical and physiological investigations of foreign and unusual PEs.

The enterobacterium Escherichia coli present in the human gut can reduce trimethylamine N-oxide (TMAO) to trimethylamine during anaerobic respiration. The TMAO reductase TorA is a monomeric, bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor-containing enzyme that belongs to the dimethyl sulfoxide reductase family of molybdoenzymes. TorA is anchored to the membrane via TorC, a pentahemic c-type cytochrome which receives the electrons from the menaquinol pool. Here, we designed an expression system for the production of a stable soluble form of multiheme-containing TorC, providing, for the first time, the purification of a soluble pentahemic cytochrome-c from E. coli. Our focus was to investigate the interaction between TorA and soluble TorC to establish the electron transfer pathway. We solved the X-ray structure of E. coli TorA and performed chemical crosslinking of TorA and TorC. Another goal was to establish an activity assay that used the physiological electron transfer pathway instead of the commonly used unphysiological electron donors methylviologen or benzylviologen. An AlphaFold model including the crosslinking sites provided insights into the electron transfer between TorCC and the active site of TorA.

Metabolic engineering of microalgae offers a promising solution for sustainable biofuel production, and rational design of engineering strategies can be improved by employing metabolic models that integrate enzyme turnover numbers. However, the coverage of turnover numbers for Chlamydomonas reinhardtii , a model eukaryotic microalga accessible to metabolic engineering, is 17-fold smaller compared to the heterotrophic cell factory Saccharomyces cerevisiae . Here we generate quantitative protein abundance data of Chlamydomonas covering 2337 to 3708 proteins in various growth conditions to estimate in vivo maximum apparent turnover numbers. Using constrained-based modeling we provide proxies for in vivo turnover numbers of 568 reactions, representing a 10-fold increase over the in vitro data for Chlamydomonas . Integration of the in vivo estimates instead of in vitro values in a metabolic model of Chlamydomonas improved the accuracy of enzyme usage predictions. Our results help in extending the knowledge on uncharacterized enzymes and improve biotechnological applications of Chlamydomonas .

According to their lifestyle, plant pathogens are divided into biotrophic and necrotrophic organisms. Biotrophic pathogens exclusively nourish living host cells, whereas necrotrophic pathogens rapidly kill host cells and nourish cell walls and cell contents. To this end, the necrotrophic fungus Botrytis cinerea secretes large amounts of phytotoxic proteins and cell wall-degrading enzymes. However, the precise role of these proteins during infection is unknown. Here, we report on the identification and characterization of the previously unknown toxic protein hypersensitive response-inducing protein 1 (Hip1), which induces plant cell death. We found the adoption of a structurally conserved folded Alternaria alternata Alt a 1 protein structure to be a prerequisite for Hip1 to exert its necrosis-inducing activity in a host-specific manner. Localization and the induction of typical plant defense responses by Hip1 indicate recognition as a pathogen-associated molecular pattern at the plant plasma membrane. In contrast to other secreted toxic Botrytis proteins, the activity of Hip1 does not depend on the presence of the receptor-associated kinases BRI1-associated kinase 1 and suppressor of BIR1-1. Our results demonstrate that recognition of Hip1, even in the absence of obvious enzymatic or pore-forming activity, induces strong plant defense reactions eventually leading to plant cell death. Botrytis hip1 overexpression strains generated by CRISPR/Cas9 displayed enhanced infection, indicating the virulence-promoting potential of Hip1. Taken together, Hip1 induces a noncanonical defense response which might be a common feature of structurally conserved fungal proteins from the Alt a 1 family.

In land plants and cyanobacteria, co-translational association of chlorophyll (Chl) to the nascent D1 polypeptide, a reaction center protein of photosystem II (PSII), requires a Chl binding complex consisting of a short-chain dehydrogenase (high chlorophyll fluorescence 244 [HCF244]/uncharacterized protein 39 [Ycf39]) and one-helix proteins (OHP1 and OHP2 in chloroplasts) of the light-harvesting antenna complex superfamily. Here, we show that an ohp2 mutant of the green alga Chlamydomonas (Chlamydomonas reinhardtii) fails to accumulate core PSII subunits, in particular D1 (encoded by the psbA mRNA). Extragenic suppressors arose at high frequency, suggesting the existence of another route for Chl association to PSII. The ohp2 mutant was complemented by the Arabidopsis (Arabidopsis thaliana) ortholog. In contrast to land plants, where psbA translation is prevented in the absence of OHP2, ribosome profiling experiments showed that the Chlamydomonas mutant translates the psbA transcript over its full length. Pulse labeling suggested that D1 is degraded during or immediately after translation. The translation of other PSII subunits was affected by assembly-controlled translational regulation. Proteomics showed that HCF244, a translation factor which associates with and is stabilized by OHP2 in land plants, still partly accumulates in the Chlamydomonas ohp2 mutant, explaining the persistence of psbA translation. Several Chl biosynthesis enzymes overaccumulate in the mutant membranes. Partial inactivation of a D1-degrading protease restored a low level of PSII activity in an ohp2 background, but not photoautotrophy. Taken together, our data suggest that OHP2 is not required for psbA translation in Chlamydomonas, but is necessary for D1 stabilization.

In Chlamydomonas (Chlamydomonas reinhardtii), the VESICLE-INDUCING PROTEIN IN PLASTIDS 1 and 2 (VIPP1 and VIPP2) play roles in the sensing and coping with membrane stress and in thylakoid membrane biogenesis. To gain more insight into these processes, we aimed to identify proteins interacting with VIPP1/2 in the chloroplast and chose proximity labeling (PL) for this purpose. We used the transient interaction between the nucleotide exchange factor CHLOROPLAST GRPE HOMOLOG 1 (CGE1) and the stromal HEAT SHOCK PROTEIN 70B (HSP70B) as test system. While PL with APEX2 and BioID proved to be inefficient, TurboID resulted in substantial biotinylation in vivo. TurboID-mediated PL with VIPP1/2 as baits under ambient and H2O2 stress conditions confirmed known interactions of VIPP1 with VIPP2, HSP70B, and the CHLOROPLAST DNAJ HOMOLOG 2 (CDJ2). Proteins identified in the VIPP1/2 proxiomes can be grouped into proteins involved in the biogenesis of thylakoid membrane complexes and the regulation of photosynthetic electron transport, including PROTON GRADIENT REGULATION 5-LIKE 1 (PGRL1). A third group comprises 11 proteins of unknown function whose genes are upregulated under chloroplast stress conditions. We named them VIPP PROXIMITY LABELING (VPL). In reciprocal experiments, we confirmed VIPP1 in the proxiomes of VPL2 and PGRL1. Our results demonstrate the robustness of TurboID-mediated PL for studying protein interaction networks in the chloroplast of Chlamydomonas and pave the way for analyzing functions of VIPPs in thylakoid biogenesis and stress responses.

Biological processes in eukaryotes depend on the spatio-temporal compartmentalization of their cells. Integrity and positioning of organelles on the other hand rely on the organization of the actin cytoskeleton. Previously, it has been shown that changes of the plants largest organelle, the vacuole, depends on a functional actin organization. The connection between actin filaments and the vacuole is established by the family of Networked (NET) 4 proteins and, consequently, altering NET4 abundance impacts vacuolar morphology. However, the precise regulatory mechanism is unknown and gene deletions of NET4 did not result in a global growth phenotype. Here we show that NET4 functions redundantly with NET3, interacting with RABG3-GTPases at the vacuole to allow for homotypic fusion or, alternatively, the generation of endoplasmic reticulum (ER) - vacuole contact sites. We found that ER-resident NET3 is able to interact with RABG3 residing at the tonoplast and that NET4 interacts with the contact site protein VAP27-1 at the ER. Generation of net3 net4 triple mutants by CRSIPR-guided mutagenesis helped us to overcome functional redundancy, resulting in impaired plant growth and development. Our results demonstrate how diversification of NET genes led to functional redundancy between different family members to create cellular plasticity of vascular plants. We hypothesize that establishment of a direct ER-vacuole connection enables direct lipid and protein transfer which is especially important in young and fast-growing cells. Availability of lipids would facilitate rapidly expanding vacuoles which are the basis for high cell elongation rates and eventually fast plant growth.

The functionality of all metabolic processes in chloroplasts depends on a balanced integration of nuclear- and chloroplast-encoded polypeptides into the plastid's proteome. The chloroplast chaperonin machinery is an essential player in chloroplast protein folding under ambient and stressful conditions, with a more intricate structure and subunit composition compared to the orthologous GroEL/ES chaperonin of Escherichia coli. However, its exact role in chloroplasts remains obscure, mainly because of very limited knowledge about the interactors. We employed the competition immunoprecipitation method for the identification of the chaperonin's interactors in Chlamydomonas reinhardtii. Co-immunoprecipitation of the target complex in the presence of increasing amounts of isotope-labelled competitor epitope and subsequent mass spectrometry analysis specifically allowed to distinguish true interactors from unspecifically co-precipitated proteins. Besides known substrates such as RbcL and the expected complex partners, we revealed numerous new interactors with high confidence. Proteins that qualify as putative substrate proteins differ from bulk chloroplast proteins by a higher content of beta-sheets, lower alpha-helical conformation and increased aggregation propensity. Immunoprecipitations targeted against a subunit of the co-chaperonin lid revealed the ClpP protease as a specific partner complex, pointing to a close collaboration of these machineries to maintain protein homeostasis in the chloroplast.

In algae and land plants, transport of fatty acids (FAs) from their site of synthesis in the plastid stroma to the endoplasmic reticulum (ER) for assembly into acyl lipids is crucial for cellular lipid homeostasis, including the biosynthesis of triacylglycerol (TAG) for energy storage. In the unicellular green alga Chlamydomonas reinhardtii, understanding and engineering of these processes is of particular interest for microalga-based biofuel and biomaterial production. Whereas in the model plant Arabidopsis thaliana, FAX (fatty acid export) proteins have been associated with a function in plastid FA-export and hence TAG synthesis in the ER, the knowledge on the function and subcellular localization of this protein family in Chlamydomonas is still scarce. Among the four FAX proteins encoded in the Chlamydomonas genome, we found Cr-FAX1 and Cr-FAX5 to be involved in TAG production by functioning in chloroplast and ER membranes, respectively. By in situ immunolocalization, we show that Cr-FAX1 inserts into the chloroplast envelope, while Cr-FAX5 is located in ER membranes. Severe reduction of Cr-FAX1 or Cr-FAX5 proteins by an artificial microRNA approach results in a strong decrease of the TAG content in the mutant strains. Further, overexpression of chloroplast Cr-FAX1, but not of ER-intrinsic Cr-FAX5, doubled the content of TAG in Chlamydomonas cells. We therefore propose that Cr-FAX1 in chloroplast envelopes and Cr-FAX5 in ER membranes represent a basic set of FAX proteins to ensure shuttling of FAs from chloroplasts to the ER and are crucial for oil production in Chlamydomonas.

The marine picocyanobacterium Prochlorococcus contributes significantly to global primary production, and its abundance and diversity is shaped in part by viral infection. Here, we identified a cyanophage-encoded MarR-type transcription factor that induces the gene expression of host Prochlorococcus MED4 endoribonuclease (RNase) E during phage infection. The increase in rne transcript levels relies on the phage (p)MarR-mediated activation of an alternative promoter that gives rise to a truncated yet enzymatically fully functional RNase E isoform. In this study, we demonstrate that pMarR binds to an atypical activator site downstream of the transcriptional start site and that binding is enhanced in the presence of Ca2+ ions. Furthermore, we show that dimeric pMarR interacts with the α subunit of RNA polymerase, and we identified amino acid residues S66, R67, and G106, which are important for Ca2+ binding, DNA binding, and dimerization of pMarR, respectively.

Botrytis cinerea is a major plant pathogen infecting more than 1400 plant species. During invasion, the fungus rapidly kills host cells, which is believed to be supported by induction of programmed plant cell death. To comprehensively evaluate the contributions of most of the currently known plant cell death inducing proteins (CDIPs) and metabolites for necrotrophic infection, an optimized CRISPR/Cas9 protocol was established which allowed to perform serial marker-free mutagenesis to generate multiple deletion mutants lacking up to 12 CDIPs. Whole genome sequencing of a 6x and 12x deletion mutant revealed a low number of off-target mutations which were unrelated to Cas9-mediated cleavage. Secretome analyses confirmed the loss of secreted proteins encoded by the deleted genes. Infection tests with the mutants revealed a successive decrease in virulence with increasing numbers of mutated genes, and varying effects of the knockouts on different host plants. Comparative analysis of mutants confirmed significant roles of two polygalacturonases (PG1, PG2) and the phytotoxic metabolites botrydial and botcinins for infection, but revealed no or only weak effects of deletion of the other CDIPs. Nicotiana benthamiana plants with mutated or silenced coreceptors of pattern recognition receptors, SOBIR1 and BAK1, showed similar susceptibility as control plants to infection by B. cinerea wild type and a 12x deletion mutant. These results raise doubts about a major role of manipulation of these plant defence regulators for B. cinerea infection. Despite the loss of most of the known phytotoxic compounds, the on planta secretomes of the multiple mutants retained substantial phytotoxic activity, proving that further, as yet unknown CDIPs contribute to necrosis and virulence. Our study has addressed for the first time systematically the functional redundancy of fungal virulence factors, and demonstrates that B. cinerea releases a highly redundant cocktail of proteins to achieve necrotrophic infection of a wide variety of host plants.