Diacylglyceryl-N,N,N-trimethylhomoserine-dependent lipid remodeling in a green alga, Chlorella kessleri

Membrane lipid remodeling contributes to the environmental acclimation of plants. In the green lineage, a betaine lipid, diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), is included exclusively among green algae and nonflowering plants. Here, we show that the green alga Chlorella kessleri synthesizes DGTS under phosphorus-deficient conditions through the eukaryotic pathway via the ER. Simultaneously, phosphatidylcholine and phosphatidylethanolamine, which are similar to DGTS in their zwitterionic properties, are almost completely degraded to release 18.1% cellular phosphorus, and to provide diacylglycerol moieties for a part of DGTS synthesis. This lipid remodeling system that substitutes DGTS for extrachloroplast phospholipids to lower the P-quota operates through the expression induction of the BTA1 gene. Investigation of this lipid remodeling system is necessary in a wide range of lower green plants for a comprehensive understanding of their phosphorus deficiency acclimation strategies.

The structure of the Physcomitrium Patens Photosystem I Reveals a Unique Lhca2 Paralogue replacing Lhca4

The moss Physcomitrium patens diverged from green algae shortly after the colonization of land by ancient plants. This colonization posed new environmental challenges which drove evolutionary processes. The photosynthetic machinery of modern flowering plants is adapted to the high light conditions on land. Red shifted Lhca4 antennae are present in the photosystem I light harvesting complex of many green lineage plants but absent from P. patens. The Cryo-EM structure of the P. patens photosystem I light harvesting complex I supercomplex (PSI-LHCI) at 2.8 Å reveals that Lhca4 is replaced by a unique Lhca2 paralogue in moss. This PSI-LHCI supercomplex also retains the PsaM subunit, present in cyanobacteria and several algal species but lost in higher plants, and the PsaO subunit responsible for binding light harvesting complex II. The blue shifted Lhca2 paralogue and chlorophyll b enrichment relative to higher plants make the P. patens PSI-LHCI spectroscopically unique among other green lineage supercomplexes. Overall, the structure represents an evolutionary intermediate PSI with the crescent shaped LHCI common in higher plants and contains a unique Lhca2 paralogue which facilitates the mosses adaptation to low light niches.

Unexpected cryptic species among streptophyte algae most distant to land plants

Streptophytes are one of the major groups of the green lineage (Chloroplastida or Viridiplantae). During one billion years of evolution, streptophytes have radiated into an astounding diversity of uni- and multicellular green algae as well as land plants. Most divergent from land plants is a clade formed by Mesostigmatophyceae, Spirotaenia spp. and Chlorokybophyceae. All three lineages are species-poor and the Chlorokybophyceae consist of a single described species, Chlorokybus atmophyticus. In this study, we used phylogenomic analyses to shed light into the diversity within Chlorokybus using a sampling of isolates across its known distribution. We uncovered a consistent deep genetic structure within the Chlorokybus isolates, which prompted us to formally extend the Chlorokybophyceae by describing four new species. Gene expression differences among Chlorokybus species suggest certain constitutive variability that might influence their response to environmental factors. Failure to account for this diversity can hamper comparative genomic studies aiming to understand the evolution of stress response across streptophytes. Our data highlight that future studies on the evolution of plant form and function can tap into an unknown diversity at key deep branches of the streptophytes.

A Comprehensive Phylogenetic Analysis of the MAP4K Family in the Green Lineage

The kinase-mediated phosphorylation impacts every basic cellular process. While mitogen-activated protein kinase technology kinase kinases (MAP4Ks) are evolutionarily conserved, there is no comprehensive overview of the MAP4K family in the green lineage (Viridiplantae). In this study, we identified putative MAP4K members from representative species of the two core groups in the green lineage: Chlorophyta, which is a diverse group of green algae, and Streptophyta, which is mostly freshwater green algae and land plants. From that, we inferred the evolutionary relationships of MAP4K proteins through a phylogenetic reconstruction. Furthermore, we provided a classification of the MAP4Ks in the green lineage into three distinct.

Identification of green lineage osmotic stress pathways

Maintenance of water homeostasis is a fundamental cellular process required by all living organisms. Here, we use the green alga Chlamydomonas reinhardtii to establish a foundational understanding of evolutionarily conserved osmotic-stress signaling pathways in the green lineage through transcriptomics, phosphoproteomics, and functional genomics approaches. Five genes acting across diverse cellular pathways were found to be essential for osmotic-stress tolerance in Chlamydomonas including cytoskeletal organization, potassium transport, vesicle trafficking, mitogen-activated protein kinase and chloroplast signaling. We show that homologs of these genes in the multicellular land plant Arabidopsis thaliana have conserved functional roles in stress tolerance and reveal a novel PROFILIN-dependent actin remodeling stage of acclimation that ensures cell survival and tissue integrity upon osmotic stress. This study highlights the conservation of the stress response in algae and land plants and establishes Chlamydomonas as a unicellular plant model system to dissect the osmotic stress signaling pathway.

A mycorrhiza-associated receptor-like kinase with an ancient origin in the green lineage

Receptor-like kinases (RLKs) are key cell signaling components. The rice ARBUSCULAR RECEPTOR-LIKE KINASE 1 (OsARK1) regulates the arbuscular mycorrhizal (AM) association postarbuscule development and belongs to an undefined subfamily of RLKs. Our phylogenetic analysis revealed that ARK1 has an ancient paralogue in spermatophytes, ARK2. Single ark2 and ark1/ark2 double mutants in rice showed a nonredundant AM symbiotic function for OsARK2. Global transcriptomics identified a set of genes coregulated by the two RLKs, suggesting that OsARK1 and OsARK2 orchestrate symbiosis in a common pathway. ARK lineage proteins harbor a newly identified SPARK domain in their extracellular regions, which underwent parallel losses in ARK1 and ARK2 in monocots. This protein domain has ancient origins in streptophyte algae and defines additional overlooked groups of putative cell surface receptors.

Diacylglyceryl-N,N,N-trimethylhomoserine-dependent Lipid Remodeling in a Green Alga, Chlorella Kessleri

Membrane lipid remodeling contributes to environmental 19 acclimation of plants. In a green lineage, a betaine lipid, diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), is included exclusively among green algae and non-flowering plants. Here we show that, a green alga, Chlorella kessleri, reported to exceptionally possess no DGTS, synthesizes it specifically under phosphorus-deficiency conditions through the eukaryotic pathway via the ER. Simultaneously, phosphatidylcholine and phosphatidylethanolamine, which are similar to DGTS in its zwitterionic property, are almost completely degraded to release 18.1% cellular phosphorus, and to provide its diacylglycerol moieties for a part of DGTS synthesis. Above lipid remodeling system that substitutes DGTS for extrachloroplast phospholipids to lower the P-quota operates through expression induction of the gene for BTA1 that is functionally identified as responsible for DGTS synthesis, and those for0 phospholipid breakdown. Investigation of this lipid remodeling is necessary in a widerange of lower green plants for a comprehensive understanding of their phosphorus deficiency acclimation strategies.

An ancient clade of Penelope-like retroelements with permuted domains is present in the green lineage and protists, and dominates many invertebrate genomes

ABSTRACTPenelope-like elements (PLEs) are an enigmatic clade of retroelements whose reverse transcriptases (RTs) share a most recent common ancestor with telomerase RTs. The single ORF of canonical EN+ PLEs encodes RT and a C-terminal GIY-YIG endonuclease (EN) that enables intrachromosomal integration, while EN–PLEs lack endonuclease and are generally restricted to chromosome termini. EN+ PLEs have only been found in animals, except for one case of horizontal transfer to conifers, while EN–PLEs occur in several kingdoms. Here we report a new, deep-branching PLE clade with a permuted domain order, whereby an N-terminal GIY-YIG endonuclease is linked to a C-terminal RT by a short domain with a characteristic Zn-finger-like motif. These N-terminal EN+ PLEs share a structural organization, including pseudo-LTRs and complex tandem/inverted insertions, with canonical EN+ PLEs from Penelope/Poseidon, Neptune and Nematis clades, and show insertion bias for microsatellites, but lack hammerhead ribozyme motifs. However, their phylogenetic distribution is much broader. The Naiad clade is found in numerous invertebrate phyla, where they can reach tens of thousands of copies per genome. Naiads in spiders and clams independently evolved to encode selenoproteins. Chlamys, which lack the CCHH motif universal to PLE endonucleases, occur in green algae, spike mosses (targeting ribosomal DNA) and the slime mold Physarum. Unlike canonical PLEs, RTs of N-terminal EN+ PLEs contain the insertion-in-fingers domain, strengthening the link between PLEs and telomerases. Additionally, we describe Hydra, a novel metazoan C-terminal EN+ clade. Overall, we conclude that PLE diversity, distribution and abundance is comparable to non-LTR and LTR-retrotransposons.

The ATG4 protease integrates redox and stress signals to regulate autophagy

Autophagy is a highly conserved degradative pathway that ensures cellular homeostasis through the removal of damaged or useless intracellular components including proteins, membranes, or even entire organelles. A main hallmark of autophagy is the biogenesis of autophagosomes, double-membrane vesicles that engulf and transport to the vacuole the material to be degraded and recycled. The formation of autophagosomes responds to integrated signals produced as a consequence of metabolic reactions or different types of stress and is mediated by the coordinated action of core autophagy-related (ATG) proteins. ATG4 is a key Cys-protease with a dual function in both ATG8 lipidation and free ATG8 recycling whose balance is crucial for proper biogenesis of the autophagosome. ATG4 is conserved in the green lineage, and its regulation by different post-translational modifications has been reported in the model systems Chlamydomonas reinhardtii and Arabidopsis. In this review, we discuss the major role of ATG4 in the integration of stress and redox signals that regulate autophagy in algae and plants.