Isoprenoid biosynthesis authenticates the classification of the green alga Mesostigma viride as an ancient streptophyte.

Land plants harbor two essential and completely different metabolic pathways for isoprenoid synthesis. The cytosolic mevalonate pathway (MVA) is shared with heterotrophic eukaryotes, whereas the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway has a cyanobacterial origin and was recruited after primary endosymbiosis. Terrestrial plants and green algae have a common evolutionary ancestry, but biochemical as well as genome analyses indicate that the cytosolic MVA pathway is generally absent from Chlorophyta. We investigated the distribution of genes for both pathways in the green alga Mesostigma viride, a key species at the basis of streptophycean (charophycean green algae, land plant) evolution. Ten of altogether twelve generally weakly expressed genes for isoprenoid biosynthesis, including three for the cytosolic MVA pathway, were amplified using a reverse transcription PCR approach with individually designed degenerate primers. Two full length cDNA clones for the first enzyme of the MVA pathway (HMGS) were additionally established from the charophycean green alga Chara vulgaris by library screening. The presence of the MVA pathway in these advanced green algae indicates a universal distribution among Streptophyta, and our phylogenetic HMGS analyses substantiate the recent classification of Mesostigma basal to charophytes and land plants. We identified each of the five cytosolic MVA genes/cDNAs in the genome of the rhodophyte Galdieria sulphuraria and, furthermore, amplified four of them from the glaucophyte Cyanophora paradoxa. Our data indicate that the MVA pathway is a characteristic trait of Plantae in general and propose that it was specifically lost in a common ancestor of Chlorophyta.

Evolution of the glucose-6-phosphate isomerase: the plasticity of primary metabolism in photosynthetic eukaryotes.

Glucose-6-phosphate isomerase (GPI) has an essential function in both catabolic glycolysis and anabolic gluconeogenesis and is universally distributed among Eukaryotes, Bacteria, and some Archaea. In addition to the cytosolic GPI, land plant chloroplasts harbor a nuclear encoded isoenzyme of cyanobacterial origin that is indispensable for the oxidative pentose phosphate pathway (OPPP) and plastid starch accumulation. We established 12 new GPI sequences from rhodophytes, the glaucophyte Cyanophora paradoxa, a ciliate, and all orders of complex algae with red plastids (haptophytes, diatoms, cryptophytes, and dinoflagellates). Our comprehensive phylogenies do not support previous GPI-based speculations about a eukaryote-to-prokaryote horizontal gene transfer from metazoa to gamma-proteobacteria. The evolution of cytosolic GPI is largely in agreement with small subunit analyses, which indicates that it is a specific marker of the host cell. A distinct subtree comprising alveolates (ciliates, apicomplexa, Perkinsus, and dinoflagellates), stramenopiles (diatoms and Phytophthora [oomycete]), and Plantae (green plants, rhodophytes, and Cyanophora) might suggest a common origin of these superensembles. Finally, in contrast to land plants where the plastid GPI is of cyanobacterial origin, chlorophytes and rhodophytes independently recruited a duplicate of the cytosolic GPI that subsequently acquired a transit peptide for plastid import. A secondary loss of the cytosolic isoenzyme and the plastid localization of the single GPI in chlorophycean green algae is compatible with physiological studies. Our findings reveal the fundamental importance of the plastid OPPP for Plantae and document the plasticity of primary metabolism.

Intron distribution in Plantae: 500 million years of stasis during land plant evolution.

Little is known about the evolution of the intron-exon organization in the more primitive groups of land plants, and the intron distribution among Plantae (glauco-, rhodo-, chloro- and streptophytes) has not been investigated so far. The present study is focused on some key species such as the liverwort Marchantia polymorpha, representing the most ancient lineage of land plants, and the streptophycean green alga Mesostigma viride, branching prior to charophycean green algae and terrestrial plants. The intron distribution of six genes for sugar phosphate metabolism was analyzed including four different glyceraldehyde-3-phosphate dehydrogenases (GAPDH), the sedoheptulose-1,7-bisphosphatase (SBP) and the glucose-6-phosphate isomerase (GPI). We established 15 new sequences including three cDNA and twelve genomic clones with up to 24 introns per gene, which were identified in the GPI of Marchantia. The intron patterns of all six genes are completely conserved among seed plants, lycopods, mosses and even liverworts. This intron stasis without any gain of novel introns seem to last for nearly 500 million years and may be characteristic for land plants in general. Some unique intron positions in Mesostigma document that a uniform distribution is no common trait of all streptophytes, but it may correlate with the transition to terrestrial habitats. However, the respective genes of chlorophycean green algae display largely different patterns, thus indicating at least one phase of massive intron rearrangement in the green lineage. We moreover included rhodophyte and glaucophyte reference sequences in our analyses and, even if the well documented monophyly of Plantae is not reflected by a uniform intron distribution, at least one GPI intron is strictly conserved for 1.5 billion years.