A spatially explicit trait-based approach uncovers changes in assembly processes under warming.

The re-assembly of plant communities during climate warming depends on several concurrent processes. Here, we present a novel framework that integrates spatially explicit sampling, plant trait information and a warming experiment to quantify shifts in these assembly processes. By accounting for spatial distance between individuals, our framework allows separation of potential signals of environmental filtering from those of different types of competition. When applied to an elevational transplant experiment in the French Alps, we found common signals of environmental filtering and competition in all communities. Signals of environmental filtering were generally stronger in alpine than in subalpine control communities, and warming reduced this filter. Competition signals depended on treatments and traits: Symmetrical competition was dominant in control and warmed alpine communities, while hierarchical competition was present in subalpine communities. Our study highlights how distance-dependent frameworks can contribute to a better understanding of transient re-assembly dynamics during environmental change.

Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging.

Sanguina nivaloides is the main alga forming red snowfields in high mountains and Polar Regions. It is non-cultivable. Analysis of environmental samples by X-ray tomography, focused-ion-beam scanning-electron-microscopy, physicochemical and physiological characterization reveal adaptive traits accounting for algal capacity to reside in snow. Cysts populate liquid water at the periphery of ice, are photosynthetically active, can survive for months, and are sensitive to freezing. They harbor a wrinkled plasma membrane expanding the interface with environment. Ionomic analysis supports a cell efflux of K+, and assimilation of phosphorus. Glycerolipidomic analysis confirms a phosphate limitation. The chloroplast contains thylakoids oriented in all directions, fixes carbon in a central pyrenoid and produces starch in peripheral protuberances. Analysis of cells kept in the dark shows that starch is a short-term carbon storage. The biogenesis of cytosolic droplets shows that they are loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress.

Tempo and drivers of plant diversification in the European mountain system.

There is still limited consensus on the evolutionary history of species-rich temperate alpine floras due to a lack of comparable and high-quality phylogenetic data covering multiple plant lineages. Here we reconstructed when and how European alpine plant lineages diversified, i.e., the tempo and drivers of speciation events. We performed full-plastome phylogenomics and used multi-clade comparative models applied to six representative angiosperm lineages that have diversified in European mountains (212 sampled species, 251 ingroup species total). Diversification rates remained surprisingly steady for most clades, even during the Pleistocene, with speciation events being mostly driven by geographic divergence and bedrock shifts. Interestingly, we inferred asymmetrical historical migration rates from siliceous to calcareous bedrocks, and from higher to lower elevations, likely due to repeated shrinkage and expansion of high elevation habitats during the Pleistocene. This may have buffered climate-related extinctions, but prevented speciation along elevation gradients as often documented for tropical alpine floras.

Altitudinal Zonation of Green Algae Biodiversity in the French Alps.

Mountain environments are marked by an altitudinal zonation of habitat types. They are home to a multitude of terrestrial green algae, who have to cope with abiotic conditions specific to high elevation, e.g., high UV irradiance, alternating desiccation, rain and snow precipitations, extreme diurnal variations in temperature and chronic scarceness of nutrients. Even though photosynthetic green algae are primary producers colonizing open areas and potential markers of climate change, their overall biodiversity in the Alps has been poorly studied so far, in particular in soil, where algae have been shown to be key components of microbial communities. Here, we investigated whether the spatial distribution of green algae followed the altitudinal zonation of the Alps, based on the assumption that algae settle in their preferred habitats under the pressure of parameters correlated with elevation. We did so by focusing on selected representative elevational gradients at distant locations in the French Alps, where soil samples were collected at different depths. Soil was considered as either a potential natural habitat or temporary reservoir of algae. We showed that algal DNA represented a relatively low proportion of the overall eukaryotic diversity as measured by a universal Eukaryote marker. We designed two novel green algae metabarcoding markers to amplify the Chlorophyta phylum and its Chlorophyceae class, respectively. Using our newly developed markers, we showed that elevation was a strong correlate of species and genus level distribution. Altitudinal zonation was thus determined for about fifty species, with proposed accessions in reference databases. In particular, Planophila laetevirens and Bracteococcus ruber related species as well as the snow alga Sanguina genus were only found in soil starting at 2,000 m above sea level. Analysis of environmental and bioclimatic factors highlighted the importance of pH and nitrogen/carbon ratios in the vertical distribution in soil. Capacity to grow heterotrophically may determine the Trebouxiophyceae over Chlorophyceae ratio. The intensity of freezing events (freezing degree days), proved also determinant in Chlorophyceae distribution. Guidelines are discussed for future, more robust and precise analyses of environmental algal DNA in mountain ecosystems and address green algae species distribution and dynamics in response to environmental changes.

Specific function of a plastid sigma factor for ndhF gene transcription.

The complexity of the plastid transcriptional apparatus (two or three different RNA polymerases and numerous regulatory proteins) makes it very difficult to attribute specific function(s) to its individual components. We have characterized an Arabidopsis T-DNA insertion line disrupting the nuclear gene coding for one of the six plastid sigma factors (SIG4) that regulate the activity of the plastid-encoded RNA polymerase PEP. This mutant shows a specific diminution of transcription of the plastid ndhF gene, coding for a subunit of the plastid NDH [NAD(P)H dehydrogenase] complex. The absence of another NDH subunit, i.e. NDHH, and the absence of a chlorophyll fluorescence transient previously attributed to the activity of the plastid NDH complex indicate a strong down-regulation of NDH activity in the mutant plants. Results suggest that plastid NDH activity is regulated on the transcriptional level by an ndhF-specific plastid sigma factor, SIG4.

Kinase Cak1 functionally interacts with the PAF1 complex and phosphatase Ssu72 via kinases Ctk1 and Bur1.

Protein kinases orthologous with Cak1 of Saccharomyces cerevisiae (ScCak1) appear specific to ascomycetes. ScCak1 phosphorylates Cdc28, the cyclin-dependent kinase (CDK) governing the cell cycle, as well as Kin28, Bur1 and Ctk1, CDKs required for the transcription process performed by RNA polymerase II (RNA Pol II). Using genetic methods, we found that Cak1 genetically interacts with Paf1 and Ctr9, two components belonging to the PAF1 elongation complex needed for histone modifications, and with Ssu72, a protein phosphatase that dephosphorylates serine-5 phosphate in the RNA Pol II C-terminal domain. We present evidence suggesting that the interactions linking Cak1 with the PAF1 complex and with Ssu72 are not direct but mediated via Ctk1 and Bur1. We discuss the possibility that Ssu72 intervenes at the capping checkpoint step of the transcription cycle.