Biology subjects

Fjellheim, S.

Publications and source records attributed to Fjellheim, S..

7 recordsLinked to original sources

bioRxiv2026

A constitutively active cortical nitrate-uptake cell state underlies nitrogen-use efficiency in perennial ryegrass

Perennial ryegrass (Lolium perenne) is the most widely sown forage grass of temperate agriculture and a major sink for fertiliser nitrogen, yet the cellular basis of its nitrogen-use efficiency (NUE) is unknown. We built a single-nucleus atlas of 76,684 root nuclei from a high- and a low-NUE genotype under matched nitrogen resupply. High-affinity nitrate uptake is concentrated in one cortical cell state that co-expresses nine NRT2 transporters and all three NAR2 partner proteins, a state that whole-root RNA-seq of the same tissue cannot resolve. Under reduced nitrogen the low-NUE genotype induces this state, whereas the high-NUE genotype does not; it expresses NRT2.3 at a higher baseline and uses different NRT2 paralogs, with one array member nearly silent. Across 44 annual and perennial Pooideae genomes, the same gene families show no life-history difference in copy number, evolutionary rate or selection intensity. Their regulation, however, varies in the same direction as between the two genotypes: the perennial Brachypodium sylvaticum expresses the NRT2.3 ortholog at the level that the annual B. distachyon reaches only under nitrogen deprivation, and an N-efficient barley genotype does not induce the transporters that an N-sensitive genotype induces. NUE differences are therefore associated with constitutive versus inducible deployment of a conserved toolkit, and the expression and presence/absence variants we identify are candidate breeding markers. The atlas, the first for a forage grass, is released as an open web resource.

plant biology↗

bioRxiv2024

Drought tolerance as an evolutionary precursor to frost and winter tolerance in grasses

Accumulating evidence is suggesting more frequent tropical-to-temperate transitions than previously thought. This raises the possibility that biome transitions could be facilitated by precursor traits. A wealth of ecological, genetic and physiological evidence suggests overlap between drought and frost stress responses, but the origin of this overlap, i.e. the evolution of these responses relative to each other, is poorly known. Here, we test whether adaptation to frost and/or severe winters in grasses (Poaceae) was facilitated by ancestral adaptation to drought. We used occurrence patterns across Koppen-Geiger climate zones to classify species as drought, frost and/or winter tolerant, followed by comparative analyses. Ancestral state reconstructions revealed different evolutionary trajectories in different clades, suggesting both drought-first and frost-first scenarios. A model of correlated evolution was not supported when transition rate heterogeneity was taken into account or compared to traits simulated under independent evolution. Our findings provide some support for ancestral drought tolerance facilitating transitions to cold, temperate biomes, at least in some clades. Different scenarios in different clades is consistent with present-day grasses being either cold or drought specialists, possibly as a consequence of trade-offs between different stress tolerance responses.

evolutionary biology↗

bioRxiv2024

Nuclear phylogenomics of grasses (Poaceae) supports current classification and reveals repeated reticulation

O_LIGrasses (Poaceae) comprise around 11,800 species and are central for human livelihoods and terrestrial ecosystems. Knowing their relationships and evolutionary history is key to comparative research and crop breeding. Advances in genome-scale sequencing allow for increased breadth and depth of phylogenomic analyses, making it possible to infer a new reference species tree of the family. C_LIO_LIWe inferred a comprehensive species tree of grasses by combining new and published sequences for 331 nuclear genes from genome, transcriptome, target enrichment and shotgun data. Our 1,153-tip tree covers 79% of grass genera (including 21 genera sequenced for the first time) and all but two small tribes. We compared it to a 910-tip plastome tree. C_LIO_LIThe nuclear phylogeny matches that of the plastome at most deep branches, with only a few instances of incongruence. Gene tree-species tree reconciliation suggests that reticulation events occurred repeatedly in the history of grasses. C_LIO_LIWe provide a robust framework for the grass tree of life to support research on grass evolution, including modes of reticulation, and genetic diversity for sustainable agriculture. C_LI

plant biology↗

bioRxiv2024

Evolution of frost and drought responses in cool season grasses (Pooideae): was drought tolerance a precursor to frost tolerance?

Frost tolerance has evolved many times independently across flowering plants. However, conservation of several frost tolerance mechanisms among distant relatives suggests that apparently independent entries into freezing climates may have been facilitated by repeated modification of existing traits ( precursor traits). One possible precursor trait for freezing tolerance is drought tolerance, because palaeoclimatic data suggest plants were exposed to drought before frost and several studies have demonstrated shared physiological and genetic responses to drought and frost stress. Here, we combine ecophysiological experiments and comparative analyses to test the hypothesis that drought tolerance acted as a precursor to frost tolerance in cool-season grasses (Pooideae). Contrary to our predictions, we measured the highest levels of frost tolerance in species with the lowest ancestral drought tolerance, suggesting that the two stress responses evolved independently in different lineages. We further show that drought tolerance is more evolutionarily labile than frost tolerance. This could limit our ability to reconstruct the order in which drought and frost responses evolved relative to each other. Further research is needed to determine whether our results are unique to Pooideae or general for flowering plants. HighlightWe tested whether drought tolerance was an evolutionary precursor to frost tolerance in grasses (Pooideae), but found these responses to be negatively correlated, suggesting they evolved independently in different lineages.

evolutionary biology↗

bioRxiv2018

The grass subfamily Pooideae: late Cretaceous origin and climate-driven Cenozoic diversification

* AimFrost is among the most dramatic stresses a plant can experience and complex physiological adaptations are needed to endure long periods of sub-zero temperatures. Due to the need for evolving these complex adaptations, transitioning from tropical to temperate climates is regarded difficult and only half of the worlds seed plant families have temperate representatives. Here, we study the transition from tropical to temperate climates in the grass subfamily Pooideae, which dominates the northern temperate grass floras. Specifically, we investigate the role of climate cooling in diversification.\n\n* LocationGlobal, temperate regions\n\n* Time periodLate Cretaceous-Cenozoic\n\n* Major taxaThe grass subfamily Pooideae\n\n* MethodsWe date a comprehensive Pooideae phylogeny and test for the impact of paleoclimates on diversification rates. Using ancestral state reconstruction, we investigate if Pooideae ancestors experienced frost and winter. To locate the area of origin of Pooideae we perform biogeographical analyses.\n\n* ResultsWe estimated a late Cretaceous origin of the Pooideae (66 million years ago (Mya)), and all major clades had already diversified at the Eocene-Oligocene transition climate cooling (34 Mya). Climate cooling was a probable driving force of Pooideae diversification. Pooideae likely evolved in mountainous regions of southwestern Eurasia in a temperate niche experiencing frost, but not long winters.\n\n* Main conclusionPooideae originated in a temperate niche and experienced cold temperatures and frost long before the expansion of temperate biomes after the Eocene-Oligocene transition. This suggests that the Pooideae ancestor had adaptations to temperate climate and that extant Pooideae grasses share responses to low temperature stress in Pooideae. Throughout the Cenozoic falling temperatures triggered diversification. However, complex mechanisms for enduring strongly seasonal climate with long, cold winters most likely evolved independently in lower taxonomic lineages. Our findings provide insight into how adaptations to historic changes in chill and frost exposure influence distribution of plant diversity today.

evolutionary biology↗

bioRxiv2017

Evolution of cold acclimation in temperate grasses (Pooideae)

In the past 50 million years climate cooling has triggered the expansion of temperate biomes. During this period, many extant plant lineages in temperate biomes evolved from tropical ancestors and adapted to seasonality and cool conditions. Among the Poaceae (grass family), one of the subfamilies that successfully shifted from tropical to temperate biomes is the Pooideae (temperate grasses). Subfamily Pooideae contains the most important crops cultivated in the temperate regions including wheat (Triticum aestivum) and barley (Hordeum vulgare). Due to the need of well-adapted cultivars, extensive research has produced a large body of knowledge about the mechanisms underlying cold adaptation in cultivated Pooideae species. Especially cold acclimation, a process which increases the frost tolerance during a period of non-freezing cold, plays an important role. Because cold adaptation is largely unexplored in lineages that diverged early in the evolution of the Pooideae, little is known about the evolutionary history of cold acclimation in the Pooideae. Here we test if several species of early diverging lineages exhibit increased frost tolerance after a period of cold acclimation. We further investigate the conservation of five well-studied gene families that are known to be involved in the cold acclimation of Pooideae crop species. Our results indicate that cold acclimation exists in early diverging lineages, but that genes involved in regulation of cold acclimation are not conserved. The investigated gene families show signs of lineage-specific evolution and support the hypothesis that gene family expansion is an important mechanism in adaptive evolution.

evolutionary biology↗

bioRxiv2017

Comparative transcriptomics reveals lineage specific evolution of cold response in Pooideae

BackgroundUnderstanding how complex traits evolve through adaptive changes in gene regulation remains a major challenge in evolutionary biology. Over the last ~50 million years, Earth has experienced climate cooling and ancestrally tropical plants have adapted to expanding temperate environments. The grass subfamily Pooideae dominates the grass flora of the temperate regions, but the role of cold-response gene regulation in the transitioning from tropical to temperate climate remains unexplored.\n\nResultsTo establish if molecular responses to cold are conserved throughout the phylogeny, we assembled the transcriptomes of five Pooideae species spanning early to later diverging lineages, and compared short- and long-term cold responsive genes using 8633 high confidence ortholog groups with resolved gene tree topologies. We found that a majority of cold responsive genes were specific to one or two lineages, an observation that we deem incompatible with a cold adapted Pooideae ancestor. However, all five species shared short-term cold response in a small set of general stress genes as well as the ability to down-regulate the photosynthetic machinery during cold temperatures.\n\nConclusionsOur observations indicate that the different Pooideae lineages have assembled cold response programs in parallel by taking advantage of a common potential for cold adaptation.

evolutionary biology↗