Biology subjects

Holmer, R.

Publications and source records attributed to Holmer, R..

7 recordsLinked to original sources

bioRxiv2026

Spatiotemporal dynamics of ethylene biosynthesis shape infection and nodule initiation in Medicago truncatula

Ethylene is a well-established negative regulator of nodulation, yet how ethylene biosynthesis and perception are spatially coordinated during early symbiotic signalling remains unresolved. Here, we investigate the dynamics of ethylene responses in Medicago truncatula using transcriptomics, promoter-reporter analyses, loss-of-function approaches and a synthetic reporter. We show that the activity of the ethylene-responsive EBSn reporter shifts from inner root tissues under non-symbiotic conditions to the outer cortex and epidermis following rhizobial inoculation, revealing a spatial reprogramming of ethylene signalling. Among the eight Medicago 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE (ACS) genes, upon rhizobia application MtACS3 is induced in outer root cell layers, while MtACS10 is repressed in the inner cortex and pericycle, mirroring the shift in ethylene perception. Functional analysis demonstrates that MtACS10 restricts nodule initiation, whereas MtACS3 modulates infection thread number, prevents nodule clustering, and contributes to radial positioning of nodule primordia. Rhizobial induced ectopic ACS expression in the root interior counteracts MtACS10 repression and blocks nodulation, highlighting the requirement for spatially confined downregulation of ethylene biosynthesis. Together, these findings establish a framework in which localized shift in ethylene biosynthesis, mediated by distinct Medicago ACS genes, balances infection and organogenesis while co-defining the spatial limits of the root susceptible zone.

plant biology↗

bioRxiv2026

Identification and Classification of Expressed Orphan Genes, Spurious Orphan Genes, and Conserved Genes in the Human Gut Microbiome

Orphan genes - genes lacking detectable homologs outside a species - are widespread in microbial genomes and are thought to contribute to their adaptation and molecular innovation. However, not all predicted orphan genes may represent novel functional coding sequences. False positive orphan genes, also called spurious orphan genes, can arise from gene prediction errors. We reason that orphan genes lacking detectable expression are more likely to be spurious. To test this, we combined large-scale metatranscriptomic profiling of the human gut microbiome with machine learning to distinguish expressed orphan genes from spurious ones and to compare them with conserved genes found in multiple species. Using nearly 5,000 metatranscriptome libraries, we identified [~]218,000 orphan genes supported by expression evidence, while [~]330,000 predicted orphan genes lacked detectable expression, and were classified as spurious. We extracted 154 features for sequence, structural, and evolutionary properties for each gene and trained XGBoost classifiers while accounting for genomic representation. The models achieved an area under the receiver operating characteristic curve (AUC) of 0.82 in distinguishing expressed orphan genes from spurious orphan genes and an AUC of 0.93 in distinguishing expressed orphan genes from conserved genes. SHAP-based interpretation revealed clear biological signals. Particularly, expressed orphans were present in more genomes than spurious ones and expressed orphan genes were shorter than conserved genes. This work improves orphan gene discovery and suggests that expressed orphan genes differ systematically from conserved genes and spurious orphan genes in sequence composition, structural constraints, and evolutionary signals. Significance statementSpecies-specific genes, also called orphan genes, are abundant in bacterial genomes and have the potential to drive evolutionary innovation. However, the study of orphan genes is confounded by annotation errors that produce spurious gene predictions. Here, we leveraged nearly 5,000 metatranscriptomes from the human gut microbiome and implemented machine learning models to distinguish expressed orphan genes from non-expressed, likely spurious, genes and conserved genes. We show that a substantial fraction of predicted orphan genes lacks expression support and they exhibit distinct sequence and evolutionary signatures from expressed orphans. By integrating expression evidence with sequence-, structure-, and evolution-based features, our work refines orphan gene catalogs. This work improves the reliability of orphan gene discovery and provides new insights into the biological characteristics of orphan genes in human microbial communities.

genomics↗

bioRxiv2024

Steep logarithmic increase of genetic variation in natural Arabidopsis thaliana accessions across geographic scales

Arabidopsis thalianas large native range across Eurasia and display of considerable genetic variation is key to its increasing use in eco-evolutionary studies. The structure and amount of this genetic variation has been studied on various geographic scales. On a continental scale, the genetic variation was postulated to follow an isolation by distance model, implying less genetic variation at smaller geographic distances. However, recent studies showed that the genetic variation is already high on small geographic scales, yet direct comparisons of the genetic variation across different geographic scales are rare. Here, we present a new local diversity panel covering 19 km2 with accessions of the Veluwe, the Netherlands. We compared the genetic variation of this local diversity panel to a national and a continental A. thaliana diversity panel. Direct comparison of these three geographic scales showed that local accessions harbour already 41.8% of the genetic variation found on a continental scale despite the substantial difference in geographic surface area covered. Moreover, a rapidly ascending logarithmic relationship between genetic and geographic distances was observed at continental, national and local scale and thus irrespective of the geographic scale considered. The high level of local genetic variation reported here poses new questions on which evolutionary forces are driving and maintaining this, and how much this constrains experimental design when using local A. thaliana populations in future eco-evolutionary studies.

genetics↗

bioRxiv2023

CYP79B2 and CYP79B3 contribute to root branching through production of the auxin precursor indole-3-acetonitrile

Lateral root placement, outgrowth and density are influenced by environmental changes, including salinity stress. CYP79B2 and B3 are two cytochrome P450 enzymes previously identified as required for root architecture remodeling in salt. They produce iAOx, a metabolite that can be converted into indole glucosinolates (IGs), camalexin and indole-3-acetic acid (IAA), a type of auxin. We report here that lateral root appearance, induced by an auxin maximum in the bending zone after gravistimulation, is delayed in the absence of CYP79B2/B3. This delay traces back to a decrease in early lateral root growth after emergence, taking place before lateral roots are macroscopically visible. We measured gene transcripts and abundance of metabolites in the iAOx pathway in root segments that are forming lateral roots. Genes involved in tryptophane and IG biosynthesis were upregulated in cyp79b2/b3 mutants, suggesting a transcriptional feedback-loop. Salt stress was found to increase the expression of genes involved in IAN biosynthesis, a precursor of both IAA and camalexin, in the root during lateral root formation. Moreover, salt increases the concentration of IAN in tissue forming lateral roots in a CYP79B2/B3 dependent manner, but these changes in IAN did not coincide with altered IAA levels. Both the reduction in lateral root density under salt and the delayed lateral root appearance in cyp79b2/b3 knock-out mutants can be complemented by exogenous application of IAN. Our results reveal a role for the iAOx pathway in regulating the timing of lateral root appearance, allowing the modulation of lateral root density under salt stress.

plant biology↗

bioRxiv2022

The genome sequence of Hirschfeldia incana, a species with high photosynthetic light-use efficiency

Photosynthesis is a biophysical and biochemical process that plays a key role in sustaining plant and human life, being the first step in the production of energy-rich molecules and oxygen in the biosphere. Improving the photosynthetic capacity of agricultural crops is highly desirable to increase their yields. While the core mechanisms of photosynthesis are highly conserved, certainly in higher plants, plants that can maintain a high photosynthetic light-use efficiency at high irradiance are exceptional and may be useful to understand and improve high irradiance photosynthesis of crops. One such exceptional species is Hirschfeldia incana, a member of the well-studied Brassicaceae family that is easy to grow under standard laboratory conditions, providing an excellent resource for studying the genetic and physiological basis of this trait. Here, we present a reference assembly of H. incana and affirm its high photosynthetic efficiency relative to the Brassicaceae species Brassica rapa, Brassica nigra, and Arabidopsis thaliana. We estimate that it diverged from B. rapa and B. nigra 10-11 million years ago and that its genome has diversified from that of the latter two species through large chromosomal rearrangements, species-specific transposon activity, and differential retention of duplicated genes. Genes present at copy numbers different from B. rapa and B. nigra include those involved in photosynthesis and/or abiotic stress, which may mediate the high photosynthetic efficiency of H. incana. We expect the reference assembly of H. incana to be a valuable genomic resource for identifying ways to enhance photosynthetic rates in crop species.

bioinformatics↗

bioRxiv2017

Parallel loss of symbiosis genes in relatives of nitrogen-fixing non-legume Parasponia

Rhizobium nitrogen-fixing nodules are a well-known trait of legumes, but nodules also occur in other plant lineages either with rhizobium or the actinomycete Frankia as microsymbiont. The widely accepted hypothesis is that nodulation evolved independently multiple times, with only a few losses. However, insight in the evolutionary trajectory of nodulation is lacking. We conducted comparative studies using Parasponia (Cannabaceae), the only non-legume able to establish nitrogen fixing nodules with rhizobium. This revealed that Parasponia and legumes utilize a large set of orthologous symbiosis genes. Comparing genomes of Parasponia and its non-nodulating relative Trema did not reveal specific gene duplications that could explain a recent gain of nodulation in Parasponia. Rather, Trema and other non-nodulating species in the Order Rosales show evidence of pseudogenization or loss of key symbiosis genes. This demonstrates that these species have lost the potential to nodulate. This finding challenges a long-standing hypothesis on evolution of nitrogen-fixing symbioses, and has profound implications for translational approaches aimed at engineering nitrogen-fixing nodules in crop plants.

plant biology↗

bioRxiv2017

Phylogenomic Synteny Network Analysis Reveals an Ancient MADS-Box Transcription Factor Tandem Duplication and Lineage-Specific Transpositions

Conserved genomic context (or synteny) provides critical information for comparative evolutionary analysis, such as the inference of ancient polyploidy events, recurrent genomic rearrangements across species and gene ancestry. With the increase of sequenced and assembled plant genomes, we now have the opportunity to use synteny to analyze the dynamics of gene family expansion and contraction across broad phylogenetic groups. Here we present an integrated approach to organize plant kingdom-wide gene synteny networks using k-clique percolation. As an example, we analyzed the gene synteny network of the MADS-box transcription factor family based on fifty-one completed plant genomes. We conclude from two massive gene clusters that one of the two Type II MADS-box gene clades evolved from an ancient tandem gene duplication likely predating the radiation of seed plants, which then expanded by polyploidy events and sub-functionalization. This gene clade now contains key regulators of major phenotypes of angiosperms including flower development. Moreover, we find lineage-specific gene clusters derived from transposition events. For example, lineage-specific clusters in the Brassicales containing genes that are well-known for their function in controlling flower morphology (AP3 and PI). Our phylogenomic synteny network approach can be applied to any group of species to gain new insights into the evolution and dynamics of any set of genes.

genomics↗