Gramine is an indole alkaloid found in certain grass species. Results of previous studies have diverged as to whether this compound might cause resistance to aphids or not. In a breeding program aiming to introduce resistance to the cereal pestRhopalosiphum padiL. in barley, a cultivar has been crossed with the progenitor of cultivated barley,Hordeum vulgaressp.spontaneum,and an F₁-derived population of doubled haploid (DH) lines was screened both for seedling gramine concentration and resistance toR. padi. The resistance was measured as individual aphid growth in the laboratory. The present study aims to determine if there is a genetic relationship between aphid growth retardation and gramine concentration. To do so, the lines were genotyped with a 384 SNP oligonucleotide pool assay and QTL analyses were performed for both traits. A previously identified aphid resistance locus on the distal part of chromosome 2HS was thereby confirmed, with resistance inherited fromH. v.ssp.spontaneum, whereas one or more QTL for gramine concentration were potentially indicated on chromosome 3H, thus corroborating that the two traits are not linked genetically.

In flowering plants, successful germinal cell development and meiotic recombination depend upon a combination of environmental and genetic factors. To gain insights into this specialised reproductive development programme we used short- and long-read RNA-sequencing (RNA-seq) to study the temporal dynamics of transcript abundance in immuno-cytologically staged barley ( Hordeum vulgare ) anthers and meiocytes. We show that the most significant transcriptional changes occur at the transition from pre-meiosis to leptotene–zygotene, which is followed by largely stable transcript abundance throughout prophase I. Our analysis reveals that the developing anthers and meiocytes are enriched in long non-coding RNAs (lncRNAs) and that entry to meiosis is characterized by their robust and significant down regulation. Intriguingly, only 24% of a collection of putative meiotic gene orthologues showed differential transcript abundance in at least one stage or tissue comparison. Changes in the abundance of numerous transcription factors, representatives of the small RNA processing machinery, and post-translational modification pathways highlight the complexity of the regulatory networks involved. These developmental, time-resolved, and dynamic transcriptomes increase our understanding of anther and meiocyte development and will help guide future research. One sentence summary: Analysis of RNA-seq data from meiotically staged barley anthers and meiocytes highlights the role of lncRNAs within a complex network of transcriptional and post-transcriptional regulation accompanied by a hiatus in differential gene expression during prophase I. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors ( www.plantcell.org ) is: Robbie Waugh ( robbie.waugh@hutton.ac.uk )

Using a positional candidate-gene approach, we show that semi-sterile desynaptic8 mutants are associated with deleterious variants of the barley homolog of XRCC2 (X-Ray Repair Cross Complementing 2). In barley XRCC2 mutants, the initial meiotic progression is normal, albeit with a small delay in initiation, with completion of synapsis. However, the absence of HvXRCC2 subsequently leads to a dramatic reduction in the number of crossovers, chromosome mis-segregation, and infertility, suggesting that HvXRCC2 plays a major role in recombination. This mutant phenotype is congruent with that reported in mammalian studies but contrasts with the XRCC2 mutant in Arabidopsis which is fertile, exhibits normal chromosome pairing and correct chromosome segregation, and is associated with an increased rate of crossovers. This observation indicates that the XRCC2 mutant phenotype in Arabidopsis is not representative of all plants and that XRCC2 is not a good candidate for the modulation of recombination in barley.

Using a positional candidate-gene approach we show that semi-sterile desynaptic8 mutants are associated with deletions in or complete knockout of the barley homolog of XRCC2 ( X-Ray Repair Cross Complementing 2 ). In barley XRCC2 mutants, the initial meiotic progression is normal, albeit with a small delay in initiation, with completion of synapsis. However, the absence of HvXRCC2 subsequently leads to a dramatic reduction in the number of crossovers, chromosome mis-segregation, and infertility, suggesting that HvXRCC2 plays a major role in recombination. This mutant phenotype is congruent with that reported in mammalian studies but contrasts with the XRCC2 mutant in Arabidopsis which is fertile, exhibits normal chromosome pairing and correct chromosome segregation, and is associated with an increased rate of crossovers. This indicates that the XRCC2 mutant phenotype in Arabidopsis is not representative of all plants and that XRCC2 is not a good candidate for the modulation of recombination in barley. Highlight: The mutants of the barley homolog of XRCC2 exhibit delays in replication leading to defective meiosis, altered RAD51 orthologue behaviour, and significant reduction in the number of crossovers as in canonical mammalian XRCC2 mutants but unlike those in Arabidopsis.

During meiosis, genetic recombination occurs via repair of DNA double-strand breaks (DSBs) as crossovers (COs) resulting in the exchange of parental genetic material (1). Crossovers are important for chromosome segregation and shuffling genetic variation, but their number and distribution are tightly regulated (2). In barley and other large genome cereals, recombination events are limited in number and mainly restricted to the ends of chromosomes (3), constraining progress in plant breeding. Recent studies have highlighted subtle differences in meiotic progression (4, 5) and the distribution of recombination events in barley compared to other plants (6-8), indicating possible evolutionary divergence of the meiotic program in large genome crops. Here we identify a spontaneous loss of function mutation in the grass specific E3 ubiquitin ligase HvST1 (Sticky Telomeres 1) which results in semi-sterility in barley. We show that abnormal synapsis in the absence of HvST1 function increases overall recombination by up to 2.5-fold and that HvST1 is capable of ubiquitinating ASY1, a key component of the lateral elements of the synaptonemal complex. Our findings shed light on a novel—and evolutionarily divergent—pathway regulating synapsis and recombination in cereals. This natural loss of function variant presents new opportunities for the modulation of recombination in large genome cereals. Significance Statement Climate change places significant strain on crop production. Crop secondary gene pools offer an excellent resource for crop improvement. However, linkage drag driven by restrictions to meiotic recombination can impose severe yield or quality penalties from introgression of traits from secondary gene pools to elite varieties. Here, we characterize a spontaneous mutation in the barley E3 ubiquitin HvST1 that leads to a significant increase in recombination. Through biochemical analysis of the wild type protein we identified a putative role for this ligase in regulating synapsis. This furthers our understanding of the control of synapsis in large genome cereals and may be of direct use in traditional barley breeding.

We quantified grain sodium (Na⁺) content across a barley GWAS panel grown under optimal conditions. We identified a strong association with a region containing two low and one high Na⁺accumulating haplotypes of a Class 1 HIGH-AFFINITY POTASSIUM TRANSPORTER (HKT1;5) known to be involved in regulating plant Na⁺homeostasis. The haplotypes exhibited an average 1.8-fold difference in grain Na⁺content. We show that an L189P substitution disrupts Na⁺transport in the high Na⁺lines, disturbs the plasma membrane localisation typical of HKT1;5 and induces a conformational change in the protein predicted to compromise function. Under NaCl stress, lines containing P189 accumulate high levels of Na⁺, but show no significant difference in biomass. P189 increases in frequency from wild-species to elite cultivars leading us to speculate that the compromised haplotype is undergoing directional selection possibly due to the value of Na⁺as a functional nutrient in non-saline environments.