A microsatellite map of white clover

The white clover ( Trifolium repens) nuclear genome (n = 2x = 16) is an important yet under-characterised genetic environment. We have developed simple sequence repeat (SSR) genetic markers for the white clover genome by mining an expressed sequence tag (EST) database and by isolation from enriched genomic libraries. A total of 2,086 EST-derived SSRs (EST-SSRs) were identified among 26,480 database accessions. Evaluation of 792 EST-SSR primer pairs resulted in 566 usable EST-SSRs. Of these, 335 polymorphic EST-SSRs, used in concert with 30 genomic SSRs, detected 493 loci in the white clover genome using 92 F1 progeny from a pair cross between two highly heterozygous genotypes--364/7 and 6525/5. Map length, as estimated using the joinmap algorithm, was 1,144 cM and spanned all 16 homologues. The R (red leaf) locus was mapped to linkage group B1 and is tightly linked to the microsatellite locus prs318c. The eight homoeologous pairs of linkage groups within the white clover genome were identified using 96 homoeologous loci. Segregation distortion was detected in four areas (groups A1, D1, D2 and H2). Marker locus density varied among and within linkage groups. This is the first time EST-SSRs have been used to build a whole-genome functional map and to describe subgenome organisation in an allopolyploid species, and T. repens is the only Trifolieae species to date to be mapped exclusively with SSRs. This gene-based microsatellite map will enable the resolution of quantitative traits into Mendelian characters, the characterisation of syntenic relationships with other genomes and acceleration of white clover improvement programmes.

Functionally associated molecular genetic marker map construction in perennial ryegrass (Lolium perenne L.)

A molecular marker-based map of perennial ryegrass (Lolium perenne L.) has been constructed through the use of polymorphisms associated with expressed sequence tags (ESTs). A pair-cross between genotypes from a North African ecotype and the cultivar Aurora was used to generate a two-way pseudo-testcross population. A selection of 157 cDNAs assigned to eight different functional categories associated with agronomically important biological processes was used to detect polymorphic EST-RFLP loci in the F(1)(NA(6) x AU(6)) population. A comprehensive set of EST-SSR markers was developed from the analysis of 14,767 unigenes, with 310 primer pairs showing efficient amplification and detecting 113 polymorphic loci. Two parental genetic maps were produced: the NA(6) genetic map contains 88 EST-RFLP and 71 EST-SSR loci with a total map length of 963 cM, while the AU(6) genetic map contains 67 EST-RFLP and 58 EST-SSR loci with a total map length of 757 cM. Bridging loci permitted the alignment of homologous chromosomes between the parental maps, and a sub-set of genomic DNA-derived SSRs was used to relate linkage groups to the perennial ryegrass reference map. Regions of segregation distortion were identified, in some instances in common with other perennial ryegrass maps. The EST-derived marker-based map provides the basis for in silico comparative genetic mapping, as well as the evaluation of co-location between QTLs and functionally associated genetic loci.

Novel Xanthomonas Species From the Perennial Ryegrass Seed Microbiome - Assessing the Bioprotection Activity of Non-pathogenic Relatives of Pathogens

The productivity of the Australian dairy industry is underpinned by pasture grasses, and importantly perennial ryegrass. The performance of these pasture grasses is supported by the fungal endophyte Epichloë spp. that has bioprotection activities, however, the broader microbiome is not well characterized. In this study, we characterized a novel bioprotectant Xanthomonas species isolated from perennial ryegrass (Lolium perenne L. cv. Alto). In vitro and in planta bioassays against key fungal pathogens of grasses (Sclerotium rolfsii, Drechslera brizae and Microdochium nivale) indicated strong bioprotection activities. A complete circular chromosome of ∼5.2 Mb was generated for three strains of the novel Xanthomonas sp. Based on the 16S ribosomal RNA gene, the strains were closely related to the plant pathogen Xanthomonas translucens, however, comparative genomics of 22 closely related xanthomonad strains indicated that these strains were a novel species. The comparative genomics analysis also identified two unique gene clusters associated with the production of bioprotectant secondary metabolites including one associated with a novel nonribosomal peptide synthetase and another with a siderophore. The analysis also identified genes associated with an endophytic lifestyle (e.g., Type VI secretion system), while no genes associated with pathogenicity were identified (e.g., Type III secretion system and effectors). Overall, these results indicate that these strains represent a novel, bioactive, non-pathogenic species of the genus Xanthomonas. Strain GW was the designated type strain of this novel Xanthomonas sp.

Transcriptome Analyses of Barley Roots Inoculated with Novel Paenibacillus sp. and Erwinia gerundensis Strains Reveal Beneficial Early-Stage Plant-Bacteria Interactions

Plant growth-promoting bacteria can improve host plant traits including nutrient uptake and metabolism and tolerance to biotic and abiotic stresses. Understanding the molecular basis of plant–bacteria interactions using dual RNA-seq analyses provides key knowledge of both host and bacteria simultaneously, leading to future enhancements of beneficial interactions. In this study, dual RNA-seq analyses were performed to provide insights into the early-stage interactions between barley seedlings and three novel bacterial strains (two Paenibacillus sp. strains and one Erwinia gerundensis strain) isolated from the perennial ryegrass seed microbiome. Differentially expressed bacterial and barley genes/transcripts involved in plant–bacteria interactions were identified, with varying species- and strain-specific responses. Overall, transcriptome profiles suggested that all three strains improved stress response, signal transduction, and nutrient uptake and metabolism of barley seedlings. Results also suggested potential improvements in seedling root growth via repressing ethylene biosynthesis in roots. Bacterial secondary metabolite gene clusters producing compounds that are potentially associated with interactions with the barley endophytic microbiome and associated with stress tolerance of plants under nutrient limiting conditions were also identified. The results of this study provided the molecular basis of plant growth-promoting activities of three novel bacterial strains in barley, laid a solid foundation for the future development of these three bacterial strains as biofertilisers, and identified key differences between bacterial strains of the same species in their responses to plants.

Implications of Seed Vault Storage Strategies for Conservation of Seed Bacterial Microbiomes

Global seed vaults are important, as they conserve plant genetic resources for future breeding to improve crop yield and quality and to overcome biotic and abiotic stresses. However, little is known about the impact of standard storage procedures, such as seed drying and cold storage on the seed bacterial community, and the ability to recover seed-associated bacteria after storage. In this study, soybean [Glycine max (L.) Merr.] seeds were analyzed to characterize changes in the bacterial community composition and culturability under varying storage conditions. The G. max bacterial microbiome was analyzed from undried seed, dried seed, and seed stored for 0, 3, 6, and 14months. Storage temperatures consisted of -20°C, 4°C, and room temperature (RT), with -20°C being commonly used in seed storage vaults globally. The seed microbiome of G. max was dominated by Gammaproteobacteria under all conditions. Undried seed was dominated by Pantoea (33.9%) and Pseudomonas (51.1%); however, following drying, the abundance of Pseudomonas declined significantly (0.9%), Pantoea increased significantly (73.6%), and four genera previously identified including Pajaroellobacter, Nesterenkonia, env.OPS_17, and Acidibacter were undetectable. Subsequent storage at RT, 4, or -20°C maintained high-abundance Genera at the majority of time points, although RT caused greater fluctuations in abundances. For many of the low-abundance Genera, storage at -20°C resulted in their gradual disappearance, whereas storage at 4°C or RT resulted in their more rapid disappearance. The changes in seed bacterial composition were reflected by cultured bacterial taxa obtained from the stored G. max seed. The main taxa were largely culturable and had similar relative abundance, while many, but not all, of the low-abundance taxa were also culturable. Overall, these results indicate that the initial seed drying affects the seed bacterial composition, suggesting that microbial isolation prior to seed drying is recommended to conserve these microbes. The standard seed storage condition of -20°C is most suitable for conservation of the bacterial seed microbiome, as this storage temperature slows down the loss of seed bacterial diversity over longer time periods, particularly low-abundance taxa.

Australian native Glycine clandestina seed microbiota hosts a more diverse bacterial community than the domesticated soybean Glycine max

BACKGROUND

Plant microbiome composition has been demonstrated to change during the domestication of wild plants and it is suggested that this has resulted in loss of plant beneficial microbes. Recently, the seed microbiome of native plants was demonstrated to harbour a more diverse microbiota and shared a common core microbiome with modern cultivars. In this study the composition of the seed-associated bacteria of Glycine clandestina is compared to seed-associated bacteria of Glycine max (soybean).

RESULTS

The seed microbiome of the native legume Glycine clandestina (crop wild relative; cwr) was more diverse than that of the domesticated Glycine max and was dominated by the bacterial class Gammaproteobacteria. Both the plant species (cwr vs domesticated) and individual seed accessions were identified as the main driver for this diversity and composition of the microbiota of all Glycine seed lots, with the effect of factor "plant species" exceeded that of "geographical location". A core microbiome was identified between the two Glycine species. A high percentage of the Glycine microbiome was unculturable [G. clandestina (80.8%) and G. max (75.5%)] with only bacteria of a high relative abundance being culturable under the conditions of this study.

CONCLUSION

Our results provided novel insights into the structure and diversity of the native Glycine clandestina seed microbiome and how it compares to that of the domesticated crop Glycine max. Beyond that, it also increased our knowledge of the key microbial taxa associated with the core Glycine spp. microbiome, both wild and domesticated. The investigation of this commonality and diversity is a valuable and essential tool in understanding the use of native Glycine spp. for the discovery of new microbes that would be of benefit to domesticated Glycine max cultivars or any other economically important crops. This study has isolated microbes from a crop wild relative that are now available for testing in G. max for beneficial phenotypes.

Insights from draft genomes of Heterodera species isolated from field soil samples

BACKGROUND

The nematode phylum includes many species key to soil food webs with trophic behaviours extending from feeding on microbes to macrofauna and plant roots. Among these, the plant parasitic cyst nematodes retain their eggs in protective cysts prolonging their survival under harsh conditions. These nematodes, including those from the genus Heterodera, cause significant economic losses in agricultural systems. Understanding of nematode diversity and ecology has expanded through application of genomic research, however, for Heterodera species there are very few available whole genome sequences. Sequencing and assembling Heterodera genomes is challenging due to various technical limitations imposed by the biology of Heterodera. Overcoming these limitations is essential for comprehensive insights into Heterodera parasitic interactions with plants, population studies, and for Australian biosecurity implications.

RESULTS

We hereby present draft genomes of six species of which Heterodera australis, H. humuli, H. mani and H. trifolii are presently recorded in Australia and two species, H. avenae and H. filipjevi, currently absent from Australia. The draft genomes were sequenced from genomic DNA isolated from 50 cysts each using an Illumina NovaSeq short read sequencing platform. The data revealed disparity in sequencing yield between species. What was previously identified as H. avenae in Australia using morphological traits is now confirmed as H. australis and may have consequences for wheat breeding programs in Australia that are breeding for resistance to H. avenae. A multigene phylogeny placed the sequenced species into taxonomic phylogenetic perspective. Genomic comparisons within the Avenae species group revealed orthologous gene clusters within the species, emphasising the shared and unique features of the group. The data also revealed the presence of a Wolbachia species, a putative bacterial endosymbiont from Heterodera humuli short read sequencing data.

CONCLUSION

Genomic research holds immense significance for agriculture, for understanding pest species diversity and the development of effective management strategies. This study provides insight into Heterodera, cyst nematode genomics and the associated symbionts and this work will serve as a baseline for further genomic analyses in this economically important nematode group.

Functionally associated molecular genetic marker map construction in perennial ryegrass (Lolium perenne L.)

A molecular marker-based map of perennial ryegrass (Lolium perenne L.) has been constructed through the use of polymorphisms associated with expressed sequence tags (ESTs). A pair-cross between genotypes from a North African ecotype and the cultivar Aurora was used to generate a two-way pseudo-testcross population. A selection of 157 cDNAs assigned to eight different functional categories associated with agronomically important biological processes was used to detect polymorphic EST-RFLP loci in the F(1)(NA(6) x AU(6)) population. A comprehensive set of EST-SSR markers was developed from the analysis of 14,767 unigenes, with 310 primer pairs showing efficient amplification and detecting 113 polymorphic loci. Two parental genetic maps were produced: the NA(6) genetic map contains 88 EST-RFLP and 71 EST-SSR loci with a total map length of 963 cM, while the AU(6) genetic map contains 67 EST-RFLP and 58 EST-SSR loci with a total map length of 757 cM. Bridging loci permitted the alignment of homologous chromosomes between the parental maps, and a sub-set of genomic DNA-derived SSRs was used to relate linkage groups to the perennial ryegrass reference map. Regions of segregation distortion were identified, in some instances in common with other perennial ryegrass maps. The EST-derived marker-based map provides the basis for in silico comparative genetic mapping, as well as the evaluation of co-location between QTLs and functionally associated genetic loci.

Corrigendum: Implications of Seed Vault Storage Strategies for Conservation of Seed Bacterial Microbiomes

[This corrects the article DOI: 10.3389/fmicb.2021.784796.].