An improved assembly of the pearl millet reference genome using Oxford Nanopore long reads and optical mapping.

Pearl millet (Pennisetum glaucum (L.)) R. Br. syn. Cenchrus americanus (L.) Morrone) is an important crop in South Asia and sub-Saharan Africa which contributes to ensuring food security. Its genome has an estimated size of 1.76 Gb and displays a high level of repetitiveness above 80%. A first assembly was previously obtained for the Tift 23D2B1-P1-P5 cultivar genotype using short-read sequencing technologies. This assembly is, however, incomplete and fragmented with around 200 Mb unplaced on chromosomes. We report here an improved quality assembly of the pearl millet Tift 23D2B1-P1-P5 cultivar genotype obtained with an approach combining Oxford Nanopore long reads and Bionano Genomics optical maps. This strategy allowed us to add around 200 Mb at the chromosome-level assembly. Moreover, we strongly improved continuity in the order of the contigs and scaffolds within the chromosomes, particularly in the centromeric regions. Notably, we added more than 100 Mb around the centromeric region on chromosome 7. This new assembly also displayed a higher gene completeness with a complete BUSCO score of 98.4% using the Poales database. This more complete and higher quality assembly of the Tift 23D2B1-P1-P5 genotype now available to the community will help in the development of research on the role of structural variants and more broadly in genomics studies and the breeding of pearl millet.

Fonio millet genome unlocks African orphan crop diversity for agriculture in a changing climate.

Sustainable food production in the context of climate change necessitates diversification of agriculture and a more efficient utilization of plant genetic resources. Fonio millet (Digitaria exilis) is an orphan African cereal crop with a great potential for dryland agriculture. Here, we establish high-quality genomic resources to facilitate fonio improvement through molecular breeding. These include a chromosome-scale reference assembly and deep re-sequencing of 183 cultivated and wild Digitaria accessions, enabling insights into genetic diversity, population structure, and domestication. Fonio diversity is shaped by climatic, geographic, and ethnolinguistic factors. Two genes associated with seed size and shattering showed signatures of selection. Most known domestication genes from other cereal models however have not experienced strong selection in fonio, providing direct targets to rapidly improve this crop for agriculture in hot and dry environments.

Abandonment of pearl millet cropping and homogenization of its diversity over a 40 year period in Senegal.

Cultivated diversity is considered an insurance against major climatic variability. However, since the 1980s, several studies have shown that climate variability and agricultural changes may already have locally eroded crop genetic diversity. We studied pearl millet diversity in Senegal through a comparison of pearl millet landraces collected 40 years apart. We found that more than 20% of villages visited in 1976 had stopped growing pearl millet. Despite this, its overall genetic diversity has been maintained but differentiation between early- and late-flowering accessions has been reduced. We also found stronger crop-to-wild gene flow than wild-to-crop gene flow and that wild-to-crop gene flow was weaker in 2016 than in 1976. In conclusion, our results highlight genetic homogenization in Senegal. This homogenization within cultivated pearl millet and between wild and cultivated forms is a key factor in genetic erosion and it is often overlooked. Improved assessment and conservation strategies are needed to promote and conserve both wild and cultivated pearl millet diversity.

A western Sahara centre of domestication inferred from pearl millet genomes.

There have been intense debates over the geographic origin of African crops and agriculture. Here, we used whole-genome sequencing data to infer the domestication origin of pearl millet (Cenchrus americanus). Our results supported an origin in western Sahara, and we dated the onset of cultivated pearl millet expansion in Africa to 4,900 years ago. We provided evidence that wild-to-crop gene flow increased cultivated genetic diversity leading to diversity hotspots in western and eastern Sahel and adaptive introgression of 15 genomic regions. Our study reconciled genetic and archaeological data for one of the oldest African crops.

New Genetic Insights into Pearl Millet Diversity As Revealed by Characterization of Early- and Late-Flowering Landraces from Senegal.

Pearl millet (Pennisetum glaucum (L.) R. Br.) is a staple food and a drought-tolerant cereal well adapted to Sub-Saharan Africa agro-ecosystems. An important diversity of pearl millet landraces has been widely conserved by farmers and therefore could help copping with climate changes and contribute to future food security. Hence, characterizing its genetic diversity and population structure can contribute to better assist breeding programs for a sustainable agricultural productivity enhancement. Toward this goal, a comprehensive panel of 404 accessions were used that correspond to 12 improved varieties, 306 early flowering and 86 late-flowering cultivated landraces from Senegal. Twelve highly polymorphic SSR markers were used to study diversity and population structure. Two genes, PgMADS11 and PgPHYC, were genotyped to assess their association to flowering phenotypic difference in landraces. Results indicate a large diversity and untapped potential of Senegalese pearl millet germplasm as well as a genetic differentiation between early- and late-flowering landraces. Further, a fine-scale genetic difference of PgPHYC and PgMADS11 (SNP and indel, respectively) and co-variation of their alleles with flowering time were found among landraces. These findings highlight new genetic insights of pearl millet useful to define heterotic populations for breeding, genomic association panel, or crosses for trait-specific mapping.

Overexpression of TaVRN1 in Arabidopsis promotes early flowering and alters development.

TaVRN1, a member of the APETALA1 (AP1) subfamily of MADS-box transcription factors, is a key gene that controls transition from vegetative to reproductive phase in wheat. The accumulation of TaVRN1 transcripts in winter wheat probably requires the down-regulation of TaVRT2, a MADS-box factor that binds and represses the TaVRN1 promoter, and of the flowering repressor TaVRN2. However, the molecular mechanisms by which TaVRN1 functions as an activator of phase transition is unknown. To address this, a combination of gene expression and functional studies was used. RNA in situ hybridization studies showed that TaVRN1 transcripts accumulate in all meristems and primordia associated with flower development. An interaction screen in yeast revealed that TaVRN1 interacts with several proteins involved in different processes of plant development such as transcription factors, kinases and a cyclophilin. Arabidopsis plants overexpressing TaVRN1 flower early and show various levels of modified plant architecture. The ectopic expression causes an overexpression of the AP1 and MAX4 genes, which are associated with flowering and auxin regulation, respectively. The induction of gene expression probably results from the binding of TaVRN1 to CArG motifs present on the AP1 and MAX4 promoters. In contrast, Arabidopsis plants that overexpress TaVRT2, which encodes a putative flowering repressor, show an opposite late flowering phenotype. Together, the data provide molecular evidence that TaVRN1 may have pleiotropic effects in various processes such as control of axillary bud growth, transition to flowering and development of floral organs.

Interaction network of proteins associated with abiotic stress response and development in wheat.

Wheat is the most widely adapted crop to abiotic stresses and considered an excellent system to study stress tolerance in spite of its genetic complexity. Recent studies indicated that several hundred genes are either up- or down-regulated in response to stress treatment. To elucidate the function of some of these genes, an interactome of proteins associated with abiotic stress response and development in wheat was generated using the yeast two-hybrid GAL4 system and specific protein interaction assays. The interactome is comprised of 73 proteins, generating 97 interactions pairs. Twenty-one interactions were confirmed by bimolecular fluorescent complementation in Nicotiana benthamiana. A confidence-scoring system was elaborated to evaluate the significance of the interactions. The main feature of this interactome is that almost all bait proteins along with their interactors were interconnected, creating a spider web-like structure. The interactome revealed also the presence of a "cluster of proteins involved in flowering control" in three- and four-protein interaction loops. This network provides a novel insight into the complex relationships among transcription factors known to play central roles in vernalization, flower initiation and abscisic acid signaling, as well as associations that tie abiotic stress with other regulatory and signaling proteins. This analysis provides useful information in elucidating the molecular mechanism associated with abiotic stress response in plants.

TaVRT-2, a member of the StMADS-11 clade of flowering repressors, is regulated by vernalization and photoperiod in wheat.

The initiation of the reproductive phase in winter cereals is delayed during winter until favorable growth conditions resume in the spring. This delay is modulated by low temperature through the process of vernalization. The molecular and genetic bases of the interaction between environmental factors and the floral transition in these species are still unknown. However, the recent identification of the wheat (Triticum aestivum L.) TaVRT-1 gene provides an opportunity to decipher the molecular basis of the flowering-time regulation in cereals. Here, we describe the characterization of another gene, named TaVRT-2, possibly involved in the flowering pathway in wheat. Molecular and phylogenetic analyses indicate that the gene encodes a member of the MADS-box transcription factor family that belongs to a clade responsible for flowering repression in several species. Expression profiling of TaVRT-2 in near-isogenic lines and different genotypes with natural variation in their response to vernalization and photoperiod showed a strong relationship with floral transition. Its expression is up-regulated in the winter genotypes during the vegetative phase and in photoperiod-sensitive genotypes during short days, and is repressed by vernalization to a level that allows the transition to the reproductive phase. Protein-protein interaction studies revealed that TaVRT-2 interacts with proteins encoded by two important vernalization genes (TaVRT-1/VRN-1 and VRN-2) in wheat. These results support the hypothesis that TaVRT-2 is a putative repressor of the floral transition in wheat.

TaVRT-1, a putative transcription factor associated with vegetative to reproductive transition in cereals.

The molecular genetics of vernalization, defined as the promotion of flowering by cold treatment, is still poorly understood in cereals. To better understand this mechanism, we cloned and characterized a gene that we named TaVRT-1 (wheat [Triticum aestivum] vegetative to reproductive transition-1). Molecular and sequence analyses indicated that this gene encodes a protein homologous to the MADS-box family of transcription factors that comprises certain flowering control proteins in Arabidopsis. Mapping studies have localized this gene to the Vrn-1 regions on the long arms of homeologous group 5 chromosomes, regions that are associated with vernalization and freezing tolerance (FT) in wheat. The level of expression of TaVRT-1 is positively associated with the vernalization response and transition from vegetative to reproductive phase and is negatively associated with the accumulation of COR genes and degree of FT. Comparisons among different wheat genotypes, near-isogenic lines, and cereal species, which differ in their vernalization response and FT, indicated that the gene is inducible only in those species that require vernalization, whereas it is constitutively expressed in spring habit genotypes. In addition, experiments using both the photoperiod-sensitive barley (Hordeum vulgare cv Dicktoo) and short or long day de-acclimated wheat revealed that the expression of TaVRT-1 is also regulated by photoperiod. These expression studies indicate that photoperiod and vernalization may regulate this gene through separate pathways. We suggest that TaVRT-1 is a key developmental gene in the regulatory pathway that controls the transition from the vegetative to reproductive phase in cereals.