Genetic architecture of flowering-time control in Brachypodium distachyon

audience: researcherControlling the timing to flowering is an essential aspect of agriculture, either by delaying flowering to maximize storage in root crops or by initiating flowering when conditions are favorable to maximize seed or fruit production. The extensive studies carried out on the model Brassicaceae Arabidopsis thaliana revealed dozens of genes whose mutation alters flowering time (Bouché et al., 2016). Comparatively, relatively little is known about the genetic mechanisms controlling flowering in Pooideae, a group of grasses that includes important crops such as wheat, barley, and oat. In recent years, Brachypodium distachyon—a small diploid grass from the Pooideae group—has been increasingly used as a model plant to study the genetic mechanisms controlling developmental processes in temperate grasses. In our lab, we combine forward and reverse genetic approaches to identify and characterize new genes controlling the timing of flowering in Brachypodium distachyon

Unraveling the molecular mechanisms controlling flowering in the model temperate grass Brachypodium distachyon

Controlling the timing to flowering is an essential aspect of agriculture, either by delaying flowering to maximize storage in root crops or by initiating flowering when conditions are favorable to maximize seed or fruit production. The extensive studies on the model Brassicaceae Arabidopsis thaliana revealed dozens of genes whose mutation alters flowering time. Comparatively, relatively little is known about the genetic mechanisms controlling flowering in Pooideae, a group of grasses that includes important crops such as wheat, barley, and oat. In recent years, Brachypodium distachyon—a small grass from the Pooideae group—has been increasingly used as a model plant to study the genetic mechanisms controlling developmental processes in temperate grasses, including flowering. Here, we report our recent contributions to the comprehension of the mechanisms controlling flowering time in Brachypodium: (i) we used a quantitative trait locus (QTL) analysis to identify genomic regions controlling flowering under a variety of photoperiod and vernalization conditions. This analysis was carried out on a recombinant inbred line population resulting from the cross between Bd21 and Bd1-1, two accessions with contrasting flowering behaviors. We identified six significant QTLs, three of which colocalized with major flowering regulators (VERNALIZATION1/PHYTOCHROME C, VERNALIZATION2, and FD), while others were not associated with known flowering-time genes. The genetic regions corresponding to these QTLs were further analyzed to identify candidate genes possibly involved in the control of flowering time. (ii) We used forward genetic screens to identify new flowering-time mutants, which are currently being analyzed. The characterization of the genes identified through these approaches will contribute to improve our knowledge of the molecular networks governing the transition to flowering in temperate grasses

Winter memory throughout the plant kingdom: different paths to flowering

peer reviewedPlants have evolved a variety of mechanisms to synchronize flowering with their environment to optimize reproductive success. Many species flower in spring when the photoperiod increases and the ambient temperatures become warmer. Winter annuals and biennials have evolved repression mechanisms that prevent the transition to reproductive development in the fall. These repressive processes can be overcome by the prolonged cold of winter through a process known as vernalization. The memory of the past winter is sometimes stored by epigenetic chromatin remodeling processes that provide competence to flower, and plants usually require additional inductive signals to flower in spring. The requirement for vernalization is widespread within groups of plants adapted to temperate climates; however, the genetic and biochemical frameworks controlling the response are distinct in different groups of plants, suggesting independent evolutionary origins. Here, we compare and contrast the vernalization pathways in different families of plants

A methyltransferase required for proper timing of the vernalization response in Arabidopsis

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Genetic Architecture of Flowering-Time Variation in Brachypodium distachyon

peer reviewedaudience: researcherThe transition to reproductive development is a crucial step in the plant life cycle, and the timing of this transition is an important factor in crop yields. Here, we report new insights into the genetic control of natural variation in flowering time in Brachypodium distachyon, a nondomesticated pooid grass closely related to cereals such as wheat (Triticum spp.) and barley (Hordeum vulgare L.). A recombinant inbred line population derived from a cross between the rapid-flowering accession Bd21 and the delayed-flowering accession Bd1-1 were grown in a variety of environmental conditions to enable exploration of the genetic architecture of flowering time. A genotyping-by-sequencing approach was used to develop SNP markers for genetic map construction, and quantitative trait loci (QTLs) that control differences in flowering time were identified. Many of the flowering-time QTLs are detected across a range of photoperiod and vernalization conditions, suggesting that the genetic control of flowering within this population is robust. The two major QTLs identified in undomesticated B. distachyon colocalize with VERNALIZATION1/PHYTOCHROME C and VERNALIZATION2, loci identified as flowering regulators in the domesticated crops wheat and barley. This suggests that variation in flowering time is controlled in part by a set of genes broadly conserved within pooid grasses

Establishment of a vernalization requirement in Brachypodium distachyon requires REPRESSOR OF VERNALIZATION1

A requirement for vernalization, the process by which prolonged cold exposure provides competence to flower, is an important adaptation to temperate climates that ensures flowering does not occur before the onset of winter. In temperate grasses, vernalization results in the up-regulation of VERNALIZATION1 (VRN1) to establish competence to flower; however, little is known about the mechanism underlying repression of VRN1 in the fall season, which is necessary to establish a vernalization requirement. Here, we report that a plant-specific gene containing a bromo-adjacent homology and transcriptional elongation factor S-II domain, which we named REPRESSOR OF VERNALIZATION1 (RVR1), represses VRN1 before vernalization in Brachypodium distachyon. That RVR1 is upstream of VRN1 is supported by the observations that VRN1 is precociously elevated in an rvr1 mutant, resulting in rapid flowering without cold exposure, and the rapid-flowering rvr1 phenotype is dependent on VRN1. The precocious VRN1 expression in rvr1 is associated with reduced levels of the repressive chromatin modification H3K27me3 at VRN1, which is similar to the reduced VRN1 H3K27me3 in vernalized plants. Furthermore, the transcriptome of vernalized wild-type plants overlaps with that of nonvernalized rvr1 plants, indicating loss of rvr1 is similar to the vernalized state at a molecular level. However, loss of rvr1 results in more differentially expressed genes than does vernalization, indicating that RVR1 may be involved in processes other than vernalization despite a lack of any obvious pleiotropy in the rvr1 mutant. This study provides an example of a role for this class of plant-specific genes

An ortholog of CURLY LEAF/ENHANCER OF ZESTE like-1 is required for proper flowering in Brachypodium distachyon

Many plants require prolonged cold exposure to acquire the competence to flower. The process by which cold exposure results in competence is known as vernalization. In Arabidopsis thaliana, vernalization leads to the stable repression of the floral repressor FLOWERING LOCUS C via chromatin modification including an increase of trimethylation on lysine 27 of histone H3 (H3K27me3) by Polycomb Repressive Complex 2 (PRC2). Vernalization in pooids is associated with the stable induction of a floral promoter, VERNALIZATION 1. From a screen for mutants with a reduced vernalization requirement in the model grass Brachypodium distachyon, we identified two recessive alleles of ENHANCER OF ZESTE-LIKE 1 (EZL1). EZL1 is orthologous to Arabidopsis CURLY LEAF 1, a gene that encodes the catalytic subunit of PRC2. B. distachyon ezl1 mutants flower rapidly without vernalization in long-day (LD) photoperiods; thus, EZL1 is required for proper maintenance of the vegetative state prior to vernalization. Transcriptomic studies in ezl1 revealed mis-regulation of thousands of genes including ectopic expression of several floral homeotic genes in leaves. Loss of EZL1 results in the global reduction of H3K27me3 and H3K27me2, consistent with this gene making a major contribution to PRC2 activity in B. distachyon. Furthermore, in ezl1 mutants, the flowering genes VRN1 and AGAMOUS (AG) are ectopically expressed and have reduced H3K27me3. Artificial microRNA knockdown of either VRN1 or AG in ezl1-1 mutants partially restores wild-type flowering behavior in non-vernalized plants suggesting that ectopic expression in ezl1 mutants may contribute to the rapid-flowering phenotype