Insights from the annotated wheat genome: Wheat is one of the major sources of food for much of the world. However, because bread wheat's genome is a large hybrid mix of three separate subgenomes, it has been difficult to produce a high-quality reference sequence. Using recent advances in sequencing, the International Wheat Genome Sequencing Consortium presents an annotated reference genome with a detailed analysis of gene content among subgenomes and the structural organization for all the chromosomes. Examples of quantitative trait mapping and CRISPR-based genome modification show the potential for using this genome in agricultural research and breeding. Ramírez-Gonzálezet al.exploited the fruits of this endeavor to identify tissue-specific biased gene expression and coexpression networks during development and exposure to stress. These resources will accelerate our understanding of the genetic basis of bread wheat.Science, this issue p.eaar7191; see also p.eaar6089

Enhancing the nutritional value of food crops is a means of improving human nutrition and health. We report here the positional cloning of Gpc-B1 , a wheat quantitative trait locus associated with increased grain protein, zinc, and iron content. The ancestral wild wheat allele encodes a NAC transcription factor ( NAM-B1 ) that accelerates senescence and increases nutrient remobilization from leaves to developing grains, whereas modern wheat varieties carry a nonfunctional NAM-B1 allele. Reduction in RNA levels of the multiple NAM homologs by RNA interference delayed senescence by more than 3 weeks and reduced wheat grain protein, zinc, and iron content by more than 30%.

Winter wheats require several weeks at low temperature to flower. This process, vernalization, is controlled mainly by the VRN1 gene. Using 6,190 gametes, we found VRN1 to be completely linked to MADS-box genes AP1 and AGLG1 in a 0.03-centimorgan interval flanked by genes Cysteine and Cytochrome B5 . No additional genes were found between the last two genes in the 324-kb Triticum monococcum sequence or in the colinear regions in rice and sorghum. Wheat AP1 and AGLG1 genes were similar to Arabidopsis meristem identity genes AP1 and AGL2 , respectively. AP1 transcription was regulated by vernalization in both apices and leaves, and the progressive increase of AP1 transcription was consistent with the progressive effect of vernalization on flowering time. Vernalization was required for AP1 transcription in apices and leaves in winter wheat but not in spring wheat. AGLG1 transcripts were detected during spike differentiation but not in vernalized apices or leaves, suggesting that AP1 acts upstream of AGLG1 . No differences were detected between genotypes with different VRN1 alleles in the AP1 and AGLG1 coding regions, but three independent deletions were found in the promoter region of AP1 . These results suggest that AP1 is a better candidate for VRN1 than AGLG1 . The epistatic interactions between vernalization genes VRN1 and VRN2 suggested a model in which VRN2 would repress directly or indirectly the expression of AP1 . A mutation in the promoter region of AP1 would result in the lack of recognition of the repressor and in a dominant spring growth habit.

Microsatellites, or tandem simple sequence repeats (SSR), are abundant across genomes and show high levels of polymorphism. SSR genetic and evolutionary mechanisms remain controversial. Here we attempt to summarize the available data related to SSR distribution in coding and noncoding regions of genomes and SSR functional importance. Numerous lines of evidence demonstrate that SSR genomic distribution is nonrandom. Random expansions or contractions appear to be selected against for at least part of SSR loci, presumably because of their effect on chromatin organization, regulation of gene activity, recombination, DNA replication, cell cycle, mismatch repair system, etc. This review also discusses the role of two putative mutational mechanisms, replication slippage and recombination, and their interaction in SSR variation.

Recently, increasingly more microsatellites, or simple sequence repeats (SSRs) have been found and characterized within protein-coding genes and their untranslated regions (UTRs). These data provide useful information to study possible SSR functions. Here, we review SSR distributions within expressed sequence tags (ESTs) and genes including protein-coding, 3′-UTRs and 5′-UTRs, and introns; and discuss the consequences of SSR repeat-number changes in those regions of both prokaryotes and eukaryotes. Strong evidence shows that SSRs are nonrandomly distributed across protein-coding regions, UTRs, and introns. Substantial data indicates that SSR expansions and/or contractions in protein-coding regions can lead to a gain or loss of gene function via frameshift mutation or expanded toxic mRNA. SSR variations in 5′-UTRs could regulate gene expression by affecting transcription and translation. The SSR expansions in the 3′-UTRs cause transcription slippage and produce expanded mRNA, which can be accumulated as nuclear foci, and which can disrupt splicing and, possibly, disrupt other cellular function. Intronic SSRs can affect gene transcription, mRNA splicing, or export to cytoplasm. Triplet SSRs located in the UTRs or intron can also induce heterochromatin-mediated–like gene silencing. All these effects caused by SSR expansions or contractions within genes can eventually lead to phenotypic changes. SSRs within genes evolve through mutational processes similar to those for SSRs located in other genomic regions including replication slippage, point mutation, and recombination. These mutational processes generate DNA changes that should be connected by DNA mismatch repair (MMR) system. Mutation that has escaped from the MMR system correction would become new alleles at the SSR loci, and then regulate and/or change gene products, and eventually lead to phenotype changes. Therefore, SSRs within genes should be subjected to stronger selective pressure than other genomic regions because of their functional importance. These SSRs may provide a molecular basis for fast adaptation to environmental changes in both prokaryotes and eukaryotes.

Genomics and domestication of wheat: Modern wheat, which underlies the diet of many across the globe, has a long history of selection and crosses among different species. Avni et al. used the Hi-C method of genome confirmation capture to assemble and annotate the wild allotetraploid wheat ( Triticum turgidum ). They then identified the putative causal mutations in genes controlling shattering (a key domestication trait among cereal crops). They also performed an exome capture–based analysis of domestication among wild and domesticated genotypes of emmer wheat. The findings present a compelling overview of the emmer wheat genome and its usefulness in an agricultural context for understanding traits in modern bread wheat. Science , this issue p. 93