CENTRORADIALIS (CEN) is a key regulator of flowering time and inflorescence architecture in plants. Natural variation in the barley (Hordeum vulgare) homolog HvCEN is important for agricultural range expansion of barley cultivation, but its effects on shoot and spike architecture and consequently yield have not yet been characterized. Here, we evaluated 23 independent hvcen, also termed mat-c, mutants to determine the pleiotropic effects of HvCEN on developmental timing and shoot and spike morphologies of barley under outdoor and controlled conditions. All hvcen mutants flowered early and showed a reduction in spikelet number per spike, tiller number, and yield in the outdoor experiments. Mutations in hvcen accelerated spikelet initiation and reduced axillary bud number in a photoperiod-independent manner but promoted floret development only under long days (LDs). The analysis of a flowering locus t3 (hvft3) hvcen double mutant showed that HvCEN interacts with HvFT3 to control spikelet initiation. Furthermore, early flowering3 (hvelf3) hvcen double mutants with high HvFT1 expression levels under short days suggested that HvCEN interacts with HvFT1 to repress floral development. Global transcriptome profiling in developing shoot apices and inflorescences of mutant and wild-type plants revealed that HvCEN controlled transcripts involved in chromatin remodeling activities, cytokinin and cell cycle regulation and cellular respiration under LDs and short days, whereas HvCEN affected floral homeotic genes only under LDs. Understanding the stage and organ-specific functions of HvCEN and downstream molecular networks will allow the manipulation of different shoot and spike traits and thereby yield.

Significance: Meristem determinacy/indeterminacy influences flower number and seed production in crops. Two closely related cool-season cereals, barley and wheat, produce variable and defined numbers of spikelets in their inflorescences, respectively. In this study, we identify a series of allelic barley mutants namedintermedium-manddouble seed1that develop wheat-like determinate inflorescences producing a terminal spikelet and a reduced number of spikelets. INT-M/DUB1 is an APETALA2-like transcription factor that promotes an active inflorescence meristem via suppression of spikelet initiation and the maintenance of meristem identity. Our work has identified key regulators that may prolong meristem activities and could be genetically engineered in barley, wheat, and other cereals to improve grain yield.

In many angiosperm plants, FLOWERING LOCUS T (FT)-like genes have duplicated and functionally diverged to control different reproductive traits or stages. Barley (Hordeum vulgare) carries several FT-like genes, the functions of which are not well understood. We characterized the role of HvFT3 in the vegetative and reproductive development of barley. Overexpression of HvFT3 accelerated the initiation of spikelet primordia and the early reproductive development of spring barley independently of the photoperiod. However, HvFT3 overexpression did not accelerate floral development, and inflorescences aborted under short days, suggesting that HvFT3 controls spikelet initiation but not floral development. Analysis of a nonfunctional HvFT3 allele supported the specific effects of this gene on spikelet initiation independent of the photoperiod. HvFT3 caused the up-regulation of the winter and spring alleles of the vernalization gene VERNALIZATION1 (VRN-H1) in nonvernalized plants and was therefore dominant over the repressive effects of the vernalization pathway. Global transcriptome analysis in developing main shoot apices of the transgenic lines showed that HvFT3 modified the expression of genes involved in hormone synthesis and response, of floral homeotic genes, and of barley row-type genes SIX-ROWED-SPIKE1 (VRS1), SIX-ROWED-SPIKE4 (VRS4), and INTERMEDIUM C. Understanding the specific functions of individual FT-like genes will allow modification of individual phases of preanthesis development and thereby adaptation to different environments and improved yield.

Spikelets are highly specialized and short-lived branches and function as a constitutional unit of the complex grass inflorescences. A series of genetic, genomic, and developmental studies across different clades of the family have called for and permitted a synthesis on the regulation and evolution of spikelets, and hence inflorescence diversity. Here, we have revisited the identity specification of a spikelet, focusing on the diagnostic features of a spikelet from morphological, developmental, and molecular perspectives. Particularly, recent studies on a collection of barley (Hordeum vulgare L.), wheat (Triticum spp.), and rice (Oryza sativa L.) mutants have highlighted a set of transcription factors that are important in the control of spikelet identity and the patterning of floral parts of a spikelet. In addition, we have endeavored to clarify some puzzling issues on the (in)determinacy and modifications of spikelets over the course of evolution. Meanwhile, genomes of two sister taxa of the remaining grass species have again demonstrated the importance of genome duplication and subsequent gene losses on the evolution of spikelets. Accordingly, we argue that changes in the orthologs of spikelet-related genes could be critical for the development and evolution of the spikelet, an evolutionary innovation in the grass family. Likewise, the conceptual discussions on the regulation of a fundamental unit of compound inflorescences could be translated into other organismal groups where compound structures are similarly formed, permitting a comparative perspective on the control of biological complexity.