MORE SPIKELETS1Is Required for Spikelet Fate in the Inflorescence of Brachypodium      

Abstract

Grasses produce florets on a structure called a spikelet, and variation in the number and arrangement of both branches and spikelets contributes to the great diversity of grass inflorescence architecture. In Brachypodium (Brachypodium distachyon), the inflorescence is an unbranched spike with a terminal spikelet and a limited number of lateral spikelets. Spikelets are indeterminate and give rise to a variable number of florets. Here, we provide a detailed description of the stages of inflorescence development in Brachypodium. To gain insight into the genetic regulation of Brachypodium inflorescence development, we generated fast neutron mutant populations and screened for phenotypic mutants. Among the mutants identified, the more spikelets1 (mos1) mutant had an increased number of axillary meristems produced from inflorescence meristem compared with the wild type. These axillary meristems developed as branches with production of higher order spikelets. Using a candidate gene approach, mos1 was found to have a genomic rearrangement disrupting the expression of an ethylene response factor class of APETALA2 transcription factor related to the spikelet meristem identity genes branched silkless1 (bd1) in maize (Zea mays) and FRIZZY PANICLE (FZP) in rice (Oryza sativa). We propose MOS1 likely corresponds to the Brachypodium bd1 and FZP ortholog and that the function of this gene in determining spikelet meristem fate is conserved with distantly related grass species. However, MOS1 also appears to be involved in the timing of initiation of the terminal spikelet. As such, MOS1 may regulate the transition to terminal spikelet development in other closely related and agriculturally important species, particularly wheat (Triticum aestivum).Plant shoots develop from the shoot apical meristem, which comprises a central region of pluripotent cells and a peripheral region where cells are recruited to form lateral organs. The shoot apical meristem is established early in embryogenesis and gives rise to all aerial portions of the plant. Following a reproductive transition, the shoot apical meristem converts to an inflorescence meristem, which may produce axillary meristems that form branches and flowers. The production, arrangement, and fate of meristems determine inflorescence architecture.Plant inflorescence architecture is vastly diverse, and one of the most dramatic reflections of this diversity is in the major cereal crop species. In maize (Zea mays), the tassel and ear inflorescence is indeterminate. Axillary meristems produced by the inflorescence in the tassel are initially indeterminate branches. Both the tassel and ear inflorescence, as well as branches of the tassel, produce spikelet pair meristems that then form determinate spikelet meristems, which terminate with production of two flowers or florets (Bortiri and Hake, 2007;Kellogg, 2007). In the rice (Oryza sativa) inflorescence or panicle, branch meristems initially produce secondary branch meristems. Both primary and secondary branches produce lateral spikelet meri...

FRIZZY PANICLE Drives Supernumerary Spikelets in Bread Wheat

“…4). These results are consistent with the report that deletion in the Bradi1g18580 promoter causes inflorescence branching (Derbyshire and Byrne, 2013). Our findings with the closely related B. distachyon as a validation system confirm the role of FZP and more precisely AP2/ERF domain function in the determination of spike architecture in Triticeae.…”

“…Derbyshire and Byrne (2013) isolated a B. distachyon mutant more spikelets1 (mos1) with similar defects in spikelet architecture; although no mutation in the coding region of FZP was found in this mutant, the authors proposed that the larger number of AxMs is due to a slightly but significantly lower expression of this AP2/ ERF transcription factor than in the wild type (20% less transcripts in the mos1 mutant). Derbyshire and Byrne (2013) suggested that this abnormal expression may be the consequence of a chromosome rearrangement affecting the promoter region of FZP. Here, we reported two new mutants sharing mutations in the highly conserved AP2/ERF domain of FZP and associated with an increased Figure 5.…”

“…The gene bd1 is the maize ortholog of FZP (Chuck et al, 2002), and bd1 mutants show a similar phenotype involving the initiation of extra spikelets in the tassel, and in the ear, spikelet meristems are replaced by branch meristems (Chuck et al, 2002). Derbyshire and Byrne (2013) isolated a B. distachyon mutant more spikelets1 (mos1) with similar defects in spikelet architecture; although no mutation in the coding region of FZP was found in this mutant, the authors proposed that the larger number of AxMs is due to a slightly but significantly lower expression of this AP2/ ERF transcription factor than in the wild type (20% less transcripts in the mos1 mutant). Derbyshire and Byrne (2013) suggested that this abnormal expression may be the consequence of a chromosome rearrangement affecting the promoter region of FZP.…”

“…Homologs of the BD1 gene have been isolated from sorghum BAC libraries and from foxtail millet (Setaria italica), common millet (Panicum miliaceum), oats (Avena sativa), and finger millet (Eleusine coracana) by PCR with degenerate primers (Chuck et al, 2002). FZP (Triticeae), bd1 (maize), and mos1 (B. distachyon) genes encode orthologous transcription factors of the AP2/ERF family (Chuck et al, 2002;Komatsu et al, 2003), showing complete amino acid identity within the AP2/ERF domain and with up to 75% identity (Chuck et al, 2002) with Bradi1g18580, 59% with BD1, and 54% with FZP (Derbyshire and Byrne, 2013).…”

Transcriptome Association Identifies Regulators of Wheat Spike Architecture

“…In another type of variation, one or more spikelets are replaced by long lateral branches, which form their own spikelets and florets. Mutations of the WFZP-A/BH t -A1 gene, encoding an AP2/ERF transcription factor, lead to such noncanonical spike branching (Derbyshire and Byrne, 2013;Dobrovolskaya et al, 2015;Poursarebani et al, 2015), which is similar to the branching produced by mutating its orthologs in maize, rice, and Brachypodium distachyon (Chuck et al, 2002;Komatsu et al, 2003). Although these recent breakthroughs shed light on the molecular mechanisms underlying rare supernumerary spikelet variations, little is known about genetic factors affecting the architecture of archetypal wheat spike, its complexity, and grain yield.…”