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).
Highlights
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
Axillary meristems at the base of the plant elongated to form multiple basal tillers that were similar to the main stem
Awns are present in other grass species, such as rice, barley, and wheat, and likely serve a function in protection against animal foraging and to promote seed dispersal, awns may have a physiological role in seed development (Grundbacher, 1963)
Summary
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. The more spikelets (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. Genes that are necessary for inflorescence development have been identified through analysis of mutants in maize and rice, and many of these genes control meristem initiation and fate (Bommert et al, 2005; Sreenivasulu and Schnurbusch, 2012). Barren stalk (ba1) in maize results in a radical decrease in the production of axillary meristems and forms an unbranched inflorescence without spikelets (Ritter et al, 2002; Gallavotti et al, 2004). The rice MONOCULM1 (MOC1) gene ( known as SMALL PANICLE) is required for initiation of axillary meristems throughout vegetative and reproductive development. moc mutants lack tillers and have few inflorescence branches and spikelets (Li et al, 2003; Oikawa and Kyozuka, 2009). barren inflorescence (bif2) in maize does not produce branches or spikelets in the inflorescence and is required for the maintenance of all types of axillary meristems (McSteen and Hake, 2001). ba, LAX1, and MOC1 encode transcription factors, whereas bif encodes a Ser/Thr protein kinase protein involved in auxin signaling (Komatsu et al, 2003b; Li et al, 2003; Gallavotti et al, 2004; McSteen et al, 2007)
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