The LAX1 and FRIZZY PANICLE 2 genes determine the inflorescence architecture of rice by controlling rachis-branch and spikelet development

We have analyzed two mutants that exhibit altered panicle architecture in rice (Oryza sativa L.). In lax1-2, which is a new and stronger allele of the previously reported lax mutant, initiation and/or maintenance of rachis-branches, lateral spikelets, and terminal spikelets was severely prevented. In situ hybridization analysis using OSH1, a rice knotted1 (kn1) ortholog, confirmed the absence of lateral meristems in lax1-2 panicles. These defects indicate that the LAX1 gene is required for the initiation/maintenance of axillary meristems in the rice panicle. In addition to its role in forming lateral meristems, the wild-type LAX1 gene acts as a floral meristem identity gene which specifies the terminal spikelet meristem. A comparison of the defects in lax1-1 and lax1-2 plants suggested that the sensitivities to reduced LAX1 activity were not uniform among different types of meristems. In the fzp2 mutant panicle, the basic branching pattern of the panicle was indistinguishable from that of the wild type; however, specification of both terminal and lateral spikelet meristems was blocked, and sequential rounds of branching occurred at the point where the spikelet meristems are initiated in the wild-type panicle. This resulted in the generation of a panicle composed of excessive ramification of rachis-branches. The lax1-1 fzp2 double mutants exhibited a novel, basically additive, phenotype, which suggests that LAX1 and FZP2 function in genetically independent pathways.     

Morphological and molecular characterization of a new frizzy panicle mutant, "fzp-9(t)", in rice (Oryza sativa L.)

The spikelet identity gene "fzp" (frizzy panicle) is required for transformation of the floral meristems to inflorescent shoots. In fzp mutants, spikelets are replaced by branches and spikelet meristems produce massive numbers of branch meristems. We have isolated and characterized a new fzp mutant derived from anther culture lines in rice and designated as fzp-9(t). The fzp-9(t) mutant showed retarded growth habit and developed fewer tillers than those of the wild-type plant. The primary and secondary rachis branches of fzp-9(t) appeared to be normal, but higher-order branches formed continuous bract-like structures without developing spikelets. The genetic segregation of fzp-9(t) showed a good fit to the expected ratio of 3: 1. The sequence analysis of fzp-9(t) revealed that there is a single nucleotide base change upstream of the ERF (ethylene-responsive element-binding factor) domain compare to wild-type plant. The mutation point of fzp-9(t) (W66G) was one of the six amino acids of the ERF domain that contributed to GCC box-specific binding. The premature formation of a stop codon at the beginning of the ERF domain might cause a non-functional product.     

Class II tassel seed mutations provide evidence for multiple types of inflorescence meristems in maize (Poaceae)

The tassel seed mutations ts4 and Ts6 of maize cause irregular branching in its inflorescences, tassels, and ears, in addition to feminization of the tassel due to the failure to abort pistils. A comparison of the development of mutant and wild-type tassels and ears using scanning electron microscopy reveals that at least four reproductive meristem types can be identified in maize: the inflorescence meristem, the spikelet pair meristem, the spikelet meristem, and the floret meristem. ts4 and Ts6 mutations affect the fate of specific reproductive meristems in both tassels and ears. ts4 mutants fail to form spikelet meristems from spikelet pair meristems. Ts6 mutants are delayed in the conversion of certain spikelet meristems into floret meristems. Once floret meristems are established in both of these mutants, they form florets that appear normal but fail to undergo pistil abortion in the tassel. The abnormal branching associated with each mutant is suppressed at the base of ears, permitting the formation of normal, fertile spikelets. The classification of the different types of reproductive meristems will be useful in interpretation of gene expression patterns in maize. It also provides a framework for understanding meristem functions that can be varied to diversify inflorescence architectures in the Gramineae.     

BRANCHED SILKLESS mediates the transition from spikelet to floral meristem during Zea mays ear development

The molecular and genetic control of inflorescence and flower development has been studied in great detail in model dicotyledonous plants such as Arabidopsis and Antirrhinum . In contrast, little is known about these important developmental steps in monocotyledonous species. Here we report the analysis of the Zea mays mutant branched silkless1–2 (bd1–2) , allelic to bd1 , which we have used as a tool to study the transition from spikelet to floret development in maize. Floret development is blocked in the female inflorescence (the ear) of bd1–2 plants, whereas florets develop almost normally in the male inflorescence (the tassel). Detailed phenotypic analyses indicate that in bd1–2 mutants ear inflorescence formation initiates normally, however, the spikelet meristems do not proceed to form floret meristems. The ear spikelets, at anthesis, contain various numbers of spikelet-like meristems and glume-like structures. Furthermore, growth of branches from the base of the ear is often observed. Expression analyses show that the floral-specific MADS box genes Zea mays AGAMOUS1 ( ZAG1 ), ZAG2 and Zea mays MADS 2 ( ZMM2 ) are not expressed in ear florets in bd1–2 mutants, whereas their expression in tassel florets is similar to that of wild type. Taken together, these data indicate that the development from spikelet to floret meristem is differentially controlled in the ear and tassel in the monoecious grass species Zea mays , and that BRANCHED SILKLESS plays an important role in regulating the transition from spikelet meristem to floral meristem during the development of the female inflorescence of maize.     

barren inflorescence2 regulates axillary meristem development in the maize inflorescence

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.     

Combinatorial control of meristem identity in maize inflorescences

The architecture of maize inflorescences, the male tassel and the female ear, is defined by a series of reiterative branching events. The inflorescence meristem initiates spikelet pair meristems. These in turn initiate spikelet meristems which finally produce the floret meristems. After initiating one meristem, the spikelet pair and spikelet meristem convert into spikelet and floret meristems, respectively. The phenotype of reversed germ orientation1 (rgo1) mutants is the production of an increased number of floret meristems by each spikelet meristem. The visible phenotypes include increased numbers of flowers in tassel and ear spikelets, disrupted rowing in the ear, fused kernels, and kernels with embryos facing the base of the ear, the opposite orientation observed in wild-type ears. rgo1 behaves as single recessive mutant. indeterminate spikelet1 (ids1) is an unlinked recessive mutant that has a similar phenotype to rgo1. Plants heterozygous for both rgo1 and ids1 exhibit nonallelic noncomplementation; these mutants fail to complement each other. Plants homozygous for both mutations have more severe phenotypes than either of the single mutants; the progression of meristem identities is retarded and sometimes even reversed. In addition, in rgo1; ids1 double mutants extra branching is observed in spikelet pair meristems, a meristem that is not affected by mutants of either gene individually. These data suggest a model for control of meristem identity and determinacy in which the progress through meristem identities is mediated by a dosage-sensitive pathway. This pathway is combinatorially controlled by at least two genes that have overlapping functions.     

Genetic control of branching patterns in grass inflorescences

Inflorescence branching in the grasses controls the number of florets and hence the number of seeds. Recent data on the underlying genetics come primarily from rice and maize, although new data are accumulating in other systems as well. This review focuses on a window in developmental time from the production of primary branches by the inflorescence meristem through to the production of glumes, which indicate the transition to producing a spikelet. Several major developmental regulatory modules appear to be conserved among most or all grasses. Placement and development of primary branches are controlled by conserved auxin regulatory genes. Subtending bracts are repressed by a network including TASSELSHEATH4, and axillary branch meristems are regulated largely by signaling centers that are adjacent to but not within the meristems themselves. Gradients of SQUAMOSA-PROMOTER BINDING-like and APETALA2-like proteins and their microRNA regulators extend along the inflorescence axis and the branches, governing the transition from production of branches to production of spikelets. The relative speed of this transition determines the extent of secondary and higher order branching. This inflorescence regulatory network is modified within individual species, particularly as regards formation of secondary branches. Differences between species are caused both by modifications of gene expression and regulators and by presence or absence of critical genes. The unified networks described here may provide tools for investigating orphan crops and grasses other than the well-studied maize and rice.

Recent work on grass inflorescence branching identifies extensive conserved regulation, but also divergence particularly in formation of secondary branches and spikelets.     

<Emphasis Type="Italic">OsREL2</Emphasis>, a rice TOPLESS homolog functions in axillary meristem development in rice inflorescence

The morphology of the rice inflorescence, called the panicle, is determined mainly by the activities of axillary meristems including primary, secondary, and spikelet meristems. Recently, in maize, the RAMOSA1 ENHANCER LOCUS2 (REL2) gene, orthologous to the Arabidopsis shoot apical meristem fate-determining TOPLESS, was shown to be involved in the regulation of axillary meristem determinacy. In order to investigate the function of the rice REL2 homolog, we identified and characterized the rice REL2 gene (OsREL2). Compared to other rice TPL homologs, OsREL2 gene expression stayed relatively low throughout panicle development. We characterized a T-DNA insertion osrel2 mutant that showed pleiotropic phenotypic defects, such as defects in panicle heading, sterile lemma elongation, and panicle development, suggesting the OsREL2 functions in multiple developmental processes. In particular, osrel2 developed shorter axillary branches and reduced numbers of lateral organs on axillary branches in comparison to the wild-type, indicating that OsREL2 is important in axillary meristem maintenance. Interestingly, osrel2 produced more primary branches and fewer secondary branches than the wild-type. These results suggest that OsREL2 is involved in branch formation regulation, presumably by suppressing primary branch formation and promoting secondary branch formation.     

MISSING FLOWERS gene controls axillary meristems initiation in sunflower

The initiation and growth of axillary meristems are fundamental components of plant architecture. Here, we describe the mutant missing flowers (mf) of Helianthus annuus characterized by the lack of axillary shoots. Decapitation experiments and histological analysis indicate that this phenotype is the result of a defect in axillary meristem initiation. In addition to shoot branching, mutation affects floral differentiation. The indeterminate inflorescence of sunflower (capitulum) is formed of a large flat meristem which produces floret primordia in multiple spirals. In wildtype plants a bisecting crease divides each primordium in two distinct bumps that adopt different fate. The peripheral (abaxial) part of the primordium becomes a small leaf-like bract and the adaxial part becomes a flower. In the mf mutant, the formation of flowers at the axil of bracts is precluded. Histological analyses show that in floret primordia of the mutant a clear subdivision in dyads is not established. The primordia progressively bend inside and only large involucral floral bracts are developed. The results suggest that the MISSING FLOWERS gene is essential to provide or perceive an appropriate signal to the initiation of axillary meristems during both vegetative and reproductive phases.     

Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

Background and Aims

The inflorescence of grass species such as wheat, rice and maize consists of a unique reproductive structure called the spikelet, which is comprised of one, a few, or several florets (individual flowers). When reproductive growth is initiated, the inflorescence meristem differentiates a spikelet meristem as a lateral branch; the spikelet meristem then produces a floret meristem as a lateral branch. Interestingly, in wheat, the number of fertile florets per spikelet is associated with ploidy level: one or two florets in diploid, two or three in tetraploid, and more than three in hexaploid wheats. The objective of this study was to identify the mechanisms that regulate the architecture of the inflorescence in wheat and its relationship to ploidy level.

Methods

The floral anatomy of diploid (Triticum monococcum), tetraploid (T. turgidum ssp. durum) and hexaploid (T. aestivum) wheat species were investigated by light and scanning electron microscopy to describe floret development and to clarify the timing of the initiation of the floret primordia. In situ hybridization analysis using Wknox1, a wheat knotted1 orthologue, was performed to determine the patterning of meristem formation in the inflorescence.

Key Results

The recessive natural mutation of tetraploid (T. turgidum ssp. turgidum) wheat, branching head (bh), which produces branched inflorescences, was used to demonstrate the utility of Wknox1 as a molecular marker for meristematic tissue. Then an analysis of Wknox1 expression was performed in diploid, tetraploid and hexaploid wheats and heterochronic development of the floret meristems was found among these wheat species.

Conclusions

It is shown that the difference in the number of floret primordia in diploid, tetraploid and hexaploid wheats is caused by the heterochronic initiation of floret meristem development from the spikelet meristem.Key words: Triticum, wheat, inflorescence, spikelet, floret, meristem, heterochrony, heterochronic development, knotted1, polyploidy     

Developmental disaster1: A novel mutation causing defects during vegetative and inflorescence development in maize (Zea mays, Poaceae)

Axillary meristems, which give rise to branches and flowers, play a critical role in plant architecture and reproduction. To understand how axillary meristems initiate, we have screened for mutants with defects in axillary meristem initiation to uncover the genes controlling this process. These mutants, called the barren class of mutants in maize (Zea mays), have defects in axillary meristem initiation during both vegetative and reproductive development. Here, we identify and characterize a new member of the barren class of mutants named Developmental disaster1 (Dvd1), due to the pleiotropic effects of the mutation. Similar to the barren mutants, Dvd1 mutants have fewer branches, spikelets, florets, and floral organs in the inflorescence due to defects in the initiation of axillary meristems. Furthermore, double mutant analysis with teosinte branched1 shows that dvd1 also functions in axillary meristems during vegetative development. However, unlike the barren mutants, Dvd1 mutants are semidwarf due to the production of shorter internodes, and they produce leaves in the inflorescence due to the outgrowth of bract leaf primordia. The suite of defects seen in Dvd1 mutants, together with the genetic interaction of Dvd1 with barren inflorescence2, suggests that dvd1 is a novel regulator of axillary meristem and internode development.     

ABERRANT PANICLE ORGANIZATION 2/RFL, the rice ortholog of Arabidopsis LEAFY, suppresses the transition from inflorescence meristem to floral meristem through interaction with APO1

The temporal and spatial control of meristem identity is a key element in plant development. To better understand the molecular mechanisms that regulate inflorescence and flower architecture, we characterized the rice aberrant panicle organization 2 (apo2) mutant which exhibits small panicles with reduced number of primary branches due to the precocious formation of spikelet meristems. The apo2 mutants also display a shortened plastochron in the vegetative phase, late flowering, aberrant floral organ identities and loss of floral meristem determinacy. Map-based cloning revealed that APO2 is identical to previously reported RFL gene, the rice ortholog of the Arabidopsis LEAFY (LFY) gene. Further analysis indicated that APO2/RFL and APO1, the rice ortholog of Arabidopsis UNUSUAL FLORAL ORGANS, act cooperatively to control inflorescence and flower development. The present study revealed functional differences between APO2/RFL and LFY. In particular, APO2/RFL and LFY act oppositely on inflorescence development. Therefore, the genetic mechanisms for controlling inflorescence architecture have evolutionarily diverged between rice (monocots) and Arabidopsis (eudicots).     

The Relationship between auxin transport and maize branching

Maize (Zea mays) plants make different types of vegetative or reproductive branches during development. Branches develop from axillary meristems produced on the flanks of the vegetative or inflorescence shoot apical meristem. Among these branches are the spikelets, short grass-specific structures, produced by determinate axillary spikelet-pair and spikelet meristems. We investigated the mechanism of branching in maize by making transgenic plants expressing a native expressed endogenous auxin efflux transporter (ZmPIN1a) fused to yellow fluorescent protein and a synthetic auxin-responsive promoter (DR5rev) driving red fluorescent protein. By imaging these plants, we found that all maize branching events during vegetative and reproductive development appear to be regulated by the creation of auxin response maxima through the activity of polar auxin transporters. We also found that the auxin transporter ZmPIN1a is functional, as it can rescue the polar auxin transport defects of the Arabidopsis (Arabidopsis thaliana) pin1-3 mutant. Based on this and on the groundbreaking analysis in Arabidopsis and other species, we conclude that branching mechanisms are conserved and can, in addition, explain the formation of axillary meristems (spikelet-pair and spikelet meristems) that are unique to grasses. We also found that BARREN STALK1 is required for the creation of auxin response maxima at the flanks of the inflorescence meristem, suggesting a role in the initiation of polar auxin transport for axillary meristem formation. Based on our results, we propose a general model for branching during maize inflorescence development.     

Evolution of floral meristem identity genes. Analysis of Lolium temulentum genes related to APETALA1 and LEAFY of Arabidopsis

Flowering (inflorescence formation) of the grass Lolium temulentum is strictly regulated, occurring rapidly on exposure to a single long day (LD). During floral induction, L. temulentum differs significantly from dicot species such as Arabidopsis in the expression, at the shoot apex, of two APETALA1 (AP1)-like genes, LtMADS1 and LtMADS2, and of L. temulentum LEAFY (LtLFY). As shown by in situ hybridization, LtMADS1 and LtMADS2 are expressed in the vegetative shoot apical meristem, but expression increases strongly within 30 h of LD floral induction. Later in floral development, LtMADS1 and LtMADS2 are expressed within spikelet and floret meristems and in the glume and lemma primordia. It is interesting that LtLFY is detected quite late (about 12 d after LD induction) within the spikelet meristems, glumes, and lemma primordia. These patterns contrast with Arabidopsis, where LFY and AP1 are consecutively activated early during flower formation. LtMADS2, when expressed in transgenic Arabidopsis plants under the control of the AP1 promoter, could partially complement the organ number defect of the severe ap1-15 mutant allele, confirming a close relationship between LtMADS2 and AP1.     

MULTI-FLORET SPIKELET1, Which Encodes an AP2/ERF Protein,Determines Spikelet Meristem Fate and Sterile Lemma Identity in Rice

The spikelet is a unique inflorescence structure of grass. The molecular mechanism that controls the development of the spikelet remains unclear. In this study, we identified a rice (Oryza sativa) spikelet mutant, multi-floret spikelet1 (mfs1), that showed delayed transformation of spikelet meristems to floral meristems, which resulted in an extra hull-like organ and an elongated rachilla. In addition, the sterile lemma was homeotically converted to the rudimentary glume and the body of the palea was degenerated in mfs1. These results suggest that the MULTI-FLORET SPIKELET1 (MFS1) gene plays an important role in the regulation of spikelet meristem determinacy and floral organ identity. MFS1 belongs to an unknown function clade in the APETALA2/ethylene-responsive factor (AP2/ERF) family. The MFS1-green fluorescent protein fusion protein is localized in the nucleus. MFS1 messenger RNA is expressed in various tissues, especially in the spikelet and floral meristems. Furthermore, our findings suggest that MFS1 positively regulates the expression of LONG STERILE LEMMA and the INDETERMINATE SPIKELET1 (IDS1)-like genes SUPERNUMERARY BRACT and OsIDS1.In the reproductive phase of angiosperms, the shoot meristem is transformed into an inflorescence meristem, which then produces a floral meristem from which floral organs begin to develop, according to the mechanism known as the ABCDE model (Coen and Meyerowitz, 1991; Coen and Nugent, 1994; Dreni et al., 2007; Ohmori et al., 2009). An inflorescence can be classified as determinate or indeterminate based on whether its apical meristem is transformed into a terminal floral meristem. In an indeterminate inflorescence, the lateral meristem is permanently differentiated from the apical meristem, which is not converted into the terminal floral meristem, as occurs during the development of the inflorescences of Arabidopsis (Arabidopsis thaliana) and snapdragon (Antirrhinum majus). In contrast, in a determinate inflorescence, the apical meristem is transformed into the terminal floral meristem after the production of a fixed number of lateral meristems, as occurs during the development of the inflorescences of tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum; Bradley et al., 1997; Ratcliffe et al., 1999; Sussex and Kerk, 2001; Chuck et al., 2008).In general, inflorescences in grasses consist of branches and spikelets (Coen and Nugent, 1994; Itoh et al., 2005; Kobayashi et al., 2010). In these organisms, the branch meristem is determinate. It produces several lateral spikelet meristems, followed by the final production of a terminal spikelet meristem. The spikelet, the specific unit of the grass inflorescence, comprises a pair of bracts and one to 40 florets; it shows determinacy or indeterminacy depending on the species (Clifford, 1987; Malcomber et al., 2006). In species with a determinate spikelet, such as rice (Oryza sativa), after the production of fixed lateral floral meristems, the spikelet meristems are converted into terminal floral meristems, resulting in termination of the spikelet meristem fate. In contrast, in species with an indeterminate spikelet, such as wheat (Triticum aestivum), the spikelet meristem fate is maintained continuously and produces a variable number of lateral floral meristems.In Arabidopsis, the gene TERMINAL FLOWER1 (TFL1) was shown to maintain indeterminacy in the fate of the inflorescence. In the tfl1 mutant, the inflorescence meristems were converted into floral meristems earlier than in the wild type, but the ectopic expression of TFL1 resulted in the transformation of floral meristems at a later stage of development to secondary inflorescence meristems (Bradley et al., 1997; Ratcliffe et al., 1999; Mimida et al., 2001). In rice, overexpression of either of the TFL1-like genes, RICE CENTRORADIALIS1 (RCN1) or RCN2, delayed the transition of branch meristems to spikelet meristems and finally resulted in the production of a greater number of branches and spikelets than in the wild type (Nakagawa et al., 2002; Rao et al., 2008).To date, no gene that acts to maintain the indeterminacy of the spikelet meristem has been reported. However, two classes of genes have been shown to be involved in termination of the indeterminacy of spikelet meristems. One of these is the group of terminal floral meristem identity genes. A grass-specific LEAFY HULL STERILE1 (LHS1) clade in the SEPALLATA (SEP) subfamily belongs to this class. LHS1-like genes were found to be expressed only in the terminal floral meristem in species with spikelet determinacy, which suggested that they exclusively determine the production of the terminal floral meristem, by which the spikelet meristem acquires determinacy (Cacharroón et al., 1999; Malcomber and Kellogg, 2004; Zahn et al., 2005). The other class comprises the INDETERMINATE SPIKELET1 (IDS1)-like genes, which belong to the APETALA2/ethylene-responsive factor (AP2/ERF) family. Unlike LHS1-like genes, this class of genes regulates the correct timing of the transition of the spikelet meristem to the floral meristem but does not specify the identity of the terminal floral meristem. In maize (Zea mays), loss of IDS1 function produces extra florets (Chuck et al., 1998). In addition, mutation of SISTER OF IDS1 (SID1), a paralog of IDS1 in maize, resulted in no defects in terms of spikelet development. However, the ids1+sid1 double mutant failed to generate floral organs and instead developed more bract-like structures than are found in wild-type plants (Chuck et al., 2008). The rice genome contains two IDS1-like genes, SUPERNUMERARY BRACT (SNB) and OsIDS1. Loss of activity of SNB or OsIDS1 produced extra rudimentary glumes, and snb+osids1 double mutant plants developed more rudimentary glumes than either of its parental mutants (Lee et al., 2007; Lee and An, 2012). These results revealed that the mutated IDS1-like genes prolonged the activity of the spikelet meristem.In most members of Oryzeae, the spikelet is distinct from those of other grasses, in that it comprises a pair of rudimentary glumes, a pair of sterile lemmas (empty glumes), and one floret (Schmidt and Ambrose, 1998; Ambrose et al., 2000; Kellogg, 2009; Hong et al., 2010). The rudimentary glumes are generally regarded as severely reduced bract organs, but the origin of sterile lemmas has been widely debated. Recent studies suggested that the sterile lemmas are the vestigial lemmas of two lateral florets. The gene LONG STERILE LEMMA (G1)/ELONGATED EMPTY GLUME1 (ELE1) is a member of a plant-specific gene family. In the g1/ele1 mutant, sterile lemmas were found to be homeotically transformed into lemmas (Yoshida et al., 2009; Hong et al., 2010). The OsMADS34 and EXTRA GLUME1 (EG1) genes were also shown to determine the identities of sterile lemmas. In the osmads34 and eg1 mutants, the sterile lemmas were enlarged and acquired the identities of lemmas (Li et al., 2009; Gao et al., 2010; Kobayashi et al., 2010). Additionally, the SEP-like gene LHS1/OsMADS1, which specifies the identities of both the lemma and the palea, was not expressed in sterile lemmas, and ectopic expression in sterile lemmas resulted in the transformation of sterile lemmas to lemmas (Jeon et al., 2000; Li et al., 2009; Tanaka et al., 2012). These findings suggest that the sterile lemma may be homologous to the lemma. Nevertheless, some researchers still considered that the sterile lemmas are instead vestigial bract-like structures similar to the rudimentary glumes (Schmidt and Ambrose, 1998; Kellogg, 2009; Hong et al., 2010).In this study, we isolated the rice MULTI-FLORET SPIKELET1 (MFS1) gene, which belongs to a clade of unknown function in the AP2/ERF gene family. The mutation of MFS1 was shown to delay the transformation of the spikelet meristem to the floral meristem and to result in degeneration of the sterile lemma and palea. These results suggest that MFS1 plays an important role in the regulation of spikelet determinacy and organ identity. Our findings also reveal that MFS1 positively regulates the expression of G1 and the IDS1-like genes SNB and OsIDS1.