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doi: 10.1242/10.1242/dev.00564

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FRIZZY PANICLE is required to prevent the formation of axillary meristems and to establish floral meristem identity in rice spikelets

¹ Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan
² Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan
³ CREST, Japan Science and Technology Corporation, Tokyo 101-0062, Japan

View larger version (18K): [in a new window]   Fig. 1. Scheme depicting the development of the shoot apical meristem (SAM) of rice. (A) Vegetative meristem. Leaf primordia emerge from the SAM in an alternate phyllotaxis. (B) Reproductive meristem at the stage of primary branch meristem (PBM) formation. PBMs emerge spirally from the SAM at 144° from the previous one. (C) PBM at the spikelet and secondary branch meristem (SBM) formation stage. Meristems emerge from the primary branches in two alternate rows. Upper meristems acquire lateral SM identity whereas lower meristems form SBMs. The PBM itself acquire terminal SM identity. (D) SM at floret formation stage. Rudimentary glumes, empty glumes, lemma and palea primordia emerge alternately from the SM. (E) Schematic representation of a mature rice spikelet. Floral organs (not represented) are enclosed by the lemma and palea. (F) Schematic representation of a mature rice inflorescence. lp, leaf primordium; pbm, primary branch meristem; tsm, terminal spikelet meristem; lsm, lateral spikelet meristem; sbm, spikelet branch meristem; org, outer rudimentary glume; irg, inner rudimentary glume; oeg, outer empty glume; ieg, inner empty glume; l, lemma; p, palea; ma, main axis; pb, primary branch; sb, secondary branch; ts, terminal spikelet; ls, lateral spikelet; dp, degenerate point.

View larger version (98K): [in a new window]   Fig. 2. Morphology of wild-type, severe and weak fzp inflorescences, spikelets and ectopic branches. (A-D) Wild type. (E-H) fzp-2. (I-L) fzp-3. The wild-type inflorescence (A) is composed of primary branches (B) and secondary branches that have terminal (C) and lateral (D) spikelets. Bracts are not observed at the junction between the spikelet pedicel and the growth axis (D arrow). (E,F) Spikelets are replaced by branches in the severe fzp-2 allele, but defective and normal spikelets can be formed at the apices of ectopic branches of the weak fzp-3 allele (I,J). fzp-2 and fzp-3 secondary branches are composed of alternate bract-like structures (F,J, insets). Tertiary branches develop at the axils of the bract-like structures (G,H,L arrowheads) in the apical part of a secondary branch and they are also composed of bract-like structures that form higher order branches at their axils (G inset). (G) The apices of a fzp-2 secondary branch is terminated with a tertiary branch. (K,L) fzp-3 tertiary branches are terminated with a spikelet. org, outer rudimentary glume; irg, inner rudimentary glume; oeg, outer empty glume; ieg, inner empty glume; l, lemma; p, palea. Scale bars: 1 cm (A,B,E,F,I,J); 1 mm (C,D,G,H,K,L).

View larger version (127K): [in a new window]   Fig. 3. SEM analysis of fzp-2 and fzp-3 ectopic branches. (A,C) fzp-2. (B,D,E) fzp-3. (A) Tertiary branch of fzp-2. Note that the direction of outgrowth of bract-like structures is at 90° to the previous branch. (B) Tertiary branches on a secondary branch of fzp-3. (C) The apex of a fzp-2 tertiary branch terminates with the formation of quaternary branches. (D,E) The majority of fzp-3 tertiary branches form defective spikelets at their tips. 2°, bract-like structure of secondary order branches; 3°, tertiary order; 4°, quaternary order. Scale bars: 100 µm.

View larger version (122K): [in a new window]   Fig. 4. SEM analysis of wild-type spikelet organs and fzp-2 bract-like structures. (A) Junction between the organs of a wild-type spikelet. (B) Border between the rudimentary glume and the empty glume of a wild-type spikelet. (C) The cells of empty glumes have a regular shape and a flat surface that result in a smooth overall appearance. (D) The cells of rudimentary glumes have a irregular shape and hairs that can be short or long, resulting in a rugous appearance. (E) Bract-like structure of a fzp-2 tertiary branch. An axillary meristem (arrow) is formed but its development is arrested. (F) The cells of bract-like structures of fzp mutants have irregular shape and hairs similar to the cells of wild-type rudimentary glumes. (G) Upper view of a bract-like structure and its degenerated axillary meristem (arrow) on the secondary branch of a fzp-2 spikelet. (H) Empty glume of a fzp-3 spikelet. org, outer rudimentary glume; irg, inner rudimentary glume; oeg, outer empty glume; ieg, inner empty glume; l, lemma; p, palea; eg, empty glume; rg, rudimentary glume. Scale bars: 100 µm (A,B,E,G,H); 10 µm (C,D,F).

View larger version (51K): [in a new window]   Fig. 5. Cloning of the FZP gene. (A) A map-based strategy was initially applied to clone the FZP gene but was discontinued with the identification of a transposon-tagged mutant. (Top) The markers flanking the FZP locus and the number of recombinants in a mapping population of 178 plants are indicated. FZP was delimited to a region of 119 kb covered by two PAC clones. (Bottom) The fzp-4 allele showed two Ac transposon insertions in a region corresponding to the AP004570 PAC clone; the distal Ac element disrupted a putative ERF-like gene. (B) Co-segregation of two Ac elements with the fzp phenotype (arrowheads) in fzp-4 seedlings. (Left) Southern hybridization of four seedlings of the parental Ac 26-6 line. (Right) Southern hybridization of four seedlings of the progeny of the Ac 26-6-1 seedling. The genotype (G) of the fzp locus disruption and phenotype (P) of each seedling are indicated below. +/-, heterozygous; -/-, homozygous mutant; +/+, homozygous wild type; wt, wild-type; fzp, frizzy panicle. M, marker: 3, 5, 6, 8, 10 kb from the bottom to the top.

View larger version (48K): [in a new window]   Fig. 6. Structure of the FZP gene and similarity to other ERF transcription factors. (A) Schematic representation of the FZP gene structure with mutant lesions. The ERF domain (black box) is localized at the N-terminal half whereas the acidic domain (hatched box) conserved in grass BD1-like proteins is closer to the C terminus. The position of the mutations in the fzp alleles are indicated above. The amino acid numbers are indicated below. The black line indicates the putative PEST sequence and the grey line indicates the bipartite nuclear localization signal. (B) Alignment of the amino acid sequences of the ERF domains of FZP and other members of the ERF family. Asterisks indicate amino acids that confer specific GCC-box binding. Mutations of the fzp-2, fzp-3, fzp-7 and fzp-8 alleles are shown. BD1, maize BRANCHED SILKLESS; BD1B, duplicate of BD1; LEP, Arabidopsis LEAFY PETIOLE; Arabidopsis ESR1, ENHANCER OF SHOOT REGENERATION1; TINY, Arabidopsis TINY. (C) Transactivation of the GAL4-LUC reporter gene by FZP. Schematics of the constructs used are indicated on the left and described in the Material and Methods section. Relative luciferase activities in Arabidopsis leaves that had been co-bombarded with reporter and effector plasmids are indicated on the right. All luciferase activities were expressed in arbitrary units relative to values obtained with the reporter construct alone (None; set arbitrarily at 1). The values shown are averages of results from three independent experiments. Error bars indicate standard deviation.

View larger version (142K): [in a new window]   Fig. 7. In situ hybridizations depicting the pattern of FZP expression (arrowheads) during the development of wild-type (A-G) and fzp-2 (H,I) spikelets. (A) Wild-type SAM at the spikelet differentiation stage. FZP RNA (arrowheads) is detected in spikelet meristems before and during the formation of rudimentary glume (RG) primordia. (B) Early stage of spikelet meristem (SM) differentiation. FZP is expressed just above the position where the RG primordium emerges. (C) SM at early inner RG differentiation stage. While the expression of FZP decreases at the outer RG axil, it increases on the iRG side. (D) SM at early palea differentiation stage. FZP expression at the inner RG axil persists weakly until the initiation of the palea primordium. Note that FZP is not expressed at the position of the lemma and palea primordia initiation or in their axils. (Inset) Higher magnification of the region expressing FZP. (E-G) FZP expression extends to a half-ring domain. (E) Lateral view. FZP expression is interrupted around the circumference of the SM. RG primordia (arrows) emerge right under the domain of FZP expression. (F,G) Frontal view of successive transverse sections. FZP is expressed at the sides but not at the center of the SM. (H) Late fzp-2 SAM. FZP is expressed at the tip of secondary branches. (I) Higher magnification of the tip of a secondary branch. FZP is expressed at the axil of a differentiating RG primordium. (J) Schematic representation of the spatial and temporal expression of FZP during the differentiation of a spikelet. org, outer rudimentary glume; irg, inner rudimentary glume; oeg, outer empty glume; ieg, inner empty glume. Scale bars: 100 µm.

View larger version (11K): [in a new window]   Fig. 8. Models representing the function of FZP during the development of rice spikelets. In wild-type plants, the inflorescence meristem (IM) generates branch meristems (BM) that in turn generate spikelet meristems (SM). Model 1, FZP acts to repress the formation of axillary meristems (AxM) from the SM and ensure that the SM acquires floral meristem (FM) identity. Model 2, FZP induces the transition from SM to FM identity. In fzp mutants, the transition to FM identity does not take place and ectopic AxMs behave as SMs.