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First published online October 27, 2004
doi: 10.1242/10.1242/dev.01441

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The gene FLORAL ORGAN NUMBER1 regulates floral meristem size in rice and encodes a leucine-rich repeat receptor kinase orthologous to Arabidopsis CLAVATA1

¹ Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
² Bioscience and Biotechnology Center, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
³ Japan Tobacco Incorporated, Aoba-ku, Yokohama 227-8512, Japan
⁴ Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan

View larger version (61K): [in a new window]   Fig. 1. Flower phenotype. (A) Wild-type flower (T65). (B) fon1-1 flower with seven stamens and two pistils. (C) fon1-2 flower with seven stamens and four pistils. (D) fon1-1 flower rescued by introducing the genomic segments containing the FON1 candidate gene. Arrows indicate pistils. Scale bars: 2 mm.

View larger version (29K): [in a new window]   Fig. 2. Number of floral organs in wild type (WT) and fon1 mutants. For each strain, 100 flowers were examined.

View larger version (124K): [in a new window]   Fig. 3. SEM images and specific expression patterns of DL and OSH1. (A) Wild-type flower at initiation of stamen primordia. (B) Wild-type flower that has started to form two styles in the pistil. (C) Almost-mature wild-type flower, in which stigmatic hairs are about to begin to grow on the styles. (D) fon1-2 flower. The floral meristem is enlarged along the palea-lemma axis. Additional stamens form in the same whorl as original stamens, and palea/lemma-like organs form ectopically on the lateral side of the meristem. (E) fon1-2 flower that develops two ectopic palea-like organs side by side. (F) fon1-2 flower that develops ectopic stamens in two whorls. (G) fon1-2 flower that has an enlarged floral meristem and develops two sets of carpels in an alternate phyllotaxy. (H,I) fon1-2 flowers that produce a number of pistils. (J) Close-up of the central region that remains undifferentiated in almost-mature fon1-2 flowers. (K) A secondary floret produced in fon1-2. The secondary flower (large arrow) has a few stamens and a carpel subtended with palea and lemma (the palea is removed to show the inside of the secondary floret). (L) Partial loss of stamen identity in fon1-2. Stigmatic hairs (black arrow) are produced at the top of the stamen instead of the anther and an undifferentiated cell mass has formed at the base of the filament (white arrow). (M,N) Spatial expression of DL. (M) Wild type; (N) fon1-2. Three sets of carpels develop in fon1-2. (O,P) Spatial expression of OSH1. (O) Wild type; (P) fon1-2. Whereas OSH1 expression disappears after carpel development in wild type, its expression is maintained in the floral meristem in the center of the flower in fon1-2. ca, carpel; eca, ectopic carpel; epl, ectopic palea/lemma-like organ; est, ectopic stamen that develops in an additional third whorl; fm, floral meristem; le, lemma; pa, palea; st, stamen; st2, stamen that develops in the secondary flower; sty, style. Scale bars: 100 µm in A-J,M-P; 500 µm in K,L.

View larger version (62K): [in a new window]   Fig. 4. Meristem sizes and inflorescences in wild-type and fon1-2. (A,C,E,G,I) Wild type. (B,D,F,H,J) fon1-2. (A,B) Floral meristems. (C,D) Expression of OSH1 in the floral meristem. The expression domain is expanded in fon1-2, but the spatial pattern does not differ from that in wild type. (E,F) Vegetative shoot apical meristems. (G,H) Inflorescence meristems. (I,J) Inflorescence phenotypes after seed production. (A,B,E-H) Micrographs viewed under Nomarski optics. Scale bars: 100 µm in A-H; 5 cm in I,J.

View larger version (64K): [in a new window]   Fig. 5. Isolation of the FON1 gene and characteristics of the FON1 protein. (A) Positional cloning of FON1. (B) Schematic representation of FON1 and CLV1. The LRR and Ser/Thr kinase domains are shown by rectangles, and the amino acid identities of these two domains are indicated. Closed triangles indicate the mutation sites of fon1-1 and fon1-2; open triangle shows the position of the intron. SP, signal peptide; TM, transmembrane domain. (C) Amino acid alignment of the LRR domain and its flanking sequence, with conserved cysteine residues indicated (asterisks). Amino acid numbers: FON1 54-628, CLV1 54-623. Sequences of the cDNA and genomic DNA of FON1 are deposited in DDBJ under accession numbers AB182388 and AB182389, respectively. (D) Amino acid alignment of the Ser/Thr kinase domain. Amino acid numbers: FON1 684-977, CLV1 672-967. Open triangle shows the position of the intron. (E) Phylogenic tree of LRR-type receptor kinases similar to FON1 and CLV1. Amino acids of the kinase domain (D) were compared and the tree was constructed by neighbor-joining methods (Saitou and Nei, 1987) using At4g20270 as a root. Numbers indicate bootstrap values. The intron positions of uncharacterized rice (FON1-like1, AC099732; FON1-like2, AC092781) and Arabidopsis (At4g20270, At3g49670, At5g65700) genes were deduced by comparing amino acid sequences and by the GT-AG rule for spliceosomal introns. Except for OsLRK1 (rice), the other known proteins are legume proteins, including those responsible for hypernodulation (HAR1, SYM29, NTS1).

View larger version (116K): [in a new window]   Fig. 6. In situ localization of the FON1 transcripts in wild-type rice. (A) Embryo. FON1 transcripts are detected in the shoot apical meristem. (B) Vegetative shoot apical meristem. (C) Inflorescence meristems at the primary branch stage. (D) Spikelets. FON1 transcripts are distributed throughout the floral meristems (the left meristem is the earliest stage of the floral meristem). In addition to floral meristems, FON1 transcripts are detected in the palea and lemma primordia. (E) A floret before carpel initiation. FON1 transcripts are detected in the lodicules, stamen primordia and the floral meristem. (F) A floret at the stage for carpel development. FON1 transcripts are expressed in the carpel primordia (arrows) and the floral meristem in addition to stamen and lodicules. (G) Inflorescences at the secondary branch stage. No transcripts are observed in the meristems. (A-F) Antisense probe; (G) sense probe. fm, floral meristem; ca, carpel, le, lemma; lo, lodicule; pa, palea; sam, shoot apical meristem; st, stamen. Scale bars: 100 µm.