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First published online 10 November 2004
doi: 10.1242/dev.01523

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Conservation of B-class floral homeotic gene function between maize and Arabidopsis

Division of Biological Sciences, Section of Cell and Developmental Biology, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116, USA

View larger version (35K): [in a new window]   Fig. 1. ABC Model for patterning maize florets and Arabidopsis flowers. (A) Diagram of the traditional ABC model. (B) ABC model adapted for the maize floret (Ambrose et al., 2000). Maize genes are indicated in their appropriate domains (an aborted lodicule is indicated by X). A photo of a maize floret, with floral organs indicated, is shown on the right. (C) Floral diagram, ABC model and photo of Arabidopsis flower.

View larger version (128K): [in a new window]   Fig. 2. Zmm16 RNA in situ hybridization of maize developing male spikelets. (A) Two young male spikelets with Zmm16 expression throughout the upper floral meristem (Uf) and absent from the emerging lower floral meristem (Lf). (B) Developing spikelet surrounded by inner (Gi) and outer (Go) glumes. In this section, Zmm16 expression appears throughout the upper floral meristem, but is absent from emerging lemma (Le) and palea (Pa) primordia. (C) Subsequent section of the same spikelet shown in B, showing absence of Zmm16 in the center of the floral meristem. (D) Zmm16 expression in the developing stamen (St), lodicule (Lo) and lower floral meristem. (E) Later stage spikelet with lower floret, reiterating the pattern of expression seen in the upper floret, shown in C. (F) Transverse section showing expression in stamens, but not in the aborting carpel (Ca).

View larger version (49K): [in a new window]   Fig. 3. In vitro DNA-binding assay of maize and Arabidopsis B-class proteins. (A) Autoradiogram of in vitro transcribed and translated SI1 and ZMM16 proteins incubated with labeled AGL5 CArG-box probe. Neither SI1 nor ZMM16 alone are capable of binding the AGL5 CArG-box, but together (SI1+ZMM16 lane) they can, as indicated by the mobility shift of labeled AGL5 CArG probe. Control lane consists of TNT lysate (without added plasmid DNA) incubated with probe. FP designates the lane loaded only with free probe. Arrows indicate background bands in the negative control caused by nonspecific binding of lysate proteins to the probe. (B) As in A, but the probe contains mutations in the AGL5 CArG-box that abolishes SI1-ZMM16 heterodimer binding (see Materials and methods for details). (C) As in A, but includes in vitro transcribed and translated AP3 and PI proteins. Weak binding to the probe in lanes containing PI (AP3+PI and PI+SI1) is due to poor in vitro expression of the PI template, as demonstrated by 35S-labelled TNT control reactions (data not shown).

View larger version (82K): [in a new window]   Fig. 4. Complementation of Arabidopsis B-class mutants ap3 and pi by their maize orthologs. (A-E) Arabidopsis flowers of (A) wild type, (B) ap3-3 mutant, (C) ap3-3 with AP3pro:Si1 transgene, (D) pi-1 mutant with AP3pro:Zmm16 transgene (note white sepal margins), and (E) ap3-3 pi-1 double mutant with both AP3pro:Si1 and AP3pro:Zmm16 transgenes (note white petals and sepals). (F-J) Scanning electron microscopy (SEM) of flowers from the same plants from which the flowers shown in A-E were obtained, in the same order. (K-O) SEM of abaxial petal or second whorl organ epidermis from flowers shown in F-J, in the same order. Cells of wild-type petal epidermis (K) are rounded and no guard cells are present. Cells of second whorl `sepal' epidermis of ap3-3 mutant (L) are elongated and irregular, with many guard cells. Rescued mutants (M-O) have epidermal cells that are intermediate in shape between those seen in K and L, with occasional guard cells. Note the lack of elongated epidermal cells characteristic of sepals. Arrows in L, N and O indicate guard cells. Scale bars: 0.5 mm in F-J; 50 µm in K-O.

View larger version (15K): [in a new window]   Fig. 5. Maize transgene expression levels compared with wild-type AP3 levels. Total RNA collected from inflorescences of strongly rescued AP3:Si1 and AP3:Zmm16 plants, was spotted onto nylon filters and hybridized with transgene specific probes or an AP3 gene-specific probe having identical specific activities. Expression levels are arbitrarily designated by the ImageQuant software after exposure to a phosphoimager screen.