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The indeterminate floral apex1 gene regulates meristem determinacy and identity in the maize inflorescence

Plant Gene Expression Center, USDA-ARS and University of California, Berkeley, 800 Buchanan Street, Albany, CA 94710, USA

View larger version (71K): [in a new window]   Fig. 1. Maize inflorescence development. (A) Different meristem types in maize. The inflorescence meristem (IM) forms spikelet pair meristems (SPM) which form two spikelet meristems (SM), each of which forms two floral meristems (FM). Tassels also form branch meristems (BM) at the base. (B,C) Schematic of a spikelet pair. Each spikelet contains two glumes and two florets. The mature floret comprises the lemma, a palea, three lodicules (one of which is rudimentary and not shown), three stamens and a pistil. The pistil aborts in male spikelets (B). All stamens and the lower floret abort in female spikelets (C). (D) Male inflorescence. The basal branches on the tassel form indeterminate BMs, which give rise to SPMs in a distichous pattern. SPMs give rise to two SMs that are side by side. Each SM forms two floral meristems (FM). (E) Female inflorescence development proceeds similarly to that of male inflorescence development, only BMs are suppressed in the ear. The boxed area contains six spikelets. In each spikelet, the upper FM and outer glume are visible. The lower FM and inner glume are obscured.

View larger version (93K): [in a new window]   Fig. 2. ifa1 mutants produce additional spikelets and extra florets. (A) A pedicellate and sessile spikelet from a normal tassel. (B) A male spikelet showing two florets within a pair of glumes. (C) In ifa1 mutants, more than two florets are observed in a spikelet. (D) Three or more spikelets form instead of a pair of spikelets. g, glume; f, floret; sp, spikelet; sp*, extra spikelet, p, pedicel.

View larger version (63K): [in a new window]   Fig. 3. Floral phenotype of ifa1 mutants. (A) Normal pistil. Two of the carpels fuse and elongate to form the silk. (B) The ovule primordium is exposed inside the fused carpels. (C) ifa1-r pistil. A nucellar-like proliferative tissue arises from the floral apex, vestiges of the carpel wall and rudimentary silk can be seen. (D) Occasionally, multiple ovule-like protrusions form at the center of the ifa1-r pistil. (E) ifa1-r spikelet showing ectopic inflorescence from both florets. This phenotype occurs at a low frequency. (F) Normal male spikelet. The stamens of the upper floret are visible, the stamens of the lower floret remain enclosed by the lemma and palea. (G) ifa1-r male floret. Nucellar-like proliferative tissue (arrow) is present inside the stamen whorl. (H) ifa1-r male floret with extra stamens at the center of the stamen whorl. (I) The dissected floret from H. Inside the stamen whorl are leaf-like organs and more stamens. si, silk; op, ovule primordium; s, stamen; lf, lower floret.

View larger version (121K): [in a new window]   Fig. 4. SEM of normal and ifa1-r spikelets and florets. (A-D) Normal floret. (F-I) ifa1-r floret. (A,F) The upper FMs have initiated three stamens, one of which is hidden. Part of the glume is removed from the spikelet in F to reveal more of the lower FM. (B,G) The carpel ridge forms on the adaxial side of the FM. (C,H) The carpel ridge begins to envelope the ovule primordium in normal (C) but not in ifa1 mutants (H). (D) The carpels have almost completely enveloped the ovule. (I) The ovule primordium in ifa1 mutants is forming organs in a distichous pattern. The lower FM is also abnormal. (E) Spikelets develop in pairs in normal plants with the outer glumes forming at the abaxial side. The upper floret, which develops ahead of the lower floret, forms at the adaxial side of the spikelet. Both the inner glume and lower floret are obscured. (F) In ifa1 mutants, an extra spikelet forms between the spikelet pair. This extra spikelet develops in a reverse orientation such that the outer glume (see small arrow) is adaxial and the inner glume and the carpel ridge of the upper floret (see large arrow) are abaxial. lf, lower floret; st, stamen; op, ovule primordium; og, outer glume; ig, inner glume.

View larger version (80K): [in a new window]   Fig. 5. kn1 gene expression in ear spikelets and florets. (A) kn1 expression in developing spikelets. A longitudinal section of a normal inflorescence shows only one spikelet of the spikelet pair. The lower floret forms at the abaxial side of each spikelet. (B) kn1 expression in developing ifa1-r spikelets. A longitudinal section reveals a portion of an extra spikelet (marked with *) that overlaps with the developing spikelet pair. The lower floret develops on the adaxial side in extra spikelets. (C) kn1 expression in an ectopic BM arising from within an ifa1-r zag1 ear floret. (D) kn1 is not expressed in the silk or ovule of normal plants. Expression is detected in the lower floral meristem and floral axis below the ovule. (E-H) Serial section of ifa1 mutant ovule primordium. kn1 is also down-regulated as the ovule primordium initiates, however, as shown in a series of 10 µm sections from the same floral apex, kn1 is expressed in a small group of cells at the center of the primordium. uf, upper floret; lf, lower floret; *, extra spikelet; ov, ovule; op, ovule primordium; ca, carpel; ov, ovule.

View larger version (107K): [in a new window]   Fig. 6. zag2 gene expression in normal and ifa1-r flower. (A) zag2 is expressed in the ovule and carpel wall. (B) In ifa1 mutants, zag2 is also expressed in the carpel wall and initiating ovule primordium. ca, carpel; op, ovule primordium.

View larger version (50K): [in a new window]   Fig. 7. ifa1 zag1 double mutant phenotype. (A) ifa1-r ear. (B) zag1-mum1 ear. (C) ifa1-r zag1-mum1 ear shows inflorescences arising from the center of every floret. (D) Close-up of (C). (E) Normal tassel. (F-H) ifa1-r zag1-mum1 tassel. (G) ifa1-r zag1-mum1 floret reveals an ectopic spikelet arising from the center of the stamen whorl where the aborted pistil should have been. (H) The ectopic spikelet from G was opened to reveal staminoid florets arising from the axils of leaf-like organs. Arrows in G and H point to one of the lodicules (lo).

View larger version (76K): [in a new window]   Fig. 8. ifa1 ids1 double mutant phenotype. Individuals are from a family segregating for the ifa1 ids1 double mutant phenotype. (A) ids1-mum1 ear. (B) ifa1-r ear. (C-F) ifa1-r ids1-mum1 ear. Each spikelet forms an ectopic inflorescence. (E) ifa1-r ids1-mum1 spikelet pair. Both spikelets in a pair are affected. (F) ifa1-r ids1-mum1 spikelet with three florets and an ectopic inflorescence. The ectopic inflorescence forms spikelet pairs along its axis (arrow points to a spikelet pair). (G-I) ifa1-r ids1-mum1 tassel. (H) Spikelet pair of an ifa1-r ids1-mum1 plant. Many more spikelets form than in normal plants. (I) A series of spikelets form inside the glumes of the spikelet. sp, spikelet; sp*, extra spikelet; f, floret; g, glume; sm*, ectopic inflorescence that arises from an SM.

View larger version (67K): [in a new window]   Fig. 9. SEM of developing spikelets. (A) Normal spikelet pair. The more advanced upper floret develops on the adaxial side and the lower floret, which is partially covered by the outer glume, forms on the abaxial side. (B) ids1-mum1 spikelet pair. One of the spikelets is developing a third floret between the upper and the lower floret. The carpel ridge (arrow) of the extra floret (f*) has an orientation opposite to that of the upper floret and similar to that of the lower floret. (C) Spikelet pair from an ifa1-r ids1-mum1 ear. An indeterminate meristem arises between two florets (sm*). The floral meristems produce additional carpel ridges as seen in the single ifa1 mutant. (D) Longitudinal row of ifa1-r zag1-mum1 spikelets. Lateral organs form from the ovule primordium (arrowheads). uf, upper floret; lf, lower floret; og, outer glume; ig, inner glume; st, stamen; sm*, indeterminate meristem.

View larger version (18K): [in a new window]   Fig. 10. Meristem identity in different mutants. IM, inflorescence meristem; BM, branch meristem; SPM, spikelet pair meristem; SM, spikelet meristem; FM, floral meristem; FA, floral apex; thin arrow, determinate meristem; thick arrow, indeterminate meristem producing extra organs or meristems.