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doi: 10.1242/10.1242/dev.00336

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Combinatorial control of meristem identity in maize inflorescences

351 Koshland Hall, University of California at Berkeley, Berkeley, CA 94720, USA

View larger version (18K): [in a new window]   Fig. 1. A model of inflorescence development. (A) The inflorescence meristem (IM, 1°) initiates (black arrows) spikelet pair meristems (SPM, 2°). The SPM then initiates a spikelet meristem (SM, 3°) on its flank, and then converts (blue arrows) into a SM. In a reiterative manner, each SM initiates one floret meristem (FM, 4°) and converts into a FM. Adapted from Irish (Irish, 1997b). (B) In the ear, wild-type spikelets (top) initiate the lower floret (1) which aborts (red) and then the spikelet converts into the upper floret (2) which develops into a functional flower (green). In rgo1 plants (bottom), the spikelet produces a third flower (3). In many cases the second flower also develops (green/red gradient). Owing to the distichous pattern of floret initiation by the spikelet, any odd numbered flower (1, 3, 5,...) will face the base of the ear, while even numbered flowers (2, 4, 6,...) face the tip of the ear. Any mutation that either increases or decreases floret number may result in a rgo phenotype.

View larger version (63K): [in a new window]   Fig. 2. rgo1 affects the ear and tassel spikelets. Side views of (A) a wild-type ear with regular rows of kernels and (B) a rgo1 ear with irregular rows of kernels. The rgo1 ear has embryos facing the base of the ear (arrow in B and D) and a fused kernel (arrowhead). Bottom views of (C) a wild-type and (D) a rgo1 ear show embryos facing the base of the ear only in rgo1 ears. (E,F) Fused kernels have two embryos (e), visible on the two faces (E and F) of this open pollinated example. Wild-type tassel spikelets (G) contain two flowers (f), while rgo1 spikelets contain three (H). Both are surrounded by leafy glumes (g).

View larger version (83K): [in a new window]   Fig. 3. Wild-type and rgo1 ear development. These images are a series from the tip to the base of developing ears. (A,C,E,G,I) Wild type and (B,D,F,H,J) rgo1. (A,B) Inflorescence meristems (im) producing multiple files of spikelet pair meristems (spm). The spikelet pair meristems produce a spikelet meristem (sm), which initiates the outer glume (og) and then the inner glume (not shown) in wild type (C) and rgo1 (D). In wild-type development, the spikelet meristem initiates one floret meristem (fm) (E), whereas in rgo1 mutants each spikelet meristem produces two floret meristems (F). The spikelet meristem then converts into a floret meristem, and initiates floral organs, including three anthers (a) and the pistil primordial (p). In wild-type spikelets the anthers are arranged with the middle anther facing the tip of the ear (G), whereas the anthers face the base of the ear in rgo1 (H). Finally, each floral meristem produces the gynoecial ridge (gr), which is in the reversed orientationin rgo1 (J) relative to wild-type (I). The second floret (arrowheads, J) often develops in rgo1 spikelets. Bars, 200 µm (A,B), 100 µm (C-J).

View larger version (192K): [in a new window]   Fig. 4. rgo1 tassel defects are similar to ear defects. (A) Wild-type tassel spikelet meristems (sm) initiate one floret meristem (fm) while (B) rgo1 spikelet meristems initiate two. Earlier events (not shown) and later events including floral organ initiation (C, wild type; D, rgo1) are unaffected. a, anther primordia; p, pistil primordia; l, lodicule. Bars, 100 µm (A), 200 µm (B), 50 µm (C,D).

View larger version (106K): [in a new window]   Fig. 5. rgo1 spikelet development has several possible outcomes. (A,E) Wild type spikelets have one silk (si), a single ovule primordium (op) and an aborting lower floret (af) surrounded by the outer glume (og) and inner glume (ig). (B-D,F-H) rgo1 mutant spikelets have three florets. The two lower florets abort in some spikelets (B,F). In other spikelets, only the first floret aborts, resulting in two functional flowers (C,G) and a pair of opposed kernels. Rarely, the two developing florets are fused (D,H), and produce a fused kernel. Bars, 100 µm (A-D), 250 µm (E-H).

View larger version (56K): [in a new window]   Fig. 6. rgo1 and ids1 exhibit nonallelic noncomplementation and have a synergistic double mutant phenotype. rgo1 and ids1 homozygous plants were crossed (top). The double heterozygote progeny had a reversed kernel and disrupted row phenotypes (middle). When these plants were self-pollinated, wild type (wt), rgo1-like (rgo), and double mutants with long branches (branched) were produced (bottom).

View larger version (84K): [in a new window]   Fig. 7. rgo1; ids1 double mutant phenotypes. (A) Double mutant plants have reversed and fused kernels (*) and SPMs that elaborate long branches (arrowhead) with spikelets (s) in a distichous pattern. (B) Individual spikelets develop multiple glumes (g) as well as multiple florets in the axils of paleas (p). (C) A SM (sm) that has elongated and reverted into a SPM, initiating another glume. These SMs can be identified by their subtending glumes (g in C,D). The spikelets that they produce, sometimes in pairs (arrowhead, D) are also surrounded by glumes. (E) A SM that has elongated and produced several spikelets. (F) Severe double mutants exhibit increased SPM and SM branching, producing a highly branched ear with no kernels and very few silks. (G) A close examination of a spikelet pair axis (sp) at the base of this ear shows at least six spikelets (s) in a distichous pattern. Each of these spikelets has initiated multiple glumes (arrowheads). Bars, in B and C 100 µm.

View larger version (27K): [in a new window]   Fig. 8. A model of rgo1 and ids1 combinatorial control of spikelet and spikelet pair meristem identity. (A) During wild-type development the inflorescence meristem (IM) produces many spikelet pair meristems (SPM). Each SPM initiates one spikelet meristem (SM) and converts into a SM. Each SM initiates one floret meristem (FM) and converts into a FM. (B) rgo1 mutants are similar to wild type except that each SM initiates two FMs before converting into an FM. (C) ids1 mutants are similar to rgo1 except that each SM can initiate multiple FMs before converting into an FM. (D) rgo1; ids1 double mutants exhibit a synergistic phenotype. SPMs initiate multiple SMs. SMs initiate multiple SMs in a reiterative fashion and can eventually produce FMs. All FMs initiate floral organs. Black and grey arrows represent meristem initiation. Blue arrows are meristem identity conversions.

View larger version (17K): [in a new window]   Fig. 9. Gene dosage model for SM and SPM identity. SPMs are wild type (green) with even one dose of ids1 or rgo1, and mutant (red) in the absence of any ids1 or rgo1. SMs are more sensitive to ids1 and rgo1 dosage, with a threshold somewhere between two and three doses.