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

Meristems may be determinate or indeterminate. In maize, the indeterminate inflorescence meristem produces three types of determinate meristems: spikelet pair, spikelet and floral meristems. These meristems are defined by their position and their products. We have discovered a gene in maize, indeterminate floral apex1 (ifa1) that regulates meristem determinacy. The defect found in ifa1 mutants is specific to meristems and does not affect lateral organs. In ifa1 mutants, the determinate meristems become less determinate. The spikelet pair meristem initiates more than a pair of spikelets and the spikelet meristem initiates more than the normal two flowers. The floral meristem initiates all organs correctly, but the ovule primordium, the terminal product of the floral meristem, enlarges and proliferates, expressing both meristem and ovule marker genes. A role for ifa1 in meristem identity in addition to meristem determinacy was revealed by double mutant analysis. In zea agamous1 (zag1) ifa1 double mutants, the female floral meristem converts to a branch meristem whereas the male floral meristem converts to a spikelet meristem. In indeterminate spikelet1 (ids1) ifa1 double mutants, female spikelet meristems convert to branch meristems and male spikelet meristems convert to spikelet pair meristems. The double mutant phenotypes suggest that the specification of meristems in the maize inflorescence involves distinct steps in an integrated process.     

Combinatorial control of meristem identity in maize inflorescences

The architecture of maize inflorescences, the male tassel and the female ear, is defined by a series of reiterative branching events. The inflorescence meristem initiates spikelet pair meristems. These in turn initiate spikelet meristems which finally produce the floret meristems. After initiating one meristem, the spikelet pair and spikelet meristem convert into spikelet and floret meristems, respectively. The phenotype of reversed germ orientation1 (rgo1) mutants is the production of an increased number of floret meristems by each spikelet meristem. The visible phenotypes include increased numbers of flowers in tassel and ear spikelets, disrupted rowing in the ear, fused kernels, and kernels with embryos facing the base of the ear, the opposite orientation observed in wild-type ears. rgo1 behaves as single recessive mutant. indeterminate spikelet1 (ids1) is an unlinked recessive mutant that has a similar phenotype to rgo1. Plants heterozygous for both rgo1 and ids1 exhibit nonallelic noncomplementation; these mutants fail to complement each other. Plants homozygous for both mutations have more severe phenotypes than either of the single mutants; the progression of meristem identities is retarded and sometimes even reversed. In addition, in rgo1; ids1 double mutants extra branching is observed in spikelet pair meristems, a meristem that is not affected by mutants of either gene individually. These data suggest a model for control of meristem identity and determinacy in which the progress through meristem identities is mediated by a dosage-sensitive pathway. This pathway is combinatorially controlled by at least two genes that have overlapping functions.     

Characterization of SaMADS D from Sinapis alba suggests a dual function of the gene: in inflorescence development and floral organogenesis

SaMADS D gene of Sinapis alba was isolated by screening a cDNA library from young inflorescences with a mixture of MADS-box genes of Antirrhinum majus (DEF, GLO, SQUA) as probe. Amino acid sequence comparison showed a high degree of similarity between the SaMADS D and AGL9, DEFH200, TM5, FBP2 and DEFH 72 gene products. Analysis of the SaMADS D gene expression by in situ hybridization reveals a novel expression pattern for a MADS-box gene and suggests a dual function for this gene: first, as a determinant in inflorescence meristem identity since it starts to be expressed directly beneath the inflorescence meristem at the time of initiation of the first floral meristem, is no longer expressed in the inflorescence meristem forced to revert to production of leafy appendages, and is expressed again when the reverted meristem resumes floral meristem initiation, and, second, as an interactor with genes specifying floral organ identity since it is expressed in the floral meristem from the stage of sepal protrusion.     

Early inflorescence and floral development in Zea mays land race Chapalote (Poaceae)

Tassel and ear primordia were collected from greenhouse-grown specimens of the Mexican maize landrace Chapalote and prepared for scanning electron microscopic (SEM) examination. Measurements of inflorescence apices and spikelet pair primordia (spp) were made from SEM micrographs. Correlation of inflorescence apex diameter with number of spikelet ranks showed no significant difference between tassels and ears, except at the two-rank level where the ear apical meristem had a significantly smaller diameter than corresponding two-ranked tassels. Within individual inflorescences, spp in different ranks enlarged at comparable rates, although the rates from one ear to the next along the stem differed. In both tassels and ears, spp divide to form paired sessile and pedicellate spikelet primordia when the spp is 150 μm wide; ear axes are significantly thicker than tassel axes at the time of bifurcation. The similarities in growth between ear and tassel primordia lend further support to the hypothesis that both the maize tassel and ear are derived from a common inflorescence pattern, a pattern shared with teosinte. Inflorescence primordial growth also suggests that a key character difference between teosinte and maize, distichous vs. polystichous arrangement of spikelets, may be related to size of the apical dome and/or rate of primordium production by the apical meristem. There appears to be more than a single morphological event in the shift from vegetative to reproductive growth. The evocation of axillary buds (ears) is independent of, and temporally separated from, the transition to flowering at the primary shoot apex (tassel).     

ABERRANT PANICLE ORGANIZATION 1 temporally regulates meristem identity in rice

We report a recessive mutation of rice, aberrant panicle organization 1 (apo1), which severely affects inflorescence architecture, floral organ identity, and leaf production rate. In the wild-type inflorescence, the main-axis meristem aborts after forming 10-12 primary branch primordia. However, in apo1, the main-axis meristem was converted to a spikelet meristem after producing a small number of branch primordia. In addition, the branch meristems in apo1 became spikelet meristems earlier than in wild type. Therefore, in the inflorescence, the apo1 mutation caused the precocious conversion of the meristem identity. In the apo1 flower, lodicules were increased at the expense of stamens, and carpels were formed indeterminately by the loss of meristem determinacy. Vegetative development is also affected in the apo1. Leaves were formed rapidly throughout the vegetative phase, indicating that APO1 is also involved in temporal regulation of leaf production. These phenotypes suggest that the APO1 plays an important role in the temporal regulation of both vegetative and reproductive development.     

Class II tassel seed mutations provide evidence for multiple types of inflorescence meristems in maize (Poaceae)

The tassel seed mutations ts4 and Ts6 of maize cause irregular branching in its inflorescences, tassels, and ears, in addition to feminization of the tassel due to the failure to abort pistils. A comparison of the development of mutant and wild-type tassels and ears using scanning electron microscopy reveals that at least four reproductive meristem types can be identified in maize: the inflorescence meristem, the spikelet pair meristem, the spikelet meristem, and the floret meristem. ts4 and Ts6 mutations affect the fate of specific reproductive meristems in both tassels and ears. ts4 mutants fail to form spikelet meristems from spikelet pair meristems. Ts6 mutants are delayed in the conversion of certain spikelet meristems into floret meristems. Once floret meristems are established in both of these mutants, they form florets that appear normal but fail to undergo pistil abortion in the tassel. The abnormal branching associated with each mutant is suppressed at the base of ears, permitting the formation of normal, fertile spikelets. The classification of the different types of reproductive meristems will be useful in interpretation of gene expression patterns in maize. It also provides a framework for understanding meristem functions that can be varied to diversify inflorescence architectures in the Gramineae.     

A putative lipase gene EXTRA GLUME1 regulates both empty-glume fate and spikelet development in rice

Recent studies have shown that molecular control of inner floral organ identity appears to be largely conserved between monocots and dicots, but little is known regarding the molecular mechanism underlying development of the monocot outer floral organ, a unique floral structure in grasses. In this study, we report the cloning of the rice EXTRA GLUME1 ( EG1 ) gene, a putative lipase gene that specifies empty-glume fate and floral meristem determinacy. In addition to affecting the identity and number of empty glumes, mutations in EG1 caused ectopic floral organs to be formed at each organ whorl or in extra ectopic whorls. Iterative glume-like structures or new floral organ primordia were formed in the presumptive region of the carpel, resulting in an indeterminate floral meristem. EG1 is expressed strongly in inflorescence primordia and weakly in developing floral primordia. We also found that the floral meristem and organ identity gene OsLHS1 showed altered expression with respect to both pattern and levels in the eg1 mutant, and is probably responsible for the pleiotropic floral defects in eg1 . As a putative class III lipase that functionally differs from any known plant lipase, EG1 reveals a novel pathway that regulates rice empty-glume fate and spikelet development.     

barren inflorescence2 regulates axillary meristem development in the maize inflorescence

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.     

Inflorescence development in the “standard exotic” maize,Argentine popcorn (Poaceae)

Argentine popcorn is an exotic race considered by some to be similar to the earliest cultivated maize. We used scanning electron microscopy to examine inflorescence development in both the tassel and ear. In our material, and under our conditions, both two-ranked central tassel spikes and two-ranked ears were observed as well as more typical four-ranked structures. Subsequent development of spikelets and florets was similar to that observed in other varieties of maize and in their close relatives—the teosintes. We suggest that the switch from two-ranked to four-ranked inflorescences (a key trait difference between teosinte and maize) may be due to a change in developmental timing allowing an additional meristem bifurcation of axillary branch primordia prior to the initiation of spikelet pair primordia.     

ABERRANT PANICLE ORGANIZATION 2/RFL, the rice ortholog of Arabidopsis LEAFY, suppresses the transition from inflorescence meristem to floral meristem through interaction with APO1

The temporal and spatial control of meristem identity is a key element in plant development. To better understand the molecular mechanisms that regulate inflorescence and flower architecture, we characterized the rice aberrant panicle organization 2 (apo2) mutant which exhibits small panicles with reduced number of primary branches due to the precocious formation of spikelet meristems. The apo2 mutants also display a shortened plastochron in the vegetative phase, late flowering, aberrant floral organ identities and loss of floral meristem determinacy. Map-based cloning revealed that APO2 is identical to previously reported RFL gene, the rice ortholog of the Arabidopsis LEAFY (LFY) gene. Further analysis indicated that APO2/RFL and APO1, the rice ortholog of Arabidopsis UNUSUAL FLORAL ORGANS, act cooperatively to control inflorescence and flower development. The present study revealed functional differences between APO2/RFL and LFY. In particular, APO2/RFL and LFY act oppositely on inflorescence development. Therefore, the genetic mechanisms for controlling inflorescence architecture have evolutionarily diverged between rice (monocots) and Arabidopsis (eudicots).     

Grass spikelets: a thorny problem

Isolation of a maize gene with strong homologies to the Arabidopsis floral organ identity gene APETALA2 has provided new insights to understanding the generation of architecture in grass inflorescences. Grass inflorescences are built of repeated units called spikelets, which consist of a pair of glumes (bracts) enclosing a cluster of one to as many as 40 flowers, the number depending on the species. The gene indeterminate spikelet (ids), isolated by Chuck and co-workers,⁽¹⁾ is involved in limiting the number of floral meristems produced by the spikelet meristem in maize inflorescences. Altered regulation of activity of ids homologues may be responsible for variation in flower number among different grasses. BioEssays 20 :789–793, 1998. © 1998 John Wiley & Sons, Inc.     

A floret by any other name: control of meristem identity in maize

The life of a plant unfolds as a series of developmental stages, with each stage defined by changes in meristem identity. In maize, there are several distinct stages: the transition from vegetative growth to flowering, the elaboration of the inflorescence, and the formation of flowers. Progress in understanding meristem identity and function has been made by analyzing maize mutants with defects at each of these stages. Recently cloned genes suggest that, although the molecular mechanisms controlling floral organ identity are conserved in maize and other model species, the control of meristem identity during earlier developmental stages might be less conserved.     

Investigation of morphogenesis of inflorescence and determination of the nature of inheritance of “supernumerary spikelets” trait of bread wheat (Triticum aestivum L.) mutant line

Using C-banding and FISH methods, the karyotype of MC1611 induced mutant of bread wheat, which develop additional spikelets at a rachis node (trait “supernumerary spikelets”) was characterized. It was determined that the mutant phenotype is not associated with aneuploidy and major chromosomal rearrangements. The results of genetic analysis showed that supernumerary spikelets of the line are caused by a mutation of the single Bh-D.1 gene, influenced by the genetic background. The mutation causes abnormalities of inflorescence morphogenesis associated with the development of ectopic spikelet meristems in place of floral meristems in the basal part of the spikelets, causing the appearance of additional spikes at a rachis node. The mutant phenotype suggests that the Bh-D gene determines the fate of the lateral meristems in ear, which develops as floral meristem and gives rise to floral organs in wild-type inflorescences. In the bh-D.1 mutant, the floral meristem identity is impaired. The characterized mutant can be used in further studies on molecular genetic basis of development of wheat inflorescence.     

Development of the mixed inflorescence in Zea diploperennis litis, Doebley & Guzman (Poaceae)

Development of the mixed inflorescence in Zea diploperennis Iltis, Doebley & Guzman (Poaceae) Mixed inflorescences of diploperennial teosinte, which terminate the main branches of the plant, arise in the same fashion as tassel spikes. The apical meristem produces bracts in a decussate arrangement. A single axillary bud primordium is initiated in the axil of each bract. Growth of the bract is retarded as the bud enlarges and divides longitudinally into two separate spikelet primordia. The paired spikelets running in two ranks on either side of the inflorescence primordium produce the four-rowed condition typical of teosinte tasselS. In the transition region between male and female portions of the inflorescence, development of the pedicellate spikelet of each spikelet pair is arrested at an early ontogenetic stage. Continued growth of the sessile spikelet and associated rachis flaps destroy the remnants of the arrested spikelet in basal portions of the inflorescence. A similar abortion of the lower floret of the sessile spikelet results in a single pistillate floret per node at anthesis. These results provide further support for the hypothesis that a tassel-like mixed inflorescence of teosinte is ancestral to the maize ear.     

BRANCHED SILKLESS mediates the transition from spikelet to floral meristem during Zea mays ear development

The molecular and genetic control of inflorescence and flower development has been studied in great detail in model dicotyledonous plants such as Arabidopsis and Antirrhinum . In contrast, little is known about these important developmental steps in monocotyledonous species. Here we report the analysis of the Zea mays mutant branched silkless1–2 (bd1–2) , allelic to bd1 , which we have used as a tool to study the transition from spikelet to floret development in maize. Floret development is blocked in the female inflorescence (the ear) of bd1–2 plants, whereas florets develop almost normally in the male inflorescence (the tassel). Detailed phenotypic analyses indicate that in bd1–2 mutants ear inflorescence formation initiates normally, however, the spikelet meristems do not proceed to form floret meristems. The ear spikelets, at anthesis, contain various numbers of spikelet-like meristems and glume-like structures. Furthermore, growth of branches from the base of the ear is often observed. Expression analyses show that the floral-specific MADS box genes Zea mays AGAMOUS1 ( ZAG1 ), ZAG2 and Zea mays MADS 2 ( ZMM2 ) are not expressed in ear florets in bd1–2 mutants, whereas their expression in tassel florets is similar to that of wild type. Taken together, these data indicate that the development from spikelet to floret meristem is differentially controlled in the ear and tassel in the monoecious grass species Zea mays , and that BRANCHED SILKLESS plays an important role in regulating the transition from spikelet meristem to floral meristem during the development of the female inflorescence of maize.     

Inflorescence development in a high-altitude annual Mexican teosinte (Poaceae)

Some have postulated that highland Mexican maize was derived from an ancient high-altitude teosinte and that later introgression between the two taxa occurred. We used scanning electron microscopy to examine the inflorescence development in both the tassel and ear of a high-altitude Toluca teosinte. One of the most interesting observations was the presence of atypical multiranked orthostiches in the central spike of some male Toluca teosinte inflorescences. Most tassels exhibited a central spike with a pure, four-ranked, tetrastichous phyllotaxy or an intermediate (distichous/tetrastichous) phyllotaxy. A few A(1) tassels had a more typical distichous (two-ranked) central spike. Most ears showed the two-rank condition expected for teosintes. However, three ears displayed an intermediate (distichous/tristichous or distichous/ tetrastichous) phyllotaxy and one ear was tetrastichous. Our analysis of spikelet and floret development in all Toluca inflorescences revealed a pattern similar to that in landrace and U.S. maize, as well as to their close relatives, the teosintes. We suggest that this investigation may reveal inflorescence development in a natural maize-teosinte hybrid. This study further supports our hypothesis that both maleness and femaleness in the Zea inflorescences are derived from a common developmental pathway and underpins a proposal that andropogonoid grasses share a common pattern of inflorescence development.     

DEVELOPMENT OF TASSEL SEED 2 INFLORESCENCES IN MAIZE

The normal pattern of maize floral development of staminate florets on the terminal inflorescence (tassel) and pistillate florets on the lateral inflorescences (ears) is disrupted by the recessive mutation tassel seed 2. Tassel seed 2 mutant plants develop pistillate florets instead of staminate florets in the tassel. In addition, the ears of tassel seed 2 plants display irregular rowing of kernels due to the development of the normally suppressed lower floret of each spikelet. The morphology of tassel and ear florets of the recessive maize mutant tassel seed 2 has been compared to those of wild-type maize through development. We have identified the earliest stages at which morphological signs of sex differentiation are evident. We find that sex determination occurs during the same stage on tassel and ear development. Early postsex determination morphology of florets in wild-type ears and in tassel seed 2 tassels and ears is identical.