Flowering and determinacy in maize

All plant organs are produced by meristems, groups of stem cells located in the tips of roots and shoots. Indeterminate meristems make an indefinite number of organs, whereas determinate meristems are consumed after making a specific number of organs. Maize is an ideal system to study the genetic control of meristem fate because of the contribution from determinate and indeterminate meristems to the overall inflorescence. Here, the latest work on meristem maintenance and organ specification in maize is reviewed. Genetic networks, such as the CLAVATA components of meristem maintenance and the ABC programme of flower development, are conserved between grasses and eudicots. Maize and rice appear to have conserved mechanisms of meristem maintenance and organ identity. Other pathways, such as sex determination, are likely to be found only in maize with its separate male and female flowers. A rich genetic history has resulted in a large collection of maize mutants. The advent of genomic tools and synteny across the grasses now permits the isolation of the genes behind inflorescence architecture and the ability to compare function across the Angiosperms.     

Competence of Immature Maize Embryos for Agrobacterium-Mediated Gene Transfer

Agrobacterium-mediated transfer of viral sequences to plant cells (agroinfection) was applied to study the susceptibility of immature maize embryos to the pathogen. The shoot apical meristem of immature embryos 10 to 20 days after pollination from four different maize genotypes was investigated for competence for agroinfection. There was a direct correlation between different morphological stages of the unwounded immature embryos and their competence for agroinfection. Agroinfection frequency was highest in the embryogenic line A188. All developmental stages tested showed Agrobacterium virulence gene-inducing activity, whereas bacteriocidal substances were produced at stages of the immature embryos competent for agroinfection. The results suggested that Agrobacterium may require differentiated tissue in the maize shoot apical meristem before wounding for successful T-DNA transfer. This requirement for the young maize embryo has implications for the possible use of Agrobacterium for maize transformation.     

Regulation of extent of vegetative development of the maize shoot meristem

In maize plants ( Zea mays L.), the extent of vegetative development in the shoot is precisely regulated such that the apical meristem produces a predictable number of leaves before converting to tassel development. In previous experiments using shoot apex culture, we showed that the developmental program that limits vegetative development in maize is not intrinsic to the shoot apical meristem. Rather, the meristem receives information from elsewhere in the plant and responds by either continuing leaf initiation or becoming determined for determinate growth and forming an inflorescence, the tassel. Here we examine leaf primordia as potential sources for that information using shoot apex culture. Our results show that the presence of the four to six youngest leaf primordia on the shoot apex is sufficient to provide such information. The ability to reset shoot development by meristem culture also allows us to examine the basis for expression of a specific phenotype at a particular developmental stage. We found that the mutation hcf106 , which is typically expressed only during seedling stages, is not re-expressed when the shoot morphogically has regained a juvenile phase.     

Developmental anatomy of the reproductive shoot in <Emphasis Type="Italic">Hydrobryum japonicum</Emphasis> (Podostemaceae)

Podostemaceae are unusual aquatic angiosperms adapting to extreme habitats, i.e., rapids and waterfalls, and have unique morphologies. We investigated the developmental anatomy of reproductive shoots scattered on crustose roots of Hydrobryum japonicum by scanning electron microscopy and using semi-thin serial sections. Two developmental patterns were observed: bracts arise either continuously from an area of meristematic cells that has produced leaves, or within differentiated root ground tissue beneath, and internal to, leaf base scars after an interruption. In both patterns, the bract primordia arise endogenously at the base of youngest bracts in the absence of shoot apical meristem, involving vacuolated-cell detachment to each bract separately. The different transition patterns of reproductive shoot development may be caused by different stages of parental vegetative shoots. The floral meristem arises between the two youngest bracts, and is similarly accompanied by cell degeneration. In contrast, the floral organs, including the spathella, arise exogenously from the meristem. Bract development, like vegetative leaf development, is unique to this podostemad, while floral-organ development is conserved.     

Regulation of cell proliferation patterns by homeotic genes during Arabidopsis floral development

The shoot apical meristem of Arabidopsis thaliana consists of three cell layers that proliferate to give rise to the aerial organs of the plant. By labeling cells in each layer using an Ac-based transposable element system, we mapped their contributions to the floral organs, as well as determined the degree of plasticity in this developmental process. We found that each cell layer proliferates to give rise to predictable derivatives: the L1 contributes to the epidermis, the stigma, part of the transmitting tract and the integument of the ovules, while the L2 and L3 contribute, to different degrees, to the mesophyll and other internal tissues. In order to test the roles of the floral homeotic genes in regulating these patterns of cell proliferation, we carried out similar clonal analyses in apetala3-3 and agamous-1 mutant plants. Our results suggest that cell division patterns are regulated differently at different stages of floral development. In early floral stages, the pattern of cell divisions is dependent on position in the floral meristem, and not on future organ identity. Later, during organogenesis, the layer contributions to the organs are controlled by the homeotic genes. We also show that AGAMOUS is required to maintain the layered structure of the meristem prior to organ initiation, as well as having a non-autonomous role in the regulation of the layer contributions to the petals.     

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.     

Genetic Interactions That Regulate Inflorescence Development in Arabidopsis

In Arabidopsis, floral meristems arise in continuous succession directly on the flanks of the inflorescence meristem. Thus, the pathways that regulate inflorescence and floral meristem identity must operate both simultaneously and in close spatial proximity. The TERMINAL FLOWER 1 (TFL1) gene of Arabidopsis is required for normal inflorescence meristem function, and the LEAFY (LFY), APETALA 1 (AP1), and APETALA 2 (AP2) genes are required for normal floral meristem function. We present evidence that inflorescence meristem identity is promoted by TFL1 and that floral meristem identity is promoted by parallel developmental pathways, one defined by LFY and the other defined by AP1/AP2. Our analysis suggests that the acquisition of meristem identity during inflorescence development is mediated by antagonistic interactions between TFL1 and LFY and between TFL1 and AP1/AP2. Based on this study, we propose a simple model for the genetic regulation of inflorescence development in Arabidopsis. This model is discussed in relation to the proposed interactions between the inflorescence and the floral meristem identity genes and in regard to other genes that are likely to be part of the genetic hierarchy regulating the establishment and maintenance of inflorescence and floral meristems.     

Cell differentiation in the longitudinal veins and formation of commissural veins in rice (Oryza sativa) and maize (Zea mays)

Vascular development is a central theme in plant science. However, little is known about the mechanism of vascular development in monocotyledons (compared with dicotyledons). Therefore, we investigated sequential processes of differentiation into various different vascular cells by carrying out detailed observations using serial sections of the bases of developing leaves of rice and maize. The developmental process of the longitudinal vascular bundles was divided into six stages in rice and five stages in maize. The initiation of differentiation into procambial progenitor cells forming the commissural vein arose in a circular layer cell that was adjacent to both a metaxylem vessel and one or a few phloem cells in stage V longitudinal vascular bundles. In most cases the differentiation of ground meristem cells into procambial progenitor cells extended in one direction, toward the next longitudinal vascular bundle, and subsequent periclinal divisions and further differentiation produced a vessel element, two companion cells and a sieve element to form a commissural vein. These results suggest the presence of an intercellular signal(s) that induces differentiation of the circular layer cell and the ground meristem cells into procambial progenitor cells, forming a commissural vein sequentially.     

An exploration of LEAFY expression in independent evolutionary origins of rosette flowering in Brassicaceae

Whereas most Brassicaceae produce flowers on an elongated inflorescence, a few lineages produce flowers directly from the vegetative rosette on elongated pedicels. Knowing the extent to which independent origins of rosette flowering involve the same developmental and genetic mechanisms could clarify the constraints acting on plant architectural evolution. Prior work in Idahoa, Ionopsidium, and Leavenworthia suggested that changes in the activity or expression of the flower meristem identity gene, LEAFY (LFY), played a role in all three origins of rosette flowering. Here we studied the developmental morphology of L. crassa and immunolocalization of LFY protein in Leavenworthia and Ionopsidium to further compare independent origins of rosette flowering. Leavenworthia crassa differs from Ionopsidium and Idahoa in producing ebracteate flowers. Flowers are, however, associated with "squamules," here interpreted as stipules of a cryptic bract. LFY was detected in L. crassa flower primordia but not in inflorescence meristems. In contrast, the rosette flowering Io. acaule accumulated LFY protein in the inflorescence meristem, whereas its inflorescence-flowering close relative, Io. prolongoi, did not. Thus, although different cases of rosette flowering likely entailed modifications of the same meristem identity program, distinct developmental genetic mechanisms appear to be involved in each case.     

Genetic control of branching patterns in grass inflorescences

Inflorescence branching in the grasses controls the number of florets and hence the number of seeds. Recent data on the underlying genetics come primarily from rice and maize, although new data are accumulating in other systems as well. This review focuses on a window in developmental time from the production of primary branches by the inflorescence meristem through to the production of glumes, which indicate the transition to producing a spikelet. Several major developmental regulatory modules appear to be conserved among most or all grasses. Placement and development of primary branches are controlled by conserved auxin regulatory genes. Subtending bracts are repressed by a network including TASSELSHEATH4, and axillary branch meristems are regulated largely by signaling centers that are adjacent to but not within the meristems themselves. Gradients of SQUAMOSA-PROMOTER BINDING-like and APETALA2-like proteins and their microRNA regulators extend along the inflorescence axis and the branches, governing the transition from production of branches to production of spikelets. The relative speed of this transition determines the extent of secondary and higher order branching. This inflorescence regulatory network is modified within individual species, particularly as regards formation of secondary branches. Differences between species are caused both by modifications of gene expression and regulators and by presence or absence of critical genes. The unified networks described here may provide tools for investigating orphan crops and grasses other than the well-studied maize and rice.

Recent work on grass inflorescence branching identifies extensive conserved regulation, but also divergence particularly in formation of secondary branches and spikelets.     

Cell lineage patterns and homeotic gene activity during Antirrhinum flower development

Background: Homeotic genes controlling the identity of flower organs have been characterized in several plant species. To determine whether cells expressing these genes are specified to follow particular developmental fates, we have studied the pattern of cell lineages in developing flowers of Antirrhinum. Each flower has four whorls of organs, and progenitor cells of these can be marked at particular stages of development using a temperature-sensitive transposon. This allows the cell lineages in the flower to be followed, as well as giving information about rates of cell division.Results We show here that, prior to the emergence of organ primordia, cells in the floral meristem have not been allocated organ identities. After this time, lineage restrictions arise between whorls, correlating with the onset of expression of genes that control organ identity. A further lineage restriction appears slightly later on, between the dorsal and ventral surfaces of the petal. Our results further suggest that the rates of cell division fluctuate during key stages of meristem development, perhaps as a consequence of meristem-identity gene expression.Conclusion The patterns of lineage restriction and organ-identity gene expression in early floral meristems are consistent with some cells being allocated specific identities at about this stage of development. Plant cells cannot move relative to each other, so lineage restrictions in plants may reflect particular orientations and/or rates of growth at boundary regions.     

Genome-Wide Identification and Analysis of Expression Profiles of Maize Mitogen-Activated Protein Kinase Kinase Kinase

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction model in animals, yeast and plants. Plant MAPK cascades have been implicated in development and stress responses. Although MAPKKKs have been investigated in several plant species including Arabidopsis and rice, no systematic analysis has been conducted in maize. In this study, we performed a bioinformatics analysis of the entire maize genome and identified 74 MAPKKK genes. Phylogenetic analyses of MAPKKKs from maize, rice and Arabidopsis have classified them into three subgroups, which included Raf, ZIK and MEKK. Evolutionary relationships within subfamilies were also supported by exon-intron organizations and the conserved protein motifs. Further expression analysis of the MAPKKKs in microarray databases revealed that MAPKKKs were involved in important signaling pathways in maize different organs and developmental stages. Our genomics analysis of maize MAPKKK genes provides important information for evolutionary and functional characterization of this family in maize.