Indeterminate Meristem Research Articles

Initiation of a legume nodule with an indeterminate meristem involves proliferating host cells that harbour infection threads.

A comparative analysis of nodule morphogenesis was carried out for three symbiotically defective pea (Pisum sativum) mutants that show abnormalities in nodule development.In the wild-type lines, resumption of cell proliferation in the pericycle and inner cortex results in the development of a nodule primordium, within which are found proliferating cells that harbour infection threads. However, this class of cell is not observed in the mutants RisFixA (sym41) and SGEFix–-2 (sym33) where nodule development is arrested at the point of formation of the apical nodule meristem. Itis proposed that the presence of proliferating cells harbouring infection threads is a prerequisite for normal formation of the nodule meristem.In mutant SGEFix–-1 (sym40), nodule development does not differ from that of wild-type plants in the early stages but is blocked at the stage after nodule meristem persistence. A scheme is proposed for the sequential functioning of pea symbiotic genes Sym33, Sym40 and Sym41 in the programme of nodule development.

Flowering-time genes modulate meristem determinacy and growth form in Arabidopsis thaliana

Plants have evolved annual and perennial life forms as alternative strategies to adapt reproduction and survival to environmental constraints. In isolated situations, such as islands, woody perennials have evolved repeatedly from annual ancestors. Although the molecular basis of the rapid evolution of insular woodiness is unknown, the molecular difference between perennials and annuals might be rather small, and a change between these life strategies might not require major genetic innovations. Developmental regulators can strongly affect evolutionary variation and genes involved in meristem transitions are good candidates for a switch in growth habit. We found that the MADS box proteins SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) not only control flowering time, but also affect determinacy of all meristems. In addition, downregulation of both proteins established phenotypes common to the lifestyle of perennial plants, suggesting their involvement in the prevention of secondary growth and longevity in annual life forms.

Class 1 KNOX genes are not involved in shoot development in the moss Physcomitrella patens but do function in sporophyte development

Although the number and form of metazoan organs are determined in the embryo, plants continuously form organs via pluripotent stem cells contained within the meristem. Flowering plants have an indeterminate meristem in their diploid generation, whereas the common ancestor of land plants is inferred to have formed an indeterminate meristem in its haploid generation, as observed in the extant basal land plants, bryophytes, including mosses. It is hypothesized that the underlying gene networks for the diploid meristem were initially present in the haploid generation of the basal land plants and were eventually co-opted for expression in the diploid generation. In flowering plants, the class 1 KNOTTED1-LIKE HOMEOBOX (KNOX) transcription factors are essential for the function of the indeterminate apical meristem. Here, we show that the class 1 KNOX orthologs function in the diploid organ, with determinate growth in the moss Physcomitrella patens, but do not function in the haploid indeterminate meristem. We propose that the genetic networks governing the indeterminate meristem in land plants are variable, and the networks governing the diploid indeterminate meristem with the class 1 KNOX genes likely evolved de novo in the flowering plant lineage.

Regulation of SHOOT MERISTEMLESS genes via an upstream-conserved noncoding sequence coordinates leaf development

The indeterminate shoot apical meristem of plants is characterized by the expression of the Class 1 KNOTTED1-LIKE HOMEOBOX (KNOX1) genes. KNOX1 genes have been implicated in the acquisition and/or maintenance of meristematic fate. One of the earliest indicators of a switch in fate from indeterminate meristem to determinate leaf primordium is the down-regulation of KNOX1 genes orthologous to SHOOT MERISTEMLESS (STM) in Arabidopsis (hereafter called STM genes) in the initiating primordia. In simple leafed plants, this down-regulation persists during leaf formation. In compound leafed plants, however, KNOX1 gene expression is reestablished later in the developing primordia, creating an indeterminate environment for leaflet formation. Despite this knowledge, most aspects of how STM gene expression is regulated remain largely unknown. Here, we identify two evolutionarily conserved noncoding sequences within the 5' upstream region of STM genes in both simple and compound leafed species across monocots and dicots. We show that one of these elements is involved in the regulation of the persistent repression and/or the reestablishment of STM expression in the developing leaves but is not involved in the initial down-regulation in the initiating primordia. We also show evidence that this regulation is developmentally significant for leaf formation in the pathway involving ASYMMETRIC LEAVES1/2 (AS1/2) gene expression; these genes are known to function in leaf development. Together, these findings reveal a regulatory point of leaf development mediated through a conserved, noncoding sequence in STM genes.

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.

Floral meristem indeterminacy depends on flower position and is facilitated by acarpellate gynoecium development in Impatiens balsamina

* Floral meristems are generally determinate. Termination of their activity varies with species, occurring after carpel or ovule development, depending on the placentation type. In terminal flowering Impatiens balsamina (cv. Dwarf Bush Flowered) some flowers exhibit meristem indeterminacy; they produce organs from the placenta after ovule development. * Here we provide a detailed description of gynoecium development in this line and explore the basis of the indeterminate nature of some of its floral meristems. * We find that the placenta is sometimes established without complete carpel fusion. Proliferative growth derives from meristematic remnants of the placenta and is more common in the terminal inflorescence. RNA in situ hybridization reveals that IbLFY (Impatiens LFY homologue) is expressed in all meristem states, even in proliferating meristems. Expression of IbAG in axillary flowers is as expected in the meristem, stamens and carpels but absent from the proliferating meristem. * We conclude that I. balsamina has cauline placentation. Incomplete suppression of inflorescence identity in flowers of the terminal inflorescence leads to floral meristem proliferation after ovule development in this species.

L1 Division and Differentiation Patterns Influence Shoot Apical Meristem Maintenance

Plant development requires regulation of both cell division and differentiation. The class 1 KNOTTED1-like homeobox (KNOX) genes such as knotted1 (kn1) in maize (Zea mays) and SHOOTMERISTEMLESS in Arabidopsis (Arabidopsis thaliana) play a role in maintaining shoot apical meristem indeterminacy, and their misexpression is sufficient to induce cell division and meristem formation. KNOX overexpression experiments have shown that these genes interact with the cytokinin, auxin, and gibberellin pathways. The L1 layer has been shown to be necessary for the maintenance of indeterminacy in the underlying meristem layers. This work explores the possibility that the L1 affects meristem function by disrupting hormone transport pathways. The semidominant Extra cell layers1 (Xcl1) mutation in maize leads to the production of multiple epidermal layers by overproduction of a normal gene product. Meristem size is reduced in mutant plants and more cells are incorporated into the incipient leaf primordium. Thus, Xcl1 may provide a link between L1 division patterns, hormonal pathways, and meristem maintenance. We used double mutants between Xcl1 and dominant KNOX mutants and showed that Xcl1 suppresses the Kn1 phenotype but has a synergistic interaction with gnarley1 and rough sheath1, possibly correlated with changes in gibberellin and auxin signaling. In addition, double mutants between Xcl1 and crinkly4 had defects in shoot meristem maintenance. Thus, proper L1 development is essential for meristem function, and XCL1 may act to coordinate hormonal effects with KNOX gene function at the shoot apex.

OsMADS22, an STMADS11-like MADS-box gene of rice, is expressed in non-vegetative tissues and its ectopic expression induces spikelet meristem indeterminacy

We report the cDNA sequence and gene expression patterns of OsMADS22, a novel member of the STMADS11-like family of MADS-box genes, from rice. In contrast to previously reported STMADS11-like genes, whose expression is detected in vegetative tissues, OsMADS22 is mainly expressed during embryogenesis and flower development. In situ hybridization analysis revealed that OsMADS22 expression is localized in the L1 layer of embryos and in developing stamen primordia. Ectopic expression of OsMADS22 in transgenic rice plants resulted in aberrant floral morphogenesis, characterized by a disorganized palea, an elongated glume, and a two-floret spikelet. The results are discussed in terms of rice spikelet development and a novel non-vegetative role for a STMADS11-like gene.

Diverse Roles of STMADS11-Like Gene in Plant Development; RMD1 is not expressed in Vegetative Tissues and Its Ectopic Expression Induces Meristem Indeterminacy in Rice Spikelets

Diverse Roles of STMADS11-Like Gene in Plant Development; RMD1 is not expressed in Vegetative Tissues and Its Ectopic Expression Induces Meristem Indeterminacy in Rice Spikelets

Developmental Dynamics of Forage Chicory

Most studies on forage chicory (Cichorium intybus L.) are empirical, and very little information exists regarding the growth of chicory as it relates to development, productivity, persistence, and stand age. Development of uncut ‘Grasslands Puna’ chicory was studied in southern West Virginia on previously established plots at the Scott Farm in 1997 and 1998 and on 1‐yr‐old plantings at the Plumley Farm in 1998 and 1999. Chicory developmental parameters were modeled as a function of growing degree days (GDD) from Calendar Day 90 using a base of 5°C. On average, only 58% of the chicory plants became reproductive in any single growing season or location after the establishment year. Mean time to initiation of bolting and first open flower occurred consistently at approximately 400 and 1030 GDD, respectively, across locations and years. The indeterminate meristem of vegetative plants continuously produced leaves of relatively uniform mature area throughout the growing season. In comparison, the determinate meristem of reproductive plants rapidly expressed progressively smaller leaves on the main stem from initiation of bolting only until the time of first open flower. The developmental data herein create a framework with which to begin optimizing strategies for pasture and livestock grazing management of forage chicory.

Separation of AG function in floral meristem determinacy from that in reproductive organ identity by expressing antisense AG RNA

The Arabidopsis floral homeotic gene AGAMOUS (AG) is a regulator of early flower development. The ag mutant phenotypes suggest that AG has two functions in flower development: (1) specifying the identity of stamens and carpels, and (2) controlling floral meristem determinacy. To dissect these two AG functions, we have generated transgenic Arabidopsis plants carrying an antisense AG construct. We found that all of the transgenic plants produced abnormal flowers, which can be classified into three types. Type I transgenic flowers are phenocopies of the ag-1 mutant flowers, with both floral meristem indeterminacy and floral organ conversion; type II flowers are indeterminate and have partial conversion of the reproductive organs; and type III flowers have normal stamens and carpels, but still have an indeterminate floral meristem inside the fourth whorl of fused carpels. The existence of type III flowers indicates that AG function can be perturbed to affect only floral meristem determinacy, but not floral organ identity. Furthermore, the fact that floral meristem determinacy is affected in all transformants, but floral organ identity only in a subset of them, suggests that the former may required a higher level of AG activity than the latter. This hypothesis is supported by the levels of AG mRNA detected in different transformants with different frequencies of distinct types of abnormal antisense AG transgenic flowers. Finally, since AG inhibits the expression of another floral regulatory gene AP1, we examined AP1 expression in antisense AG flowers, and found that AP1 is expressed at a relatively high level in the center of type II flowers, but very little or below detectable levels in the inner whorls of type III flowers. These results provide further insights into the interaction of AG and AP1 and how such an interaction may control both organ identity and floral meristem determinacy.