The pattern of histone H4 expression in the tomato shoot apex changes during development

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon esculentum Mill.) shoot-tip cDNA library using a heterologous probe from barley (Hordeum vulgare L.). Both cDNAs, which are 81% identical in the coding region, are polyadenylated and belong to a small gene family in the tomato genome. Histone H4 message is abundant in young tissues and rare in older tissues. In the shoot apical meristem, the distribution of H4-expressing cells changes during development. In a juvenile vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical meristem, H4-expressing cells are localized in the peripheral zone extending into the provascular strands and the rib meristem whereas the central zone is almost devoid of H4 mRNA. After floral transition, H4 mRNA is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-expressing cells were found to occur frequently in clusters, which may indicate a partial synchronization of cell divisions in the shoot apex.     

Mechanisms of bamboo witches' broom symptom development caused by endophytic/epiphytic fungi

Aciculosporium take causes continuous shoot growth but maintains normal leaf-arrangement and branching patterns in the host plant, which eventually resulting in witches'' broom disease of bamboo. An in situ hybridization technique with a species-specific oligonucleotide probe was recently used to demonstrate that endophytic mycelia of A. take is predominantly distributed in the intercellular spaces of the shoot apical meristem of the host. Endophytic hyphae in meristematic tissues, which may produce auxin, are responsible for continuous primordium initiation within the shoot apex. Here I examine another bamboo witches'' broom causal fungus, Heteroepichloë sasae. Both species are biotrophic and belong to family Clavicipitaceae: however, H. sasae does not cause continuous shoot growth. Histological study showed that H. sasae mycelia were distributed superficially, even on shoot apical meristems. These observations suggest that when their stromata develop, endophytic A. take destroys shoot apical meristem and epiphytic H. sasae chokes the shoot apex of the host. Stromata formation consequently causes lateral bud out-growth because of release from apical dominance. This process repeats and eventually results in the witches'' broom symptoms.Key words: apical dominance, endophyte, epiphyte, histology, shoot development, witches'' broom symptom     

Axillary Meristem Formation in Rice Requires the WUSCHEL Ortholog TILLERS ABSENT1

Axillary shoot formation is a key determinant of plant architecture. Formation of the axillary shoot is regulated by initiation of the axillary meristem or outgrowth of the axillary bud. Here, we show that rice (Oryza sativa) TILLERS ABSENT1 (TAB1; also known as Os WUS), an ortholog of Arabidopsis thaliana WUS, is required to initiate axillary meristem development. We found that formation of the axillary meristem in rice proceeds via a transient state, which we term the premeristem, characterized by the expression of OSH1, a marker of indeterminate cells in the shoot apical meristem. In the tab1-1 (wus-1) mutant, however, formation of the axillary meristem is arrested at various stages of the premeristem zone, and OSH1 expression is highly reduced. TAB1/WUS is expressed in the premeristem zone, where it shows a partially overlapping pattern with OSH1. It is likely, therefore, that TAB1 plays an important role in maintaining the premeristem zone and in promoting the formation of the axillary meristem by promoting OSH1 expression. Temporal expression patterns of WUSCHEL-RELATED HOMEOBOX4 (WOX4) indicate that WOX4 is likely to regulate meristem maintenance instead of TAB1 after establishment of the axillary meristem. Lastly, we show that the prophyll, the first leaf in the secondary axis, is formed from the premeristem zone and not from the axillary meristem.     

CnFL,a FLORICAULA/LEAFY homolog in Chrysanthemum nankingense is dramatically upregulated in induced shoot apical meristems

A FLORICOULA/LEAFY (FLO/LFY) homolog CnFL gene was isolated from the flower bud of a short-day Chrysanthemum nankingense plant during the flowering induction period. The sequence of CnFL contained a 1236 bp open reading frame that encoded a putative protein of 412 amino acids, which shared 68.67% homology with FLO and 60.23% homology with LFY. The spatial expression patterns of CnFL were analyzed using quantitative real-time PCR in different tissues and the apical meristem during short-day flowering induction. The results indicated that CnFL was highly expressed in the flower buds while its expression was also detected in the stems, young leaves, and vegetative apical meristem. During the period of flowering induction, CnFL expression increased remarkably and reached its highest levels after 15 days of induction. The expression of CnFL in the apical shoot after short-day flowering induction indicates that CnFL regulation is controlled primarily by photoperiodicity.     

Expression of two HOOKLESS genes in peas (Pisum sativum L.)

The apical hook of dark-grown dicotyledonous plants results from asymmetric growth of its inner and outer sides. It is a protective structure that prevents damage to the shoot apical meristem and the young leaves as the seedling pushes through the soil. Two phytohormones, ethylene and auxin, are thought to be involved in regulating apical hook formation. HOOKLESS1 (HLS1) of Arabidopsis was recognized as an ethylene-response gene whose product is required for hook formation. We cloned two cDNAs from peas, Ps-HLS1 and Ps-HLS2, whose products are functional homologs of HLS1. Both Ps-HLS1 and Ps-HLS2 complement the hls1 mutation in Arabidopsis. Expression of Ps-HLS1 is enhanced by ethylene and by IAA. Because the effect of ethylene is counteracted by 2,5-norbornadiene, an inhibitor of ethylene action, it appears that the primary factor in apical hook formation in peas is ethylene.     

A complex case of simple leaves: indeterminate leaves co-express ARP and KNOX1 genes

The mutually exclusive relationship between ARP and KNOX1 genes in the shoot apical meristem and leaf primordia in simple leaved plants such as Arabidopsis has been well characterized. Overlapping expression domains of these genes in leaf primordia have been described for many compound leaved plants such as Solanum lycopersicum and Cardamine hirsuta and are regarded as a characteristic of compound leaved plants. Here, we present several datasets illustrating the co-expression of ARP and KNOX1 genes in the shoot apical meristem, leaf primordia, and developing leaves in plants with simple leaves and simple primordia. Streptocarpus plants produce unequal cotyledons due to the continued activity of a basal meristem and produce foliar leaves termed “phyllomorphs” from the groove meristem in the acaulescent species Streptocarpus rexii and leaves from a shoot apical meristem in the caulescent Streptocarpus glandulosissimus. We demonstrate that the simple leaves in both species possess a greatly extended basal meristematic activity that persists over most of the leaf’s growth. The area of basal meristem activity coincides with the co-expression domain of ARP and KNOX1 genes. We suggest that the co-expression of ARP and KNOX1 genes is not exclusive to compound leaved plants but is associated with foci of meristematic activity in leaves.     

Gene NANA regulates cell proliferation in Arabidopsis thaliana shoot apical meristem without interaction with CLV1, CLV2, CLV3

A constancy of stem cell pool in shoot apical meristem of Arabidopsis thaliana is provided by a genetic regulation system with negative feedback loop based on the interaction of the gene WUS, which maintains indeterminate state of cells, with CLV genes, which restrict the level of WUS expression and stem cell pool size. clv mutations lead to an increase in the pool of stem cells in the apical and floral meristems and wus mutation leads to the opposite effect. Mutation na (nana), like wus mutation, causes premature termination of shoot apical meristem function, although it does not affect the activity of the flower meristem. To elucidate the role of NA in the control of shoot apical meristem functioning, the interaction of NA with CLV genes were investigated. Additive phenotype of double mutants na clv1, na clv2-1, and na clv3-2 indicates that the NA gene makes an independent contribution to the functioning of the shoot apical meristem. It is assumed that the NA gene controls apical meristem cell proliferation during the transition to the reproductive phase of plant development, acting much later and independently of the genes WUS-CLV.     

Two-Step Regulation of LAX PANICLE1 Protein Accumulation in Axillary Meristem Formation in Rice

Axillary meristem (AM) formation is an important determinant of plant architecture. In rice (Oryza sativa), LAX PANICLE1 (LAX1) function is required for the generation of AM throughout the plant''s lifespan. Here, we show a close relationship between AM initiation and leaf development; specifically, the plastochron 4 (P4) stage of leaf development is crucial for the proliferation of meristematic cells. Coincident with this, LAX1 expression starts in the axils of leaves at P4 stage. LAX1 mRNA accumulates in two to three layers of cells in the boundary region between the initiating AM and the shoot apical meristem. In lax1 mutants, the proliferation of meristematic cells is initiated but fails to progress into the formation of AM. The difference in sites of LAX1 mRNA expression and its action suggests non-cell-autonomous characteristics of LAX1 function. We found that LAX1 protein is trafficked to AM in a stage- and direction-specific manner. Furthermore, we present evidence that LAX1 protein movement is required for the full function of LAX1. Thus, we propose that LAX1 protein accumulates transiently in the initiating AM at P4 stage by a strict regulation of mRNA expression and a subsequent control of protein trafficking. This two-step regulation is crucial to the establishment of the new AM.     

The shoot apex of Podostemaceae: De novo structure or reduction of the conventional type?

The paper reviews and discusses various interpretations of the shoot apex of Podostemaceae with special reference to subfamily Podostemoideae. Main questions concern (1) the proposed absence of a shoot apical meristem (SAM) in apical “meristemless” shoot tips of Podostemoideae and, as the consequence, the endogenous inception of leaf-borne leaves and branches and (2) the predicted stem bifurcation below a “terminal” dithecous (double-sheathed) leaf positioned instead of a shoot apex, as it is reported for subfamily Podostemoideae. Does the “meristemless” shoot apex represent a true evolutionary novelty? Does the view of stem bifurcation represent a new ramification pattern with the consequence that the “classical root–shoot model” of angiosperms is not valid for Podostemaceae? Both interpretations do not conform to previous studies that are complemented here by new data on the SAM of Zeylanidium olivaceum and Thelethylax minutiflora (Podostemoideae). Although a SAM is difficult to observe in the vegetative shoots of many Podostemoideae, it becomes well visible when the shoot passes into the flowering stage approaching the conspicuous shoot apex of floriferous shoots. The arguments of the absence of a SAM in vegetative shoots are not convincing and the endogenous origin of “leaf-borne leaves” appears questionable. Consequently, the “meristemless” shoot apex cannot be considered as a structure having evolved de novo. In the less advanced subfamilies Tristichoideae and Weddellinoideae, the leaf primordia develop only from a few apical cells of the outer shoot layer. This allows the conclusion that the surface layer of the apex in these subfamilies corresponds to the horizontally spread single-layered apical meristem of subfamily Podostemoideae. Similarly, the view of shoot bifurcation does not conform to the diachsial–sympodial branching pattern occurring in the cymose inflorescences of many Podostemoideae. This fact contradicts the presence of a terminal leaf.     

Multiple MONOPTEROS-dependent pathways are involved in leaf initiation

Initiation of leaves at the flanks of the shoot apical meristem occurs at sites of auxin accumulation and pronounced expression of auxin-inducible PIN-FORMED1 (PIN) genes, suggesting a feedback loop to progressively focus auxin in concrete spots. Because PIN expression is regulated by auxin response factor activity, including MONOPTEROS (MP), it appeared possible that MP affects leaf formation as a positive regulator of PIN genes and auxin transport. Here, we analyze a novel, completely leafless phenotype arising from simultaneous interference with both auxin signaling and auxin transport. We show that mp pin1 double mutants, as well as mp mutants treated with auxin-efflux inhibitors, display synergistic abnormalities not seen in wild type regardless of how strongly auxin transport was reduced. The synergism of abnormalities indicates that the role of MP in shoot meristem organization is not limited to auxin transport regulation. In the mp mutant background, auxin transport inhibition completely abolishes leaf formation. Instead of forming leaves, the abnormal shoot meristems dramatically increase in size, harboring correspondingly enlarged expression domains of CLAVATA3 and SHOOTMERISTEMLESS, molecular markers for the central stem cell zone and the complete meristem, respectively. The observed synergism under conditions of auxin efflux inhibition was further supported by an unrestricted PIN1 expression in mp meristems, as compared to a partial restriction in wild-type meristems. Auxin transport-inhibited mp meristems also lacked detectable auxin maxima. We conclude that MP promotes the focusing of auxin and leaf initiation in part through pathways not affected by auxin efflux inhibitors.     

Shoot apical meristem and cotyledon formation during Arabidopsis embryogenesis: interaction among the CUP-SHAPED COTYLEDON and SHOOT MERISTEMLESS genes

The shoot apical meristem and cotyledons of higher plants are established during embryogenesis in the apex. Redundant CUP-SHAPED COTYLEDON 1 (CUC1) and CUC2 as well as SHOOT MERISTEMLESS (STM) of Arabidopsis are required for shoot apical meristem formation and cotyledon separation. To elucidate how the apical region of the embryo is established, we investigated genetic interactions among CUC1, CUC2 and STM, as well as the expression patterns of CUC2 and STM mRNA. Expression of these genes marked the incipient shoot apical meristem as well as the boundaries of cotyledon primordia, consistent with their roles for shoot apical meristem formation and cotyledon separation. Genetic and expression analyses indicate that CUC1 and CUC2 are redundantly required for expression of STM to form the shoot apical meristem, and that STM is required for proper spatial expression of CUC2 to separate cotyledons. A model for pattern formation in the apical region of the Arabidopsis embryo is presented.     

The effects of 1-naphthaleneacetic acid and N6-benzyladenine on the growth of Cymbidium forrestii rhizomes in vitro

An Asiatic orchid, Cymbidium forrestii, was clonally propagated using seed-derived rhizomes as explants. The rhizomes were cultured and proliferated on Murashige and Skoog medium supplemented with various growth substances. Auxins stimulated rhizome growth by increasing branching and fresh weight of the explant, with 1-naphthaleneacetic acid (NAA) being the most effective auxin. All auxins tested suppressed normal shoot formation. The apical meristem of the rhizome reacted to exogenously applied auxin by reducing the cytoplasmic zone of the apical meristem and causing meristem derivatives to rapidly differentiate into vacuolated parenchyma cells. Leaf formation and development was retarded in the presence of auxin. Cytokinins generally reduced rhizome growth and the number of branches, but benzyladenine (BA) can induce shoot formation in vitro. BA induced the cytoplasmic zone of the apical meristem to enlarge and enhanced leaf development. A 5% (w/v) sucrose concentration was most effective in shoot induction when combined with 5 mg1^-1 BA. Activated charcoal promoted rhizome growth; however, shoot formation was inhibited.     

The effect of cadmium on shoot apical meristems of barley

We studied the effects of cadmium acetate at various concentrations (200 to 800 mg/kg substrate) on growth and development of shoot apical meristem of the barley plants (Hordeum vulgare L.) under the conditions of vegetative experiment. It was shown that in the presence of increasing cadmium concentrations in the soil substrate, the apex length and number of inflorescence elements decreased and the rate of organogenesis slowed down, thus affecting the spike potential productivity and morphological parameters of barley plants at the flowering stage. It is possible that the negative effect of cadmium on the shoot apical meristem is associated with its influence on division of the apex cells.     

Duplicated STM-like KNOX I genes act in floral meristem activity in Eschscholzia californica (Papaveraceae)

In angiosperms, the shoot apical meristem is at the origin of leaves and stems and is eventually transformed into the floral meristem. Class I knotted-like homeobox (KNOX I) genes are known as crucial regulators of shoot meristem formation and maintenance. KNOX I genes maintain the undifferentiated state of the apical meristem and are locally downregulated upon leaf initiation. In Arabidopsis, KNOX I genes, especially SHOOTMERISTEMLESS (STM), have been shown to regulate flower development and the formation of carpels. We investigated the role of STM-like genes in the reproductive development of Eschscholzia californica, to learn more about the evolution of KNOX I gene function in basal eudicots. We identified two orthologs of STM in Eschscholzia, EcSTM1 and EcSTM2, which are predominantly expressed in floral tissues. In contrast, a KNAT1/BP-like and a KNAT2/6-like KNOX I gene are mainly expressed in vegetative organs. Virus-induced gene silencing (VIGS) was used to knockdown gene expression, revealing that both EcSTM genes are required for the formation of reproductive organs. Silencing of EcSTM1 resulted in the loss of the gynoecium and a reduced number of stamens. EcSTM2-VIGS flowers had reduced and defective gynoecia and a stronger reduction in the number of stamen than observed in EcSTM1-VIGS. Co-silencing of both genes led to more pronounced phenotypes. In addition, silencing of EcSTM2 alone or together with EcSTM1 resulted in altered patterns of internodal elongation and sometimes in other floral defects. Our data suggest that some aspects of STM function present in Arabidopsis evolved already before the basal eudicots diverged from core eudicots.