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PubMed2002

Combined SHOOT MERISTEMLESS and WUSCHEL trigger ectopic organogenesis in Arabidopsis.

Almost all aerial parts of plants are continuously generated at the shoot apical meristem (SAM). To maintain a steady pool of undifferentiated cells in the SAM while continuously generating new organs, it is necessary to balance the rate of cell division with the rate of entrance into differentiation pathways. In the Arabidopsis meristem, SHOOT MERISTEMLESS (STM) and WUSCHEL (WUS) are necessary to keep cells undifferentiated and dividing. Here, we tested whether ectopic STM and WUS functions are sufficient to revert differentiation and activate cell division in differentiating tissues. Ectopic STM and WUS functions interacted non-additively and activated a subset of meristem functions, including cell division, CLAVATA1 expression and organogenesis, but not correct phyllotaxy or meristem self-maintenance. Our results suggest that WUS produces a non-cell autonomous signal that activates cell division in combination with STM and that combined WUS/STM functions can initiate the progression from stem cells to organ initiation.

Arabidopsis↗

PubMed2005

Plant stem cell niches.

Stem cells are required to support the indeterminate growth style of plants. Meristems are a plants stem cell niches that foster stem cell survival and the production of descendants destined for differentiation. In shoot meristems, stem cell fate is decided at the populational level. The size of the stem cell domain at the meristem tip depends on signals that are exchanged with cells of the organizing centre underneath. In root meristems, individual stem cells are controlled by direct interaction with cells of the quiescent centre that lie in the immediate neighbourhood. Analysis of the interactions and signaling processes in the stem cell niches has delivered some insights into the molecules that are involved and revealed that the two major niches for plant stem cells are more similar than anticipated.

Arabidopsis↗

PubMed2001

Native Lygus spp. (Heteroptera: Miridae) damaging introduced Hibiscus cannabinus in Italy.

Kenaf, Hibiscus cannabinus L, an introduced tropical fiber crop, is attacked in central Italy by the native mirid bugs Lygus rugulipennis Poppius and L. pratensis (L.), thus establishing novel insect-plant associations. Feeding by Lygus bugs damages the apical meristem, with consequent development of secondary stems and leaf tattering. In a laboratory experiment, both species caused apical meristem damage on potted kenaf. In a field experiment with caged plots, the percentage of damaged plants was significantly higher in plots artificially infested with multiple releases of Lygus spp. compared with a single release and with the protected control without Lygus spp. In naturally infested control plots, the percentage of damaged plants was intermediate. Plant height decreased as a function of the number of damaged meristems. Mean plant height was significantly lower in the multiple-release treatment compared with the single-release treatment and the protected control. Dry biomass was significantly lower in the multiple-release plots compared with the protected control, and intermediate in the single-release plots. In a field experiment with three kenaf cultivars naturally infested by Lygus spp., plant height and stem diameter were significantly lower in damaged plants compared with healthy plants. Dry weights of plants, stems, and basts were also lower, depending on the cultivar, whereas the bast/core ratio was not affected. Stem lesions developed due to feeding or incisions made by ovipositing females. Lesions and eggs were significantly more abundant on plants with damaged apical meristems compared with healthy plants. Damage mechanisms by Lygus spp. and consequences of new crop introductions are discussed.

Animals↗

PubMed1995

Isolation and characterization of the tomato homeobox gene THOM1.

We have screened a cDNA library prepared from tomato (Lycopersicon esculentum Mill. cv. VFNT Cherry) shoot tips for homeobox-containing clones. The isolated cDNA clone THOM1 (tomato homeobox gene 1) contains a homeobox, a leucine zipper motif and and an acidic region. These features support the hypothesis that the THOM1 gene product acts as a transcriptional activator. THOM1 is highly expressed in the vegetative shoot apical meristem, the floral meristem and axillary meristems. Young derivatives of these meristems show similar levels of THOM1 transcripts which decrease with increasing age of the respective tissue. The THOM1 gene is located in the centromere region of chromosome 1 and belongs to a small gene family of the tomato genome.

Amino Acid Sequence↗

PubMed2005

The formation and patterning of leaves: recent advances.

Leaves, the plant's major photosynthetic organs, form through the activity of groups of pluripotent cells, termed shoot apical meristems (SAMs), located at the growing tips of plants. Leaves develop with a dorso-ventral asymmetry, with the adaxial surface adjacent to the meristem and the abaxial surface developing at a distance from it. Molecular genetic studies have shown that the correct specification of adaxial/abaxial polarity requires communication between the incipient leaf and the meristem, and that the juxtaposition of adaxial/abaxial fates is necessary for lamina outgrowth (Waites and Hudson 1995; McConnell et al. 2001). Over the last few years, a number of factors that control cell fate specification in the apex have been identified. This review will focus on recent advances on distinct but overlapping aspects of leaf development, namely, the transition from meristem to leaf fate and the specification of abaxial/adaxial polarity and its possible role in leaf growth.

Gene Expression Regulation, Developmental↗

PubMed2000

The SCHIZOID gene regulates differentiation and cell division in Arabidopsis thaliana shoots.

Cell division and cell differentiation are key processes in shoot development. The Arabidopsis thaliana (L.) Heynh. SCHIZOID (SHZ) gene appears to influence cell differentiation and cell division in the shoot. The shz-2 mutant is notable in that distinct phenotypes develop, depending on the environment in which the plants are grown. When shz-2 mutants are grown in petri dishes, callus develops from the petiole and hypocotyl. In contrast, when the mutants are grown on soil, shoots appear externally stunted with malformed leaves. However, detailed examination of soil-grown mutants shows that the two phenotypes are related. Soil-grown mutants form adventitious meristems, produce a large amount of vascular tissues and have aberrant cell divisions in the meristem. Cells with abnormal cell-division patterns were found in the apical and vascular meristems, suggesting SHZ influences cell division. Development of callus in petri dishes, development of adventitious meristems and aberrations in leaves on soil suggest that SHZ influences cell differentiation. The distinct, but related phenotypes on soil and in petri dishes suggests that SHZ normally functions to regulate differentiation and/or cell division in a manner that is responsive to environmental conditions.

Arabidopsis↗

PubMed2005

Developmental morphology of the Asian one-leaf plant, Monophyllaea glabra (Gesneriaceae) with emphasis on inflorescence morphology.

We examined the developmental morphology of the tropical Asian one-leaf plant Monophyllaea glabra, which is believed to have diverged first in the phylogenetic tree of the genus. The embryo within the seed consists of two cotyledons and a hypocotyl with no shoot or root apical meristems. The endogenous root meristem is formed nearer the hypocotyl end than in other examined Monophyllaea species. One of the cotyledons grows to form the macrocotyledon by means of the basal meristem. The groove meristem arises between the anisocotyledons, shifts toward the macrocotyledon, and is transformed to the inflorescence apex, which produces inflorescence axes in the axils of all ventral bracts of two rows, and secondary inflorescences in the axils of the lower dorsal bracts of the other two rows. The macrocotyledon may act as a ventral bract for the first inflorescence axis at the reproductive stage. This organization suggests that a common ancestor of Monophyllaea and Whytockia with decussate inflorescences diverged in one direction to become Monophyllaea and in another to become Whytockia.

Biological Evolution↗

PubMed2006

Effects of postharvest storage and dormancy status on ABA content, metabolism, and expression of genes involved in ABA biosynthesis and metabolism in potato tuber tissues.

At harvest, and for an indeterminate period thereafter, potato tubers will not sprout and are physiologically dormant. Abscisic acid (ABA) has been shown to play a critical role in tuber dormancy control but the mechanisms controlling ABA content during dormancy as well as the sites of ABA synthesis and catabolism are unknown. As a first step in defining the sites of synthesis and cognate processes regulating ABA turnover during storage and dormancy progression, gene sequences encoding the ABA biosynthetic enzymes zeaxanthin epoxidase (ZEP) and 9-cis-epoxycarotenoid dioxygenase (NCED) and three catabolism-related genes were used to quantify changes in their relative mRNA abundances in three specific tuber tissues (meristems, their surrounding periderm and underlying cortex) by qRT-PCR. During storage, StZEP expression was relatively constant in meristems, exhibited a biphasic pattern in periderm with transient increases during early and mid-to-late-storage, and peaked during mid-storage in cortex. Expression of two members of the potato NCED gene family was found to correlate with changes in ABA content in meristems (StNCED2) and cortex (StNCED1). Conversely, expression patterns of three putative ABA-8'-hydroxylase (CYP707A) genes during storage varied in a tissue-specific manner with expression of two of these genes rising in meristems and periderm and declining in cortex during storage. These results suggest that ABA synthesis and metabolism occur in all tuber tissues examined and that tuber ABA content during dormancy is the result of a balance of synthesis and metabolism that increasingly favors catabolism as dormancy ends and may be controlled at the level of StNCED and StCYP707A gene activities.

Abscisic Acid↗

PubMed1992

Development states associated with the floral transition.

Floral initiation can be analyzed from a developmental perspective by focusing upon how developmental fates are imprinted, remembered, and expressed. This is not an altogether new perspective, since people studying flowering have been concerned for a long time with the commitment of meristems to form flowers and the morphological, cellular, and molecular changes associated with this commitment. What is novel is the emphasis on developmental states as opposed to physiological processes. This developmental focus indicates that there appear to be at least three major developmental states that are acquired and expressed in the process of a meristem initiating floral morphogenesis. The meristem cells must first become competent to respond to a developmental signal that evokes them into a florally determined state. The leaves are the usual source of this signal and a specific leaf may or may not have the capacity to be inductively active. When a leaf does develop the capacity for inductive activity, this capacity is usually correlated with the ontogeny of the leaf. Inductive activity, however, may be continually expressed as in some day-neutral plants or may be latent as in plants where the photoperiod is the external cue for activity. Competent shoot apical meristems respond to inductive leaf signal by being evoked into a florally determined state. Under permissive conditions this florally determined state is expressed as the initiation of floral morphogenesis. Many mechanisms have evolved to regulate entry into and expression of these developmental states. As we learn more about the developmental states associated with flowering and how they are acquired and expressed, we will understand better how the various patterns of flowering are related to one another as well as which developmental processes are common to all angiosperms.

Morphogenesis↗

PubMed1994

Regulation of the arabidopsis floral homeotic gene APETALA1.

The Arabidopsis floral homeotic gene APETALA1 (AP1) encodes a putative transcription factor that acts locally to specify the identity of the floral meristem and to determine sepal and petal development. RNA tissue in situ hybridization studies show that AP1 RNA accumulates uniformly throughout young floral primordia, but is absent from the inflorescence meristem. Later in development, AP1 RNA is excluded from cells that will give rise to the two inner whorls of organs. Here we show that AP1 expression is under the control of two negative regulators: the meristem identity gene TERMINAL FLOWER represses AP1 RNA accumulation in the inflorescence meristem, and the organ identity gene AGAMOUS prevents AP1 RNA accumulation in the two inner whorls of wild-type flowers. These and other data presented here lead to a revised model for the regulatory interactions among the genes specifying floral organ identity in Arabidopsis.

Arabidopsis↗

PubMed1985

Increased replication in plant nuclei, whose spread chromatin contains enlarged nucleosomal linkers, is only partially due to a rise in the number of DNA elongating chains.

Dormant and proliferating meristems of Allium cepa L. roots were compared. In vivo DNA synthesis was nine times higher in proliferating meristems, partially due to a 2.5 times increase in the relative number of DNA elongating chains, as estimated after the assay for 3H-TTP incorporation in permeabilized cells. Stereology done on conventional electronmicrographs showed that nucleoplasm was nearly three times larger in nuclei of proliferating meristems, while the amount of compact chromatin did not diminish but redistributed instead, resulting that the chromatin/nucleoplasm interface increased. Finally, the nucleosomal linker -as seen after the spreading of the chromatin fibres- was larger and more variable in proliferating than in dormant meristems.

Cell Division↗

PubMed2005

A putative CENTRORADIALIS/TERMINAL FLOWER 1-like gene, Ljcen1, plays a role in phase transition in Lotus japonicus.

CENTRORADIALIS/TERMINAL FLOWER 1 (CEN/TFL1) genes play an important role in the phase transition of plant flowering. Here we characterized the expression pattern of a CEN/TFL1-like gene, Ljcen1, from Lotus japonicus. Sequence analysis revealed that Ljcen1 shared 67-76% identity to its homologs from a variety of plant species. Ljcen1 transcripts could be detected at the young root tip and reproductive shoot apical meristem of L. japonicus. RNA in situ hybridization analysis revealed that Ljcen1 was continuously expressed in the sub-domain of the primary inflorescence meristem and transiently expressed in the secondary inflorescence meristem. The ectopic expression of Ljcen1 in Arabidopsis driven by double CaMV 35S promoter delayed the flowering. These results suggested that Ljcen1 gene was involved in a conserved CEN/TFL1 pathway that functions in phase transition of shoot apical meristem in L. japonicus.

Amino Acid Sequence↗

PubMed2002

Establishment of polarity in angiosperm lateral organs.

In seed plants, lateral organs such as leaves and floral organs are formed from the flanks of apical meristems. Therefore, they have an inherent positional relationship: organ primordia have an adaxial side next to the meristem, and an abaxial one away from the meristem. Surgical and genetic studies suggest that a morphogenetic gradient, which originates in the meristem, converts the inherent polarity into a functional one. Once an adaxial-abaxial axis of polarity is established within organ primordia, it provides cues for proper lamina growth and asymmetrical development. Several key participants in this process have been identified, and analyses of these genes support and refine our views of axis formation in plants. The complex relationships between and within various members of these plant-specific gene families (class III HD-ZIPs, YABBYs and KANADIs) might account for a substantial part of the morphological variation in lateral organs of seed plants.

Arabidopsis Proteins↗

PubMed1998

Cortical tissue-specific accumulation of the root-specific ns-LTP transcripts in the bean (Phaseolus vulgaris) seedlings.

The characterization of a cDNA clone encoding non-specific lipid transfer protein (PvLTP, formerly named PVR3) in the roots of bean seedlings has been previously reported. In this study, we examined the temporal and spatial accumulation of PvLTP mRNA and the effect of the auxin naphthaleneacetic acid (NAA) on the accumulation of PvLTP mRNA during root development. In situ hybridization showed that accumulation of PvLTP mRNA is highly tissue-specific. Accumulation was detected in the cortical tissue, but not in other tissues of root, including the quiescent center and root cap. Within the cortical tissue, accumulation of PvLTP mRNA was developmentally regulated; accumulation of PvLTP mRNA was high in the cortical tissue of the proximal and ground meristem and declined as cortical tissue developed further. Since the appropriate distribution of auxin is an important factor responsible for the maintenance of root meristem organization. We examined effect of auxin on the accumulation of PvLTP mRNA in relation to the development of cortical tissue. In bean seedlings grown on medium supplemented with 5 microM NAA, morphological alternations, including radial root expansion and abnormal tissue organization in the root apical meristem, were observed. Only faint accumulation signals of PvLTP mRNA were observed in the cortical tissue of proximal meristem region, indicating that cortical tissue development was repressed by exogenous NAA. However, our results suggest that the change in accumulation of PvLTP mRNA is not direct regulatory effect but reflective effect of altered development of cortical tissue that was induced by exogenous NAA. The temporal and spatial accumulation of PvLTP mRNA indicates that PvLTP is a useful marker for the development of cortical tissue in the root tip in bean seedlings.

Carrier Proteins↗

PubMed2001

Bimodal patterns of floral gene expression over the two seasons that kiwifruit flowers develop.

Polymerase chain reaction fragments with homology to the Arabidopsis floral meristem identity genes LEAFY and APETALA1 have been isolated from kiwifruit (Actinidia deliciosa [A. Chev.] C. F. Liang and A. R. Ferguson) and have been named ALF and AAP1, respectively. Northern hybridisation analyses have shown that ALF and AAP1 have bimodal patterns of annual expression in developing first-order axillary buds and their subsequent shoots. This pattern of expression is consistent with the 2-year cycle of axillary bud, flower and fruit development observed in kiwifruit. The first period of expression was early in first-order bud development (late spring of the first growing season), when second-order meristems are initiated, and the second, approximately 10 months later, when those meristems differentiate flowers (late spring of the second growing season). In situ hybridisation analyses on axillary buds collected during late spring of the first growing season have shown ALF expression throughout the developing first-order buds and AAP1 expression was localised in developing second-order axillary meristems. During the spring of the second growing season, transcript accumulation for both ALF and AAP1 is localised in differentiating flowers. Our results show that important developmental events are occurring very early in kiwifruit first-order axillary bud development (spring of the first growing season) and it is likely that this includes floral commitment (evocation).

Journal Article↗

PubMed1993

Effect of lateral suppressor on petal initiation in tomato.

Flowers developing on tomato (Lycopersicon esculentum) plants homozygous for the lateral suppressor (ls) mutation lack petals. Scanning electron micrographs revealed that in ls plants no second whorl organs were initiated. The initiation of first, third, and fourth whorl organs were unaffected by this mutation. To investigate interactions between the cells in different layers of the floral meristem during organ initiation, a periclinal chimera between wild-type and ls tomato was generated. Flowers of the chimera having ls cells in the outer meristem layer (L1) and wild-type cells in internal layers (L2 and L3) developed normally, including the initiation of organ primordia that differentiated as petals in normal positions within the second whorl. L1 of the chimera developed in a non-autonomous manner during petal development. Thus, wild-type cells occupying the internal meristem layers provided developmental cues necessary for initiation of petal primordia at appropriate positions on the floral meristem. L1 cells carrying the lateral suppressor mutation were fully capable of responding to this information and differentiated appropriately.

Chimera↗

PubMed1995

Gibberellin promotes histone H1 kinase activity and the expression of cdc2 and cyclin genes during the induction of rapid growth in deepwater rice internodes.

Partial submergence or treatment with either ethylene or gibberellin (GA) promotes rapid internodal growth in deepwater rice (Oryza sativa L.). Earlier work has shown that GA is the immediate hormonal signal for this growth response, which involves induction of the cell cycle at the G2/M phase transition and subsequent enhancement in the rate of DNA synthesis. In all eukaryotes, onset of mitosis is regulated by the p34cdc2/CDC28 protein kinase, whose activity is assayed by in vitro phosphorylation of histone H1. It was found that GA enhanced the activity of p34cdc2/CDC28-like histone H1 kinase in the intercalary meristem of rice internodes. The enzyme activity showed a sharp peak that correlated with a decrease in the population of cells in the G2 phase during the first 4 h of GA treatment but not with changes in DNA synthesis. The level of histone H1 kinase activity increased again when cell division activity in the intercalary meristem is known to be high. The expression of two cdc2 homologs was examined. The mRNA level of one of these, cdc2Os-2, was increased after 1 h of GA treatment, whereas the mRNA level of the other, cdc2Os-1, was not affected. Two cDNAs, cycOs1 and cycOs2, which show high homology to cyclin cDNAs, were cloned from rice. They share 75.1% sequence identity at the amino acid level, and both of them are encoded by mRNAs of 1.6 kb. Expression of the two corresponding cyclin genes was enhanced by GA, and the time course of the induction was compatible with a role for both cyclins in regulating the G2/M phase transition. The cyclins were expressed in the intercalary meristem and the elongation zone of the internode, but the GA-induced increase in transcript levels was restricted to the meristem only. The results support the hypothesis that induction of mitosis by GA is brought about by increased p34cdc2/CDC28 protein kinase activity, which may be the result of transcriptional activation of the cdc2Os-2, cycOs1 and cycOs2 genes.

Amino Acid Sequence↗

PubMed2000

Arabidopsis thaliana SHAGGY-related protein kinases (AtSK11 and 12) function in perianth and gynoecium development.

In higher plants, the correct patterning of the floral meristem in terms of organ type, number and form is the result of a concerted expression of a network of genes. We describe phenotypes of flower patterning, resulting from a reduction of transcript levels of the Arabidopsis SHAGGY-related protein kinase genes AtSK11(ASKalpha) and AtSK12(ASKgamma). The AtSK genes are plant homologues of the Drosophila shaggy (SGG) gene and the mammalian Glycogen-Synthase Kinase-3 (GSK-3). The SGG protein kinase is a key component of the wingless signalling pathway and is required for the establishment of tissue patterning and cell fate determination. The expression patterns of the AtSK11(ASKalpha) and AtSK12(ASKgamma) genes during wild-type Arabidopsis inflorescence development, detected by in situ hybridisation, have been shown to be consistent with a possible role in floral meristem patterning. AtSK11(ASKalpha) and AtSK12(ASKgamma) transcripts were detected at the periphery of the inflorescence meristem and in the floral meristem. At later stages the expression of the AtSK genes became localised in specific regions of developing flower organ primordia. Furthermore, we have obtained and analysed transgenic plants containing AtSK11(ASKalpha) and AtSK12(ASKgamma) gene specific antisense constructs. These plants developed flowers showing a higher number of perianth organs and an alteration of the apical-basal patterning of the gynoecium.

Arabidopsis↗