Cellular Parameters of the Shoot Apical Meristem in Arabidopsis

197

223

Telomerase is an essential enzyme that maintains telomeres on eukaryotic chromosomes. In mammals, telomerase is required for the lifelong proliferative capacity of normal regenerative and reproductive tissues and for sustained growth in a dedifferentiated state. Although the importance of telomeres was first elucidated in plants 60 years ago, little is known about the role of telomeres and telomerase in plant growth and development. Here we report the cloning and characterization of the Arabidopsis telomerase reverse transcriptase (TERT) gene, AtTERT. AtTERT is predicted to encode a highly basic protein of 131 kDa that harbors the reverse transcriptase and telomerase-specific motifs common to all known TERT proteins. AtTERT mRNA is 10 -20 times more abundant in callus, which has high levels of telomerase activity, versus leaves, which contain no detectable telomerase. Plants homozygous for a transfer DNA insertion into the AtTERT gene lack telomerase activity, confirming the identity and function of this gene. Because telomeres in wild-type Arabidopsis are short, the discovery that telomerase-null plants are viable for at least two generations was unexpected. In the absence of telomerase, telomeres decline by approximately 500 bp per generation, a rate 10 times slower than seen in telomerase-deficient mice. This gradual loss of telomeric DNA may reflect a reduced rate of nucleotide depletion per round of DNA replication, or the requirement for fewer cell divisions per organismal generation. Nevertheless, progressive telomere shortening in the mutants, however slow, ultimately should be lethal.transfer DNA ͉ telomeres ͉ plant ͉ callus T he groundbreaking studies of Barbara McClintock (1, 2) and Hermann Muller (3) demonstrated that genome stability depends on the integrity of the telomere complex at the ends of eukaryotic chromosomes. Although alternative strategies have been reported (4), telomere synthesis by telomerase is the primary mechanism for sustaining chromosome ends in eukaryotes. Telomeres and their maintenance by telomerase comprise a biological clock that influences cellular lifespan in mammals (5). Telomerase expression is confined primarily to the germ line and permanently regenerating tissues of the adult soma. In other cells, telomerase is turned off and telomeres progressively shorten with each division. Once telomeres shorten below a critical length, a DNA damage checkpoint is activated, leading to cellular senescence and death. Telomerase is reactivated in about 85% of all human tumors and telomere function is maintained indefinitely (6).Telomerase is a ribonucleoprotein reverse transcriptase (7). The RNA subunit contains a templating sequence complementary to the G-rich strand of the telomere, whereas the telomerase reverse transcriptase (TERT) harbors the catalytic activity for telomere repeat synthesis. Characterization of TERT subunits from a variety of protozoa, yeasts, and mammals has revealed several distinct reverse transcriptase motifs that comprise the polymerase active site as well ...

Section: Figmentioning

confidence: 99%

Disruption of the telomerase catalytic subunit gene from Arabidopsis inactivates telomerase and leads to a slow loss of telomeric DNA

Fitzgerald

Říha

Gao

et al. 1999

197

223

“…Spatial analysis of cell division patterns in meristems has revealed that stem cells, the ultimate source of all shoot and root cells, proliferate very slowly. Their immediate progeny, which form the flanks of the shoot meristem from which lateral organ primordia arise, or the initials in the root that generate the cell types of the root, proliferate slightly faster (3)(4)(5). Subsequently, in newly initiated leaf primordia and in the domain distal to the initials in the root, a rapid increase of cell division rates is observed (5-7).…”

mentioning

confidence: 97%

Arabidopsis TCP20 links regulation of growth and cell division control pathways

Potuschak²,

Colón‐Carmona³

et al. 2005

312

287

During postembryonic plant development, cell division is coupled to cell growth. There is a stringent requirement to couple these processes in shoot and root meristems. As cells pass through meristems, they transit through zones with high rates of cell growth and proliferation during organogenesis. This transition implies a need for coordinate regulation of genes underpinning these two fundamental cell functions. Here, we report a mechanism for coregulation of cell division control genes and cell growth effectors. We identified a GCCCR motif necessary and sufficient for high-level cyclin CYCB1;1 expression at G 2/M. This motif is overrepresented in many ribosomal protein gene promoters and is required for high-level expression of the S27 and L24 ribosomal subunit genes we examined. p33 TCP20 , encoded by the Arabidopsis TCP20 gene, binds to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes in vitro and in vivo. We propose a model in which organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting teosinte-branched, cycloidea, PCNA factor (TCP) genes in the distal meristem boundary zone where cells become mitotically quiescent before expansion and differentiation.

“…Thus, numerous studies report the occurrence of nonanticlinal divisions in the tunica layers as indicative of the site of presumptive leaf formation (3). This altered pattern of cell division may also be accompanied by an increase in cell proliferation (4). However, whether the observed changes in cell division pattern are causally involved in leaf initiation remains debatable.…”

mentioning

confidence: 99%

Cell division pattern influences gene expression in the shoot apical meristem

Wyrzykowska

Fleming

2003

The shoot apical meristem of angiosperms shows a highly conserved cellular architecture in which a change of cell division orientation correlates with early events of leaf initiation. However, the causal role of this altered cellular parameter in leaf formation is debatable. We have used the dynamin-like protein phragmoplastin as a tool to modify the pattern of cell division within the apical meristem. Taking a microinduction approach, we show that local alteration in cell division orientation is not sufficient to induce morphogenesis in the meristem. Surprisingly, an altered cell division pattern did lead to an altered pattern of expression of genes implicated in aspects of leaf formation. Our data identify inducible expression of phragmoplastin as a tool to manipulate cell division pattern. Furthermore, they indicate that a mechanism exists by which cells in the meristem can respond at the level of gene expression to altered parameters of cell division. These data are discussed in the context of a model linking leaf morphogenesis and differentiation.