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Activation of E2F transcription factors at the G1-to-S phase boundary, with the resultant expression of genes needed for DNA synthesis and S-phase, is due to phosphorylation of the retinoblastoma-related (RBR) protein by cyclin D-dependent kinase (CYCD-CDK), particularly CYCD3-CDKA. Arabidopsis has three canonical E2F genes, of which E2Fa and E2Fb are proposed to encode transcriptional activators and E2Fc a repressor. Previous studies have identified genes regulated in response to high-level constitutive expression of E2Fa and of CYCD3;1, but such plants display significant phenotypic abnormalities. We have sought to identify targets that show responses to lower level induced changes in abundance of these cell cycle regulators. Expression of E2Fa, E2Fc or CYCD3;1 was induced using dexamethasone and the effects analysed using microarrays in a time course allowing short and longer term effects to be observed. Overlap between CYCD3;1 and E2Fa modulated genes substantiates their action in a common pathway with a key role in controlling the G1/S transition, with additional targets for CYCD3;1 in chromatin modification and for E2Fa in cell wall biogenesis and development. E2Fc induction led primarily to gene downregulation, but did not antagonise E2Fa action and hence E2Fc appears to function outside the CYCD3-RBR pathway, does not have a direct effect on cell cycle genes, and promoter analysis suggests a distinct binding site preference.

The mechanisms by which plants modulate their growth rate in response to environmental and developmental conditions are unknown, but are presumed to involve specialized regions called meristems where cell division is concentrated. The possible role of cell division in influencing meristem activity and overall plant growth rate is controversial, with a prevailing view that cell division is secondary to higher order meristem controls. Here we show that a reduction in the length of the cell-cycle G1 phase and faster cell cycling occur when the rate of cell division in transgenic tobacco plants is increased by the plant D-type cyclin CycD2 (ref. 8). The plants have normal cell and meristem sizes, but elevated overall growth rates, an increased rate of leaf initiation and accelerated development in all stages from seedling to maturity. We conclude that cell division is a principal determinant of meristem activity and overall growth rate, and propose that modulation of plant growth rate is achieved through regulation of G1.

In essence, the mitotic cell cycle in eukaryotes involves the duplication and separation of chromosomes, coupled to the process of dividing one cell into two. Cytokinesis is therefore the culmination of a series of events that were triggered during G1 phase, and brings the daughter cells back to the starting position in G1 for another possible round of division. In all eukaryotes, progression through the cell cycle is controlled by cyclin-dependent kinases that bind to positive regulators called cyclins. This review explores some of the pathways that trigger the plant cell cycle, with emphasis on the G1 phase. Examples include signalling pathways involving glutathione and cellular redox potential, the possible existence of a G1 DNA-damage checkpoint, and the plant hormones auxin and cytokinin. Progress in understanding the link between cell proliferation, cell differentiation and the cell-cycle machinery in a developmental context is discussed.

The plant embryo is a relatively simple structure consisting of a primordial shoot and root, whose development is frozen in the form of a seed. Most development of the mature plant takes place post-embryonically, and is the consequence of cell division and organogenesis in small regions known as meristems, which originate in the embryonic shoot and root apices. Significant recent progress has been made in understanding the mechanisms that control the plant cell cycle at a molecular level, and the first attempts have been made to control plant growth through modulation of cell-cycle genes. These results suggest that there is significant potential to control plant growth and architecture through manipulation of cell division rates. However, a full realisation of the promise of such strategies will probably require a much greater understanding of cell division control and how its upstream regulation is co-ordinated by spatial relationships between cells and by environmental signals.

lin-26, which encodes a unique Zn-finger protein, is required for differentiation of nonneuronal ectodermal cells in Caenorhabditis elegans. Here, we show that the two genes located immediately upstream of lin-26 encode LIN-26-like Zn-finger proteins; hence their names are lir-1 and lir-2 (lin-26 related). lir-2, lir-1, and lin-26 generate several isoforms by alternative splicing and/or trans-splicing at different positions. On the basis of their trans-splicing pattern, their intergenic distances, and their expression, we suggest that lir-2, lir-1, and lin-26 form two overlapping transcriptional operons. The first operon, which is expressed in virtually all cells, includes lir-2 and long lir-1 isoforms. The second operon, which is expressed in the nonneuronal ectoderm, includes short lir-1 isoforms, starting at exon 2 and lin-26. This unusual genomic organization has been conserved in C. briggsae, as shown by cloning the C. briggsae lir-2, lir-1, and lin-26 homologs. Particularly striking is the sequence conservation throughout the first lir-1 intron, which is very long in both species. Structural conservation is functionally meaningful as C. briggsae lin-26 is also expressed in the nonneuronal ectoderm and can complement a C. elegans lin-26 null mutation.

The Caenorhabditis elegans LIN-26 protein is required to specify and/or maintain the fates of all non-neuronal ectodermal cells. Here we show that lin-26 is expressed until the somatic gonad primordium stage in all cells of the somatic gonad, except in distal tip cells, and later in all uterine cells. To determine if lin-26 functions in the somatic gonad, we have generated gonad-specific lin-26 alleles obtained by integration of lin-26 promoter deletion derivatives into a lin-26 null mutant background. In this way, we rescued the lethal phenotype imparted by lin-26 null mutations and uncovered a highly penetrant sterile phenotype. Specifically, the strongest of these new alleles was characterized by the absence of lin-26 expression in the somatic gonad, the presence of endomitotic oocytes, decreased germline proliferation, a protruding vulva and a less penetrant absence of gonad arms. Lineage analysis of mutant somatic gonads and examination of several markers expressed in the spermatheca, sheath cells, distal tip cells and the uterus, suggest that LIN-26 is required in sheath, spermatheca and uterine precursors, and in uterine cells. We conclude that lin-26 performs a similar function in the non-neuronal ectoderm and the somatic gonad, a mesoderm derivative, and we speculate that lin-26 is required to express epithelial characteristics.

Little is known about the signals that govern the network of meristem and organ identity genes that control flower development. In Arabidopsis, we can induce a heterochronic switch from flower to shoot development, a process known as floral meristem reversion, by manipulating photo-period in the floral homeotic mutant agamous and in plants heterozygous for the meristem identity gene leafy. The transformation from flower to shoot meristem is suppressed by hy1, a mutation blocking phytochrome activity, by spindly, a mutation that activates basal gibberellin signal transduction in a hormone independent manner, or by the exogenous application of gibberellins. We propose that LFY and AG play an important role in the maintenance of flower meristem identity and that floral meristem reversion in heterozygous lfy and in ag flowers is regulated by a phytochrome and gibberellin signal transduction cascade.

APETALA2 (AP2) plays a central role in the establishment of the floral meristem, the specification of floral organ identity, and the regulation of floral homeotic gene expression in Arabidopsis. We show here that in addition to its functions during flower development, AP2 activity is also required during seed development. We isolated the AP2 gene and found that it encodes a putative nuclear protein that is distinguished by an essential 68-amino acid repeated motif, the AP2 domain. Consistent with its genetic functions, we determined that AP2 is expressed at the RNA level in all four types of floral organs--sepals, petals, stamens, and carpels--and in developing ovules. Thus, AP2 gene transcription does not appear to be spatially restricted by the floral homeotic gene AGAMOUS as predicted by previous studies. We also found that AP2 is expressed at the RNA level in the inflorescence meristem and in nonfloral organs, including leaf and stem. Taken together, our results suggest that AP2 represents a new class of plant regulatory proteins that may play a general role in the control of Arabidopsis development.

We have isolated two nitrate reductase genes and their corresponding cDNAs from Arabidopsis thaliana. Sequences of the two cDNAs, when compared to a sequence of a barley cDNA clone, confirm their identity as nitrate reductase clones and show that they are closely related. The two genes have been mapped using restriction fragment length polymorphisms; gNR2 is close to the previously identified chl-3 locus and is probably identical to it, while gNR1 maps to a new locus (NIA1) on chromosome 1, near gl-2.