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Agricultural management practices influence soil structure, but the characterization of these modifications and consequences are still not completely understood. In this study, we aim at improving water retention and hydraulic conductivity curves using both classical soil techniques and X-ray microtomography in the context of tillage simplification. We show a good match for retention and conductivity functions between macroscopic measurements and microtomographic information. Microtomography highlights the presence of a secondary pore system. Analysis of structural parameters for these pores appears to be significant and offers additional clues for objects differentiation. We show that relatively fast scans supply not only good results, but also enhance near saturation characterization, making microtomography a highly competitive instrument for routine soil characterization. [ABSTRACT FROM AUTHOR]

Semaphorins are a large family of secreted and membrane-associated proteins necessary for wiring of the brain. Semaphorin 5A (SEMA5A) acts as a bifunctional guidance cue, exerting both attractive and inhibitory effects on developing axons. Previous studies have suggested that SEMA5A could be a susceptibility gene for autism spectrum disorders (ASDs). We first identified a de novo translocation t(5;22)(p15.3;q11.21) in a patient with ASD and intellectual disability (ID). At the translocation breakpoint on chromosome 5, we observed a 861-kb deletion encompassing the end of the SEMA5A gene. We delineated the breakpoint by NGS and observed that no gene was disrupted on chromosome 22. We then used Sanger sequencing to search for deleterious variants affecting SEMA5A in 142 patients with ASD. We also identified two independent heterozygous variants located in a conserved functional domain of the protein. Both variants were maternally inherited and predicted as deleterious. Our genetic screens identified the first case of a de novo SEMA5A microdeletion in a patient with ASD and ID. Although our study alone cannot formally associate SEMA5A with susceptibility to ASD, it provides additional evidence that Semaphorin dysfunction could lead to ASD and ID. Further studies on Semaphorins are warranted to better understand the role of this family of genes in susceptibility to neurodevelopmental disorders.

Ductal plate malformations (DPM) present with a wide phenotypic spectrum comprising Von Meyenburg complexes (VMC), Caroli disease (CD), Caroli syndrome (CS), and autosomal recessive polycystic kidney disease (ARPKD). Variants in PKHD1 are responsible for ARPKD and CS with a high inter- and intra-familial phenotypic variability. Rare familial cases of CD had been reported and exceptional cases of CD are associated with PKHD1 variants. In a family of three siblings presenting with a wide spectrum of severity of DPM, we performed whole exome sequencing and identified two PKHD1 compound heterozygous variants (c.10444G>A; p.Arg3482Cys and c.5521C>T; p.Glu1841Lys), segregating with the symptoms. Two compound heterozygous PKHD1 variants, including one hypomorphic variant, were identified in two other familial cases of DPM with at least one patient presenting with CD. This report widens the phenotypic variability of PKHD1 variants to VMC, and others hepatic bile ducts malformations with inconstant renal phenotype in adults and highlights the important intra-familial phenotypic variability. It also showed that PKHD1 might be a major gene for CD. This work adds an example of the contribution of exome sequencing, not only in the discovery of new genes but also in expanding the phenotypic spectrum of well-known disease-associated genes, using reverse phenotyping., (© 2015 Wiley Periodicals, Inc.)

SASH1 (SAM and SH3 domain-containing protein 1) is a tumor suppressor gene involved in the tumorigenesis of a spectrum of solid cancers. Heterozygous SASH1 variants are known to cause autosomal-dominant dyschromatosis. Homozygosity mapping and whole-exome sequencing were performed in a consanguineous Moroccan family with two affected siblings presenting an unclassified phenotype associating an abnormal pigmentation pattern (hypo- and hyperpigmented macules of the trunk and face and areas of reticular hypo- and hyperpigmentation of the extremities), alopecia, palmoplantar keratoderma, ungueal dystrophy and recurrent spinocellular carcinoma. We identified a homozygous variant in SASH1 (c.1849G>A; p.Glu617Lys) in both affected individuals. Wound-healing assay showed that the patient's fibroblasts were better able than control fibroblasts to migrate. Following the identification of SASH1 heterozygous variants in dyschromatosis, we used reverse phenotyping to show that autosomal-recessive variants of this gene could be responsible for an overlapping but more complex phenotype that affected skin appendages. SASH1 should be added to the list of genes responsible for autosomal-dominant and -recessive genodermatosis, with no phenotype in heterozygous patients in the recessive form, and to the list of genes responsible for a predisposition to skin cancer.

Objectives: Conradi-Hünermann-Happle [X-linked dominant chondrodysplasia punctata 2 (CDPX2)] syndrome is a rare X-linked dominant skeletal dysplasia usually lethal in men while affected women show wide clinical heterogeneity. Different EBP mutations have been reported. Severe female cases have rarely been reported, with only six antenatal presentations., Methods: To better characterize the phenotype in female fetuses, we included nine antenatally diagnosed cases of women with EBP mutations. All cases were de novo except for two fetuses with an affected mother and one case of germinal mosaicism., Results: The mean age at diagnosis was 22 weeks of gestation. The ultrasound features mainly included bone abnormalities: shortening (8/9 cases) and bowing of the long bones (5/9), punctuate epiphysis (7/9) and an irregular aspect of the spine (5/9). Postnatal X-rays and examination showed ichthyosis (8/9) and epiphyseal stippling (9/9), with frequent asymmetric short and bowed long bones. The X-inactivation pattern of the familial case revealed skewed X-inactivation in the mildly symptomatic mother and random X-inactivation in the severe fetal case. Differently affected skin samples of the same fetus revealed different patterns of X-inactivation., Conclusion: Prenatal detection of asymmetric shortening and bowing of the long bones and cartilage stippling should raise the possibility of CPDX2 in female fetuses, especially because the majority of such cases involve de novo mutations., (© 2015 John Wiley & Sons, Ltd.)

Centrioles are microtubule-based, barrel-shaped structures that initiate the assembly of centrosomes and cilia. How centriole length is precisely set remains elusive. The microcephaly protein CPAP (also known as MCPH6) promotes procentriole growth, whereas the oral-facial-digital (OFD) syndrome protein OFD1 represses centriole elongation. Here we uncover a new subtype of OFD with severe microcephaly and cerebral malformations and identify distinct mutations in two affected families in the evolutionarily conserved C2CD3 gene. Concordant with the clinical overlap, C2CD3 colocalizes with OFD1 at the distal end of centrioles, and C2CD3 physically associates with OFD1. However, whereas OFD1 deletion leads to centriole hyperelongation, loss of C2CD3 results in short centrioles without subdistal and distal appendages. Because C2CD3 overexpression triggers centriole hyperelongation and OFD1 antagonizes this activity, we propose that C2CD3 directly promotes centriole elongation and that OFD1 acts as a negative regulator of C2CD3. Our results identify regulation of centriole length as an emerging pathogenic mechanism in ciliopathies.

Cohen syndrome (CS) is a rare autosomal recessive disorder with multisytemic clinical features due to mutations in the VPS13B gene, which has recently been described encoding a mandatory membrane protein involved in Golgi integrity. As the Golgi complex is the place where glycosylation of newly synthesized proteins occurs, we hypothesized that VPS13B deficiency, responsible of Golgi apparatus disturbance, could lead to glycosylation defects and/or mysfunction of this organelle, and thus be a cause of the main clinical manifestations of CS. The glycosylation status of CS serum proteins showed a very unusual pattern of glycosylation characterized by a significant accumulation of agalactosylated fucosylated structures as well as asialylated fucosylated structures demonstrating a major defect of glycan maturation in CS. However, CS transferrin and α1-AT profiles, two liver-derived proteins, were normal. We also showed that intercellular cell adhesion molecule 1 and LAMP-2, two highly glycosylated cellular proteins, presented an altered migration profile on SDS-PAGE in peripheral blood mononuclear cells from CS patients. RNA interference against VPS13B confirmed these glycosylation defects. Experiments with Brefeldin A demonstrated that intracellular retrograde cell trafficking was normal in CS fibroblasts. Furthermore, early endosomes were almost absent in these cells and lysosomes were abnormally enlarged, suggesting a crucial role of VPS13B in endosomal-lysosomal trafficking. Our work provides evidence that CS is associated to a tissue-specific major defect of glycosylation and endosomal-lysosomal trafficking defect, suggesting that this could be a new key element to decipher the mechanisms of CS physiopathology.

Oral-facial-digital syndrome type VI (OFD VI) is a recessive ciliopathy defined by two diagnostic criteria: molar tooth sign (MTS) and one or more of the following: (1) tongue hamartoma (s) and/or additional frenula and/or upper lip notch; (2) mesoaxial polydactyly of one or more hands or feet; (3) hypothalamic hamartoma. Because of the MTS, OFD VI belongs to the "Joubert syndrome related disorders". Its genetic aetiology remains largely unknown although mutations in the TMEM216 gene, responsible for Joubert (JBS2) and Meckel-Gruber (MKS2) syndromes, have been reported in two OFD VI patients. To explore the molecular cause(s) of OFD VI syndrome, we used an exome sequencing strategy in six unrelated families followed by Sanger sequencing. We identified a total of 14 novel mutations in the C5orf42 gene in 9/11 families with positive OFD VI diagnostic criteria including a severe fetal case with microphthalmia, cerebellar hypoplasia, corpus callosum agenesis, polydactyly and skeletal dysplasia. C5orf42 mutations have already been reported in Joubert syndrome confirming that OFD VI and JBS are allelic disorders, thus enhancing our knowledge of the complex, highly heterogeneous nature of ciliopathies.

Over one hundred VPS13B mutations are reported in Cohen syndrome (CS). Most cases exhibit a homogeneous phenotype that includes intellectual deficiency (ID), microcephaly, facial dysmorphism, slender extremities, truncal obesity, progressive chorioretinal dystrophy, and neutropenia. We report on a patient carrying two VPS13B splicing mutations with an atypical phenotype that included microcephaly, retinopathy, and congenital neutropenia, but neither obesity nor ID. RNA analysis of the IVS34+2T_+3AinsT mutation did not reveal any abnormal splice fragments but mRNA quantification showed a significant decrease in VPS13B expression. RNA sequencing analysis up- and downstream from the IVS57+2T>C mutation showed abnormal splice isoforms. In contrast to patients with typical CS, who express only abnormal VPS13B mRNA and truncated protein, a dose effect of residual normal VPS13B protein possibly explains the incomplete phenotype in the patient. This observation emphasizes that VPS13B analysis should be performed in cases of congenital neutropenia associated with retinopathy, even in the absence of ID, therefore extending the VPS13B phenotype spectrum., (© 2013 Wiley Periodicals, Inc.)

Cohen syndrome (CS) is a rare autosomal recessive condition caused by mutations and/or large rearrangements in the VPS13B gene. CS clinical features, including developmental delay, the typical facial gestalt, chorioretinal dystrophy (CRD) and neutropenia, are well described. CS diagnosis is generally raised after school age, when visual disturbances lead to CRD diagnosis and to VPS13B gene testing. This relatively late diagnosis precludes accurate genetic counselling. The aim of this study was to analyse the evolution of CS facial features in the early period of life, particularly before school age (6 years), to find clues for an earlier diagnosis. Photographs of 17 patients with molecularly confirmed CS were analysed, from birth to preschool age. By comparing their facial phenotype when growing, we show that there are no special facial characteristics before 1 year. However, between 2 and 6 years, CS children already share common facial features such as a short neck, a square face with micrognathia and full cheeks, a hypotonic facial appearance, epicanthic folds, long ears with an everted upper part of the auricle and/or a prominent lobe, a relatively short philtrum, a small and open mouth with downturned corners, a thick lower lip and abnormal eye shapes. These early transient facial features evolve to typical CS facial features with aging. These observations emphasize the importance of ophthalmological tests and neutrophil count in children in preschool age presenting with developmental delay, hypotonia and the facial features we described here, for an earlier CS diagnosis.

Oral-facial-digital syndrome type VI (OFD VI) is characterized by the association of malformations of the face, oral cavity and extremities, distinguished from the 12 other OFD syndromes by cerebellar and metacarpal abnormalities. Cerebellar malformations in OFD VI have been described as a molar tooth sign (MTS), thus, including OFD VI among the "Joubert syndrome related disorders" (JSRD). OFD VI diagnostic criteria have recently been suggested: MTS and one or more of the following: 1) tongue hamartoma(s) and/or additional frenula and/or upper lip notch; 2) mesoaxial polydactyly of hands or feet; 3) hypothalamic hamartoma. In order to further delineate this rare entity, we present the neurological and radiological data of 6 additional OFD VI patients. All patients presented oral malformations, facial dysmorphism and distal abnormalities including frequent polydactyly (66%), as well as neurological symptoms with moderate to severe mental retardation. Contrary to historically reported patients, mesoaxial polydactyly did not appear to be a predominant clinical feature in OFD VI. Sequencing analyzes of the 14 genes implicated in JSRD up to 2011 revealed only an OFD1 frameshift mutation in one female OFD VI patient, strengthening the link between these two oral-facial-digital syndromes and JSRD., (Copyright © 2013 Elsevier Masson SAS. All rights reserved.)

Background: DYRK1A plays different functions during development, with an important role in controlling brain growth through neuronal proliferation and neurogenesis. It is expressed in a gene dosage dependent manner since dyrk1a haploinsufficiency induces a reduced brain size in mice, and DYRK1A overexpression is the candidate gene for intellectual disability (ID) and microcephaly in Down syndrome. We have identified a 69 kb deletion including the 5' region of the DYRK1A gene in a patient with growth retardation, primary microcephaly, facial dysmorphism, seizures, ataxic gait, absent speech and ID. Because four patients previously reported with intragenic DYRK1A rearrangements or 21q22 microdeletions including only DYRK1A presented with overlapping phenotypes, we hypothesised that DYRK1A mutations could be responsible for syndromic ID with severe microcephaly and epilepsy., Methods: The DYRK1A gene was studied by direct sequencing and quantitative PCR in a cohort of 105 patients with ID and at least two symptoms from the Angelman syndrome spectrum (microcephaly < -2.5 SD, ataxic gait, seizures and speech delay)., Results: We identified a de novo frameshift mutation (c.290&#95;291delCT; p.Ser97Cysfs*98) in a patient with growth retardation, primary severe microcephaly, delayed language, ID, and seizures., Conclusion: The identification of a truncating mutation in a patient with ID, severe microcephaly, epilepsy, and growth retardation, combined with its dual function in regulating the neural proliferation/neuronal differentiation, adds DYRK1A to the list of genes responsible for such a phenotype. ID, microcephaly, epilepsy, and language delay are the more specific features associated with DYRK1A abnormalities. DYRK1A studies should be discussed in patients presenting such a phenotype.

Shprintzen-Goldberg syndrome (SGS) is characterized by severe marfanoid habitus, intellectual disability, camptodactyly, typical facial dysmorphism, and craniosynostosis. Using family-based exome sequencing, we identified a dominantly inherited heterozygous in-frame deletion in exon 1 of SKI. Direct sequencing of SKI further identified one overlapping heterozygous in-frame deletion and ten heterozygous missense mutations affecting recurrent residues in 18 of the 19 individuals screened for SGS; these individuals included one family affected by somatic mosaicism. All mutations were located in a restricted area of exon 1, within the R-SMAD binding domain of SKI. No mutation was found in a cohort of 11 individuals with other marfanoid-craniosynostosis phenotypes. The interaction between SKI and Smad2/3 and Smad 4 regulates TGF-β signaling, and the pattern of anomalies in Ski-deficient mice corresponds to the clinical manifestations of SGS. These findings define SGS as a member of the family of diseases associated with the TGF-β-signaling pathway., (Copyright © 2012 The American Society of Human Genetics. Published by Elsevier Inc. All rights reserved.)

Poikiloderma occurs in a number of hereditary syndromes, the best known of which is Rothmund-Thomson syndrome (RTS). Differential diagnoses include Dyskeratosis Congenita (DC) with high genetic heterogeneity and Clericuzio-type Poikiloderma with Neutropenia (CPN) due to mutations in the C16orf57 gene. Mutations in the RECQL4 gene are only observed in two thirds of RTS patients. In this study, 10 patients referred for syndromic poikiloderma and negative for RECQL4 sequencing analysis were investigated for C16orf57 mutations. Two C16orf57 heterozygous nonsense mutations (p.W81X and p.Y89X) were identified in a 5-year-old female child presenting with generalized poikiloderma, dental dysplasia, gingivitis, nail dystrophy, palmoplantar keratoderma and pachyonychia of the great toenails. Previously undetected and silent neutropenia was evidenced after C16orf57 molecular analysis. Neutropenia was absent in the C16orf57-negative patients. This report confirms that neutrophil count should be performed in all patients with poikiloderma to target the C16orf57 gene sequencing analysis, prior to RECQL4 analysis., (Copyright © 2011 Elsevier Masson SAS. All rights reserved.)

Background: Cinnamoyl CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) catalyze the final steps in the biosynthesis of monolignols, the monomeric units of the phenolic lignin polymers which confer rigidity, imperviousness and resistance to biodegradation to cell walls. We have previously shown that the Eucalyptus gunnii CCR and CAD2 promoters direct similar expression patterns in vascular tissues suggesting that monolignol production is controlled, at least in part, by the coordinated transcriptional regulation of these two genes. Although consensus motifs for MYB transcription factors occur in most gene promoters of the whole phenylpropanoid pathway, functional evidence for their contribution to promoter activity has only been demonstrated for a few of them. Here, in the lignin-specific branch, we studied the functional role of MYB elements as well as other cis-elements identified in the regulatory regions of EgCAD2 and EgCCR promoters, in the transcriptional activity of these gene promoters., Results: By using promoter deletion analysis and in vivo footprinting, we identified an 80 bp regulatory region in the Eucalyptus gunnii EgCAD2 promoter that contains two MYB elements, each arranged in a distinct module with newly identified cis-elements. A directed mutagenesis approach was used to introduce block mutations in all putative cis-elements of the EgCAD2 promoter and in those of the 50 bp regulatory region previously delineated in the EgCCR promoter. We showed that the conserved MYB elements in EgCAD2 and EgCCR promoters are crucial both for the formation of DNA-protein complexes in EMSA experiments and for the transcriptional activation of EgCAD2 and EgCCR promoters in vascular tissues in planta. In addition, a new regulatory cis-element that modulates the balance between two DNA-protein complexes in vitro was found to be important for EgCAD2 expression in the cambial zone., Conclusions: Our assignment of functional roles to the identified cis-elements clearly demonstrates the importance of MYB cis-elements in the transcriptional regulation of two genes of the lignin-specific pathway and support the hypothesis that MYB elements serve as a common means for the coordinated regulation of genes in the entire lignin biosynthetic pathway.

Oral-facial-digital type I syndrome (OFDI) is characterised by an X-linked dominant mode of inheritance with lethality in males. Clinical features include facial dysmorphism with oral, dental and distal abnormalities, polycystic kidney disease and central nervous system malformations. Considerable allelic heterogeneity has been reported within the OFD1 gene, but DNA bi-directional sequencing of the exons and intron-exon boundaries of the OFD1 gene remains negative in more than 20% of cases. We hypothesized that genomic rearrangements could account for the majority of the remaining undiagnosed cases. Thus, we took advantage of two independent available series of patients with OFDI syndrome and negative DNA bi-directional sequencing of the exons and intron-exon boundaries of the OFD1 gene from two different European labs: 13/36 cases from the French lab; 13/95 from the Italian lab. All patients were screened by a semiquantitative fluorescent multiplex method (QFMPSF) and relative quantification by real-time PCR (qPCR). Six OFD1 genomic deletions (exon 5, exons 1-8, exons 1-14, exons 10-11, exons 13-23 and exon 17) were identified, accounting for 5% of OFDI patients and for 23% of patients with negative mutation screening by DNA sequencing. The association of DNA direct sequencing, QFMPSF and qPCR detects OFD1 alteration in up to 85% of patients with a phenotype suggestive of OFDI syndrome. Given the average percentage of large genomic rearrangements (5%), we suggest that dosage methods should be performed in addition to DNA direct sequencing analysis to exclude the involvement of the OFD1 transcript when there are genetic counselling issues., ((c) 2008 Wiley-Liss, Inc.)

Summary EgMYB2, a member of a new subgroup of the R2R3 MYB family of transcription factors, was cloned from a library consisting of RNA from differentiating Eucalyptus xylem. EgMYB2 maps to a unique locus on the Eucalyptus grandis linkage map and co-localizes with a quantitative trait locus (QTL) for lignin content. Recombinant EgMYB2 protein was able to bind specifically the cis-regulatory regions of the promoters of two lignin biosynthetic genes, cinnamoyl-coenzyme A reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD), which contain MYB consensus binding sites. EgMYB2 was also able to regulate their transcription in both transient and stable expression assays. Transgenic tobacco plants over-expressing EgMYB2 displayed phenotypic changes relative to wild-type plants, among which were a dramatic increase in secondary cell wall thickness, and an alteration of the lignin profiles. Transcript abundance of genes encoding enzymes specific to lignin biosynthesis was increased to varying extents according to the position of individual genes in the pathway, whereas core phenylpropanoid genes were not significantly affected. Together these results suggest a role for EgMYB2 in the co-ordinated control of genes belonging to the monolignol-specific pathway, and therefore in the biosynthesis of lignin and the regulation of secondary cell wall formation.

Intron-bearing replacement histone H3 genes in Arabidopsis and other plants are highly and constitutively expressed. We demonstrate that the introns located within the 5'-untranslated regions (5'-UTR) of the two Arabidopsis replacement H3 genes will abolish the cell cycle dependence of an endogenous histone H4 promoter. We demonstrate that these introns, functionally combined with their endogenous promoters, could produce the high and constitutive expression of the replacement H3 genes observed in planta. They strongly increase gene expression whatever the promoter, from the strong 35S CaMV promoter to complete and resected promoters of cell cycle-dependent and replacement histone genes. Quantitative analysis of the extent of reporter gene enhancement in different parts of developing transgenic plantlets, ranging from 2-fold to 70-fold, supports the notion that trans-acting factors are responsible for this effect. Such factors appear most abundant in roots.

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Although the basic mechanisms which control the progression through the cell cycle appear to be conserved in all higher eukaryotes, the unique features of the plant developmental programme must be somehow reflected in a plant-specific regulation of the factors which control cell division. In the past few years, considerable progress has been achieved in identifying the major components of the cell cycle machinery in plants, especially the cyclin-dependent kinases (CDKs) and their regulatory subunits, the cyclins. The question of how these components direct expression of specific genes at specific stages of the cell cycle, and how they are themselves regulated, constitutes a challenge for the present and for the years to come. This review summarizes our current knowledge of a particular class of plant cyclins, the A-type cyclins, which can be further subdivided into three structural groups. The putative functions of these A-type cyclins are discussed in relation to the presence of remarkable motifs in their amino acid sequences, and to their specific transcriptional regulation, protein amount and subcellular localization.

Protein kinase CK2 is an ubiquitous Ser/Thr kinase essential for cell growth. We have used the highly synchronizable tobacco BY-2 cell line to investigate whether CK2 activity and expression are regulated in a cell cycle phase-dependent manner in higher plants. Specific cDNA probes for tobacco CK2alpha and beta subunits, respectively, and polyclonal antibodies recognising alpha and beta subunits separately, were obtained to determine mRNA and protein levels of both subunits. Our results show that CK2 activity oscillates throughout the cell cycle, peaking at G1/S and M phases, due to a post-translational regulation of the tetrameric enzyme. Additional levels of control of CK2 expression operate in relation to the proliferative state of the cells, including differential accumulation of alpha and beta transcripts and post-transcriptional regulation of protein levels (beta subunit). Moreover, in vivo inhibition of CK2 activity corroborates the requirement of the functional CK2 to progress through the cell division cycle, and suggests that CK2 might play an important role at the G2/M checkpoint.

Although the basic mechanisms which control the progression through the cell cycle appear to be conserved in all higher eukaryotes, the unique features of the plant developmental programme must be somehow reflected in a plant-specific regulation of the factors which control cell division. In the last few years, considerable progress has been achieved in identifying the major components of the cell cycle in plants. The question of how these components direct expression of specific genes at specific stages of the cell cycle, and how they are themselves regulated, constitutes a challenge for the present and the next years. This review summarizes our current knowledge at molecular and biochemical levels of cell cycle-regulated expression in the model system, the synchronized tobacco BY2 cell suspension, and discusses the results in comparison to those obtained by different methods and in other plant systems.