DEFECTIVE EMBRYO AND MERISTEMS genes are required for cell division and gamete viability in Arabidopsis

The DEFECTIVE EMBRYO AND MERISTEMS 1 (DEM1) gene encodes a protein of unknown biochemical function required for meristem formation and seedling development in tomato, but it was unclear whether DEM1’s primary role was in cell division or alternatively, in defining the identity of meristematic cells. Genome sequence analysis indicates that flowering plants possess at least two DEM genes. Arabidopsis has two DEM genes, DEM1 and DEM2, which we show are expressed in developing embryos and meristems in a punctate pattern that is typical of genes involved in cell division. Homozygous dem1 dem2 double mutants were not recovered, and plants carrying a single functional DEM1 allele and no functional copies of DEM2, i.e. DEM1/dem1 dem2/dem2 plants, exhibit normal development through to the time of flowering but during male reproductive development, chromosomes fail to align on the metaphase plate at meiosis II and result in abnormal numbers of daughter cells following meiosis. Additionally, these plants show defects in both pollen and embryo sac development, and produce defective male and female gametes. In contrast, dem1/dem1 DEM2/dem2 plants showed normal levels of fertility, indicating that DEM2 plays a more important role than DEM1 in gamete viability. The increased importance of DEM2 in gamete viability correlated with higher mRNA levels of DEM2 compared to DEM1 in most tissues examined and particularly in the vegetative shoot apex, developing siliques, pollen and sperm. We also demonstrate that gamete viability depends not only on the number of functional DEM alleles inherited following meiosis, but also on the number of functional DEM alleles in the parent plant that undergoes meiosis. Furthermore, DEM1 interacts with RAS-RELATED NUCLEAR PROTEIN 1 (RAN1) in yeast two-hybrid and pull-down binding assays, and we show that fluorescent proteins fused to DEM1 and RAN1 co-localize transiently during male meiosis and pollen development. In eukaryotes, RAN is a highly conserved GTPase that plays key roles in cell cycle progression, spindle assembly during cell division, reformation of the nuclear envelope following cell division, and nucleocytoplasmic transport. Our results demonstrate that DEM proteins play an essential role in cell division in plants, most likely through an interaction with RAN1.

Up to half of the genes predicted from genome projects lack a known biological and biochemical function. Many of these genes are likely to play essential roles but it is difficult to reveal their function because minor changes in the genetic sequence can result in lethality and genetic redundancy can obscure analysis. Genome projects predict that flowering plants have at least two DEM genes that encode a protein of unknown cellular and biochemical function. In this paper, we use multiple combinations of dem mutants in Arabidopsis to show that DEM genes are essential for cell division and gamete viability. Interestingly, gamete viability depends not only on the number of functional copies of DEM genes in the gametes, but also on the number of functional copies of DEM genes in the parent plant that produces the gametes. We also show that DEM proteins interact with RAN, a highly conserved protein that controls cell division in all eukaryotic organisms.

Differential role of two VDR coactivators, DRIP205 and SRC-3, in keratinocyte proliferation and differentiation

Cell programs such as proliferation and differentiation involve the selective activation and repression of gene expression. The vitamin D receptor (VDR), through 1,25(OH)(2)D(3), controls the proliferation and differentiation of keratinocytes. Previously, we have identified two VDR binding coactivator complexes. In proliferating keratinocytes VDR bound preferentially to the DRIP complex, whereas in differentiated keratinocytes the SRC complex was preferred. We proposed that different coactivators are required for sequential gene regulation in the transition from proliferation to differentiation. Here we examined the roles of DRIP205 and SRC-3 in this transition. Silencing of DRIP205 and VDR caused hyperproliferation of keratinocytes, demonstrated by increased XTT and BrdU incorporation. SRC-3 silencing, on the other hand, did not have an effect on proliferation. In contrast, SRC-3 as well as DRIP205 and VDR silencing blocked keratinocyte differentiation as shown by decreased expression of keratin 1 and filaggrin. These results are consistent with the differential localization of DRIP205 and SRC-3 in skin. These results indicate that DRIP205 is required for keratinocyte proliferation. Both DRIP205 and SRC-3 are required for the keratinocyte differentiation. These results support the concept that the selective use of coactivators by VDR underlies the selective regulation of gene expression in keratinocyte proliferation and differentiation.

Regulation of human epidermal keratinocyte differentiation by the vitamin D receptor and its coactivators DRIP205, SRC2, and SRC3

It has long been known that the active metabolite of vitamin D, 1,25 dihydroxyvitamin D(3), stimulates differentiation and inhibits proliferation in epidermal keratinocytes through interaction with the vitamin D receptor (VDR). VDR functions through the coordinate binding of vitamin D response elements in the DNA and specific coactivator proteins which help to initiate transcription. It was recently observed that VDR binds to two major coactivator complexes, DRIP (VDR-interacting protein) and SRC (steroid receptor coactivator), during keratinocyte differentiation. To determine the role of VDR and its coactivators in mediating keratinocyte differentiation, we developed an adenoviral system to knock down, or in the case of VDR, overexpress these genes. In order to study all stages of keratinocyte development, we employed an advanced differentiated normal human keratinocyte culture system that produces a multilayer phenotype similar to that of normal skin. These studies have shown that VDR, DRIP, and SRC are all required for promotion of both early and late keratinocyte differentiation. Additionally, each individual differentiation marker that was assayed has a different specificity for the coactivators that regulate its expression.

Roles for Class III HD-Zip and KANADI genes in Arabidopsis root development

Meristems within the plant body differ in their structure and the patterns and identities of organs they produce. Despite these differences, it is becoming apparent that shoot and root apical and vascular meristems share significant gene expression patterns. Class III HD-Zip genes are required for the formation of a functional shoot apical meristem. In addition, Class III HD-Zip and KANADI genes function in patterning lateral organs and vascular bundles produced from the shoot apical and vascular meristems, respectively. We utilize both gain- and loss-of-function mutants and gene expression patterns to analyze the function of Class III HD-Zip and KANADI genes in Arabidopsis roots. Here we show that both Class III HD-Zip and KANADI genes play roles in the ontogeny of lateral roots and suggest that Class III HD-Zip gene activity is required for meristematic activity in the pericycle analogous to its requirement in the shoot apical meristem.

Radial Patterning of Arabidopsis Shoots by Class III HD-ZIP and KANADI Genes

BACKGROUND

Shoots of all land plants have a radial pattern that can be considered to have an adaxial (central)-abaxial (peripheral) polarity. In Arabidopsis, gain-of-function alleles of PHAVOLUTA and PHABULOSA, members of the class III HD-ZIP gene family, result in adaxialization of lateral organs. Conversely, loss-of-function alleles of the KANADI genes cause an adaxialization of lateral organs. Thus, the class III HD-ZIP and KANADI genes comprise a genetic system that patterns abaxial-adaxial polarity in lateral organs produced from the apical meristem.

RESULTS

We show that gain-of-function alleles of REVOLUTA, another member of the class III HD-ZIP gene family, are characterized by adaxialized lateral organs and alterations in the radial patterning of vascular bundles in the stem. The gain-of-function phenotype can be obtained by changing only the REVOLUTA mRNA sequence and without changing the protein sequence; this finding indicates that this phenotype is likely mediated through an interference with microRNA binding. Loss of KANADI activity results in similar alterations in vascular patterning as compared to REVOLUTA gain-of-function alleles. Simultaneous loss-of-function of PHABULOSA, PHAVOLUTA, and REVOLUTA abaxializes cotyledons, abolishes the formation of the primary apical meristem, and in severe cases, eliminates bilateral symmetry; these phenotypes implicate these three genes in radial patterning of both embryonic and postembryonic growth.

CONCLUSIONS

Based on complementary vascular and leaf phenotypes of class III HD-ZIP and KANADI mutants, we propose that a common genetic program dependent upon miRNAs governs adaxial-abaxial patterning of leaves and radial patterning of stems in the angiosperm shoot. This finding implies that a common patterning mechanism is shared between apical and vascular meristems.