Aviva Katz

Mutations in the FIE and MEA Genes That Encode Interacting Polycomb Proteins Cause Parent-of-Origin Effects on Seed Development by Distinct Mechanisms

In flowering plants, two cells are fertilized in the haploid female gametophyte. Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses with the central cell nucleus, which replicates to generate the endosperm, a tissue that supports embryo development. The FERTILIZATION-INDEPENDENT ENDOSPERM (FIE) and MEDEA (MEA) genes encode WD and SET domain polycomb proteins, respectively. In the absence of fertilization, a female gametophyte with a loss-of-function fie or mea allele initiates endosperm development without fertilization. fie and mea mutations also cause parent-of-origin effects, in which the wild-type maternal allele is essential and the paternal allele is dispensable for seed viability. Here, we show that FIE and MEA polycomb proteins interact physically, suggesting that the molecular partnership of WD and SET domain polycomb proteins has been conserved during the evolution of flowering plants. The overlapping expression patterns of FIE and MEA are consistent with their suppression of gene transcription and endosperm development in the central cell as well as their control of seed development after fertilization. Although FIE and MEA interact, differences in maternal versus paternal patterns of expression, as well as the effect of a recessive mutation in the DECREASE IN DNA METHYLATION1 (DDM1) gene on mutant allele transmission, indicate that fie and mea mutations cause parent-of-origin effects on seed development by distinct mechanisms.

Polycomb Group Complexes Self-Regulate Imprinting of the Polycomb Group Gene MEDEA in Arabidopsis

Fertilization in flowering plants initiates the development of the embryo and endosperm, which nurtures the embryo. A few genes subjected to imprinting are expressed in endosperm from their maternal allele, while their paternal allele remains silenced. Imprinting of the FWA gene involves DNA methylation. Mechanisms controlling imprinting of the Polycomb group (Pc-G) gene MEDEA (MEA) are not yet fully understood. Here we report that MEA imprinting is regulated by histone methylation. This epigenetic chromatin modification is mediated by several Pc-G activities during the entire plant life cycle. We show that Pc-G complexes maintain MEA transcription silenced throughout vegetative life and male gametogenesis. In endosperm, the maternal allele of MEA encodes an essential component of a Pc-G complex, which maintains silencing of the paternal MEA allele. Hence, we conclude that a feedback loop controls MEA imprinting. This feedback loop ensures a complete maternal control of MEA expression from both parental alleles and might have provided a template for evolution of imprinting in plants.

Class 1 Reversibly Glycosylated Polypeptides Are Plasmodesmal-Associated Proteins Delivered to Plasmodesmata via the Golgi Apparatus

SE-WAP41, a salt-extractable 41-kD wall-associated protein that is associated with walls of etiolated maize (Zea mays) seedlings and is recognized by an antiserum previously reported to label plasmodesmata and the Golgi, was cloned, sequenced, and found to be a class 1 reversibly glycosylated polypeptide (C1RGP). Protein gel blot analysis of cell fractions with an antiserum against recombinant SE-WAP41 showed it to be enriched in the wall fraction. RNA gel blot analysis along the mesocotyl developmental axis and during deetiolation demonstrates that high SE-WAP41 transcript levels correlate spatially and temporally with primary and secondary plasmodesmata (Pd) formation. All four of the Arabidopsis thaliana C1RGP proteins, when fused to green fluorescent protein (GFP) and transiently expressed in tobacco (Nicotiana tabacum) epidermal cells, display fluorescence patterns indicating they are Golgi- and plasmodesmal-associated proteins. Localization to the Golgi apparatus was verified by colocalization of transiently expressed AtRGP2 fused to cyan fluorescence protein together with a known Golgi marker, Golgi Nucleotide Sugar Transporter 1 fused to yellow fluorescent protein (GONST1:YFP). In transgenic tobacco, AtRGP2:GFP fluorescence is punctate, is present only in contact walls between cells, and colocalizes with aniline blue–stained callose present around Pd. In plasmolyzed cells, AtRGP2:GFP remains wall embedded, whereas GONST1:YFP cannot be found embedded in cell walls. This result implies that the targeting to Pd is not due to a default pathway for Golgi-localized fusion proteins but is specific to C1RGPs. Treatment with the Golgi disrupting drug Brefeldin A inhibits Pd labeling by AtRGP2:GFP. Integrating these data, we conclude that C1RGPs are plasmodesmal-associated proteins delivered to plasmodesmata via the Golgi apparatus.

Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution

The Polycomb group (PcG) complex is involved in the epigenetic control of gene expression profiles. In flowering plants, PcG proteins regulate vegetative and reproductive programs. Epigenetically inherited states established in the gametophyte generation are maintained after fertilization in the sporophyte generation, having a profound influence on seed development. The gametophyte size and phase dominance were dramatically reduced during angiosperm evolution, and have specialized in flowering plants to support the reproductive process. The moss Physcomitrella patens is an ideal organism in which to study epigenetic processes during the gametophyte stage, as it possesses a dominant photosynthetic gametophytic haploid phase and efficient homologous recombination, allowing targeted gene replacement. We show that P. patens PcG protein FIE (PpFIE) accumulates in haploid meristematic cells and in cells that undergo fate transition during dedifferentiation programs in the gametophyte. In the absence of PpFIE, meristems overproliferate and are unable to develop leafy gametophytes or reach the reproductive phase. This aberrant phenotype might result from failure of the PcG complex to repress proliferation and differentiation of three-faced apical stem cells, which are designated to become lateral shoots. The PpFIE phenotype can be partially rescued by FIE of Arabidopsis thaliana, a flowering plant that diverged >450 million years ago from bryophytes. PpFIE can partially complement the A. thaliana fie mutant, illustrating functional conservation of the protein during evolution in regulating the differentiation of meristematic cells in gametophyte development, both in bryophytes and angiosperms. This mechanism was harnessed at the onset of the evolution of alternating generations, facilitating the establishment of sporophytic developmental programs.

A dysfunctional movement protein of tobacco mosaic virus interferes with targeting of wild-type movement protein to microtubules.

The Tobacco mosaic virus (TMV) movement protein (MPTMV) mediates cell-to-cell viral trafficking by altering properties of the plasmodesmata (Pd) in infected cells. During the infection cycle, MPTMV becomes transiently associated with endomembranes, microfilaments, and microtubules (MT). It has been shown that the cell-to-cell spread of TMV is reduced in plants expressing the dysfunctional MP mutant MPNT-1. To expand our understanding of the MP function, we analyzed events occurring during the intracellular and intercellular targeting of MPTMV and MPNT-1 when expressed as a fusion protein to green fluorescent protein (GFP), either by biolistic bombardment in a viral-free system or from a recombinant virus. The accumulation of MPTMV:GFP, when expressed in a viral-free system, is similar to MPTMV:GFP in TMV-infected tissues. Pd localization and cell-to-cell spread are late events, occurring only after accumulation of MP:GFP in aggregate bodies and on MT in the target cell. MPNT-1:GFP localizes to MT but does not target to Pd nor does it move cell to cell. The spread of transiently expressed MPTMV:GFP in leaves of transgenic plants that produce MPNT-1 is reduced, and targeting of the MPTMV:GFP to the cytoskeleton is inhibited. Although MPTMV:GFP targets to the Pd in these plants, it is partially impaired for movement. It has been suggested that MPNT-1 interferes with host-dependent processes that occur during the intracellular targeting program that makes MP movement competent.

A 41 kDa protein isolated from maize mesocotyl cell walls immunolocalizes to plasmodesmata

SummaryPlasmodesmata, dynamic pore structures that traverse plant cell walls, function in cytoplasmic transport between contiguous cells. Cell walls containing embedded plasmodesmata were isolated from mesocotyls of etiolated maize seedlings. Proteins associated with the isolated walls were separated by SDS-PAGE and antibodies were generated against a 41 kDa protein, one of several associated with this wall fraction. Immunoblot analysis showed that the 41 kDa polypeptide was also associated with other subcellular fractions obtained following tissue homogenization and differential centrifugation. The wall associated 41 kDa protein is apparently a peripheral membrane protein since it could be extracted by high salt and high pH. Silver-enhanced immunogold light microscopy showed that the 41 kDa protein was associated with the cell walls of cells both in the stele and cortex. The immunolabeling pattern was transwall and punctate. Electron microscopic immuno-gold labeling localized the polypeptide to plasmodesmata and to electron dense cytoplasmic structures that are apparently Golgi membranes. The significance of the presence of this protein in the Golgi is discussed.

A calcium-dependent protein kinase is associated with maize mesocotyl plasmodesmata

Summary Plasmodesmata (Pd) are trans-wall membrane lined tunnels that regulate cell-to-cell cytoplasmic movement. It has been suggested that Pd conductivity may be regulated by a phosphorylation mechanism. In a maize ( Zea mays L.) mesocotyl cell wall fraction, a Ca 2+ -dependent protein kinase (CDPK) is present that phosphorylates approximately 8 of 20 wall-associated proteins. The kinase is membrane-associated and is not extracted by EGTA, NaCl, up to 4mol/L LiCl, Triton X-100, or Na 2 CO 3 (pH 11), but is fully extracted with SDS or 8 mol/L LiCl. Two polypeptides in the cell wall fraction, with apparent molecular masses of 51 and 56 kD, cross-react with an Arabidopsis CDPK anti-serum and undergo in situ Ca 2+ -dependent autophosphorylation on nitrocellulose. The molecular masses of the CDPKs extracted by 8 mol/L LiCl from the cell wall fraction are different from those extracted from the cell membrane fraction, suggesting that the wall-associated CDPK is unique to the cell wall fraction. Immuno-fluorescence microscopy with isolated walls localizes CDPK to discrete punctate loci in the cell wall. Isolated Pd challenged with CDPK anti-serum show a pattern of cross-reactivity similar to the cell wall fraction. These data suggest that the cell wall-associated CDPK is a putative plasmodesmal-associated membrane protein and may be involved in regulating Pd conductivity.

Interaction of FIE, a polycomb protein, with pRb: a possible mechanism regulating endosperm development.

AbstractInactivation of the Arabidopsis protein FERTILIZATION INDEPENDENT ENDOSPERM (FIE) induces division of the central cell of the embryo sac, leading to endosperm development in the absence of fertilization. The mechanism whereby FIE regulates this process is unknown. We postulated that activation of central cell division in fie mutant plants might involve the retinoblastoma protein (pRb), a cell cycle regulatory element. Pull-down and surface plasmon resonance assays demonstrated that FIE interacts in-vitro with the pRb homologues from Arabidopsis (AtRb), maize (ZmRb) and human (HuRb). The interaction of FIE with ZmRB and HuRb in the yeast two-hybrid system supports the possibility that a FIE-pRb interaction may occur also in planta. Mutational analysis showed that this interaction does not occur via the LxCxE motif of the FIE protein nor via the pocket B domain of pRb. These results suggest that FIE may inhibit premature division of the central cell of the embryo sac, at least partly, through interaction with pRb, and suppression of pRb-regulated genes.

A single homeobox gene triggers phase transition, embryogenesis and asexual reproduction.

Plants characteristically alternate between haploid gametophytic and diploid sporophytic stages. Meiosis and fertilization respectively initiate these two different ontogenies1. Genes triggering ectopic embryo development on vegetative sporophytic tissues are well described2,3; however, a genetic control of embryo development from gametophytic tissues remains elusive. Here, in the moss Physcomitrella patens we show that ectopic overexpression of the homeobox gene BELL1 induces embryo formation and subsequently reproductive diploid sporophytes from specific gametophytic cells without fertilization. In line with this, BELL1 loss-of-function mutants have a wild-type phenotype, except that their egg cells are bigger and unable to form embryos. Our results identify BELL1 as a master regulator for the gametophyte-to-sporophyte transition in P. patens and provide mechanistic insights into the evolution of embryos that can generate multicellular diploid sporophytes. This developmental innovation facilitated the colonization of land by plants about 500 million years ago4 and thus shaped our current ecosystems.

A single CMT methyltransferase homolog is involved in CHG DNA methylation and development of Physcomitrella patens

C-5 DNA methylation is an essential mechanism controlling gene expression and developmental programs in a variety of organisms. Though the role of DNA methylation has been intensively studied in mammals and Arabidopsis, little is known about the evolution of this mechanism. The chromomethylase (CMT) methyltransferase family is unique to plants and was found to be involved in DNA methylation in Arabidopsis, maize and tobacco. The moss Physcomitrella patens, a model for early terrestrial plants, harbors a single homolog of the CMT protein family designated as PpCMT. Our phylogenetic analysis suggested that the CMT family is unique to embryophytes and its earliest known member PpCMT belongs to the CMT3 subfamily. Thus, P. patens may serve as a model to study the ancient functions of the CMT3 family. We have generated a ΔPpcmt deletion mutant which demonstrated that PpCMT is essential for P. patens protonema and gametophore development and is involved in CHG methylation as demonstrated at four distinct genomic loci. PpCMT protein accumulation pattern correlated with proliferating cells and was sub-localized to the nucleus as predicted from its function. Taken together, our results suggested that CHG DNA methylation mediated by CMT has been employed early in land plant evolution to control developmental programs during both the vegetative and reproductive haploid phases along the plant life cycle.