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The haploid multicellular male gametophyte of plants, the pollen grain, is a terminally differentiated structure whose function ends at fertilization. Unlike pollen grains, the immature gametophyte retains its capacity for totipotent growth when cultured in vitro. Haploid embryo production from cultured immature male gametophytes is a widely used plant breeding and propagation technique that was described nearly 50 years ago, but one that is poorly understood at the mechanistic level. Using a chemical approach, we show that the switch to haploid embryogenesis is controlled by the activity of histone deacetylases (HDACs). Blocking HDAC activity with trichostatin A (TSA) in cultured immature male gametophytes of Brassica napus leads to a large increase in the proportion of cells that switch from pollen to embryogenic growth. Embryogenic growth is enhanced by, but not dependent on, the high temperature stress that is normally used to induce haploid embryogenesis in B. napus. The immature male gametophyte of Arabidopsis thaliana, which is recalcitrant for haploid embryo development in culture, also forms embryogenic cell clusters after TSA treatment. TSA treatment of immature male gametophytes for as little as eight hours was accompanied by hyperacetylation of histones H3 and H4, and by the upregulation of genes involved in cell-cycle progression, the auxin pathway and cell wall catabolism pathways. We propose that the totipotency of the immature male gametophyte in planta is kept in check by an HDAC-dependent mechanism, and that high temperature or other stresses used to induce haploid embryo development in culture impinge on this HDAC-dependent pathway.

AINTEGUMENTA-LIKE (AIL) transcription factors are key regulators of cell proliferation and meristem identity. Although AIL functions have been well described, the direct signalling components of this pathway are largely unknown.We show that BABY BOOM(BBM) and other AIL proteins physically interact with multiple members of the L1-expressed HOMEODOMAIN GLABROUS (HDG) transcription factor family, including HDG1, HDG11 and HDG12. Overexpression of HDG1, HDG11 and HDG12 restricts growth due to root and shoot meristem arrest, which is associated with reduced expression of genes involved in meristem development and cell proliferation pathways, whereas downregulation of multiple HDG genes promotes cell overproliferation. These results suggest a role for HDG proteins in promoting cell differentiation. We also reveal a transcriptional network in which BBM andHDG1regulate several common target genes, and whereBBM/AIL and HDG regulate the expression of each other. Taken together, these results suggest opposite roles for AIL and HDG proteins, with AILs promoting cell proliferation and HDGs stimulating cell differentiation, and that these functions are mediated at both the protein-protein interaction and transcriptional level.

Vanadium doped ZnO thin films (Zn1−xVxO, where x=0.05 or x=0.13) were grown on c-cut sapphire substrates using pulsed laser deposition technique. Their structure and magnetic properties were examined in relation to the doping concentration. All deposited films were highly oriented along the c-axis and exhibited ferromagnetic behavior with a Curie temperature up to 300 K. The crystal structure was found to be better for layers with lower vanadium concentration. The films had a porous fine-grained microstructure and a column-like character as the V concentration was reduced. A weak dependence of magnetization on temperature was observed. The saturation magnetization was found to be strongly dependent on the crystal structure, grain size and V-ion concentration.

We report structural, transport, and thermal properties of carrier-tuned Ba_8Ga_<16>Sn_<30> single crystals with the type-1 clathrate structure (β phase), demonstrating that Ba_8Ga_<16>Sn_<30> is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure (α phase) and of A_8Ga_<16>Ge_<30> (A=Sr,Eu). Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520±3 °C. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43–0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/mol K^2 for both n- and p-type carriers in the β phase, four times larger than that for the α phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the α phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities κL for the β phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in κL(T) is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the β phase is significantly larger than that for Sr_8Ga_<16>Ge_<30>.

A glucocorticoid-regulated BBM protein (35S:BBM-GR) was used in combination with microarray analysis to identify genes directly activated by BBM. We employed the system described by (Lloyd et al., 1994) in which dexamethasone (DEX) and cycloheximide (CHX) are applied together to respectively, induce nuclear localization of the BBM-GR protein and prevent translation of the primary targets mRNAs. In this way it is possible to identify direct targets of a transcriptional activator by comparing gene expression profiles between DEX+CHX-treated transgenic and wild-type tissues. The ability of the 35S:BBM GR construct to induce somatic embryogenesis in Arabidopsis seedlings was determined by phenotypic observation of 35S:BBM GR seeds germinated and grown in the presence of 10 µM dexamethasone (DEX). As in 35S:BBM seedlings, we observed somatic embryo formation on the cotyledons, first leaves and shoot meristem of DEX-treated 35S:BBM GR seedlings. We identified a set of 20 genes (including BBM itself) and our analysis indicates that BBM directly activates a signaling pathway comprising transcription factors and other signaling molecules, but which does not initially include genes known to induce somatic embryogenesis, such as LEC1, LEC2 or WUS. The functions of the BBM target genes are unknown, however a number of them have recently been identified in microarray screens for meristem-expressed genes. The identification of BBM-interacting partners and downstream targets provides new tools for unraveling pathways related to plant cell growth and organogenesis.