Nucleoli and nucleolus-associated chromatin (NAC) of radicle cells have been three-dimensionally reconstructed from serial ultrathin sections during early germination of Zea mays. As a preliminary, the effect of 5 methods of fixation on the ultrastructure of the active NAC were tested qualitatively and quantitatively. It appeared that paraformaldehyde best preserved the fibrillar centres (FCs) and was consequently used for the 3-D reconstructions. In quiescent cells, the NAC forms either 2 short internal strands about 0.7 μm thick running within the nucleolus or 2 peripheral knobs of the same diameter. Whatever its morphology, the NAC was composed of one clear zone, i.e., secondary constriction (SC) of the nucleolar organizer region (NOR), and one electron-opaque zone, i.e., heterochromatic segment (HS). During germination the NAC was always connected to the nuclear envelope (NE) by a bridge of dense chromatin. The NAC strands or knobs of the quiescent cells are likely to be the counterpart of the 2 NORs of this species. 10–12 hr after onset of germination, one or several networks of nucleolar vacuoles were formed within which the whole NAC was located. Chromatin fibers about 12 nm thick emerged from unfolding portions of the NAC within these “nucleolar chromatin dispersal vacuoles” (NCDV). At 24 hr, the NAC appeared as 2 dichotomous strands. Seventy-two hr after germination both the stretching out and branching of the NAC were more pronounced. After 120 hr, the transcribing ribosomal genes for each NAC strand together with the newly synthesized RNP transcripts formed a layer of dense fibrillar component surrounding a thin axis which was composed mainly of pale material. Together these formed a typical plant nucleolonema.
Dopamine (DA) facilitates male sexual behavior and modulates aromatase activity in the quail preoptic area (POA). Aromatase neurons in the POA receive dopaminergic inputs, but the anatomical substrate that mediates the behavioral and endocrine effects of DA is poorly understood. Intracellular recordings showed that 100 μmDA hyperpolarizes most neurons in the medial preoptic nucleus (80%) by a direct effect, but depolarizes a few others (10%). DA-induced hyperpolarizations were not blocked by D1 or D2 antagonists (SCH-23390 and sulpiride). Extracellular recordings confirmed that DA inhibits the firing of most cells (52%) but excites a few others (24%). These effects also were not affected by DA antagonists (SCH-23390 and sulpiride) but were blocked by α2-(yohimbine) and α1-(prazosin) noradrenergic receptor antagonists, respectively. Two dopamine-β-hydroxylase (DBH) inhibitors (cysteine and fusaric acid) did not block the DA-induced effects, indicating that DA is not converted into norepinephrine (NE) to produce its effects. The pKB of yohimbine for the receptor involved in the DA- and NE-induced inhibitions was similar, indicating that the two monoamines interact with the same receptor. Together, these results demonstrate that the effects of DA in the POA are mediated mostly by the activation of α2 (inhibition) and α1(excitation) adrenoreceptors. This may explain why DA affects the expression of male sexual behavior through its action in the POA, which contains high densities of α2-noradrenergic but limited amounts of DA receptors. This study thus clearly demonstrates the existence of a cross talk within CNS catecholaminergic systems between a neurotransmitter and heterologous receptors.
Metallic micronutrients are essential throughout the plant life cycle. Maintaining metal homeostasis in plant tissues requires a highly complex and finely tuned network controlling metal uptake, transport, distribution and storage. Zinc and cadmium hyperaccumulation, such as observed in the model plant Arabidopsis halleri, represents an extreme evolution of this network. Here, non-ectopic overexpression of the A. halleri ZIP6 (AhZIP6) gene, encoding a zinc and cadmium influx transporter, in Arabidopsis thaliana enabled examining the importance of zinc for flower development and reproduction. We show that AhZIP6 expression in flowers leads to male sterility resulting from anther indehiscence in a dose-dependent manner. The sterility phenotype is associated to delayed tapetum degradation and endothecium collapse, as well as increased magnesium and potassium accumulation and higher expression of the MHX gene in stamens. It is rescued by the co-expression of the zinc efflux transporter AhHMA4, linking the sterility phenotype to zinc homeostasis. Altogether, our results confirm that AhZIP6 is able to transport zinc in planta and highlight the importance of fine-tuning zinc homeostasis in reproductive organs. The study illustrates how the characterization of metal hyperaccumulation mechanisms can reveal key nodes and processes in the metal homeostasis network.
