ALBERT

Notes: Abstract Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum-Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific-type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward-dipping subduction zone(s). The 650-500 Ma arc-related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490-470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present-day western Pacific. At 480-470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460-430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc-continent collision(s). From 435-400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430-400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro-Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390-370 Ma collision of Gondwana-derived Aquitaine-Cantabrian blocks with Eastern Avalonia-Baltica and removed the part of the Iapetus south of Baltica.Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present-day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid-Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a 〈 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably 〈 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present-day Mediterranean.

Notes: Raphanus sativus L. Chinese Radish Jumbo Scarlet has characteristics that make it an excellent plant model for vernalization studies. This study further characterizes flower induction of R. sativus Chinese Radish Jumbo Scarlet. Seed were imbibed in distilled water containing 0, 10−5M or 10−3M GA3 for 24 h and were then exposed to 6 ± 0.5°C (vernalized) for 0, 2, 4, 6, or 8 days. Seedlings were then grown under a short- (8 h) or long-day photoperiod (8 h with or without a 4-h night interruption; 2200–0200 h). Of unvernalized plants grown under long- and short-day conditions, 45 and 3% flowered, respectively. Saturation of the vernalization response occurred after a 4- or 8-day vernalization treatment when plants were placed under long- or short-days, respectively. Basal leaf number and days to anthesis decreased when seeds were cooled for 2 or 4 days and were imbibed with 10−3M GA3 compared to distilled water only. These data indicate that R. sativus Chinese Jumbo Scarlet has principally an obligate vernalization requirement when grown under short-days. GA3 application only facilitated flowering when the length of the vernalization treatment was marginal. Taken together, these data support the use of this plant as a model plant for identifying vernalization responses under short-day conditions.

Notes: Numerous studies have shown that auxin induces adventitious root initiation in stem explants from a variety of species, including tobacco. A dominant, monogenic mutation previously identified in tobacco (Nicotiana tabacum cv. Xanthii), rac, confers tenfold auxin resistance to mesophyll-derived cell suspensions and an impaired primary root development phenotype to seedlings. Results presented here demonstrate that adventitious root formation does not occur when heterozygous and homozygous rac stem cuttings are treated in vitro with indole-3-butyric acid (IBA) concentrations ranging from 0.5 μM to 500 μM. Histological analysis showed that some phloem parenchyma or inner cortical parenchyma cells in wild-type stem cuttings undergo adventitious root morphogenesis when they are treated with 5 μM IBA. The same cell types in heterozygous and homozygous rac stem cuttings undergo mitoses in response to auxin, but never form adventitious root meristems. The lack of adventitious root initiation in rac stem cuttings is phenotypically distinct from the aberrant primary root development in rac seedlings. The rac mutation appears to block an essential process for auxin induction of adventitious root initiation but not cell division in phloem parenchyma or inner cortical parenchyma cells. Comparisons of rac heterozygous and homozygous seedling primary root length and callus formation in response to auxin in stem cuttings indicate that rac copy number is correlated to the degree of expression of these two phenotypes.

Notes: Cell division and root initiation of excised juvenile, mature and half-expanded mature (My) leaf petioles of Hedera helix L. cultured in vitro were studied to determine whether these processes were correlated with localized expression of a proline-rich protein (PRP) gene. Petioles of all three types showed cell divisions at day 5 of culture in auxin-treated petioles but not in non-auxin-treated petioles. No cell division occurred in non-auxin-treated petioles even after day 9 of culture. Juvenile and one population of My auxin-treated petioles formed root primordia after 9 days of culture. Mature petioles and another population of My petioles formed only callus in response to auxin. The spatial and temporal expression pattern of a gene encoding a PRP was analyzed by in situ hybridization. The PRP mRNA was not detectable in petioles of any developmental phase immediately after excision. In both juvenile and mature petioles the PRP mRNA preferentially accumulated in the phloem parenchyma, the inner cortex adjacent to the phloem, and in cells surrounding ducts. Cell division was not required for PRP gene expression since both auxin-treated and non-treated juvenile and mature petioles had expression. Steady state levels of PRP mRNA were much lower in juvenile relative to mature petioles cultured in vitro. Auxin treatment reduced the steady state levels of PRP mRNA in My petioles but not in mature or juvenile petioles. These data are consistent with an inverse relationship between competence to form adventitious roots and PRP mRNA levels in the specific cell types from which root primordia form. Alternatively, the PRP mRNA level may serve as a molecular marker for developmental plasticity for root initiation.

Notes: Summary The mitochondrial genome of Black Mexican Sweet maize consists of the principal genome, a 2.3 kb minilinear DNA, a 1,913 by (1.9 kb) and a 1,445 bp (1.4 kb) minicircular DNA. The three extrachromosomal DNAs exhibit characteristics of autonomous replication in cell suspension culture. The complete sequence of the 1.4 kb minicircle was determined. It has 61 by of near perfect sequence homology to the 1.9 kb minicircle. Both minicircular DNAs are transcriptionally active; the longest open reading frame of the 1.4 kb minicircle was 231 bp. A putative origin of replication was identified as a high A+T sequence. These minicircles were present in some but not all of 20 maize lines surveyed. None of the lines examined carried the 1.4 kb minicircle without the 1.9 kb minicircle. Nuclear DNA of one line of the seven examined carried homology to both DNAs.

Notes: Abstract Juvenile-phase English ivy (Hedera helix L.) accumulates anthocyanin pigment in the hypodermis of stems and petioles, whereas mature-phase ivy does not. Lamina tissue of both phases of ivy accumulate flavonols, another class of the flavonoids, in response to sucrose and light treatment in vitro. However, juvenile- but not mature-phase lamina tissue accumulates anthocyanin in response to sucrose and light. The lack of anthocyanin accumulation in mature phase tissue is due to a lack of dihydroflavonol reductase (DFR) activity, which catalyzes a reaction late in the anthocyanin biosynthetic pathway. The objective of this work was to determine the level of regulation of gene expression that limits DFR activity in mature phase tissue. There was an induction of DFR transcription and accumulation of DFR mRNA in juvenile-phase lamina tissue treated with sucrose and light. In contrast, transcription and mRNA accumulation of DFR was not detectable in treated mature-phase lamina tissue. The induction of DFR transcription in juvenile tissue required the combination of sucrose and light. There was an induction of transcription of chalcone synthase, which catalyzes the first committed reaction of flavonoid biosynthesis, in both juvenile- and maturephase lamina tissue, indicating that mature-phase tissue is responsive to sucrose and light treatment.

Notes: Abstract A critical stage in pollen development is the dissolution of the four products of meiosis, the tetrads, into free microspores. The tetrads are surrounded by a thick callose wall composed of β-1,3-glucan. At the completion of meiosis, the tetrads are released into the anther locule after hydrolysis of the callose by a β-1,3-glucanase. Using the polymerase chain reaction, we have amplified and subsequently cloned a cDNA corresponding to a β-1,3-glucanase, tobacco (Nicotiana tabacum cv. Samsun) anther glucanase (Tag 1), which is expressed exclusively in anthers from meiosis to the free microspore stage of pollen development. The identity of the clone was determined by DNA and deduced protein sequence similarity to other known β-1,3-glucanases. Several regions strictly conserved among four classes of glucanases are also conserved in the Tag 1 protein. Tag 1 represents a novel class of β-1,3-glucanase based on phylogenetic analysis and RNA expression pattern. Tag 1 RNA was detected in situ only in the tapetum, with maximal expression just prior to tetrad dissolution. Due to its expression pattern and sequence similarity to other β-1,3-glucanases, we believe Tag 1 may be involved in tetrad dissolution.