Proper-motion age dating of the progeny of Nova Scorpii AD 1437.

'Cataclysmic variables' are binary star systems in which one star of the pair is a white dwarf, and which often generate bright and energetic stellar outbursts. Classical novae are one type of outburst: when the white dwarf accretes enough matter from its companion, the resulting hydrogen-rich atmospheric envelope can host a runaway thermonuclear reaction that generates a rapid brightening. Achieving peak luminosities of up to one million times that of the Sun, all classical novae are recurrent, on timescales of months to millennia. During the century before and after an eruption, the 'novalike' binary systems that give rise to classical novae exhibit high rates of mass transfer to their white dwarfs. Another type of outburst is the dwarf nova: these occur in binaries that have stellar masses and periods indistinguishable from those of novalikes but much lower mass-transfer rates, when accretion-disk instabilities drop matter onto the white dwarfs. The co-existence at the same orbital period of novalike binaries and dwarf novae-which are identical but for their widely varying accretion rates-has been a longstanding puzzle. Here we report the recovery of the binary star underlying the classical nova eruption of 11 March AD 1437 (refs 12, 13), and independently confirm its age by proper-motion dating. We show that, almost 500 years after a classical-nova event, the system exhibited dwarf-nova eruptions. The three other oldest recovered classical novae display nova shells, but lack firm post-eruption ages, and are also dwarf novae at present. We conclude that many old novae become dwarf novae for part of the millennia between successive nova eruptions.

Exchanging sequence domains between S-RNases from Nicotiana alata disrupts pollen recognition.

In self-incompatible plants of the Solanaceae, the specificity of pollen rejection is controlled by a single multiallelic S-locus. Pollen tube growth is inhibited in the style when its single S-allele matches either S-allele present in the diploid pistil. Each S-allele encodes an S-RNase with a unique sequence. S-RNases are secreted into the extra-cellular matrix of the transmitting tract which guides pollen tubes toward the ovary. Although it is known that S-RNases are the determinants of S-allele specificity in the pistil, it is not known how allele-specific information is encoded in the sequence. Therefore, we exchanged domains between S-RNases with different recognition specificities and expressed the chimeric proteins in transgenic plants to determine their effects on pollination behavior. Nine chimeric constructs were prepared in which domains from Nicotiana alata SA2- and SC10-RNases were exchanged. Among these nine constructs, the entire S-RNase sequence was sampled by exchanging single variable domains as well as larger blocks of contiguous sequences. The chimeric S-RNases retained enzymatic activity and were expressed at levels comparable to control transformants expressing SA2- and SC10-RNases. However, none of the chimeric S-RNases caused rejection of either SA2- or SC10-pollen. We conclude that the recognition function of S-RNases can be disrupted by alterations in many parts of the sequence. It appears that the recognition function of S-RNase is not localized to a specific domain.

S RNase and Interspecific Pollen Rejection in the Genus Nicotiana: Multiple Pollen-Rejection Pathways Contribute to Unilateral Incompatibility between Self-Incompatible and Self-Compatible Species.

In self-incompatible (SI) plants, the S locus acts to prevent growth of self-pollen and thus promotes outcrossing within the species. Interspecific crosses between SI and self-compatible (SC) species often show unilateral incompatibility that follows the SI x SC rule: SI species reject pollen from SC species, but the reciprocal crosses are usually compatible. The general validity of the SI x SC rule suggests a link between SI and interspecific pollen rejection; however, this link has been questioned because of a number of exceptions to the rule. To clarify the role of the S locus in interspecific pollen rejection, we transformed several Nicotiana species and hybrids with genes encoding SA2 or SC10 RNase from SI N. alata. Compatibility phenotypes in the transgenic plants were tested using pollen from three SC species showing unilateral incompatibility with N. alata. S RNase was implicated in rejecting pollen from all three species. Rejection of N. plumbaginifolia pollen was similar to S allele-specific pollen rejection, showing a requirement for both S RNase and other genetic factors from N. alata. In contrast, S RNase-dependent rejection of N. glutinosa and N. tabacum pollen proceeded without these additional factors. N. alata also rejects pollen from the latter two species through an S RNase-independent mechanism. Our results implicate the S locus in all three systems, but it is clear that multiple mechanisms contribute to interspecific pollen rejection.

Selective use of gingival-toned ceramics: case reports.

Use of periodontal surgical procedures should generally be considered, prior to prosthodontic treatment, for ridge augmentation and correction of minor soft tissue deficiencies. However, in certain instances surgery may be contraindicated or undesirable. An adequate functional and cosmetic result can often be achieved by incorporating pink ceramic material in the patient's fixed prosthesis. Five case reports are reviewed to demonstrate some appropriate uses of this material.

Investigation of Gene Expression, Growth Kinetics, and Wall Extensibility during Brassinosteroid-Regulated Stem Elongation.

Brassinosteroids promote stem elongation in a variety of plants but little is known about the mechanism of action of these plant growth regulators. We investigated a number of physiological and molecular parameters associated with brassinosteroid-enhanced elongation. Continuous growth recordings of soybean (Glycine max L. cv Williams 82) epicotyls showed that there was a 45-min lag before 0.1 [mu]M brassinolide (BR) exerted a detectable effect on elongation. BR caused a marked increase in Instron-measured plastic extensibility, suggesting that BR may promote elongation in part by altering mechanical properties of the cell wall (wall loosening). Structure-function studies suggested that the dimensions of the brassinosteroid side chain were critical for promotion of elongation and expression of BRU1, a gene regulated specifically by active brassinosteroids. Auxin-BR interactions were examined by using small auxin up RNA (SAUR) gene probes and the auxin-insensitive diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.). We have shown that in wild-type tomato, which elongates in response to exogenous auxin, a transcript of identical size to the soybean SAUR 15A is strongly induced within 1 h by 50 [mu]M 2,4-dichlorophenoxyacetic acid or indoleacetic acid, whereas in the dgt mutant, which does not elongate in response to auxin, no transcript is expressed. Furthermore, BR promotes equal elongation of hypocotyls in both wild-type and dgt tomatoes but does not rapidly induce the SAUR 15A homolog in either genotype. BR does not cause rapid induction of SAUR 6B in elongating soybean epicotyls but does lead to increased expression after 18 h. This late BR activation of SAUR 6B is controlled, at least in part, at the transcriptional level and is not accompanied by an increase of free indoleacetic acid in the tissue. We conclude that although both BR and auxin affect wall relaxation processes, BR-promoted elongation in soybean and tomato stems acts via a mechanism that most likely does not proceed through the auxin signal transduction pathway.

Molecular cloning and characterization of a brassinosteroid-regulated gene from elongating soybean (Glycine max L.) epicotyls.

Brassinosteroids promote elongation and regulate gene expression in soybean (Glycine max L.) stems. We constructed a cDNA library from brassinosteroid-treated soybean epicotyls and used differential hybridization to isolate a cDNA (pBRU1) corresponding to a transcript whose abundance is increased by brassinosteroid treatment. Sequence analysis of pBRU1 revealed an open reading frame of 283 amino acids with a putative signal peptide of 29 amino acids. The sequence had extensive homology (77% identity, 89% similarity) over 114 contiguous amino acids to the meri-5 gene of Arabidopsis thaliana (J.I. Medford, J.S. Elmer, H.J. Klee [1991] Plant Cell 3: 359-370), and significant homology (48% identity, 62% similarity) to a xyloglucan endotransglycosylase localized in the cell walls of nasturtium (J. de Silva, C.D. Jarman, D.A. Arrowsmith, M.S. Stronach, S. Chengappa, C. Sidebottom, J.S. Reid [1993] Plant J 3: 701-711). RNase protection studies showed that BRU1 transcript levels are not increased by 1.0 microM auxins, cytokinins, abscisic acid, or gibberellic acid and that BRU1 expression is highest in stem tissue. Findings from studies with run-on transcripts from isolated soybean nuclei most likely indicate that the regulation of BRU1 by brassinosteroids is largely posttranscriptional. The elevated levels of BRU1 transcripts in elongating tissue and the homology with a xyloglucan endotransglycosylase suggest a possible role for the BRU1 protein in brassinosteroid-stimulated elongation.

Effect of brassinolide on gene expression in elongating soybean epicotyls.

We have studied the effect of brassinolide (BR), a plant steroidal lactone, on the expression of auxin-regulated genes in soybean (Glycine max L. cv Williams 82) epicotyls. BR caused up to 4-fold increases in epicotyl length during extended assays at 10(-7)m, in the absence of added auxin. Structurally related steroids failed to induce elongation or to alter the BR effect. Northern blot analysis, using sequences corresponding to auxin-regulated genes as probes, has shown that the molecular mechanism of BR-induced elongation is likely to differ from that of auxin-induced elongation in this system. BR does not rapidly induce members of the GH, SAUR, or JCW auxin-inducible gene families before the onset of elongation. BR enhances SAUR and GH1 transcripts after 18 h but has no effect on JCW1 or GH3 transcripts at any time examined. We have shown by two-dimensional gel analysis of in vitro translated mRNA that a submicromolar concentration of BR alters the pattern of gene expression in elongating soybean epicotyls.