Post-transcriptional silencing of plant genes using anti-sense or co-suppression constructs usually results in only a modest proportion of silenced individuals. Recent work has demonstrated the potential for constructs encoding self-complementary 'hairpin' RNA (hpRNA) to efficiently silence genes. In this study we examine design rules for efficient gene silencing, in terms of both the proportion of independent transgenic plants showing silencing, and the degree of silencing. Using hpRNA constructs containing sense/anti-sense arms ranging from 98 to 853 nt gave efficient silencing in a wide range of plant species, and inclusion of an intron in these constructs had a consistently enhancing effect. Intron-containing constructs (ihpRNA) generally gave 90-100% of independent transgenic plants showing silencing. The degree of silencing with these constructs was much greater than that obtained using either co-suppression or anti-sense constructs. We have made a generic vector, pHANNIBAL, that allows a simple, single PCR product from a gene of interest to be easily converted into a highly effective ihpRNA silencing construct. We have also created a high-throughput vector, pHELLSGATE, that should facilitate the cloning of gene libraries or large numbers of defined genes, such as those in EST collections, using an in vitro recombinase system. This system may facilitate the large-scale determination and discovery of plant gene functions in the same way as RNAi is being used to examine gene function in Caenorhabditis elegans.

Summary Barley yellow dwarf virus-PAV (BYDV-PAV) is the most serious and widespread virus of cereals worldwide. Natural resistance genes against this luteovirus give inadequate control, and previous attempts to introduce synthetic resistance into cereals have produced variable results. In an attempt to generate barley with protection against BYDV-PAV, plants were transformed with a transgene designed to produce hairpin (hp)RNA containing BYDV-PAV sequences. From 25 independent barley lines transformed with the BYDV-PAV hpRNA construct, nine lines showed extreme resistance to the virus and the majority of these contained a single transgene. In the progeny of two independent transgenic lines, inheritance of a single transgene consistently correlated with protection against BYDV-PAV. This protection was rated as immunity because the virus could not be detected in the challenged plants by ELISA nor recovered by aphid feeding experiments. In the field, BYDV-PAV is sometimes associated with the related luteovirus Cereal yellow dwarf virus-RPV (CYDV-RPV). When the transgenic plants were challenged with BYDV-PAV and CYDV-RPV together, the plants were susceptible to CYDV-RPV but immune to BYDV-PAV. This shows that the immunity is virus-specific and not broken down by the presence of CYDV. It suggests that CYDV-RPV does not encode a silencing-suppressor gene or that its product does not protect BYDV-PAV against the plant's RNAi-like defence mechanism. Either way, our results indicate that the BYDV-PAV immunity will be robust in the field and is potentially useful in minimizing losses in cereal production worldwide.

view Abstract Citations (31) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photospheres of Hot Stars. IV. Spectral Type O4 Bohannan, Bruce ; Voels, Stephen A. ; Hummer, David G. ; Abbott, David C. Abstract The basic stellar parameters of a supergiant (Zeta Pup) and two main-sequence stars, 9 Sgr and HD 46223, at spectral class O4 are determined using line profile analysis. The stellar parameters are determined by comparing high signal-to-noise hydrogen and helium line profiles with those from stellar atmosphere models which include the effect of radiation scattered back onto the photosphere from an overlying stellar wind, an effect referred to as wind blanketing. At spectral class O4, the inclusion of wind-blanketing in the model atmosphere reduces the effective temperature by an average of 10 percent. This shift in effective temperature is also reflected by shifts in several other stellar parameters relative to previous O4 spectral-type calibrations. It is also shown through the analysis of the two O4 V stars that scatter in spectral type calibrations is introduced by assuming that the observed line profile reflects the photospheric stellar parameters. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169526 Bibcode: 1990ApJ...365..729B Keywords: Early Stars; O Stars; Photosphere; Radiative Transfer; Stellar Atmospheres; Supergiant Stars; Abundance; Helium; Line Spectra; Signal To Noise Ratios; Stellar Luminosity; Stellar Mass; Stellar Spectra; Astrophysics; LINE PROFILES; RADIATIVE TRANSFER; STARS: ATMOSPHERES; STARS: EARLY-TYPE; STARS: SUPERGIANTS full text sources ADS | data products SIMBAD (7) Related Materials (3) Part 1: 1985ApJ...294..286A Part 2: 1986ApJ...308..728B Part 3: 1989ApJ...340.1073V

view Abstract Citations (76) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Non--LTE Model Calculations for SN 1987A and the Extragalactic Distance Scale Schmutz, W. ; Abbott, D. C. ; Russell, R. S. ; Hamann, W. -R. ; Wessolowski, U. Abstract We present model atmospheres for the first week of SN 1987A, based on the luminosity and density/velocity structure from hydrodynamic models of Woosley. The models account for line blanketing, expansion, sphericity, and departures from LTE in hydrogen and helium and differ from previously published efforts because our models represent ab initio calculations, i.e., they contain essentially no free parameters. The formation of the UV spectrum is dominated by the effects of line blanketing. In the absorption troughs, the Balmer line profiles were fit well by our models, but the observed emissions are significantly stronger than predicted, perhaps due to clumping. The generally good agreement between our synthetic spectra and observations provides independent support for the overall accuracy of the hydrodynamic models of Woosley. We address the question of the accuracy of the Baade-Wesselink method in a detailed discussion of its approximations. While the application of the standard method produces a distance within an uncertainty of 20% in the case of SN 1987A, systematic errors up to a factor of 2 are possible, particularly if the precursor was a red supergiant. The key parameter that determines the systematic error of the method is the density/velocity structure, which can vary for each individual supernova. The supernova- based extragalactic distance scale is not yet precise enough to be of practical interest, owing to the large possible systematic error in the Hubble constant, H_0_ ~ 60^times 2^_divided by 2_ km s^-1^ Mpc^-1^. Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168759 Bibcode: 1990ApJ...355..255S Keywords: Nonequilibrium Thermodynamics; Stellar Atmospheres; Stellar Models; Supernova 1987a; Balmer Series; Stellar Spectra; Ultraviolet Spectra; Astrophysics; STARS: ATMOSPHERES; STARS: INDIVIDUAL ALPHANUMERIC: SN 1987A; STARS: SUPERNOVAE full text sources ADS | data products NED (2) SIMBAD (1) MAST (1)

Some LBV’s undergo brightness variations in the visual of about 1–2 magnitudes on time-scales of decades. These outbursts occur at approximately constant bolometric luminosity with the stellar radius and the temperature changing correspondingly. This subgroup of the LBV’s is generally called Hubble-Sandage variables or S Doradus stars and will be the focus of this talk. We present self-consistent radiation-hydrodynamic calculations to model the observations from minimum to maximum states.

We study the relation between photospheric and wind parameters of LBV′s on the basis of self-consistent atmospheric models. Our theoretical model consists of a spherically extended, dynamical NLTE atmosphere for the radiative transfer and a modified CAK code for the hydrodynamics. These codes are combined in an iterative scheme to achieve self-consistency.

view Abstract Citations (66) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Atmospheric Models for Luminous Blue Variables Leitherer, Claus ; Schmutz, Werner ; Abbott, David C. ; Hamann, Wolf-Rainer ; Wessolowski, Ulf Abstract The photospheres and stellar winds of luminous blue variables are studied theoretically. Non-LTE calculations of radiation transfer in spherically extended, expanding atmospheres are combined with a hydrodynamics code for radiation-driven winds. Models are calculated which are typical for luminous blue variables in minimum and maximum states. The recombination of iron group elements from double to singly ionized stages, which occurs when the effective temperature falls below 10,000 K, can explain why the mass-loss rates increase when luminous blue variables approach their maximum states; the singly charged ions provide a much larger number of strong lines which can intercept the radiation pressure. However, the mass-loss increase by itself cannot account for the drop of the effective temperature in a self-consistent way. The corresponding increase of the wind opacity causes only a comparativley small change in the radius of continuum formation. It is concluded that the observed variability of the photospheric radius must be induced from deeper, subphotospheric regions. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/168072 Bibcode: 1989ApJ...346..919L Keywords: Atmospheric Models; Blue Stars; Stellar Atmospheres; Stellar Luminosity; Stellar Mass Ejection; Variable Stars; Radiative Transfer; Stellar Winds; Supergiant Stars; Astrophysics; STARS: ATMOSPHERES; STARS: MASS LOSS; STARS: SUPERGIANTS; STARS: VARIABLES full text sources ADS | data products SIMBAD (2)

view Abstract Citations (109) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photospheres of Hot Stars. III. Luminosity Effects at Spectral Type O9.5 Voels, Stephen A. ; Bohannan, Bruce ; Abbott, David C. ; Hummer, D. G. Abstract Hydrogen and helium line profiles with high signal-to-noise ratios were obtained for four stars of spectral type 09.5 (Alpha Cam, Xi Ori A, Delta Ori A,AE Aur) that form a sequence in luminosity: Ia, Ib, II, V. The basic stellar parameters of these stars are determined by fitting the observed line profiles of weak photospheric absorption lines with profiles from models which include the effect of radiation scattered back onto the photosphere from their stellar winds, an effect referred to as wind blanketing. For these stars, the inclusion of wind blanketing is significant only for the most luminous star, Alpha Cam, for which the effective temperature was shifted about -2000 K relative to an unblanketed model. Publication: The Astrophysical Journal Pub Date: May 1989 DOI: 10.1086/167459 Bibcode: 1989ApJ...340.1073V Keywords: Hot Stars; O Stars; Photosphere; Stellar Luminosity; Stellar Spectra; Absorption Spectra; Abundance; Line Spectra; Stellar Atmospheres; Stellar Winds; Astrophysics; LINE PROFILES; STARS: ABUNDANCES; STARS: ATMOSPHERES; STARS: EARLY-TYPE full text sources ADS | data products SIMBAD (5) Related Materials (3) Part 1: 1985ApJ...294..286A Part 2: 1986ApJ...308..728B Part 4: 1990ApJ...365..729B

Analytical solutions for radiation-driven winds of hot stars including the important finite cone angle effect (see Pauldrach et al., 1986; Friend and Abbott, 1986) are derived which approximate the detailed numerical solutions of the exact wind equation of motion very well. They allow a detailed discussion of the finite cone angle effect and provide for given line force parameters k, alpha, delta definite formulas for mass-loss rate M and terminal velocity v-alpha as function of stellar parameters.

view Abstract Citations (26) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Failure of Continuum Methods for Determining the Effective Temperature of Hot Stars Hummer, D. G. ; Abbott, David C. ; Voels, Stephen A. ; Bohannan, Bruce Abstract The authors demonstrate that for hot stars (Teff > 30,000K) methods based on the integrated continuum flux are completely unreliable discriminators of the effective temperature. They show that the observed continuum flux distribution of ζ Pup (O4f) can be fitted to within the observational errors by photospheric models having a wide range of effective temperatures and gravities. In contrast, absorption line profiles provide much more accurate values of these parameters. As an example, profiles of weak lines of H I, He I, and He II are computed from standard non-LTE models at the effective temperature and surface gravity inferred by Underhill from the continuous energy flux. These profiles show a severe lack of agreement with observed profiles. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166328 Bibcode: 1988ApJ...328..704H Keywords: Early Stars; Hot Stars; Line Spectra; Stellar Atmospheres; Stellar Temperature; Energy Distribution; Error Analysis; O Stars; Thermodynamic Equilibrium; Astrophysics; LINE PROFILES; SPECTROPHOTOMETRY; STARS: ATMOSPHERES; STARS: EARLY-TYPE full text sources ADS | data products SIMBAD (4)

view Abstract Citations (437) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Theory of Radiatively Driven Stellar Winds. III. Wind Models with Finite Disk Correction and Rotation Friend, David B. ; Abbott, David C. Abstract A model of a radiation-driven stellar wind is presented that includes a correction factor for the finite size of the star as well as the centrifugal force due to the star's rotation. The addition of the finite disk factor reduces the line force near the star and increases it far from the star, which lowers the mass-loss rate, increases the terminal velocity, and makes the velocity law more gradual. These changes bring the wind model into much closer agreement with observations of hot star winds. The decrease in the ratio of terminal velocity to escape velocity in the B supergiants can be explained as due to a change in the mixture of driving lines. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164809 Bibcode: 1986ApJ...311..701F Keywords: Stellar Mass Ejection; Stellar Models; Stellar Rotation; Stellar Winds; Centrifugal Force; Equations Of Motion; Hot Stars; Wind Velocity; Astrophysics; STARS: EARLY-TYPE; STARS: ROTATION; STARS: WINDS full text sources ADS | Related Materials (2) Part 1: 1980ApJ...242.1183A Part 2: 1982ApJ...259..282A

Observations of thermal emission at radio wavelengths provides a powerful diagnostic of the rate of mass loss and temperature of the winds of early-type stars. Some winds are also strong sources of nonthermal emission. Case studies of known thermal and nonthermal sources provide empirical criteria for classifying the observed radio radiation. Mass loss rates are derived for 37 OB and Wolf-Rayet stars considered “definite” or “probable” thermal wind sources by these criteria. The rate of mass loss is strongly linked to stellar luminosity in OB stars and probably linked to stellar mass in Wolf-Rayet stars, with no measurable correlation with any other stellar property. A few late-type giants and supergiants also have detectable thermal emission, which arises from extended, accelerating, partially-ionized chromospheres.

The radio flux of P Cygni is variable on a time scale of months or shorter. The variations are interpreted as variations in the degree of ionization of the wind due to the ejection of shells which shield the wind from the ionizing radiation of the star. The observed time scale agrees with the recombination time scale of the wind.

Wolf-Rayet (WR) stars are the helium-burning remnants of massive stars (initial mass ≲30 M©), which have lost their outer hydrogen-rich layers through the processes of Roche lobe overflow to a companion or mass loss by a strong stellar wind. The characteristic emission-line spectrum which defines the WR spectral type is produced by a stellar wind that is so dense and opaque, that the radiation of all lines and continua arise from material in the wind. Because the wind completely screens any radiation emitted by the hydrostatic core of the star, the spectra of WR stars are nearly impossible to interpret quantitatively, and the basic parameters — such as mass, luminosity, temperature, and chemical composition — are poorly determined.

Observations of a distance-limited sample of 44 Wolf-Rayet (WR) and 25 OB stars were made at 6 cm with the VLA. The stronger sources were reobserved at 2 cm and 6 cm. Based on the criteria of source strength, variability, spectral index, and spatial extent, we conclude that the emission from 5 OB stars and 2 WR stars is produced by nonthermal processes. The ratio of probable nonthermal emitters to probable thermal emitters in our sample is 0.6 in the OB stars and 0.2 in the WR stars.

Peter Conti has a tradition of always talking about 0-type stars at Wolf-Rayet symposia, and Wolf-Rayet stars at 0 star symposia. Since there is no well-developed theory for the origin of the winds of WR stars, it is my pleasure to join Peter's tradition, and to talk mainly about the theory of radiation driven winds in OB stars. The advantage of OB stars is that there exists a fairly complete wind theory, which agrees well with the available observations. The question is, can the mass loss observed from Wolf-Rayet stars be explained by a version of this wind theory which is scaled to the conditions found in the envelopes of Wolf-Rayet stars? The topics I consider are: —The calculated radiation pressure in OB stars, and its dependence on temperature, density, and chemical composition.—A comparison between predicted and observed mass loss rates and terminal velocities for OB stars.—The applicability of the standard radiation driven wind models to Wolf-Rayet stars.—Speculations on how Wolf-Rayet stars achieve their enormous mass loss rates within the context of the radiation pressure mechanism.