Host plant resistance in Brachiaria grasses to the spittlebug Zulia colombiana

Twelve forage grass accessions including 11 accessions of Brachiaria Griseb, were evaluated in a glasshouse for host plant resistance to nymphs and tolerance to feeding damage caused by adults of Zulia colombiana (Lallemand) (Homoptera: Cercopidae). Resistance to nymphs was evaluated with a technique that provided uniform environmental conditions and abundant feeding sites. B. brizantha Stapf (cv. Marandú) was the most resistant of the accessions tested based on nymphal mortality, duration of nymphal stadia, and weight of adult females. Andropogon gayanus Kunth, resistant to spittlebug attack in the field, was susceptible under the conditions of this study. While growth habit and rooting characteristics may contribute to field resistance, other resistance factors are present within the genus Brachiaria, particularly in the case of B. brizantha cv. Marandú. The number of insect-days causing severe damage in the most tolerant species (B. dictyoneura Stapf and B. humidicola Schweick) was approximately six times greater than that necessary to cause the same level of damage to the most susceptible species (B. ruziziensis Germain & Evrard and B. decumbens Stapf). No difference was found in regrowth capacity between infested and noninfested plants within accessions. There was a significant positive correlation between number of insect-days causing severe damage (tolerance) and regrowth of infested plants.

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Résumé Brachiaria est une graminée fourragère prometteuse pour les sols tropicaux acides, saturés d'aluminium. Z. colombiana est un Cercopidae très répandu, limitant l'utilisation de Brachiaria en Amérique Latine. La résistance (antibiose et tolérance) à Z. colombiana, de Brachiaria d'origines diverses a été examinée. B. brizantha cv. Marandù s'est révélé le plus résistant d'après la forte mortalité larvaire, la prolongation du développement larvaire, et le poids réduit des femelles adultes de Z. colombiana. Andropogon gayanus, résistant dans la nature, s'est révélé sensible. Ces résultats suggèrent que cette résistance de A. gayanus dans la nature pourrait être due à la structure du végétal et à son mode de croissance. Dans le cas de B. brizantha cv. Marandù, des facteurs supplémentaires de résistance, mis en évidence à partir de différents modes de croissance, ont été éliminés, de façon à identifier les mécanismes de l'antibiose présents chez Brachiaria. Une grande gamme de résistance aux attaques alimentaires a été observée chez Brachiaria. Les plus résistants ont besoin de 6 fois plus de jours d'attaque par Z. colombiana pour provoquer les dégâts observés sur individus sensibles.

     

Resistance to low temperature photoinhibition is not associated with isolated thylakoid membranes of winter rye

In vivo measurements of chlorophyll a fluorescence indicate that cold-hardened winter rye (Secale cereale L. cv Musketeer) develops a resistance to low temperature-induced photoinhibition compared with nonhardened rye. After 7.2 hours at 5°C and 1550 micromoles per square meter per second, the ratio of variable fluorescence/maximum fluorescence was depressed by only 23% in cold-hardened rye compared with 46% in nonhardened rye. We have tested the hypothesis that the principal site of this resistance to photoinhibition resides at the level of rye thylakoid membranes. Thylakoids were isolated from cold-hardened and nonhardened rye and exposed to high irradiance (1000-2600 micromoles per square meter per second) at either 5 or 20°C. The photoinhibitory response measured by room temperature fluorescence induction, photosystem II electron transport, photoacoustic spectroscopy, or [¹⁴C]atrazine binding indicates that the differential resistance to low temperature-induced photoinhibition in vivo is not observed in isolated thylakoids. Similar results were obtained whether isolated rye thylakoids were photoinhibited or thylakoids were isolated from rye leaves preexposed to a photoinhibitory treatment. Thus, we conclude that increased resistance to low temperature-induced photoinhibition is not a property of thylakoid membranes but is associated with a higher level of cellular organization.     

Compartmental analysis of 45Ca2+ efflux in perfused rat liver: effects of hormonal stimulation.

The kinetics of calcium movements in the isolated perfused rat liver were examined using compartmental analysis of the efflux profiles of 45Ca2+ from 45Ca(2+)-equilibrated livers under a variety of calcium concentrations and hormonal treatments. From the 45Ca2+ efflux profiles, we determined that a three compartment model was appropriate to describe the movements of calcium in the liver on the time scale of the experiments. Hormonal treatment with the alpha-adrenergic agonist, phenylephrine, or the vasoactive peptide, vasopressin, during the efflux period lowered significantly the rate of transfer of Ca2+ between the internal compartments at all of the calcium concentrations employed. Also, phenylephrine treatment leads to increased transfer of Ca2+ into the liver from the perfusate. The temporal characteristics of the phenylephrine and vasopressin sensitive Ca2+ pools were examined by pulsing livers, loaded for variable periods of time with 45Ca2+, with the two hormones during the efflux of 45Ca2+ to measure the kinetics of Ca2+ exchange in the hormone-sensitive pools. Results from these experiments indicate that the rate of unstimulated Ca2+ efflux, k2, for the phenylephrine and vasopressin sensitive Ca2+ pools, modeled as a one compartment system, are the same, 0.074 and 0.078 min-1 for phenylephrine and vasopressin respectively, corresponding to half times for turnover of the pool(s) of 9.3 and 8.9 min, respectively.     

Pentylenetetrazole inhibits glutamate dehydrogenase and aspartate aminotransferase, and stimulates GABA aminotransferase in homogenates from rat cerebral cortex

The mechanism by which pentylenetetrazole provokes convulsions in animals has been investigated by measuring its influence in vitro on the activities of several enzymes of glutamate metabolism in rat brain homogenates. Pentylenetetrazole does not affect the specific activities of glutamine synthetase, glutaminase, or glutamate decarboxylase; it inhibits those of glutamate dehydrogenase and aspartate aminotransferase, and stimulates that of gamma-aminobutyric acid (GABA) aminotransferase. The overall consequence of the action of pentylenetetrazole on the activities of these enzymes should be an increase in the concentration of glutamate and a decrease in that of GABA. This modulation of glutamate and GABA metabolism by pentylenetetrazole could contribute to the triggering of convulsions.     

The monomeric glutamyl-tRNA synthetase from Bacillus subtilis 168 and its regulatory factor. Their purification, characterization, and the study of their interaction

The glutamyl-tRNA synthetase from Bacillus subtilis has been purified to homogeneity. It is a monomer of Mr = 65,500 whose NH2-terminal sequence is Met-Asn-Glu-Val-Arg-Val-Arg-Tyr-Ser-Pro-Ser-Pro-Thr-Gly-His-Leu. The number of tryptic peptides indicates the absence of a significant amount of sequence duplication. Under certain conditions, this monomeric enzyme is co-purified with a polypeptide beta of Mr = 46,000, which increases the affinity of the enzyme about 10-fold for glutamate and for ATP, and stabilizes it against heat inactivation. gamma-Globulins prepared against the monomeric enzyme can inhibit completely the glutamyl-tRNA synthetase activity of a B. subtilis extract and precipitate from this extract both the monomeric enzyme and the regulatory factor beta. These anti-alpha immunoglobulins do nt precipitate pure beta. These results show that the glutamyl-tRNA synthetase of B. subtilis has a structure similar to that of the Escherichia coli enzyme (Lapointe, J., and S?ll, D. (1972) J. Biol. Chem. 247, 4966-4974) and indicate that the beta factor has a function in the regulation of glutamyl-tRNA biosynthesis in vivo.