Arachidonic acid-induced hypersensitive cell death as an assay of downy mildew resistance in pearl millet

Arachidonic acid (AA) induces hypersensitive response (HR) on coleoptile/root regions of two-day-old pearl millet seedlings. The response is comparable to the HR induced by the downy mildew pathogen, Sclerospora graminicola. A time gap in the appearance of cell necrosis among genotypes of pearl millet was related to the degree of resistance to downy mildew. Based on the time required for the development of necrotic spots induced by AA, the pearl millet genotypes were categorised as highly resistant/resistant (HR in 3–6 h), susceptible (HR in 7–12 h) and highly susceptible (HR in 13 h and above). The percentage disease incidence in each genotype was compared with the time required for the development of AA-induced HR. The appearance of hypersensitive cell necrosis was rapid in genotypes having high resistance to downy mildew and was slow in genotypes with high susceptibility. This simple method of screening various pearl millet genotypes in the absence of the pathogen aids in identifying the downy mildew resistant/susceptible host cultivars without the risk of introducing the virulent race of the pathogen.     

The Possible Involvement of Lipoxygenase in Downy Mildew Resistance in Pearl Millet

The downy mildew disease, incited by Sclerospora graminicola,is a major biotic constraint for pearl millet production inthe semi-arid tropics. Sources of resistance to this diseasehave been identified. However, the mechanism of host resistancestill remains obscure. The enzyme lipoxygenase (LOX) is knownto play a role in disease resistance in many host-pathosystems.In the present study, LOX activity was tested in seeds of differentgenotypes of pearl millet with different susceptibility to downymildew. The LOX assay of the seeds indicated a good correlationbetween enzyme activity and their downy mildew reaction in thefield. Maximum activity was recorded in seeds of highly resistantgenotypes and minimum activity was found in the highly susceptiblegenotypes. Seeds obtained from plants recovered from the downymildew disease had more LOX activity than that of the originalparent seeds. Thus, in seeds, the LOX activity can be used asa biochemical marker for screening different genotypes of pearlmillet for downy mildew. The study, carried out in the susceptiblegenotype of pearl millet seedlings, showed that LOX activitydecreased after inoculating with S. graminicola zoospores whencompared with uninoculated controls. However, a significantincrease in the enzyme activity was observed on the second andthird days after inoculation in resistant seedlings. The possiblerole of LOX in conferring resistance to downy mildew infectionof pearl millet is discussed. Key words: Lipoxygenase, pearl millet, downy mildew     

Gel Electrophoretic Analysis of Histones in Lens Epithelium, Lens Fiber, Liver, Brain, and Erythrocytes of Late Chick Embryos

Histones from 19-day-old chick embryo lens epithelium, lens fibers, liver, brain, and erythrocytes were electrophoresed in polyacrylamide gels using buffers containing sodium dodecylsulfate, acetic acid urea, or mixtures of Triton X-100 acetic acid urea. In the last two buffer systems, histone bands were characterized by their apparent molecular weights determined by electrophoresis in the second dimension in sodium dodecylsulfate containing polyacrylamide gels. From the densitograms of the stained gels, the relative proportion of protein in different histone bands was estimated. With the exception of the erythrocyte-specific histone H5, all histones from different tissues examined in any of the gel systems migrated with the same mobilities. In lens epithelium and lens fibers, all histones were present in identical proportions. As compared to liver and brain, the total amount of histone H1 was significantly lower in lens cells and erythrocytes, possibly reflecting differences between the differentiated states. However, no tissue-specific differences were found in the relative distribution of histone H1 I and H1 II among lens epithelium, lens fiber, liver and, brain, but a threefold higher H1 I: H1 II ratio (0.5–0.7) was found in erythrocytes.     

Hypersensitive response, cell death and histochemical localisation of hydrogen peroxide in host and non-host seedlings infected with the downy mildew pathogen Sclerospora graminicola

Hypersensitive response, cell death and release of hydrogen peroxide as measures of host and non‐host defense mechanisms upon inoculation with the downy mildew pathogen Sclerospora graminicola were studied histochemically at the light microscopy level. The materials consisted of coleoptile tissues of the highly susceptible (cv. HB3), highly resistant (cv. IP18293) and induced resistant pearl millet host seedlings and non‐host sorghum (cv. SGMN10/8) and cotyledon of french bean (cv. S9). Resistance up to 80% protection against the downy mildew pathogen was induced in the highly susceptible HB3 cultivar of pearl millet by treating the seeds with 2% aqueous leaf extract of Datura metel for 3 h. Time course study with the pathogen inoculated highly resistant pearl millet cultivar revealed the appearance of hypersensitive response in 20% of seedlings as necrotic spots as early as 2 h after inoculation. In contrast, a similar reaction was observed in the highly susceptible pearl millet cultivar only 8 h after inoculation with the pathogen. In induced resistant seedlings, appearance of hypersensitive response was recorded 4 h after inoculation. Delayed hypersensitive response was observed in both the non‐host species at 10 h after inoculation. Hypersensitive response in the seedlings of the highly resistant pearl millet cultivar 24 h after inoculation showed 100% hypersensitive response, which was not observed in susceptible and non‐host species, although the induced resistant seedlings showed 90% hypersensitive response after that period of time. Cell death in the tissues of the test seedlings was also observed to change with time. Statistical analysis revealed that the tissues of highly resistant pearl millet seedlings required 2.9 h to attain 50% cell death. Tissues of induced resistant and highly susceptible pearl millet seedlings required 4.65 and 6.50 h respectively. In non‐hosts, 50% cell death was not recorded. Quantification of hydrogen peroxide in the tissue periplasmic spaces of the test seedlings revealed 2.94 h as the time required for 50% hydrogen peroxide accumulation in the tissues of highly resistant pearl millet seedlings. Tissues of induced resistant and highly susceptible pearl millet seedlings needed 3.76 and 5.5 h respectively. Fifty percent hydrogen peroxide localisation in non‐hosts could not be recorded. These results suggested the involvement of hydrogen peroxide, cell death and hypersensitive response in pearl millet host defense against S. graminicola.     

Sclerospora graminicola- and arachidonic acid-induced autofluorescence in downy mildew resistant and susceptible genotypes of pearl millet

Autofluorescence of downy mildew resistant and susceptible cells of pearl millet seedlings undergoing hypersensitive reaction (HR) upon Sclerospora graminicola-inoculation and arachidonic acid (AA)-treatment was studied. Two-day-old seedlings of a highly resistant (IP 18296) and a highly susceptible (23D₂B) genotype of pearl millet were either inoculated with zoospore suspension of S. graminicola or treated with AA for 24 h. The coleoptiles with hypersensitive necrotic spots were processed by the standard procedure, and the tissues were subjected to fluorescence microscopy. A differential accumulation of autofluor-escent compounds in resistant and susceptible pearl millet genotypes was observed with most accumulation occurring in resistant cells treated with AA. The variation in the degree of fluorescence and the spatial accumulation of autofluorescent compounds among the two inoculated/treated genotypes is discussed.     

Variability of dermal bones and other parameters in the skull of Amia calva

A sample of 84 dried skulls of Amia calva has been examined for variations in dermal bones and other parameters. One specimen was found with a single parietal and another with incomplete division of the parietals. Observations on parietal asymmetry, disposition of sutures, number and disposition of postorbitals, infraorbitals, number of dentary and premaxillary teeth, and tooth row/jaw length relations are quantified. Statistical analysis of five paired characters and dispersion analysis of 12 characters shows that the skull growth is isometric. Given a few measurements on a skull it is possible to predict several others.     

Phenylalanine Ammonia Lyase Activity in Pearl Millet Seedlings and its Relation to Downy Mildew Disease Resistance

Phenylalanine ammonia lyase (PAL) activity was studied in differentgenotypes of pearl millet with varying degrees of susceptibilityto downy mildew disease, after inoculating with Pathotype 1of Sclerospora graminicola. In resistant genotypes, the enzymeactivity significantly increased 24 h after fungal inoculationwhile in the susceptible genotypes, the activity decreased.The increase or decrease in enzyme activity was well-correlatedwith the degree of host resistance to the pathogen. A time-courseof change in activity of PAL after inoculation showed a considerabledifference between resistant and susceptible genotypes. Studieson the activity of PAL in different parts of pearl millet seedlingsrevealed that in the resistant genotype, enzyme activity significantlyincreased at 24 h post-inoculation only in the shoot portion,whereas in mesocotyl and root the activity decreased. In susceptibleseedlings, enzyme activity decreased at 24 h post-inoculationin shoot, mesocotyl and root. The activity of PAL was also foundto be pathotype-specific. Histochemical tests for lignin werepositive in infected cells in the resistant genotypes. The roleof PAL in imparting resistance to pearl millet against downymildew disease is discussed. Key words: Sclerospora graminicola, resistance screening, enzyme activity     

The influence of natural selection and sexual selection on the tails of sea-snakes (Laticauda colubrina)

Many phenotypic traits perform more than one function, and so can influence organismal fitness in more than one way. Sexually dimorphic traits offer an exceptional opportunity to clarify such complexity, especially if the trait involved is subject to natural as well as sexual selection, and if the sexes differ in ecology as well as reproductive behaviour. Relative tail length in sea-snakes fulfils these conditions. Our field studies on a Fijian population of yellow-lipped sea kraits ( Laticauda colubrina ) show that relative tail lengths in male sea kraits have strong consequences for individual fitness, both via natural and sexual selection. Males have much longer tails (relative to snout-vent length) than do females. Mark-recapture studies revealed a trade-off between growth and survival: males with relatively longer tails grew more slowly, but were more likely to survive, than were shorter-tailed males. A male snake's tail length relative to body length influenced not only his growth rate and probability of survival, but also his locomotor ability and mating success. Relative tail length in male sea kraits was thus under a complex combination of selective forces. These forces included directional natural selection (through effects on survival, growth and swimming speed) as well as stabilizing natural selection (males with average-length tails swam faster) and stabilizing sexual selection (males with average-length tails obtained more matings). In contrast, our study did not detect significant selection on relative tail length in females. This sex difference may reflect the fact that females use their tails primarily for swimming, whereas males also must frequently use the tail in terrestrial locomotion and in courtship as well as for swimming.     

Cerebroside as an elicitor for induced resistance against the downy mildew pathogen in pearl millet

Innate defence mechanisms in plants can be triggered and enhanced by certain agents, which are referred to as inducers. Inducing resistance against a broad spectrum of pathogens in otherwise susceptible plants is seen as a potentially safer alternative to other methods of chemical control of plant diseases. Cerebrosides, which are glycosphingolipids extracted from various plant pathogens, have been reported as resistance elicitors in the rice‐pathogen system. In the present study, cerebroside elicited resistance against downy mildew disease (caused by Sclerospora graminicola) of pearl millet (Pennisetum glaucum) that was highly significant. The resistance was of systemic nature and the time required for the resistance to build up was from 2 days onwards. There was a significant yield enhancement due to disease suppression by cerebroside treatment. Promising results were obtained in a preliminary field trial.     

Cellular and Biochemical Parameters of Growth of Chick Blastoderms during Early Morphogenesis

Avian blastoderms can be cultured in vitro during the early developmental period involving the process of gastrulation, primary induction, and the establishment of the embryonic axis. Very little information is available on the biochemical and molecular events related to gene expression in this system. To undertake such studies we have determined cell numbers and amounts of DNA, RNA and protein in early chick blastoderms during the developmental period of 0–52 h of incubation (stages 1–13).
The cell population growth is logarithmic, with two distinct phases characterized by an average cell-doubling time of 6 h during the first phase (stage 2–9) and 12 h during the second (stages 9–13). The average amount of DNA/cell changes from 2.33 pg in the first period to 2.21 pg in the second, in accordance with the increased average cell-doubling time. The period when doubling time increases coincides with the folding and fusion of cell sheets, as well as with the establishment of defined organ primordia, and the increase is suggested to be due to a combination of various factors, such as an appearance of heterogeneity in different cell populations, localized cell death and cessation of cellproliferative activity.
Gastrulation is accompanied by a substantial increase in both RNA and protein: cell ratio, reaching a maximum value at the full primitive streak stage. Thereafter the concentration of both macromolecular classes decreases progressively.