Rearrangement of the Cytoskeleton in Triticum aestivum Cells during Cold Hardening and after Treatment with Abscisic Acid

Immunocytochemical study of the basic characteristics of the tubulin and actin cytoskeleton (total content, orientation, structure, and stability) was performed for various root zones of the seedlings of winter wheat cultivars contrasting in their freezing tolerance. Plant cold hardening (3°C, 7 days) and ABA treatment (30 M, 3 days) increased the stability of tubulin microtubules (MT), that is, reduced the depolymerizing action of oryzalin in vivo. However, the mechanisms of hardening and ABA stabilizing action on the cytoskeleton were different: low temperature enhanced spatial MT aggregation and resulted in the formation of a dense network of thick MT bundles, whereas ABA reduced the content of tubulin components and induced microfilament (MF) depolymerization. Most pronounced temperature- and ABA-induced cytoskeleton changes were observed in the differentiation zone, which indicates an important role of this root zone in plant adaptation and development of root freezing tolerance. Low temperatures reduced the hormonal effect on the structural arrangement and stability of MT and MF in wheat cultivars of high and moderate freezing tolerance but increased hormonal effects in the slightly tolerant cultivar. MF depolymerization and an increase in the proportion of stable MT are supposed to be a necessary condition for seedling growth retardation after their treatment with ABA and for seedlings at the initial phase of their adaptation to low temperature. At the final phase of cold hardening, some growth acceleration is evidently determined by the accumulation of highly labile MT and greater actin polymerization.     

Effect of modification of cell calcium status on lectin activity

Effects of oryzalin (10 microM), an inhibitor of microtubule polymerization, on the activity of soluble and cell wall lectins were studied in 7 day-old seedlings of unhardened (23 degrees C) and cold acclimated (7 days at 2-3 degrees C) winter wheat (Triticum aestivum L.). Seedlings were grown in the presence of 25 microM and 1 mM Ca2+, 500 microM verapamil, 250 microM chlorpromazine or without modifiers of calcium status in the medium. Inhibitor of the microtubule polymerization inhibitor, likely as inhibitors of Ca(2+)-signal, decreased the activity of soluble lectins and increased that of cell wall lectins. Apparently, injury of microtubule phosphorylation results in a more considerable microtubule disorganization, than that observed after oryzalin effect. A low Ca2+ concentration (25 microM) depressed, while a high concentration (1 mM) prompted microtubule sensibility to oryzalin. Such an effect of high Ca2+ concentration may be related to destabilizative action of Ca(2+)-calmodulin in these conditions, because chlorpromazine decreased oryzalin-induced increase in the activity of cell wall lectins with 1 mM Ca2+. It is concluded that the activity of cell wall lectins depends on the microtubule status that is regulated by calcium signal.     

Reorganization of the tubulin and actin cytoskeleton under acclimation and abscisic acid treatment of Triticum aestivum L. plants

Only scanty and contradictory data are available concerning effects of low temperatures and ABA on the structural organization of microtubules (MTs) and microfilaments (MFs), and no information exists on the interaction of these parameters at cold acclimation of plants. Therefore, in cold acclimate and ABA-treated winter wheat plants, a comparative study was made of the state (localization, orientation, structure) and stability of actin and tubulin cytoskeleton in root cells taken from different zones, using indirect immunofluorescent microscope. The plant cold acclimation caused MT aggregation, the rise of MT and MF fluorescence, and the increase of their stability (a decrease of oryzalin effect) mainly in the root differentiation zone, that may testify to the strengthening of contacts between MTs and MFs. Like the cold acclimation, ABA induced the formation of MT bunches only in meristem and elongation zone cells. However in the zone of differentiation, the hormone stimulated the increase of tubulin structure stability, well correlating with a decrease in MT content, aggregation degree, and immunofluorescence, and, in addition with a complete depolymerization of MFs. Low temperatures removed the hormone effect on the structural organization of tubulin and actin cytoskeleton in the zone of differentiation. It is suggested that MT destruction, the decrease of instable MT populations, and the increase of stable MT populations may slow down growth processes in ABA-treated plants, similarly as in seedlings being on the initial stages of cold acclimation. By the end of this process, the induction of plant growth is determined evidently by the increase in the number of instable, highly labile MT populations, and in the status of MF polymerization.     

Host discrimination by two desert fleas using an odour cue

We compared the responses of two fleas, Xenopsylla dipodilli and Parapulex chephrenis simultaneously exposed to the odours of their rodent hosts, Gerbillus dasyurus (specific host ofX. dipodilli ) and Acomys cahirinus (specific host of P. chephrenis). We hypothesized that fleas are able to discriminate between host species by using an odour cue and predicted that X. dipodilli andP. chephrenis would select an odour of an appropriate host species. Xenopsylla dipodilli choseG. dasyurus significantly more often than A. cahirinus, whereas P. chephrenis choseA. cahirinus significantly more often than G. dasyurus. The ability to select an appropriate host species did not differ significantly either between flea species or between individuals of different sex or age classes within flea species. No X. dipodilli, but 67 of 150 P. chephrenis, refused to choose a host. The latency to move in an experimental maze was significantly shorter for X. dipodilli than P. chephrenis. The flea species also differed in the time taken from the beginning of the movement to the choice of a host, withX. dipodilli being faster than P. chephrenis. Neither flea sex nor age affected this parameter in either species. Females of both flea species produced significantly more eggs when they fed on their specific host than when they fed on the other host species. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Changes in fatty acids contents and growth characteristics in transformed oilseed rape (Brassica napus)

Spring oilseed rapeBrassica napus L. ssp.oleifera cv. HM-81 was transformed with T_L-DNA of the Ri plasmid of the agropine strainAgrobacterium rhizogenes 15834. Selfed progenies (R₂ and R₃ generations) were studied for changes in values of growth characteristics and fatty acids contents. Transformants are ‘homozygous’ for T_L-DNA. Both generations of transformants differed significantly from the nontransformed control plants in reduced length, lower number of pods per plant, lower total mass of seeds and the higher number of branches. The contents of palmitic, linoleic and linolenic acids were significantly higher in transformants when compared with the control. On the contrary, the contents of both stearic and oleic acids were in most of transformants significantly lower. Only traces of erucic acid (less than 0.05 % ) were found, both in transformed and nontransformed plants.     

Diversification of ectoparasite assemblages and climate: an example with fleas parasitic on small mammals

Aim We studied the relationships between the numbers of species and numbers of higher taxa (genera, tribes, subfamilies and families) in flea assemblages of small mammalian hosts with the aims of: (a) comparing these relationships across different regions, and (b) testing the hypothesis that flea assemblages in warmer regions diversify mainly via intrahost speciation, whereas those in colder regions diversify mainly via host switching. Location The study used previously published data on flea assemblages on small mammalian hosts from 25 different regions of the Holarctic. Methods The number of flea genera, tribes, subfamilies or families in an assemblage (host species) was plotted against the number of flea species in this assemblage for each region separately, and a power function was fitted to the resulting relationships. Then, the values of the exponent of the power function for a region were regressed against the mean annual temperature in this region, across all regions. Results The relationships between the number of flea species and the numbers of flea genera, tribes, subfamilies or families on a host species in each region were found to be well described by simple power functions. The exponent of the power function of the relationship between the number of flea species and the number of flea genera per host tended to decrease with increasing local mean annual temperature. When two apparent outliers from the trend (corresponding to regions where sampling was not performed as in other regions) were omitted from the analysis, the negative relationship between temperature and the exponent of the power function between the number of flea species and number of flea genera per host became highly significant. No relationship was found between the values of the exponents of the power functions between the number of flea species and the number of flea tribes, subfamilies or families per host, and the mean local annual temperature. Main conclusions The results suggest that the diversification of flea assemblages is associated with climatic variables. In warm regions, the greater number of congeneric species per flea assemblage, reflected by the lower exponent of the power function, may well be the outcome of intrahost speciation. This indicates that, as regional temperature increases, intrahost speciation becomes a relatively more important mode of diversification than acquisition of fleas via host switching.     

Sampling fleas: the reliability of host infestation data

The use of measures of host infestation as a reliable indicator of a flea population size to be used in interspecific comparisons was considered. The abundance of fleas collected from host bodies and collected from host burrows was compared among 55 flea species, controlling for the effect of flea phylogeny. The mean number of fleas on host bodies correlated positively with the mean number of fleas in host burrows/nests both when the entire data pool was analysed and for separate subsets of data on 'fur' fleas and 'nest' fleas. This was also true for a within-host (Microtus californicus) between-flea comparison. The results of this study demonstrate that, in general, the index of host body infestation by fleas can be used reliably as an indicator of the entire population size.     

Lectin and mitotic activities in root meristems of winter wheat under oryzalin effect

The effect of oryzalin, a microtubule polymerization inhibitor (10 MM), on lectin and mitotic activities (mitotic index and duration of mitotic phases) was studied in unhardened (23 degrees C) and hardened (7 days, 2-3 degrees C) winter wheat seedlings. Three wheat cultivars differing in their frost tolerance were compared. Oryzalin treatment (3 h) decreased activity of soluble lectins, increased activity of cell wall lectin mitotic index. Under these conditions, prolongation of anaphases and disappearance of telophases were detected. Plant hardening reduced the sensitivity of cell wall lectins and mitotic activity to the cytoskeleton inhibitor due, presumably, to the appearance of cold-stable microtubules. Plant growing and hardening with oryzalin stopped mitoses and caused the appearance of polyploid cells and cells with micronuclei. These abnormalities were preserved after hardening. The results obtained demonstrate an important role of microtubules in adaptation of plants to low temperature.     

The effect of vegetation cover on vigilance and foraging tactics in the fat sand rat Psammomys obesus

The combination of the visual obstruction and protection properties of vegetation is considered to be one of the most important factors determining the trade-off between vigilance and foraging in a prey species. In the Negev desert, diurnal fat sand rats construct their burrows in the ephemeral river beds ("wadis"), under dense and tall shrubs of Atriplex halimus, or on the open first fluvial terrace, covered with scattered low shrubs of Anabasis articulata. We tested the hypothesis that properties of the vegetation would affect the time budget of female sand rats. Sand rats spent more time aboveground, rested more, were less vigilant, and moved more slowly under the dense cover in the wadi than at the open terrace. No differences in the total foraging time were revealed, but foraging tactics varied between habitats: individuals in the wadi mainly fed aboveground, whereas those at the terrace mainly hoarded. Our results indicate that sand rats perceive the dense vegetation cover as good protection despite its visual obstructive nature, and that vigilance in Psammomys obesus is performed at the cost of resting rather than at the cost of foraging. Received: January 15, 2001 / Accepted: April 17, 2001     

Actin-regulated water permeability of two transport channels of plasmodesmata in roots of winter wheat cultivars varying in drought resistance

In roots of 5-6-day old seedlings of three cultivars of the winter wheat, varying in drought-resistance: Bezostaya 1 (low resistant), Mironovskaya 808 (resistant), and Albidum 114 (highly resistant) water permeability of two transport channels of plasmodesmata was studied at the action of cytochalasin B, which is known to inhibit polymerization of cytoskeleton actin filaments, by a pulse method of NMR, on the background of increasing water loss in the seedlings. It has been found that the registered coefficients of water self diffusion, two of which (D2 and D3) depend on the water permeability of different transport channels of plasmodesmata, differ in opposite directions. This may suggest that in roots of drought-resistant plants, after a moderate water loss, a diffusive water flow through the cytoplasmic symplast increases (demonstrated by an increase of D2), while that through the vacuolar symplast decreases (seen by an increase of D3). After a high water loss in seedlings, we noticed an even greater increase in water permeability of the cytoplasmic symplast, and a decrease in water permeability of the vacuolar symplast, however, in the roots of low resistant cultivars these changes were poorly expressed, if at all. Under stress-less conditions cytochalasin B would result in an increased water transport through the cytoplasmic channel of plasmodesmata due apparently to a destruction of their actin-myosin sphincters. Both weak and average degrees of water loss would strengthen the cytochalasin B exerted influence on plasmodesmal water conductance, that may testify to a synergetic action of these two factors. After a significant water loss this action was kept only partially, because the inhibitor, on blocking the cytoplasmic channel, did increase at the same time the effect of water stress, limiting water flows through the vacuolar symplast and, simultaneously, raising the water inflow to the apoplast.