Regulation of root hair density by phosphorus availability in Arabidopsis thaliana

We characterized the response of root hair density to phosphorus (P) availability in Arabidopsis thaliana. Arabidopsis plants were grown aseptically in growth media with varied phosphorus concentrations, ranging from 1 mmol m⁻³ to 2000 mmol m⁻³ phosphorus. Root hair density (number of root hairs per mm of root length) was analysed starting at 7 d of growth. Root hair density was highly regulated by phosphorus availability, increasing significantly in roots exposed to low-phosphorus availability. The initial root hairs produced by the radicle were not sensitive to phosphorus availability, but began to respond after 9 d of growth. Root hair density was about five times greater in low phosphorus (1 mmol m⁻³) than in high phosphorus (1000 mmol m⁻³) media. Root hair density decreased logarithmically in response to increasing phosphorus concentrations within that range. Root hair density also increased in response to deficiencies of several other nutrients, but not as strongly as to low phosphorus. Indoleacetic acid (IAA), the auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (CMPA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the ethylene synthesis inhibitor amino-oxyacetic acid (AOA) all increased root hair density under high phosphorus but had very little effect under low phosphorus. Low phosphorus significantly changed root anatomy, causing a 9% increase in root diameter, a 31% decrease in the cross-sectional area of individual trichoblasts, a 40% decrease in the cross-sectional area of individual atrichoblasts, and 45% more cortical cells in cross-section. The larger number of cortical cells and smaller epidermal cell size in low phosphorus roots increased the number of trichoblast files from eight to 12. Two-thirds of increased root hair density in low phosphorus roots was caused by increased likelihood of trichoblasts to form hairs, and 33% of the increase was accounted for by changes in low phosphorus root anatomy resulting in an increased number of trichoblast files. These results show that phosphorus availability can fundamentally alter root anatomy, leading to changes in root hair density, which are presumably important for phosphorus acquisition.     

Problems with the use of terracotta clay saucers as phosphorus-diffusing substrata to assess nutrient limitation of epilithic algae

1. We examined the diffusion properties of terracotta clay saucers, of types often used as components in phosphorus-diffusing substrata for investigating nutrient limitation of epilithic algae.
2. Laboratory experiments showed that phosphorus diffusion was low and inconsistent (0.06–2.6 mg P day⁻¹) through clay saucers filled with agar containing orthophosphate. Similarly, in situ release of phosphorus from two types of terracotta clay saucers (Australian and Italian) was variable (2–8 mg P day⁻¹; 5–25%) under three flow regimes (0, 0.1 and 0.3 m s⁻¹) over 30 days, with most phosphorus being released during the first day. Clogging of pores by agar appears to prevent the diffusion of phosphorus through the terracotta clay saucers. However, the two types of terracotta clay saucer also irreversibly sorbed large quantities of phosphorus (40–140 mg P).
3. Individual saucers can have markedly different physical and chemical properties both within and among terracotta types. Terracotta saucers also contain large quantities of iron, calcium and aluminium that are known binding agents for phosphorus. Variability in saucer composition and diffusion properties makes treatments difficult to replicate and weakens comparisons between studies that have used terracotta with different clay composition and manufacture.
4. We recommend that phosphorus-diffusing substrata should not be constructed from terracotta clay components.     

Fungal rhodopsins and opsin-related proteins: eukaryotic homologues of bacteriorhodopsin with unknown functions.

In the last decade, several genome sequencing projects revealed the existence of previously unknown photoreceptors. Among those are eukaryotic rhodopsins of haloarchaeal type, mostly represented by fungal sequences. We have classified and analyzed seventy-seven of these fungal proteins, which show a high similarity of their putative transmembrane regions to those of bacteriorhodopsin. Those sequences can be divided into the two subgroups, fungal rhodopsins (RDs) and opsin-related proteins (ORPs), the latter lacking the lysine residue necessary for retinal binding. We have analyzed the conservation pattern of the residues known to have functional or structural importance in bacteriorhodopsin and discussed dramatic differences in the conservation between RDs and ORPs. We found many cases of multiple forms of RDs and/or ORPs and examined possible reasons for such multiplicity. For some species the reason may lie in functional photobiological diversification, while for the others it follows the pattern of evolutionary recent genome duplication and possible functional redundancy.     

SEASONAL VARIATION IN SEXUAL SELECTION IN THE MOTTLED SCULPIN

Seasonal variation in sexual and natural selection in male mottled sculpins (Cottus bairdi) can be evaluated by calculating selection differentials, which measure the magnitude of phenotypic change resulting from selection, and by calculating indices of the opportunity for selection, which indicate the potential for phenotypic selection in a given interval. Selection differentials are high at the beginning of the breeding season and decline throughout the breeding season. The magnitude and direction of selection differentials depend on when spawning occurs and are independent of the size or age of the females that spawn. Annual selection differentials due to differences in mating success (female choice) are nearly constant between years. Annual selection differentials associated with hatching success are variable. Opportunities for selection (I = fitness variance/[mean fitness]²) show clear seasonal patterns. They are highest at the beginning and at the end of the spawning season. However, this variation is dependent on the mean used to calculate I, and hence variation in I values does not indicate a significant change in the variance of male fitness.     

Copper resistance determinants in bacteria.

Copper is an essential trace element that is utilized in a number of oxygenases and electron transport proteins, but it is also a highly toxic heavy metal, against which all organisms must protect themselves. Known bacterial determinants of copper resistance are plasmid-encoded. The mechanisms which confer resistance must be integrated with the normal metabolism of copper. Different bacteria have adopted diverse strategies for copper resistance, and this review outlines what is known about bacterial copper resistance mechanisms and their genetic regulation.     

Stressful life events, social support networks, and the physical and mental health of urban black adults

The direct and the buffering effects of social support networks have been documented, but few studies have examined their health outcomes for blacks and, specifically, the differences in physical and mental health. This study uses six measures of social support to examine the relationships of social support to health outcomes for black males and females. The data are from a community sample of 451 urban black adults. Results show significant differences by sex according to source of social support and particular health outcomes. Number of nearby relatives and perceived social support have direct and buffering effects, respectively, on mental health for black females, but no significant influences emerge for black males. On the other hand, the number of confidants for black males and the degree of religiosity among black females were inversely related to physical health, suggesting differences in conceptual models of social support for mental and physical health.     

Flow cytometric analysis and sorting of plant chromosomes from Petunia hybrida protoplasts

A method to obtain a high metaphase index and thereafter a plant chromosome suspension is described for Petunia hybrida (2n = 14). Mesophyll protoplast cultures have been used, giving easily disrupted cell walls and a high percentage of dividing cells after 42 h. On 2.5 mM colchicine-treated cells, metaphase indexes reaching 10% were routinely obtained. The lysis medium in which the protoplast-derived cells were disrupted was a simplified culture medium. After chromosome release, samples were stained with Hoechst 33342 dye and analysed by flow cytofluorometry. The histogram of fluorescence intensities included three peaks of metaphase chromosomes and a duplication of this flow karyotype provoked by "monochromatid chromosome." This interpretation was established after flow sorting; micronuclei could also be observed and sorted. Of the 7 chromosomes, only the largest formed a distinct peak while the others were incompletely resolved, due to the similar DNA content of various chromosomes. Model distributions of Petunia hybrida chromosomes have been computed according to the relative chromosome length. The theoretical histograms indicated that low variability is indispensable for resolving distinctive chromosome peaks. The experimental flow karyotype was consistent with one of the models having CV of 2.5%.     

The influence of 4-hydroxy-4-androstene-3,17-dione on androgen metabolism and action in cultured human foreskin fibroblasts

4-hydroxy-4-androstene-3,17-dione (4-OHA) has been shown to be a potent inhibitor of aromatase activity. It is effective in the control of estrogen-dependent processes in female subjects and may potentially be useful in the treatment of estrogen-dependent processes in men. Human foreskin fibroblasts grown in cell culture provide a model to investigate the effects of 4-OHA on extraglandular aromatase activity as well as the ability of the compound to influence androgen receptor binding and the 5 alpha-reduction of testosterone (T). Initial experiments were carried out to determine the potency of 4-OHA in genital skin fibroblasts by incubating cells with 4-OHA over a range of concentrations. When aromatase activity was determined at a substrate concentration close to the apparent Km of the enzyme, a 44% inhibition of enzyme activity occurred at a mean concentration of 5 nM 4-OHA. Enzyme kinetic studies analyzed by Eadie-Hofstee plots demonstrated competitive inhibition by 4-OHA with a mean apparent Ki of 2.7 nM. When 5 alpha-reductase activity was determined in the presence of 200 nM [3H]T, in the absence or presence of 4-OHA, a 50% inhibition of enzyme activity occurred at an inhibitor concentration of 3 microM. In androgen receptor binding studies, 4-OHA possessed 1% of the affinity of dihydrotestosterone (DHT) for [3H]DHT binding sites. In summary: 4-OHA is a potent and specific inhibitor of aromatase activity in human genital skin fibroblasts, the affinity of the enzyme for 4-OHA being greater than its affinity for the substrate, androstenedione. The influence of 4-OHA on 5 alpha-reductase activity and androgen receptor binding is minimal.