The ecological significance of toxic nectar

Although plant-herbivore and plant-pollinator interactions have traditionally been studied separately, many traits are simultaneously under selection by both herbivores and pollinators. For example, secondary compounds commonly associated with herbivore defense have been found in the nectar of many plant species, and many plants produce nectar that is toxic or repellent to some floral visitors. Although secondary compounds in nectar and toxic nectar are geographically and phylogenetically widespread, their ecological significance is poorly understood. Several hypotheses have been proposed for the possible functions of toxic nectar, including encouraging specialist pollinators, deterring nectar robbers, preventing microbial degradation of nectar, and altering pollinator behavior. All of these hypotheses rest on the assumption that the benefits of toxic nectar must outweigh possible costs; however, to date no study has demonstrated that toxic nectar provides fitness benefits for any plant. Therefore, in addition to these adaptive hypotheses, we should also consider the hypothesis that toxic nectar provides no benefits or is tolerably detrimental to plants, and occurs due to previous selection pressures or pleiotropic constraints. For example, secondary compounds may be transported into nectar as a consequence of their presence in phloem, rather than due to direct selection for toxic nectar. Experimental approaches are necessary to understand the role of toxic nectar in plant-animal interactions.     

Activity of the natural algicide, cyanobacterin, on eukaryotic microorganisms

Abstract The natural product cyanobacterin has been shown to be toxic to most cyanobacteria at a concentration of approx. 5 μM. We demonstrate here that cyanobacterin will also inhibit the growth of most eukaryotic algae at a similar concentration. Some algae, such as Euglena gracilis , are resistant because they are able to maintain themselves by heterotrophic nutrition. Others, such as Chlamydomonas reinhardtii , can apparently induce a detoxification mechanism to maintain photosynthesis in the presence of low concentrations of the inhibitor. Non-photosynthetic microorganisms are not affected by cyanobacterin.     

Heme binding and polymerization by Plasmodium falciparum histidine rich protein II: influence of pH on activity and conformation.

The histidine rich protein II (HRPII) from Plasmodium falciparum has been implicated as a heme polymerase which detoxifies free heme by its polymerization to inactive hemozoin. Histidine-iron center coordination is the dominant mechanism of interaction between the amino acid and heme. The protein also contains aspartate allowing for ionic/coordination interactions between the carboxylate side chain and the heme metal center. The pH profile of heme binding and polymerization shows the possibility of these two types of binding sites being differentiated by pH. Circular dichroism studies of the protein show that pH and heme binding cause a change in conformation above pH 6 implying the involvement of His-His+ transitions. Heme binding at pHs above 6 perturbs HRPII conformation, causing an increase in helicity.     

Rational construction of a 2-hydroxyacid dehydrogenase with new substrate specificity

Using site-directed mutagenesis on the lactate dehydrogenase gene from Bacillus stearothermophilus, three amino acid substitutions have been made at sites in the enzyme which we suggest in part determine specificity toward different hydroxyacids (R-CHOH-COOH). To change the preferred substrates from the pyruvate/lactate pair (R = -CH3) to the oxaloacetate/malate pair (R = -CH2-COO-), the volume of the active site was increased (thr 246----gly), an acid was neutralized (asp-197----asn) and a base was introduced (gln-102 - greater than arg). The wild type enzyme has a catalytic specificity for pyruvate over oxaloacetate of 1000 whereas the triple mutant has a specificity for oxaloacetate over pyruvate of 500. Despite the severity and extent of these active site alterations, the malate dehydrogenase so produced retains a reasonably fast catalytic rate constant (20 s-1 for oxaloacetate reduction) and is still allosterically controlled by fructose-1,6-bisphosphate.     

NOTES ON THE ECOLOGY OF A SPECIES OF ZYGOGONIUM (KÜTZ.) IN YELLOWSTONE NATIONAL PARK

A species of Zygogonium forms extensive dark purple mats in Yellowstone National Park in acidic habitats adjacent to thermal areas. These mats range up to 6 cm in thickness and up to 3000 m² in areal extent. Temperatures in the mats varied from 20–31 C and pH varied from 2.4–3.1. These mats form on soil in areas where a moist surface is created by the presence of small acidic springs or seeps. The effect of light, temperature, and pH on photosynthesis was studied in the field by use of ¹⁴CO₂. Photosynthesis increased in rate up to full sunlight; light inhibition was not observed. Temperature optimum for photo-synthesis was 25 C. A broad pH optimum was found between 1.0 and 5.0. The Zygogonium mats have a high water holding capacity and create a moist habitat in which Euglena and Chlamydomonas develop. The mats also serve as repositories for the eggs of the brine fly Ephydra bruesi and both larvae and adults of this fly probably consume Zygogonium filaments as their main food source.