Ferricrocin synthesis in Magnaporthe grisea and its role in pathogenicity in rice

Iron is an essential element for the growth of nearly all organisms. In order to overcome the problem of its low bioavailability, microorganisms (including fungi) secrete siderophores, high-affinity iron chelators. As the acquisition of iron is also a key step in infection processes, siderophores have been considered as potential virulence factors in several host–pathogen interactions. Most fungi produce siderophores of the hydroxamate-type, which are synthesized by non-ribosomal peptide synthetases (NRPSs). Magnaporthe grisea , the causal agent of rice blast disease, produces ferricrocin as intracellular storage siderophore and excretes coprogens. In the M. grisea genome we identified SSM1 , an NRPS gene, and a gene encoding an l -ornithine N5-monooxygenase ( OMO1 ) that is clustered with SSM1 and responsible for catalysing the first step in siderophore biosynthesis, the N⁵ hydroxylation of ornithine. Disruption of SSM1 confirmed that the gene encodes ferricrocin synthetase. Pathogenicity of these mutants towards rice was reduced, suggesting a role of this siderophore in pathogenicity of M. grisea .     

Stratification and diel activity of arthropods in a lowland rainforest in Gabon

The abundance, activity and species richness of arthropods, particularly of insect herbivores, were investigated in the upper canopy and understorey of a lowland rainforest at La Makandé, Gabon. In total 14 161 arthropods were collected with beating, flight interception and sticky traps, from six canopy sites, during the day and at night, from mid-January to mid-March 1999. The effects of stratum were most important, representing between 40 and 70% of the explained variance in arthropod distribution. Site effects represented between 20 and 40% of the variance and emphasized the néed for replication of sampling among canopy sites. Time effects (diel activity) explained a much lower percentage of variance (6–9%). The density and abundance of many arthropod taxa and species were significantly higher in the upper canopy than in the understorey. Arthropod activity was also higher during the day than at night. In particular, insect herbivores were 2.5 times more abundant and twice as speciose in the upper canopy than in the understorey, a probable response to the greater and more diverse food resources in the former stratum. Faunal overlap between the upper canopy and understorey was low. The most dissimilar herbivore communities foraged in the understorey at night and the upper canopy during the day. Further, a taxonomic study of a species-rich genus of herbivore collected there (Agrilus , Coleoptera Buprestidae) confirmed that the fauna of the upper canopy was different, diverse and very poorly known in comparison to that of the understorey. Herbivore turnover between day and night was rather high in the upper canopy and no strong influx of insect herbivores from lower foliage to the upper canopy was detected at night. This suggests that insect herbivores of the upper canopy may be resident and well adapted to environmental conditions there.     

Kinetic characterization of barley root plasma membrane-bound Ca2+- and Mg2+-dependent adenosine triphosphatase activities

A plasma membrane-rich microsome fraction isolated from barley (Hordeum vulgare L. cv. Conquest) roots contained considerable divalent cation-dependent ATPase activity when assayed at 16°C. The maximal divalent cation-stimulation of the apparent basal ATPase activity varied as Ca²⁺ > Mg²⁺ > Mn²⁺= Zn²⁺ > Co²⁺ > Ni²⁺, with all other divalent cations tested being inhibitory. Double reciprocal plots of the Ca²⁺- and Mg²⁺-dependent ATPase velocities as a function of substance concentration were nonlinear, suggesting the presence of multiple catalytic sites. Both MgATP^2- and CaATP^2- served as the true substrates and apparently bind to the same catalytic sites. Free ATP and Ca²⁺ could inhibitit the Ca²⁺- and Mg²⁺-dependent ATPase. Increasing free Mg²⁺ levels enhanced the affinity of the Mg²⁺-dependent ATPase for MgATP^2-, while slightly inhibiting the V_max values. Other divalent cation-nucleoside triphosphate complexes produced maximal enzyme velocities equal to or greater than those generated by CaATP^2- and MgATP^2-. However, the ATPase had significantly higher affinities for CaATP^2- and MgATP^2-, than for the alternative substrates. The high and low affinity components of the Ca²⁺- and Mg²⁺-dependent ATPase exhibited optimal V_max values at pH 5 and 6, respectively. Analysis of the pH-dependence of the enzyme K_m values indicated enzyme-substrate binding with charge neutralization at neutral and alkaline pH's. Nonlinear double reciprocal plots were obtained at all assay temperatures. However, the complexity of the enzyme kinetics became less apparent at the higher assay temperatures. The kinetics of the barley root divalent cation-dependent ATPase activities are discussed in terms of the kinetics of ATPases from other plants and the methods used to obtain them, and compared to the kinetics of ion transport ATPases from animal membranes.     

The structure of the cells of the mesophyll and parenchymatous bundle sheath of the Gramineae

By a comparison of the ultrastructural features of the parenchymatous bundle sheath and the mesophyll it can be shown that the grasses can be divided into a number of groups roughly corresponding to those proposed by Brown, viz. Pooid, Eragrostoid, Panicoid, Arundinoid, Bambusoid and Aristidoid. In particular the ultrastructural features strengthen the grounds for separating the eragrostoid type of leaf anatomy from the panicoid type. These ultrastructural differences seem to be correlated with differences in photosynthetic pathways, and, where they do not correlate with the taxonomic divisions, neither do the photosynthetic pathways. The correlation with a taxonomy based on microcharacters as well as macroscopic characters is generally good except in the case of "Phragmitiform" grasses, and in particular the tribe Danthonieae, which seems to require a serious detailed extensive investigation.     

Multiple Pleistocene refugia and Holocene range expansion of an abundant southwestern American desert plant species (Melampodium leucanthum,Asteraceae)

Pleistocene climatic fluctuations had major impacts on desert biota in southwestern North America. During cooler and wetter periods, drought‐adapted species were isolated into refugia, in contrast to expansion of their ranges during the massive aridification in the Holocene. Here, we use Melampodium leucanthum (Asteraceae), a species of the North American desert and semi‐desert regions, to investigate the impact of major aridification in southwestern North America on phylogeography and evolution in a widespread and abundant drought‐adapted plant species. The evidence for three separate Pleistocene refugia at different time levels suggests that this species responded to the Quaternary climatic oscillations in a cyclic manner. In the Holocene, once differentiated lineages came into secondary contact and intermixed, but these range expansions did not follow the eastwardly progressing aridification, but instead occurred independently out of separate Pleistocene refugia. As found in other desert biota, the Continental Divide has acted as a major migration barrier for M. leucanthum since the Pleistocene. Despite being geographically restricted to the eastern part of the species’ distribution, autotetraploids in M. leucanthum originated multiple times and do not form a genetically cohesive group.