Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

Biodiversity is rapidly declining, and this may negatively affect ecosystem processes, including economically important ecosystem services. Previous studies have shown that biodiversity has positive effects on organisms and processes across trophic levels. However, only a few studies have so far incorporated an explicit food-web perspective. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades.     

Acyl-CoA thioesterase 9 (ACOT9) in mouse may provide a novel link between fatty acid and amino acid metabolism in mitochondria

Acyl-CoA thioesterase (ACOT) activities are found in prokaryotes and in several compartments of eukaryotes where they hydrolyze a wide range of acyl-CoA substrates and thereby regulate intracellular acyl-CoA/CoA/fatty acid levels. ACOT9 is a mitochondrial ACOT with homologous genes found from bacteria to humans and in this study we have carried out an in-depth kinetic characterization of ACOT9 to determine its possible physiological function. ACOT9 showed unusual kinetic properties with activity peaks for short-, medium-, and saturated long-chain acyl-CoAs with highest V _max with propionyl-CoA and (iso) butyryl-CoA while K _cat/K _m was highest with saturated long-chain acyl-CoAs. Further characterization of the short-chain acyl-CoA activity revealed that ACOT9 also hydrolyzes a number of short-chain acyl-CoAs and short-chain methyl-branched CoA esters that suggest a role for ACOT9 in regulation also of amino acid metabolism. In spite of markedly different K _ms, ACOT9 can hydrolyze both short- and long-chain acyl-CoAs simultaneously, indicating that ACOT9 may provide a novel regulatory link between fatty acid and amino acid metabolism in mitochondria. Based on similar acyl-CoA chain-length specificities of recombinant ACOT9 and ACOT activity in mouse brown adipose tissue and kidney mitochondria, we conclude that ACOT9 is the major mitochondrial ACOT hydrolyzing saturated C₂-C₂₀-CoA in these tissues. Finally, ACOT9 activity is strongly regulated by NADH and CoA, suggesting that mitochondrial metabolic state regulates the function of ACOT9.     

Structure of the guide-strand-containing argonaute silencing complex

The slicer activity of the RNA-induced silencing complex is associated with argonaute, the RNase H-like PIWI domain of which catalyses guide-strand-mediated sequence-specific cleavage of target messenger RNA. Here we report on the crystal structure of Thermus thermophilus argonaute bound to a 5'-phosphorylated 21-base DNA guide strand, thereby identifying the nucleic-acid-binding channel positioned between the PAZ- and PIWI-containing lobes, as well as the pivot-like conformational changes associated with complex formation. The bound guide strand is anchored at both of its ends, with the solvent-exposed Watson-Crick edges of stacked bases 2 to 6 positioned for nucleation with the mRNA target, whereas two critically positioned arginines lock bases 10 and 11 at the cleavage site into an unanticipated orthogonal alignment. Biochemical studies indicate that key amino acid residues at the active site and those lining the 5'-phosphate-binding pocket made up of the Mid domain are critical for cleavage activity, whereas alterations of residues lining the 2-nucleotide 3'-end-binding pocket made up of the PAZ domain show little effect.     

Recruitment of entomopathogenic nematodes by insect-damaged maize roots

Plants under attack by arthropod herbivores often emit volatile compounds from their leaves that attract natural enemies of the herbivores. Here we report the first identification of an insect-induced belowground plant signal, (E)-beta-caryophyllene, which strongly attracts an entomopathogenic nematode. Maize roots release this sesquiterpene in response to feeding by larvae of the beetle Diabrotica virgifera virgifera, a maize pest that is currently invading Europe. Most North American maize lines do not release (E)-beta-caryophyllene, whereas European lines and the wild maize ancestor, teosinte, readily do so in response to D. v. virgifera attack. This difference was consistent with striking differences in the attractiveness of representative lines in the laboratory. Field experiments showed a fivefold higher nematode infection rate of D. v. virgifera larvae on a maize variety that produces the signal than on a variety that does not, whereas spiking the soil near the latter variety with authentic (E)-beta-caryophyllene decreased the emergence of adult D. v. virgifera to less than half. North American maize lines must have lost the signal during the breeding process. Development of new varieties that release the attractant in adequate amounts should help enhance the efficacy of nematodes as biological control agents against root pests like D. v. virgifera.     

SUMO-modified PCNA recruits Srs2 to prevent recombination during S phase

Damaged DNA, if not repaired before replication, can lead to replication fork stalling and genomic instability; however, cells can switch to different damage bypass modes that permit replication across lesions. Two main bypasses are controlled by ubiquitin modification of proliferating cell nuclear antigen (PCNA), a homotrimeric DNA-encircling protein that functions as a polymerase processivity factor and regulator of replication-linked functions. Upon DNA damage, PCNA is modified at the conserved lysine residue 164 by either mono-ubiquitin or a lysine-63-linked multi-ubiquitin chain, which induce error-prone or error-free replication bypasses of the lesions. In S phase, even in the absence of exogenous DNA damage, yeast PCNA can be alternatively modified by the small ubiquitin-related modifier protein SUMO; however the consequences of this remain controversial. Here we show by genetic analysis that SUMO-modified PCNA functionally cooperates with Srs2, a helicase that blocks recombinational repair by disrupting Rad51 nucleoprotein filaments. Moreover, Srs2 displays a preference for interacting directly with the SUMO-modified form of PCNA, owing to a specific binding site in its carboxy-terminal tail. Our finding suggests a model in which SUMO-modified PCNA recruits Srs2 in S phase in order to prevent unwanted recombination events of replicating chromosomes.     

A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera

Myeloproliferative disorders are clonal haematopoietic stem cell malignancies characterized by independency or hypersensitivity of haematopoietic progenitors to numerous cytokines. The molecular basis of most myeloproliferative disorders is unknown. On the basis of the model of chronic myeloid leukaemia, it is expected that a constitutive tyrosine kinase activity could be at the origin of these diseases. Polycythaemia vera is an acquired myeloproliferative disorder, characterized by the presence of polycythaemia diversely associated with thrombocytosis, leukocytosis and splenomegaly. Polycythaemia vera progenitors are hypersensitive to erythropoietin and other cytokines. Here, we describe a clonal and recurrent mutation in the JH2 pseudo-kinase domain of the Janus kinase 2 (JAK2) gene in most (> 80%) polycythaemia vera patients. The mutation, a valine-to-phenylalanine substitution at amino acid position 617, leads to constitutive tyrosine phosphorylation activity that promotes cytokine hypersensitivity and induces erythrocytosis in a mouse model. As this mutation is also found in other myeloproliferative disorders, this unique mutation will permit a new molecular classification of these disorders and novel therapeutical approaches.