Mapping of QTL for Resistance against the Crucifer Specialist Herbivore Pieris brassicae in a New Arabidopsis Inbred Line Population, Da(1)-12×Ei-2

In Arabidopsis thaliana and other crucifers, the glucosinolate-myrosinase system contributes to resistance against herbivory by generalist insects. As yet, it is unclear how crucifers defend themselves against crucifer-specialist insect herbivores.We analyzed natural variation for resistance against two crucifer specialist lepidopteran herbivores, Pieris brassicae and Plutella xylostella, among Arabidopsis thaliana accessions and in a new Arabidopsis recombinant inbred line (RIL) population generated from the parental accessions Da(1)-12 and Ei-2. This RIL population consists of 201 individual F(8) lines genotyped with 84 PCR-based markers. We identified six QTL for resistance against Pieris herbivory, but found only one weak QTL for Plutella resistance. To elucidate potential factors causing these resistance QTL, we investigated leaf hair (trichome) density, glucosinolates and myrosinase activity, traits known to influence herbivory by generalist insects. We identified several previously unknown QTL for these traits, some of which display a complex pattern of epistatic interactions.Although some trichome, glucosinolate or myrosinase QTL co-localize with Pieris QTL, none of these traits explained the resistance QTL convincingly, indicating that resistance against specialist insect herbivores is influenced by other traits than resistance against generalists

The GC-Rich Mitochondrial and Plastid Genomes of the Green Alga Coccomyxa Give Insight into the Evolution of Organelle DNA Nucleotide Landscape

Most of the available mitochondrial and plastid genome sequences are biased towards adenine and thymine (AT) over guanine and cytosine (GC). Examples of GC-rich organelle DNAs are limited to a small but eclectic list of species, including certain green algae. Here, to gain insight in the evolution of organelle nucleotide landscape, we present the GC-rich mitochondrial and plastid DNAs from the trebouxiophyte green alga Coccomyxa sp. C-169. We compare these sequences with other GC-rich organelle DNAs and argue that the forces biasing them towards G and C are nonadaptive and linked to the metabolic and/or life history features of this species. The Coccomyxa organelle genomes are also used for phylogenetic analyses, which highlight the complexities in trying to resolve the interrelationships among the core chlorophyte green algae, but ultimately favour a sister relationship between the Ulvophyceae and Chlorophyceae, with the Trebouxiophyceae branching at the base of the chlorophyte crown

Arabidopsis thaliana encodes a bacterial-type heterodimeric isopropylmalate isomerase involved in both Leu biosynthesis and the Met chain elongation pathway of glucosinolate formation

The last steps of the Leu biosynthetic pathway and the Met chain elongation cycle for glucosinolate formation share identical reaction types suggesting a close evolutionary relationship of these pathways. Both pathways involve the condensation of acetyl-CoA and a 2-oxo acid, isomerization of the resulting 2-malate derivative to form a 3-malate derivative, the oxidation-decarboxylation of the 3-malate derivative to give an elongated 2-oxo acid, and transamination to generate the corresponding amino acid. We have now analyzed the genes encoding the isomerization reaction, the second step of this sequence, in Arabidopsis thaliana. One gene encodes the large subunit and three encode small subunits of this enzyme, referred to as isopropylmalate isomerase (IPMI) with respect to the Leu pathway. Metabolic profiling of large subunit mutants revealed accumulation of intermediates of both Leu biosynthesis and Met chain elongation, and an altered composition of aliphatic glucosinolates demonstrating the function of this gene in both pathways. In contrast, the small subunits appear to be specialized to either Leu biosynthesis or Met chain elongation. Green fluorescent protein tagging experiments confirms the import of one of the IPMI small subunits into the chloroplast, the localization of the Met chain elongation pathway in these organelles. These results suggest the presence of different heterodimeric IPMIs in Arabidopsis chloroplasts with distinct substrate specificities for Leu or glucosinolate metabolism determined by the nature of the different small subunit

A Naturally Occurring Plant Cysteine Protease Possesses Remarkable Toxicity against Insect Pests and Synergizes Bacillus thuringiensis Toxin

When caterpillars feed on maize (Zea maize L.) lines with native resistance to several Lepidopteran pests, a defensive cysteine protease, Mir1-CP, rapidly accumulates at the wound site. Mir1-CP has been shown to inhibit caterpillar growth in vivo by attacking and permeabilizing the insect's peritrophic matrix (PM), a structure that surrounds the food bolus, assists in digestion and protects the midgut from microbes and toxins. PM permeabilization weakens the caterpillar defenses by facilitating the movement of other insecticidal proteins in the diet to the midgut microvilli and thereby enhancing their toxicity. To directly determine the toxicity of Mir1-CP, the purified recombinant enzyme was directly tested against four economically significant Lepidopteran pests in bioassays. Mir1-CP LC50 values were 1.8, 3.6, 0.6, and 8.0 ppm for corn earworm, tobacco budworm, fall armyworm and southwestern corn borer, respectively. These values were the same order of magnitude as those determined for the Bacillus thuringiensis toxin Bt-CryIIA. In addition to being directly toxic to the larvae, 60 ppb Mir1-CP synergized sublethal concentrations of Bt-CryIIA in all four species. Permeabilization of the PM by Mir1-CP probably provides ready access to Bt-binding sites on the midgut microvilli and increases its activity. Consequently, Mir1-CP could be used for controlling caterpillar pests in maize using non-transgenic approaches and potentially could be used in other crops either singly or in combination with Bt-toxins

Dissecting the Genetic Architecture of Host–Pathogen Specificity

International audienc

Drought-Adaptation Potential in Fagus sylvatica: Linking Moisture Availability with Genetic Diversity and Dendrochronology

<div><h3>Background</h3><p>Microevolution is essential for species persistence especially under anticipated climate change scenarios. Species distribution projection models suggested that the dominant tree species of lowland forests in Switzerland, European beech (<em>Fagus sylvatica</em> L.), might disappear from most areas due to expected longer dry periods. However, if genotypes at the moisture boundary of the species climatic envelope are adapted to lower moisture availability, they can serve as seed source for the continuation of beech forests under changing climates.</p> <h3>Methodology/Principal Findings</h3><p>With an AFLP genome scan approach, we studied neutral and potentially adaptive genetic variation in <em>Fagus sylvatica</em> in three regions containing a dry and a mesic site each (<em>n</em>_ind. = 241, <em>n</em>_markers = 517). We linked this dataset with dendrochronological growth measures and local moisture availabilities based on precipitation and soil characteristics. Genetic diversity decreased slightly at dry sites. Overall genetic differentiation was low (<em>F</em>_st = 0.028) and Bayesian cluster analysis grouped all populations together suggesting high (historical) gene flow. The Bayesian outlier analyses indicated 13 markers with three markers differing between all dry and mesic sites and the others between the contrasting sites within individual regions. A total of 41 markers, including seven outlier loci, changed their frequency with local moisture availability. Tree height and median basal growth increments were reduced at dry sites, but marker presence/absence was not related to dendrochronological characteristics.</p> <h3>Conclusion and Their Significance</h3><p>The outlier alleles and the makers with changing frequencies in relation to moisture availability indicate microevolutionary processes occurring within short geographic distances. The general genetic similarity among sites suggests that ‘preadaptive’ genes can easily spread across the landscape. Yet, due to the long live span of trees, fostering saplings originating from dry sites and grown within mesic sites might increase resistance of beech forests during the anticipated longer dry periods.</p> </div