Attenuation of phytofungal pathogenicity of Ascomycota by autophagy modulators.

Autophagy in eukaryotes functions to maintain homeostasis by degradation and recycling of long-lived and unwanted cellular materials. Autophagy plays important roles in pathogenicity of various fungal pathogens, suggesting that autophagy is a novel target for development of antifungal compounds. Here, we describe bioluminescence resonance energy transfer (BRET)-based high-throughput screening (HTS) strategy to identify compounds that inhibit fungal ATG4 cysteine protease-mediated cleavage of ATG8 that is critical for autophagosome formation. We identified ebselen (EB) and its analogs ebselen oxide (EO) and 2-(4-methylphenyl)-1,2-benzisothiazol-3(2H)-one (PT) as inhibitors of fungal pathogens Botrytis cinerea and Magnaporthe oryzae ATG4-mediated ATG8 processing. The EB and its analogs inhibit spore germination, hyphal development, and appressorium formation in Ascomycota pathogens, B. cinerea, M. oryzae, Sclerotinia sclerotiorum and Monilinia fructicola. Treatment with EB and its analogs significantly reduced fungal pathogenicity. Our findings provide molecular insights to develop the next generation of antifungal compounds by targeting autophagy in important fungal pathogens.

A plant balancing act: Meshing new and existing metabolic pathways towards an optimized system.

Specialized metabolic pathways evolve from existing pathways, creating new functionality potentially boosting fitness. However, how these pathways are integrated into a pre-existing working and well-balanced metabolic system is unclear. They could be integrated to the system as a functional appendage, or they could be fully embedded into primary metabolism by establishing new biochemical and regulatory connections. A full integration into the primary metabolic system requires substantial system re-wiring and because of this complexity, the latter is often not experimentally pursued. New studies provide evidence that some specialized metabolic pathways are fully embedded in primary metabolism with extensive new regulatory and biochemical connections. This suggests, that we should consider whether other specialized metabolic pathways could be fully integrated rather than being simple appendages. In this mini review, we survey compelling evidence supporting that some specialized metabolic pathways are fully integrated and ask if these metabolites now act as de-facto primary metabolites?

A pair of receptor-like kinases is responsible for natural variation in shoot growth response to mannitol treatment in Arabidopsis thaliana.

Growth is a complex trait that adapts to the prevailing conditions by integrating many internal and external signals. Understanding the molecular origin of this variation remains a challenging issue. In this study, natural variation of shoot growth under mannitol-induced stress was analyzed by standard quantitative trait locus mapping methods in a recombinant inbred line population derived from a cross between the Col-0 and Cvi-0 Arabidopsis thaliana accessions. Cloning of a major QTL specific to mannitol-induced stress condition led to identification of EGM1 and EGM2, a pair of tandem-duplicated genes encoding receptor-like kinases that are potentially involved in signaling of mannitol-associated stress responses. Using various genetic approaches, we identified two non-synonymous mutations in the EGM2[Cvi] allele that are shared by at least ten accessions from various origins and are probably responsible for a specific tolerance to mannitol. We have shown that the enhanced shoot growth phenotype contributed by the Cvi allele is not linked to generic osmotic properties but instead to a specific chemical property of mannitol itself. This result raises the question of the function of such a gene in A. thaliana, a species that does not synthesize mannitol. Our findings suggest that the receptor-like kinases encoded by EGM genes may be activated by mannitol produced by pathogens such as fungi, and may contribute to plant defense responses whenever mannitol is present.

Quantitative genomics: analyzing intraspecific variation using global gene expression polymorphisms or eQTLs.

Scientific inquiries in fields ranging from ecology to plant breeding assess phenotypic variation within a plant species either to explain its presence or utilize its consequences. Frequently this natural genetic variation is studied via mapping quantitative trait loci (QTLs); however, elucidation of the underlying molecular mechanisms is a continuing bottleneck. The genomic analysis of transcripts as individual phenotypes has led to the emerging field of expression QTL analysis. This field has begun both to delve into the ecological/evolutionary significance of this transcript variation as well as to use specific eQTLs to speed up our analysis of the molecular basis of quantitative traits. This review introduces eQTL analysis and begins to illustrate how these data can be applied to multiple research fields.

Distinct roles of jasmonates and aldehydes in plant-defense responses.

BACKGROUND: Many inducible plant-defense responses are activated by jasmonates (JAs), C(6)-aldehydes, and their corresponding derivatives, produced by the two main competing branches of the oxylipin pathway, the allene oxide synthase (AOS) and hydroperoxide lyase (HPL) branches, respectively. In addition to competition for substrates, these branch-pathway-derived metabolites have substantial overlap in regulation of gene expression. Past experiments to define the role of C(6)-aldehydes in plant defense responses were biased towards the exogenous application of the synthetic metabolites or the use of genetic manipulation of HPL expression levels in plant genotypes with intact ability to produce the competing AOS-derived metabolites. To uncouple the roles of the C(6)-aldehydes and jasmonates in mediating direct and indirect plant-defense responses, we generated Arabidopsis genotypes lacking either one or both of these metabolites. These genotypes were subsequently challenged with a phloem-feeding insect (aphids: Myzus persicae), an insect herbivore (leafminers: Liriomyza trifolii), and two different necrotrophic fungal pathogens (Botrytis cinerea and Alternaria brassicicola). We also characterized the volatiles emitted by these plants upon aphid infestation or mechanical wounding and identified hexenyl acetate as the predominant compound in these volatile blends. Subsequently, we examined the signaling role of this compound in attracting the parasitoid wasp (Aphidius colemani), a natural enemy of aphids. PRINCIPAL FINDINGS: This study conclusively establishes that jasmonates and C(6)-aldehydes play distinct roles in plant defense responses. The jasmonates are indispensable metabolites in mediating the activation of direct plant-defense responses, whereas the C(6)-aldehyes are not. On the other hand, hexenyl acetate, an acetylated C(6)-aldehyde, is the predominant wound-inducible volatile signal that mediates indirect defense responses by directing tritrophic (plant-herbivore-natural enemy) interactions. SIGNIFICANCE: The data suggest that jasmonates and hexenyl acetate play distinct roles in mediating direct and indirect plant-defense responses. The potential advantage of this "division of labor" is to ensure the most effective defense strategy that minimizes incurred damages at a reduced metabolic cost.

The glucosinolate-myrosinase system in an ecological and evolutionary context.

Functional analysis of natural variation in the model species Arabidopsis thaliana has enabled the cloning of many glucosinolate biosynthesis and hydrolysis genes. Variation in these genes is central to understanding the ecological role of the glucosinolate-myrosinase defense system, and allows us to dissect the evolutionary and ecological forces that shape polymorphism at underlying loci. These same genes are also variable in other crucifer species, suggesting the presence of recurring selection, possibly mediated by insects. By utilizing the genomic tools available in A. thaliana to investigate these loci fully, it might be possible to generate detailed evolutionary or ecological models to apply to other species.