Pathogen-triggered ethylene signaling mediates systemic-induced susceptibility to herbivory in Arabidopsis.

Multicellular eukaryotic organisms are attacked by numerous parasites from diverse phyla, often simultaneously or sequentially. An outstanding question in these interactions is how hosts integrate signals induced by the attack of different parasites. We used a model system comprised of the plant host Arabidopsis thaliana, the hemibiotrophic bacterial phytopathogen Pseudomonas syringae, and herbivorous larvae of the moth Trichoplusia ni (cabbage looper) to characterize mechanisms involved in systemic-induced susceptibility (SIS) to T. ni herbivory caused by prior infection by virulent P. syringae. We uncovered a complex multilayered induction mechanism for SIS to herbivory. In this mechanism, antiherbivore defenses that depend on signaling via (1) the jasmonic acid-isoleucine conjugate (JA-Ile) and (2) other octadecanoids are suppressed by microbe-associated molecular pattern-triggered salicylic acid (SA) signaling and infection-triggered ethylene signaling, respectively. SIS to herbivory is, in turn, counteracted by a combination of the bacterial JA-Ile mimic coronatine and type III virulence-associated effectors. Our results show that SIS to herbivory involves more than antagonistic signaling between SA and JA-Ile and provide insight into the unexpectedly complex mechanisms behind a seemingly simple trade-off in plant defense against multiple enemies.

When females produce sperm: genetics of C. elegans hermaphrodite reproductive choice.

Reproductive behaviors have manifold consequences on evolutionary processes. Here, we explore mechanisms underlying female reproductive choice in the nematode Caenorhabditis elegans, a species in which females have evolved the ability to produce their own self-fertilizing sperm, thereby allowing these "hermaphrodites" the strategic choice to self-reproduce or outcross with males. We report that hermaphrodites of the wild-type laboratory reference strain N2 favor self-reproduction, whereas a wild isolate CB4856 (HW) favors outcrossing. To characterize underlying neural mechanisms, we show that N2 hermaphrodites deficient in mechanosensation or chemosensation (e.g., mec-3 and osm-6 mutants) exhibit high mating frequency, implicating hermaphrodite perception of males as a requirement for low mating frequency. Within chemosensory networks, we find opposing roles for different sets of neurons that express the cyclic GMP-gated nucleotide channel, suggesting both positive and negative sensory-mediated regulation of hermaphrodite mating frequency. We also show that the ability to self-reproduce negatively regulates hermaphrodite mating. To map genetic variation, we created recombinant inbred lines and identified two QTL that explain a large portion of N2 × HW variation in hermaphrodite mating frequency. Intriguingly, we further show that ∼40 wild isolates representing C. elegans global diversity exhibit extensive and continuous variation in hermaphrodite reproductive outcome. Together, our findings demonstrate that C. elegans hermaphrodites actively regulate the choice between selfing and crossing, highlight the existence of natural variation in hermaphrodite choice, and lay the groundwork for molecular dissection of this evolutionarily important trait.

Identification of a copper-inducible promoter for use in ectopic expression in the fungal pathogen Histoplasma capsulatum.

Despite the existence of a number of genetic tools to study the fungal pathogen Histoplasma capsulatum, strategies for conditional gene expression have not been developed. We used microarray analysis to identify genes that are transcriptionally induced or repressed by the addition of copper sulfate (CuSO(4)) to H. capsulatum yeast cultures. One of these genes, CRP1, encodes a putative copper efflux pump that is significantly induced in the presence of CuSO(4). The upstream regulatory region of CRP1 was sufficient to drive copper-regulated expression of two reporter genes, lacZ and the gene encoding green fluorescent protein. Microarray experiments were performed to determine a copper concentration that triggers accumulation of the CRP1 transcript without significant perturbation of global gene expression. These studies show that the CRP1 upstream regulatory region can be used for ectopic expression of heterologous genes in H. capsulatum. Furthermore, they demonstrate the strategic use of microarrays to identify conditional promoters that confer induction in the absence of large-scale shifts in gene expression.

Pseudomonas syringae manipulates systemic plant defenses against pathogens and herbivores.

Many pathogens are virulent because they specifically interfere with host defense responses and therefore can proliferate. Here, we report that virulent strains of the bacterial phytopathogen Pseudomonas syringae induce systemic susceptibility to secondary P. syringae infection in the host plant Arabidopsis thaliana. This systemic induced susceptibility (SIS) is in direct contrast to the well studied avirulence/R gene-dependent resistance response known as the hypersensitive response that elicits systemic acquired resistance. We show that P. syringae-elicited SIS is caused by the production of coronatine (COR), a pathogen-derived functional and structural mimic of the phytohormone jasmonic acid (JA). These data suggest that SIS may be a consequence of the previously described mutually antagonistic interaction between the salicylic acid and JA signaling pathways. Virulent P. syringae also has the potential to induce net systemic susceptibility to herbivory by an insect (Trichoplusia ni, cabbage looper), but this susceptibility is not caused by COR. Rather, consistent with its role as a JA mimic, COR induces systemic resistance to T. ni. These data highlight the complexity of defense signaling interactions among plants, pathogens, and herbivores.

Identifying phase-specific genes in the fungal pathogen Histoplasma capsulatum using a genomic shotgun microarray.

A fundamental feature of the fungal pathogen Histoplasma capsulatum is its ability to shift from a mycelial phase in the soil to a yeast phase in its human host. Each form plays a critical role in infection and disease, but little is understood about how these two morphologic phases are established and maintained. To identify phase-regulated genes of H. capsulatum, we carried out expression analyses by using a genomic shotgun microarray representing approximately one-third of the genome, and identified 500 clones that were differentially expressed. Genes induced in the mycelial phase included several involved in conidiation, cell polarity, and melanin production in other organisms. Genes induced in the yeast phase included several involved in sulfur metabolism, extending previous observations that sulfur metabolism influences morphology in H. capsulatum. Other genes with increased expression in the yeast phase were implicated in nutrient acquisition and cell cycle regulation. Unexpectedly, differential regulation of the site of transcript initiation was also observed in the two phases. These findings identify genes that may determine some of the major characteristics of the mycelial and yeast phases.