Xanthomonas campestris pathovar campestris (Xcc) is the causative agent of crucifer black rot disease, which causes severe losses in agricultural yield world-wide. This bacterium is a model organism for studying plant-bacteria interactions. We sequenced the complete genome of Xcc 8004 (5,148,708 bp), which is highly conserved relative to that of Xcc ATCC 33913. Comparative genomics analysis indicated that, in addition to a significant genomic-scale rearrangement cross the replication axis between two IS1478 elements, loss and acquisition of blocks of genes, rather than point mutations, constitute the main genetic variation between the two Xcc strains. Screening of a high-density transposon insertional mutant library (16,512 clones) of Xcc 8004 against a host plant (Brassica oleraceae) identified 75 nonredundant, single-copy insertions in protein-coding sequences (CDSs) and intergenic regions. In addition to known virulence factors, full virulence was found to require several additional metabolic pathways and regulatory systems, such as fatty acid degradation, type IV secretion system, cell signaling, and amino acids and nucleotide metabolism. Among the identified pathogenicity-related genes, three of unknown function were found in Xcc 8004-specific chromosomal segments, revealing a direct correlation between genomic dynamics and Xcc virulence. The present combination of comparative and functional genomic analyses provides valuable information about the genetic basis of Xcc pathogenicity, which may offer novel insight toward the development of efficient methods for prevention of this important plant disease.

It has been predicted that the Arctic Ocean will sequester much greater amounts of carbon dioxide (CO2) from the atmosphere as a result of sea ice melt and increasing primary productivity. However, this prediction was made on the basis of observations from either highly productive ocean margins or ice-covered basins before the recent major ice retreat. We report here a high-resolution survey of sea-surface CO2 concentration across the Canada Basin, showing a great increase relative to earlier observations. Rapid CO2 invasion from the atmosphere and low biological CO2 drawdown are the main causes for the higher CO2, which also acts as a barrier to further CO2 invasion. Contrary to the current view, we predict that the Arctic Ocean basin will not become a large atmospheric CO2 sink under ice-free conditions.

The combined effects of anthropogenic and biological CO2 inputs may lead to more rapid acidification in coastal waters compared to the open ocean. It is less clear, however, how redox reactions would contribute to acidification. Here we report estuarine acidification dynamics based on oxygen, hydrogen sulfide (H2S), pH, dissolved inorganic carbon and total alkalinity data from the Chesapeake Bay, where anthropogenic nutrient inputs have led to eutrophication, hypoxia and anoxia, and low pH. We show that a pH minimum occurs in mid-depths where acids are generated as a result of H2S oxidation in waters mixed upward from the anoxic depths. Our analyses also suggest a large synergistic effect from river–ocean mixing, global and local atmospheric CO2 uptake, and CO2 and acid production from respiration and other redox reactions. Together they lead to a poor acid buffering capacity, severe acidification and increased carbonate mineral dissolution in the USA’s largest estuary.

Strains of the chestnut blight fungus Cryphonectria parasitica harboring RNA viruses of the genus Hypovirus exhibit significantly reduced levels of virulence (called hypovirulence). The accumulation of a heterotrimeric GTP-binding protein (G protein) a subunit of the G1 class was found to be reduced in hypovirus-containing C. parasitica strains. Transgenic cosuppression, a phenomenon frequently observed in transgenic plants, reduced the accumulation of this cr subunit in virus-free fungal strains. Significantly, the resulting transgenic fungal strains were also hypovirulent. These results indicate a crucial role for G-protein-linked signal transduction in fungal pathogenesis and suggest a molecular basis for virus-mediated attenuation of fungal virulence.Strains of the chestnut blight fungus Cryphonectria parasitica stably infected with members of the genus Hypovirus exhibit reduced levels of virulence (called hypovirulence) (1, 2). Since these viral elements are transmissible to vegetatively compatible fungal strains through fused hyphae (anastomoses), application of an appropriate hypovirulent strain can result in conversion of a recipient virulent strain providing the basis for biological control (1, 3). The development of an infectious cDNA copy of a hypovirus RNA (2) has further enhanced the potential for effective biological control by leading to the construction of engineered hypovirulent fungal strains that exhibit modes of hypovirus transmission not observed for natural hypovirulent strains (4). It is also possible to modify hypovirus genetic information in the context of the infectious viral cDNA to generate hypovirulent strains altered in specific phenotypic traits (5). Chen et at (6) recently demonstrated that transfection-mediated introduction of infectious hypovirus RNA transcripts into several other fungal pathogens not previously shown to harbor hypoviruses also results in profound phenotypic changes including hypovirulence, thus broadening the potential application of virus-mediated hypovirulence to other fungal pathogens.It is now well established that hypovirus infection modifies the gene expression profile of the fungal host (7-13). Such alterations presumably cause the array of phenotypic changes observed in hypovirulent strains, including virulence attenuation. Thus, an understanding of the mechanisms involved in virus-mediated alteration of host gene expression is likely to provide insights into fungal virulence. Several recent studies have shown that hypovirus infection suppresses expression of the gene lac-1, encoding the enzyme laccase, at the level of transcriptional promoter activity and that the mechanism involves perturbation of signal transduction pathways that normally govern lac-1 transcription in virus-free strains (10, 11). Although a role for laccase in fungal infection remains to be demonstrated, it is tempting, by way of extrapolation, to speculate that hypovirus-mediated attenuation of fungal vir-The publication costs of this article were defrayed in part by page ch...

Noninfectious, cytoplasmically transmissible viral double-stranded RNAs of the genus Hypovirus cause reduced virulence (hypovirulence) in the chestnut blight fungus Cryphonectria parasitica, providing the basis for virus-mediated biological control of a fungal disease. Synthetic transcripts corresponding to a full-length hypovirus RNA coding strand are infectious when introduced into fungal spheroplasts by electroporation. Hypovirus infections were readily established in Cryphonectria parasitica and in related fungal species not previously reported to harbor viruses. These results demonstrate the use of a synthetic mycovirus transcript to expand fungal host range, thereby broadening the potential application of virus-mediated hypovirulence to control fungal pathogenesis.