Symptom development proceeds at different rates in susceptible and partially resistant cereal seedlings infected with Fusarium pseudograminearum

Variation in the reaction of cereal cultivars to crown rot caused by Fusarium spp., in particular Fusarium pseudograminearum, was identified over 50 yrs ago, however the parameters and pathways of infection by F. pseudograminearum remain poorly understood. Seedlings of wheat, barley and oat genotypes that differ in susceptibility to crown rot were inoculated with a mixture of F. pseudograminearum isolates. Seedlings were harvested from 7 to 42 days after inoculation and expanded plant parts were rated for severity of visible disease symptoms. Individual leaf sheaths were placed onto nutrient media and fungal colonies emerging from the leaf sheathes were counted to estimate the degree of fungal spread within the host tissue. Significant differences in both the timing and the severity of disease symptoms were observed in the leaf sheath tissues of different host genotypes. Across all genotypes and plant parts examined, the development of visible symptoms closely correlated with the spread of the fungus into that tissue. The degree of infection of the coleoptile and sub-crown internode varied between genotypes, but was unrelated to the putative resistance of the host. In contrast leaf sheath tissues of the susceptible barley cv. Tallon and bread wheat cv. Puseas scored higher disease ratings and consistently showed faster, earlier spread of the fungus into younger tissues than infections of the oat cv. Cleanleaf or the wheat lines 2-49 and CPI 133814. While initial infections usually spread upwards from near the base of the first leaf sheath, the pathogen did not appear to invade younger leaf sheaths only from the base, but rather spread laterally across from older leaf sheaths into younger, subtended leaf sheaths, particularly as disease progressed. Early in the infection of each leaf sheath, disease symptoms in the partially resistant genotypes were less severe than in susceptible genotypes, however as infected leaf sheaths aged, differences between genotypes lessened as disease symptoms approached maximum values. Hence, while visual scoring of disease symptoms on leaf sheaths is a reliable comparative measure of the degree of fungal infection, differences between genotypes in the development of disease symptoms are more reliably assessed using the most recently expanded leaf sheaths.

Investigation of the microbial metabolism of carbon dioxide and hydrogen in the kangaroo foregut by stable isotope probing

Kangaroos ferment forage material in an enlarged forestomach analogous to the rumen, but in contrast to ruminants, they produce little or no methane. The objective of this study was to identify the dominant organisms and pathways involved in hydrogenotrophy in the kangaroo forestomach, with the broader aim of understanding how these processes are able to predominate over methanogenesis. Stable isotope analysis of fermentation end products and RNA stable isotope probing (RNA-SIP) were used to investigate the organisms and biochemical pathways involved in the metabolism of hydrogen and carbon dioxide in the kangaroo forestomach. Our results clearly demonstrate that the activity of bacterial reductive acetogens is a key factor in the reduced methane output of kangaroos. In in vitro fermentations, the microbial community of the kangaroo foregut produced very little methane, but produced a significantly greater proportion of acetate derived from carbon dioxide than the microbial community of the bovine rumen. A bacterial operational taxonomic unit closely related to the known reductive acetogen Blautia coccoides was found to be associated with carbon dioxide and hydrogen metabolism in the kangaroo foregut. Other bacterial taxa including members of the genera Prevotella, Oscillibacter and Streptococcus that have not previously been reported as containing hydrogenotrophic organisms were also significantly associated with metabolism of hydrogen and carbon dioxide in the kangaroo forestomach.The ISME Journal advance online publication, 13 March 2014; doi:10.1038/ismej.2014.25.