Comparison of rhizosphere bacterial communities in Arabidopsis thaliana mutants for systemic acquired resistance.

Systemic acquired resistance (SAR) is an inducible systemic plant defense against a broad spectrum of plant pathogens, with the potential to secrete antimicrobial compounds into the soil. However, its impact on rhizosphere bacteria is not known. In this study, we examined fingerprints of bacterial communities in the rhizosphere of the model plant Arabidopsis thaliana to determine the effect of SAR on bacterial community structure and diversity. We compared Arabidopsis mutants that are constitutive and non-inducible for SAR and verified SAR activation by measuring pathogenesis-related protein activity via a beta-glucoronidase (GUS) reporter construct driven by the beta-1-3 glucanase promoter. We used terminal restriction fragment length polymorphism (T-RFLP) analysis of MspI- and HaeIII-digested 16S rDNA to estimate bacterial rhizosphere community diversity, with Lactobacillus sp. added as internal controls. T-RFLP analysis showed a clear rhizosphere effect on community structure, and diversity analysis of both rhizosphere and bulk soil operational taxonomic units (as defined by terminal restriction fragments) using richness, Shannon-Weiner, and Simpson's diversity indices and evenness confirmed that the presence of Arabidopsis roots significantly altered bacterial communities. This effect of altered soil microbial community structure by plants was also seen upon multivariate cluster analysis of the terminal restriction fragments. We also found visible differences in the rhizosphere community fingerprints of different Arabidopsis SAR mutants; however, there was no clear decrease of rhizosphere diversity because of constitutive SAR expression. Our study suggests that SAR can alter rhizosphere bacterial communities, opening the door to further understanding and application of inducible plant defense as a driving force in structuring soil bacterial assemblages.

Aeroallergen prevalence in the northern New Jersey-New York City metropolitan area: a 15-year summary.

BACKGROUND: Elevated environmental pollen levels result in allergic and asthmatic symptoms in sensitive individuals. OBJECTIVE: To present data collected during a 15-year period demonstrating the seasonal pollen variation in a metropolitan area. METHODS: Pollen was collected daily except for weekends. Pollen counts were counted using light microscopy and were used to calculate the average daily pollen count per month between March 1 and October 31 of each calendar year. The month in which each class of pollen reached the highest level (peak) was analyzed across the sampling period. Spearman p correlation coefficients were calculated to show changes in peak pollen levels across time. RESULTS: The average daily pollen level (tree, grasses, and weeds) for each month was analyzed (1987-2002). Tree pollen peaked in May and composed 98.7% of the measurable pollen between March and May. Grass pollen had a biphasic peak (June and September), representing 42.9% of measurable pollen in July and 6.4% in September. Weed and ragweed levels peaked in September. Total weed pollen constituted 93.5% of the measurable pollen between August and October. The combined total pollen levels peaked in May. The highest annual peak tree pollen count was observed between 1992 and 1997, with a linear relationship between tree and total pollen (R2 = 0.97); highest levels of grass pollen were observed between 1993 and 1997; and highest levels of weed pollen were observed between 1993 and 1995. A trend toward declining levels of total pollen was observed between 1993 and 2002. This declining trend was most pronounced for weed pollen. CONCLUSIONS: Aeroallergens pollinate sequentially, starting with trees in the spring, grass throughout the summer, and weeds in late summer to early fall. Pollen levels have declined from 1993 to the present. The most pronounced drop has been in weed pollen levels. Grass pollen demonstrates a biphasic pattern. Tree pollen composes most annual pollen measured in the northern New Jersey-New York City area.