|Yamasaki, Hideo - UNIV. OF RYUKYUS, OKINAWA|
Submitted to: Free Radicals in Biology and Medicine
Publication Type: Other
Publication Acceptance Date: June 1, 2003
Publication Date: December 18, 2003
Citation: Cohen, M.F., Yamasaki, H., Mazzola, M. 2003. Role of enhanced nitric oxide production by soil bacteria in suppression of the fungal plant pathogen rhizoctonia solani. Free Radicals in Biology and Medicine. 35:S177. Technical Abstract: Prior amendment of soils with Brassica napus (rape) seed meal (RSM) effectively suppresses infection of apple seedling by root rot disease-causing fungi, including Rhizoctonia solani. In year 2002 field trials at two locations, application of RSM reduced R. solani root infection from 18.6% to 3.1% and 12.5% to 4.2%, for Gala/M26 and Golden Delicious/M7, respectively. Streptomyces spp. populations represented 31.1% of the total culturable bacteria population from RSM-treated soils but never exceeded 10% from non-treated control soil over the course of the study. In greenhouse (1) and field experiments RSM-amendment of soils stimulated 3- to 5-fold increases in apple root colonization by actinomycetes (predominatly Streptomyces spp.). Decades ago, the degree of disease suppressiveness obtained through soil amendments was reported to be positively correlated with increases in actinomycete populations and in soil nitrification activity. Soil bacteria release nitic oxide via nitrification and the activity of nitric oxide synthase (NOS) an enzyme found in many actinomycetes. Certain plant defense mechanisms are enhanced by exposure to nitric oxide. We thus hypothesize that at least some of the disease suppressive effects of RSM amendment may be explained by increases in bacterial nitric oxide production. In vitro experiments demonstrate dramatically higher (>100-fold) rates of nitrogen oxide production in RSM-amended soils compared to control or glucose-amended soils. Also, NOS activity has been found in several actinomycetes isolated from RSM soils. Presently, we are investigating the relative importance of bacterial NO derived from nitrification versus NOS on root protection against fungal disease. (1) Mazzola, M. et al. (2001) Phytopathology 91:637-679.