Serological and pathological evaluations of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight of rice

Abstract

Six new serological groups IIb, VI, V, VII, VIII and IV were delineated with new monoclonal antibodies (MAbs) Xoo-7, Xoo-8, Xoo-9, Xoo-10 and Xoo-11 made to Asian strains and tested against 299 strains of Xanthomonas oryzae pv. oryzae representing various rice growing areas of the world. Serologically distinct populations were recognized by new MAbs among Indian and Nepalese strains of X o. oryzae. No relationship between serogroups and virulence or serogroup and race was obtained, except that previously reported serogroups III, IV and V contained only weakly virulent, atypical strains of X o. oryzae from the United States. Ninety seven percent of the typical X o. oryzae strains reacted either with MAb Xoo-7 or a previously reported MAb, Xco-2, but no strain reacted with both. These MAbs identified lipopolysaccharide antigens and gave bright fluorescence In indirect immunofluorescence microscopy (IF). An immunofluorescence colony staining technique (IFC) also was developed for detection of X. o. oryzae from rice seed extracts with fluorescein isothyocyanate (FITC) conjugated MAbs Xoo-2 and Xoo-7 after enrichment in a semiselective medium E. Colonies of typical strains of X. o. oryzae on medium E formed 3 to 4 days earlier than on other semiselective media. IFC enabled detection of as few as 44 cfu in 100 µl extract from 100 rice seeds (1% infestation rate) even when plates were crowded with contaminants. Since both antibodies give bright immunofluorescence, a mixture of these MAbs can be used with IFC to detect 97% of the typical strains of X. o. oryzae. Enrichment on medium E followed by identification of X. o. oryzae by IFC with two pathovar specific MAbs enhanced sensitivity of detection, and the method is useful for epidemiological and seed transmission studies of this pathogen.