Colonization of roots of wheat seedlings by bacilli proposed as biocontrol agents against take-all

Abstract

Colonization of wheat roots by bacteria reported antagonistic to Gaeumannomyces graminis (Sacc.) Arx et Olivier var. tritici Walker, the cause of take-all of wheat, was studied. Under gnotobiotic conditions in sand, where the sole carbon source was supplied from roots, bacteria inoculated onto wheat grain spread down roots in the absence of watering. A motile strain, Bacillus pumilus Meyer and Gottheil, colonized the root system rapidly and extensively, reaching population of 10'-10' colony forming units (cfu) g −1 dry root. Colonization was enhanced when roots were simultaneously infected with G. graminis and impeded when the rooting medium contained clay. A non-motile strain. Bacillus cereus var. mycoides (Flugge) Smith (syn. B. mycoides Flugge), spread more slowly down roots to reach a population of 10 6 cfu g −1 dry root on only the oldest parts of the root system, closest to the point of inoculation. Colonization by B. mycoides was inhibited by G. graminis and stimulated by clay. Mutant strains of B. pumilus and B. mycoides, marked with antibiotic resistance, were used to show that the pattern of spread down roots in non-sterile soil was similar to that of gnotobiotic root colonization and to thus demonstrate a potential application for the method in screening for root-colonizing biocontrol agents.