Investigation on the North American isolate of Pasteuria sp., a biological control agent of the soybean cyst nematode, Heterodera glycines.

摘要

A four-year study was conducted on an undescribed species of Pasteuria, a gram-positive, mycelial, and endospore-forming bacterium that parasitizes the soybean cyst nematode, Heterodera glycines. The objectives of the study were (i) to investigate the population dynamics of both H. glycines and Pasteuria sp., (ii) to assess the potential of Pasteuria sp. as a biological control agent of H. glycines, (iii) to elucidate the life cycle, ultrastructure, and host specificity of Pasteuild sp., and (iv) to determine the phylogenetic position of Pasteuria among the gram-positive eubacteria. During the soil sampling phase of the study, which was conducted for two years in naturally infested microplots, numbers of cysts per 250 cm3 soil declined markedly from 20 at the beginning to 9 at the end of soil sampling. Several years before Pasteuria was discovered in those microplots, H. glycines cysts averaged 300/250 cm3 soil. Numbers of second-stage juveniles, J2, of H. glycines also decreased exponentially with increasing numbers of endospores per J2, from 288 J2/250 cm3 to an equilibrium density of 67 J2/250 cM3 soil, consistent with the predictions of the Lotka-Volterra model of population dynamics.;Microscopic examinations of H. glycines adults and cysts extracted from the rhizosphere of soybean plants grown in infested soil, and of juvenile stages excised from the roots of the same plants revealed that the endospores that attached to the cuticle of J2 in soil did not germinate until the nematodes invaded soybean roots. Subsequently, the bacterium developed and completed its life cycle only in females. Transmission electron microscopic examinations of ultrathin sections of parasitized cysts showed that the North American isolate of Pasteuria and P. nishizawae, the only other Pasteuria known to attack H. glycines and which is native to Japan, have similar morphometrics and ultrastructure. The two Pasteuria differed, however, in the nature of the mesosome-like bodies present in earlier stages of endosporogenesis, in the degree of differentiation of the outer coat, and in the appearance of the fibers lining the basal adhesion layer and the exosporium of mature endospores. In attachment tests, endospores from parasitized cysts of H. glycines attached to J2 of H. schachtii, H. trifolii, and H. lespedezae, but not to Meloidogyne arenaria race 1, Tylenchorhynchus nudus, and Lahronema sp. Phylogenetic analysis of the 16S rDNA sequences placed the North American Pasteuria and its relative, P. ramosa, at the base of a clade that contained Alicyclobacillus spp.;The research presented herein indicates that, given sufficient time following introduction into a field, Pasteuria may increase to levels that would be effective as one component in an integrated pest management program to control H. glycines.