A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice (Oryza sativa L.). Rice seedlings inhibited the growth of cress (Lepidium sativum L.) and lettuce (Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 microM, respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.
A new species of Acinetubacter, Acinetubacter radioresistens, is proposed for three radiation-resistant Acinetobacter strains which were isolated from samples of cotton and soil. This species is phenotypically, genetically, and enzymatically distinguished from other Acinetubacter species. Strains of this species are gram-negative, oxidase-negative, nonsporeforming, nonmotile, nonfermentative, aerobic, pleomorphic coccobacilli and produce no acid from saccharides; they assimilated 16 of the 52 carbon sources which we examined. The radiation-resistant Acinetubacter strains had little deoxyribonucleic acid homology (15 to 44%) with other Acinetubacter strains and showed a different pattern from other Acinetubacter strains on electrophoretic analysis of enzymes. The guanine-plus-cytosine contents of the deoxyribonucleic acids are 44.1 to 44.8 mol%. The major cellular fatty acids are C,8:1, c 1 6 : 1 and c 1 6 : 0 , and the ubiquinone system is Q-9. The type strain of this species is strain FO-1 (= IAM 13186). Since Brisou and Prevot (2) in 1954 established the genusAcinetobacter by separating the nonmotile species from the genus "Achromobacter," the taxonomy of Acinetobacter has been widely studied, and a number of named species have been proposed. However, only two species (Acinetobacter calcoaceticus and Acinetohacter lwofii) appear in the Approved Lists of Bacterial Names (16), and only one species (A. calcoaceticus) is described in Bergey 's Manual of Systematic Bacteriology (5). Recently, Bouvet and Grimont (1) described 12 hybridization groups and gave new names to four of these genospecies (Acinetobacter baumannii, Acinetobacter haemolyticus, Acinetobacter johnsonii, and Acinetobacter junii).We have isolated acinetobacters from samples of soil and cotton. One of these organisms, strain FO-lT (T = type strain), was isolated as a radiation-resistant organism from cotton sterilized by gamma radiation (7) and was considered to be a separate species from A . calcoaceticus on the basis of phenotypic characteristics (11). We also reported the differential characteristics of the outer membrane in a comparison of strain FO-lT and A . calcoaceticus type strain IAM 12087 (= ATCC 23055) (10). Furthermore, we grouped our collection of acinetobacters, both these isolates and authentic strains, on the basis of deoxyribonucleic acid (DNA)-DNA hybridization (13) and electrophoretic patterns of enzymes (Y. Nishimura, H. Kanzaki, and H. Iizuka, J. Basic Microbiol., in press). These studies showed that two isolates (G82012 and G82076) from soils belong to the same group as strain FO-lT; these two isolates were also resistant to gamma rays (13). The radiation susceptibilities reported previously (6, 7, 13) for the three radiation-resistant Acinetobacter (RRA) strains and the reference Acinetobacter strains are shown in Table 1. The RRA strains showed D,, values which were ca. 19 to 34 times that of the type strain of A . calcoaceticus, measured under air equilibrium in 0.067 M phosphate buffer, and their survival curves showed no...
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