We previously reported that extra-slow-growing Bradyrhizobium japonicum isolates obtained from three field sites in Japan, designated as HRS (highly reiterated sequence-possessing) strains, have high copy numbers of the insertion sequences RSalpha and RSbeta. When strain collections in the USA, Japan, Korea, Thailand and China were examined by Southern hybridization using RSalpha, RSbeta and IS1631 as probes, HRS strains were found in the Japanese, Chinese, and American collections, but not in the Korean and Thai ones. Copy number analyses of RSalpha and RSbeta, calibrated with the copy number of rrs (16S rRNA gene), indicated that the HRS stains can be divided into two major groups. Group A is comprised of members with a high copy number of RSalpha (mean+/-S.D., 139+/-27), a low number of RSbeta (mean+/-S.D., 30+/-13) sequences, and extremely slow growth rates (mean doubling time+/-S.D., 27+/-9 h). In contrast, group B is comprised of strains with a high copy number of RSbeta (mean+/-S.D., 93+/-6) and a lower number of RSalpha. These groupings of HRS strains were well correlated with phylogenetic clusters based on rrs, gyrB and serogroups (110/122 and 123/135). Growth rate of B. japonicum strains was also correlated exclusively with RSalpha copy number. The ecological, evolutionary and biotechnological implications of the findings are discussed.

The rapid expansion of mandarin (Citrus reticulata L.) production areas with high agrochemical input in the highland areas of northern Thailand has resulted in negative effects in terms of production, environment, soil quality, and public health. The use of microorganisms as plant growth promoters is an alternative method to reduce agrochemical input. Thus, we studied the diversity of endophytic actinomycetes in mandarin and their potential as plant growth promoters. A total of 252 endophytic actinomycete isolates were recovered from mandarin. Based on spore chain morphology, cell wall type, and 16S rRNA gene sequence, the isolates were classified into six genera: Streptomyces, Nocardia, Nocardiopsis, Spirillospora, Microbispora and Micromonospora. The most frequent isolates recovered were members of Streptomyces (85.3%). Selected isolates (64 isolates) from these genera were evaluated for their indole-3-acetic acid (IAA) production potential in a medium with 2 mg mL −1 tryptophan, and all the selected isolates showed the potential to produce IAA, with average values of IAA production of 13.34, 3.36, 140.38, 12.55, 1.40, and 6.19 µg IAA mL −1 , respectively. Isolates of genus Nocardiopsis showed a very high ability to produce IAA that was the highest among all the genera, with values ranging from 62.23 to 222.75 µg mL −1 . Twelve isolates selected from these genera were inoculated onto mandarin seedlings, and the results indicated that the shoot height, fresh shoot weight and fresh root weight of the seedlings were promoted by the inoculation of endophytic actinomycetes, with values ranging from 20.2 to 49.1%, 14.9 to 53.6%, and 1.6 to 102% over the control, respectively.

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Over 1500 root-nodule bacteria were isolated from a range of uninoculated soybeans, and one cowpea, 'trap-hosts', sown in 1985 into traditional soybean-growing areas of soybean-growing areas of northern Thailand. Most isolates were slow-growing Bradyrhizobium japonicum. Using a modified 'bottle-jar' technique, 586 of the isolates were tested with a range of soybean hosts and one cowpea host. The results indicated: (a) a very high level of infectiveness, with only one isolate failing to nodulate one host, and 95%forming >20 nodules per plant; (b) a high level of effectiveness of fixation of nitrogen by the local rhizobium populations with soybeans; (c) evidence of selection of effective strains by both soybean and cowpea hosts in the field;These results strongly suggest that there is no need for selection of soybean germplasm for compatibility with the local rhizobium populations. 1500 isolates were examined serologically using antisera to both US and Thai strains. While there was greater compatibility between the local isolates and Thai antisera, there was no obvious geographic relationship within or between isolates at the local or regional level. Inter-site differences were as great as inter-regional differences.Antibiotic resistance patterns, examined on 116 isolates, were useful for separating isolates for effectiveness testing, but the degree of variability was great, and the patterns differed greatly from published data on US collections.The considerable superiority in effectiveness of fixation of this collection, over that of a collection made from comparable trap hosts in Nigeria, and the lack of common serological and antibiotic resistance patterns with strains of US origin, illustrate the uniqueness of this collection, and emphasise the value of intensive study of natural populations.

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