AFLP, Pathogenicity, and VCG Analyses of Fusarium oxysporum and Fusarium pseudocircinatum from Acacia koa.

Acacia koa (koa), a native tree in Hawaii, suffers from a dieback caused by Fusarium oxysporum. Pathogenicity tests, vegetative compatibility group (VCG) tests, and amplified fragment length polymorphisms (AFLP) analyses were conducted on Fusarium isolates recovered from diseased koa. Koa seedling mortality with individual strains ranged from 0 to 85%, with 42% of the strains killing no seedlings. Thus, strains of F. oxysporum recovered from dying koa trees may or may not be virulent. In addition to F. oxysporum, F. pseudocircinatum strains were isolated from diseased koa; however, they were either nonvirulent or had weak virulence. This is the first report of F. pseudocircinatum in Hawaii. The 46 strains of F. oxysporum and F. pseudocircinatum were grouped into 16 VCGs, but 86% of the highly virulent strains belonged to VCG 2. In AFLP analyses, strains from the same VCG generally clustered with one another. Identification of the same set of strains using VCG, AFLP, and pathogenicity tests showed that the highly virulent strains are genetically close and that high virulence toward koa is not a property of all strains of F. oxysporum. Thus, VCG 2 with the corresponding AFLP data is a significant biological entity for which we propose the name F. oxysporum f. sp. acaciae to reflect its virulence on koa.

Nodulation in black locust by the Gammaproteobacteria Pseudomonas sp. and the Betaproteobacteria Burkholderia sp.

Nodulation abilities of bacteria in the subclasses Gammaproteobacteria and Betaproteobacteria on black locust (Robinia pseudoacacia) were tested. Pseudomonas sp., Burkholderia sp., Klebsiella sp., and Paenibacillus sp. were isolated from surface-sterilized black locust nodules, but their nodulation ability is unknown. The aims of this study were to determine if these bacteria are symbiotic. The species and genera of the strains were determined by RFLP analysis and DNA sequencing of 16S rRNA gene. Inoculation tests and histological studies revealed that Pseudomonas sp. and Burkholderia sp. formed nodules on black locust and also developed differentiated nodule tissue. Furthermore, a phylogenetic analysis of nodA and a BLASTN analysis of the nodC, nifH, and nifHD genes revealed that these symbiotic genes of Pseudomonas sp. and Burkholderia sp. have high similarities with those of rhizobial species, indicating that the strains acquired the symbiotic genes from rhizobial species in the soil. Therefore, in an actual rhizosphere, bacterial diversity of nodulating legumes may be broader than expected in the Alpha-, Beta-, and Gammaproteobacteria subclasses. The results indicate the importance of horizontal gene transfer for establishing symbiotic interactions in the rhizosphere.