16S rDNA fingerprinting of rhizosphere bacterial communities associated with healthy and Phytophthora infected avocado roots

Molecular techniques employing 16S rDNA profiles generated by PCR-DGGE were used to detect changes in bacterial community structures of the rhizosphere of avocado trees during infection by Phytophthora cinnamomi and during repeated bioaugmentation with a disease suppressive fluorescent pseudomonad. When the 16S rDNA profiles were analyzed by multivariate analysis procedures, distinct microbial communities were shown to occur on healthy and infected roots. Bacterial communities from healthy roots were represented by simple DNA banding profiles, suggestive of colonization by a few predominant species, and were approximately 80% similar in structure. In contrast, roots that were infected with Phytophthora, but which did not yet show visible symptoms of disease, were colonized by much more variable bacterial communities that had significantly different community structures from those of healthy roots. Root samples from trees receiving repeated applications of the disease suppressive bacterium Pseudomonas fluorescens st. 513 were free of Phytophthora infection, and had bacterial community structures that were similar to those of nontreated healthy roots. Sequence analysis of clones generated from four predominant bands cut from the DGGE gels revealed the presence of pseudomonads, as well as several previously unidentified bacteria. Differentiation of 16S rDNA profiles for healthy and infected roots suggests that rhizosphere bacterial community structure may serve as an integrative indicator of changes in chemical and biological conditions in the plant rhizosphere during the infection process.