A study of bacterial endophytes of Cocoloba uvifera at Cabo Rojo, Puerto Rico.

摘要

All plants contain microorganisms called endophytes that live within their tissues without causing apparent signs of disease. Most of the endophytes that have been studied are fungi living in grasses and crops of agricultural importance in temperate regions. A significantly less amount of information is available on bacterial endophytes of neotropical trees. In the present study, bacterial endophytes of the coastal tree Coccoloba uvifera (sea grape) were characterized near a hypersaline solar saltern and a beach site in Cabo Rojo, Puerto Rico in September, 2008 (wet season) and March, 2009 (dry season). It is hypothesized that the frequency of colonization and bacterial diversity will differ in both ecosystems sampled between the wet and dry seasons. Two trees were selected at a site near the solar saltern fed by the Fraternidad lagoon and two trees in Playuela. On each sampling, four healthy leaves from each tree were obtained. Leaves were surface sterilized and ten fragments measuring 2mm x 2mm were chosen randomly from each leaf. The fragments were inoculated on 50% Tryptic Soy Agar (TSA) until colony growth was observed. Pure isolates were transferred to 50% TSA Petri dishes. Cultures of bacteria were separated into morphotypes depending on Gram stains and physicalappearance. DNA was extracted from isolates of different morphotypes and characterized by 16S rDNA sequencing. Sixty-one cultures were sequenced and they were characterized as 42 different strains within 14 genera of endophytic bacteria. Endophytes belonging to the Gammaproteobacteria have been found with higher frequency, but members of Betaproteobacteria and Bacilli have also been encountered in this study. Some commonly encountered genera are Stenotrophomonas sp, Pseudomonas sp, Bacillus sp, and Burkholderia sp. Chi-square and Fisher's exact test demonstrated that the frequency of colonization did not significantly differ between the trees studied in the salterns and Playuela on both seasons sampled. However, significant differences in the colonization frequencies at the Playuela site were observed. This indicates that seasonality (precipitation) does not affect the frequency of bacterial endophytes recovered. However, according to the evidence shown here, the communities of bacterial endophytes do change depending on the season.