Cactus Roots Harbor an Endophytic Growth Promoting Pseudomonas Species

Abstract:

The prickly pear cactus, Opuntia sp., is a xerophytic desert plant which is distributed in various parts of the United States. Previous studies of the Opuntia rhizosphere revealed that the roots were primarily colonized by fungi, with few bacteria. In this study, we isolated below-ground microbiota of an Eastern Prickly Pear cactus found in the wild using serial dilution plating and identified them using 16S rRNA sequencing. Of the sixteen culturable bacterial isolates, we found that the four endophytes were exclusively gram negative, with a fluorescent /Pseudomonas/ sp. (strain IF1) found as most abundant. The rhizosphere immediately surrounding the roots were colonized by a higher diversity of bacteria, mostly gram negative. The bacteria in the surrounding sand were mostly gram positive, including Bacillus sp. These results indicate that the cactus roots actively select and colonize specific taxa of bacteria. The bacteria in the three environments were found to be biochemically distinct. Inclusion of the Pseudomonas strain IF1 dramatically improved the growth of two different cacti species based on the number of de novo padlet sprouts. Thus, wild cacti and endophytic Pseudomonas could benefit from a mutualistic interaction.