Azospirillum in the cloudy boundaries of bacterial endophytes

Abstract

The vast scientific community that has been studying for years thecomplex interaction between plants and Azospirillum, perhaps the superstar of plant growth promoting rhizobacteria (PGPR), should celebrate(at least we celebrate) a recent publication of Malinich and Bauer(2018) in Symbiosis journal. These authors combined culture-dependentand culture-independent techniques, and in that way, they first provedthat 7 out of 9 commercial seeds analyzed (belonging to 9 differentplant species, including important crops such as wheat, maize, rice,tomato and bean) harbored Azospirillum cells. Based on sequencing oftotal prokaryotic transcriptome of common bean seeds, they found thatAzospirillum was the fourth most abundant genus in mature seeds of thisleguminous plant (making up approximately 6.3% of total prokaryoticRNA). Through multiple and modern experimental approaches (includingthe use of gfp-tagged strains and qPCR of NifA genes), they then demonstrated (among other interesting points) that inoculated Azospirillumbrasilense strains became part of the inner plant bacterial community of Phaseolus vulgaris, not only at root level (its most recognized habitat), but also inside aerial plant parts, specially insideseeds and pods, reflecting a notorious population density shift of introduced A. brasilense cells toward seeds as P. vulgaris developed. And what is much more important: they provide evidence that Azospirillumcells residing inside bean seeds remained viable, as vertical transmission of this microorganism to the next generation effectively occurred. Time to upgrade Azospirillum to the cloudy category of natural endophytes