The plant growth promoting bacterium Azospirillum brasilense is vertically transmitted in Phaseolus vulgaris (common bean)

Azospirillum brasilense is a plant growth-promoting bacterium that is utilized as a bio-fertilizer worldwide. Inoculation of crops with A. brasilense can lead to a substantial boost in crop yield, although this enhancement is variable. Sometimes crop production is enhanced as much as 70% and other times enhancement does not occur at all. To solve this problem, a greater understanding of A. brasilense - plant interactions is needed. While the root/rhizosphere ecology of A. brasilense has been studied extensively there are no significant studies that address whether A. brasilense populates above ground plant tissues. In this study, we use a combination of qPCR, transcriptomics, confocal microscopy, and culturing techniques to map the in-planta distribution of a GFP-expressing A. brasilense in Phaseolus vulgaris (common bush bean). We show that A. brasilense constitute part of the plant bacterial microflora well above the root system. During P. vulgaris development A. brasilense was shown to actively move to developing bean seeds where it forms a significant intercellular population. GFP-expressing A. brasilense was shown to vertically transmit to successive plant generations demonstrating that A. brasilense in the seed constitute an effective inoculum. The ability of A. brasilense to not only colonize roots but to also actively migrate to developing seeds indicates the existence of a more complex symbiotic relationship than previously understood.