Methylobacterium sp. 2A influences root development through auxin signalling

Abstract

The plant root system determines the capacity to acquire nutrients and water and plays a critical role in adaptation. Auxins are the “root forming hormones of plants” and some root-associated beneficial bacteria can stimulate root growth secreting indole acetic acid (IAA). Methylobacterium sp. 2A is a pink-pigmented rhizobacterium that can enhance Arabidopsis growth and lateral root density. Two proposed IAA biosynthesis pathways were predicted in its genome: the indole-3-acetamide (IAM) and the indole-3-acetonitrile (IAN) pathways. Moreover, it can produce high levels of IAA in vitro. In this work, we aim to shed light on the influence of this isolate on root behavior. For this, DR5:GUS tomato plants were grown in vitro on MS medium and roots were inoculated with 10 μl of a bacterial suspension (OD600 0.05 in 0.85% NaCl). Histochemical and fluorometric analyses were conducted at 10 dpi. Also, roots of Arabidopsis DR5:GUS plants were inoculated at stage 1.0 with 2 μl of the above-mentioned suspension and pictures were taken at 7 dpi. Furthermore, an Arabidopsis auxin mutant, iaa19, which is defective in lateral root formation, was inoculated and the number of lateral roots was quantified at 4 and 7 dpi. Upon inoculation, the expression of the auxin-responsive promoter DR5 was induced in the roots of tomato and Arabidopsis plants, and lateral root formation was improved in iaa19 plants (37 and 44% at 4 and 7 dpi, respectively). Our results suggest that Methylobacterium sp. 2A promotes lateral root formation through modulating auxin signaling.