Galactosides in the rhizosphere: Utilization by Sinorhizobium meliloti and development of a biosensor

Abstract

Identifying the types and distributions of organic substrates that support microbial activities around plant roots is essential for a full understanding of plant-microbe interactions and rhizosphere ecology. We have constructed a strain of the soil bacterium Sinorhizobium meliloti containing a gfp gene fused to the melA promoter which is induced on exposure to galactose and galactosides. We used the fusion strain as a biosensor to determine that galactosides are released from the seeds of several different legume species during germination and are also released from roots of alfalfa seedlings growing on artificial medium. Galactoside presence in seed wash and sterile root washes was confirmed by HPLC. Experiments examining microbial growth on ␣-galactosides in seed wash suggested that ␣-galactoside utilization could play an important role in supporting growth of S. meliloti near germinating seeds of alfalfa. When inoculated into microcosms containing legumes or grasses, the biosensor allowed us to visualize the localized presence of galactosides on and around roots in unsterilized soil, as well as the grazing of fluorescent bacteria by protozoa. Galactosides were present in patches around zones of lateral root initiation and around roots hairs, but not around root tips. Such biosensors can reveal intriguing aspects of the environment and the physiology of the free-living soil S. meliloti before and during the establishment of nodulation, and they provide a nondestructive, spatially explicit method for examining rhizosphere soil chemical composition.Rhizobium ͉ GFP ͉ protozoa ͉ sugars

Mapping the Sinorhizobium meliloti 1021 solute-binding protein-dependent transportome

“…We found that an integrated reporter gene in SMb21644, encoding an ABC subunit, and downstream of SMb21647 was induced, not only by raffinose and galactose, but by the tetra-saccharide (␣-galactoside) stachyose (Gal␣1-6Gal␣1-6Glu␣1-␤2Fruf), and by the galactose derivatives, galactosamine and dulcitol (galactitol). This is consistent with the results of Bringhurst et al (24) and Gage and Long (23).…”

“…A plasmid reporter fusion to the region upstream of SMb21647 was not expressed under any of the conditions tested, so it is likely that expression of all of the transporter components is under the control of the promoter of melA. Bringhurst et al (24) found that a reporter gene fused to melA was induced by galactose and ␣-galactosides melibiose and raffinose, and (to a lesser extent) by ␤-galactosides lactose and lactulose. We found that an integrated reporter gene in SMb21644, encoding an ABC subunit, and downstream of SMb21647 was induced, not only by raffinose and galactose, but by the tetra-saccharide (␣-galactoside) stachyose (Gal␣1-6Gal␣1-6Glu␣1-␤2Fruf), and by the galactose derivatives, galactosamine and dulcitol (galactitol).…”

Selective progressive response of soil microbial community to wild oat roots

“…As roots move through soil, they impact its physical, chemical and biotic characteristics, and these changes are accompanied by alterations in microbial community activity (Bringhurst et al, 2001;DeAngelis et al, 2008). Soil that is directly influenced by roots, the rhizosphere, can make up a substantial volume of temperate zone soils in the top 10-15 cm, though root influence may extend to meters of depth (Lynch and Whipps, 1990).…”

“…Root movement through soil creates dynamic environmental gradients that are constantly reiterated with new root growth. A root moving through 'bulk soil' introduces labile carbon and nutrients, creates water conduits, and deposits antimicrobial compounds and hormones (Hawes et al, 1998;Brimecombe et al, 2001;Bringhurst et al, 2001;DeAngelis et al, 2005;Hawkes et al, 2007) across temporal scales of hours to days (Jaeger et al, 1999;Lubeck et al, 2000). As many soil microbes are carbon limited (Paul and Clark, 1996), they may be expected to respond quickly to root-induced changes in soil chemistry and nutrient status by reproducing and increasing in activity (Heijnen et al, 1995;Jaeger et al, 1999;Herman et al, 2006).…”

Control of Inducer Accumulation Plays a Key Role in Succinate-Mediated Catabolite Repression in Sinorhizobium meliloti

“…S. meliloti can utilize ␣-galactosides in laboratory medium and also when growing in the rhizospheres of host and nonhost plants (4). The utilization of ␣-galactosides requires genes which are part of an operon located on pSymB, a 1.7-Mb plasmid, in S. meliloti ( Fig.…”