Substrate utilization by endophytic bacteria Paenibacillus polymyxa P2b-2R that may facilitate bacterial entrance and survival inside diverse plant hosts

Henry Yang,Akshit Puri,Kiran Preet Padda,Chris P Chanway,Elena P Ivanova

doi:10.1139/facets-2016-0031

Abstract

Bacterial endophytes are thought to enter plants either through pre-existing openings in plant tissues or by creating openings by hydrolyzing major plant cell wall components. A lodgepole endophyte, Paenibacillus polymyxa P2b-2R, consistently formed endophytic colonies when inoculated in diverse plant hosts, viz., lodgepole pine, western red cedar, corn, canola, and tomato. We were interested to know, whether or not this bacterial strain possesses enzymes that can hydrolyze three major plant cell wall components namely cellulose, xylan, and pectin to facilitate entrance into the host plants. Using a BIOLOG assay, we also tested this bacterial strain’s ability to utilize carbon sources that might facilitate its entrance and hence its survival inside host plants. Paenibacillus polymyxa P2b-2R hydrolyzed sodium carboxymethylcellulose, beechwood xylan, and sodium polypectate and utilized 39 of the 95 carbon sources (41%) tested. Of the 39 carbon substrates oxidized by P2b-2R, the “carbohydrates” group represents the largest source of utilizable carbon (23 out of 39). Thus, it can be concluded that P. polymyxa P2b-2R is able to degrade major cell wall components (cellulose, xylan, and pectin) and utilize some of the available carbon substrates, possibly to gain entry and survive inside the plant and form endophytic colonies thereafter.

Highlights

The term “bacterial endophyte” is used to describe a bacterium that colonizes internal plant tissues without inducing disease symptoms (Hallmann et al 1997; Azevedo et al 2000)
Bacteria that are capable of endophytic colonization may enter roots passively via existing cracks such as emergence sites of lateral roots (James et al 1994) or, more actively, by using hydrolytic enzymes to break down plant cell wall components and metabolize organic compounds in the apoplast (Hurek et al 1994)
The latter mode of entry has been reported for plant–microbe interactions ranging from the highly specific symbiosis between Rhizobium leguminosarum biovar trifolii and white clover (Mateos et al 1992) to more casual associations such as those between Azoarcus sp. and grasses (Hurek et al 1994)

Summary

Introduction

The term “bacterial endophyte” is used to describe a bacterium that colonizes internal plant tissues without inducing disease symptoms (Hallmann et al 1997; Azevedo et al 2000). Gluconacetobacter diazotrophicus was found to possess cellulolytic enzymes (Mateos et al 1992; Adriano-Anaya et al 2005) and was able to grow on media supplemented with 10% sucrose and tolerate low pH (Boddey et al 1991). These characteristics were thought to facilitate endophytic colonization of sugarcane tissues by G. diazotrophicus, which was confirmed in studies where it reached high population densities (Döbereiner et al 1988; Munoz-Rojas and Caballero-Mellado 2003)

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Conclusion