Mining the phytomicrobiome to understand how bacterial coinoculations enhance plant growth.

Maskit Maymon,Jérôme Ventosa,Teni Anbarchian,Minxia Chou,Tiffany Sung,Erin R Sanders,Tyler Ice,Pilar Martínez-Hidalgo,Johannes Sikorski,Ann M Hirsch,Karena Craemer,Nancy A Fujishige,Kym F Faull,William Villella,Lin H Hwang,Stephen S Tran

doi:10.3389/fpls.2015.00784

Abstract

In previous work, we showed that coinoculating Rhizobium leguminosarum bv. viciae 128C53 and Bacillus simplex 30N-5 onto Pisum sativum L. roots resulted in better nodulation and increased plant growth. We now expand this research to include another alpha-rhizobial species as well as a beta-rhizobium, Burkholderia tuberum STM678. We first determined whether the rhizobia were compatible with B. simplex 30N-5 by cross-streaking experiments, and then Medicago truncatula and Melilotus alba were coinoculated with B. simplex 30N-5 and Sinorhizobium (Ensifer) meliloti to determine the effects on plant growth. Similarly, B. simplex 30N-5 and Bu. tuberum STM678 were coinoculated onto Macroptilium atropurpureum. The exact mechanisms whereby coinoculation results in increased plant growth are incompletely understood, but the synthesis of phytohormones and siderophores, the improved solubilization of inorganic nutrients, and the production of antimicrobial compounds are likely possibilities. Because B. simplex 30N-5 is not widely recognized as a Plant Growth Promoting Bacterial (PGPB) species, after sequencing its genome, we searched for genes proposed to promote plant growth, and then compared these sequences with those from several well studied PGPB species. In addition to genes involved in phytohormone synthesis, we detected genes important for the production of volatiles, polyamines, and antimicrobial peptides as well as genes for such plant growth-promoting traits as phosphate solubilization and siderophore production. Experimental evidence is presented to show that some of these traits, such as polyamine synthesis, are functional in B. simplex 30N-5, whereas others, e.g., auxin production, are not.

Highlights

Rhizosphere bacteria function as a consortium, synergistically protecting plants from disease (Kloepper et al, 2004), providing plants with essential nutrients (Pradhan and Sukla, 2005; Martínez-Hidalgo et al, 2014), and stimulating plant growth by producing growth-promoting factors (El-Tarabily et al, 2008; Merzaeva and Shirokikh, 2010)
B. simplex has been shown in a number of reports to be an effective Plant Growth Promoting Bacterial (PGPB) (Ertruk et al, 2010; Hassen and Labuschagne, 2010; Schwartz et al, 2013)
We show that B. simplex strains 30N-5 and II3b11 are phylogenetically and genetically different from the known PGPB bacilli

Summary

Introduction

Rhizosphere bacteria function as a consortium, synergistically protecting plants from disease (Kloepper et al, 2004), providing plants with essential nutrients (Pradhan and Sukla, 2005; Martínez-Hidalgo et al, 2014), and stimulating plant growth by producing growth-promoting factors (El-Tarabily et al, 2008; Merzaeva and Shirokikh, 2010). The microbial composition of the root microbiomes for many plants is known (Schlaeppi et al, 2014), defining the mechanisms driving the microbe/plant synergism in the soil is challenging This is because soil is complex and the experiments are difficult to perform. Simpler models have been employed, such as using microcosms or rhizotrons and limiting the number of plant and microbial species to be studied This is especially true for specific interactions such as those involved in nitrogen fixation, where investigations of the interactions between nitrogen-fixing bacteria and other soil bacteria or fungi consist of coinoculating a legume plant with a rhizobium and a single plant growth promoting bacterial (PGPB) species. Such interactions usually result in an enhancement of plant growth over inoculation solely with rhizobia (see references in Schwartz et al, 2013)

Methods

Results

Conclusion