Accelerated Growth Rate and Increased Drought Stress Resilience of the Model Grass Brachypodium distachyon Colonized by Bacillus subtilis B26

RESEARCH ARTICLE Accelerated Growth Rate and Increased Drought Stress Resilience of the Model Grass Brachypodium distachyon Colonized by Bacillus subtilis B26 François Gagné-Bourque1☯, Boris F. Mayer1☯, Jean-Benoit Charron1*, Hojatollah Vali2, Annick Bertrand3, Suha Jabaji1* a11111 1 Department of Plant Science, Macdonald Campus of McGill University, 21,111 Lakeshore Rd. Ste-Annede-Bellevue, Québec, CANADA, H9X 3V9, 2 Facility of Electron Microscopy Research (FEMR) McGill University, 3640 University Street, Montréal, Québec, CANADA, H3A 0C7, 3 Soils and Crops Research Development Center, Agriculture and Agri-Food Canada, 2560 Hochelaga Boulevard, Québec City, Québec, CANADA, G1V 2J3 ☯ These authors contributed equally to this work. * (SJ); (JBC) OPEN ACCESS Citation: Gagné-Bourque F, Mayer BF, Charron J-B, Vali H, Bertrand A, Jabaji S (2015) Accelerated Growth Rate and Increased Drought Stress Resilience of the Model Grass Brachypodium distachyon Colonized by Bacillus subtilis B26. PLoS ONE 10(6): e0130456. doi:10.1371/journal. pone.0130456 Academic Editor: Zhulong Chan, Chinese Academy of Sciences, CHINA Received: March 10, 2015 Accepted: May 20, 2015 Published: June 23, 2015 Copyright: © 2015 Gagné-Bourque et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by The natural Sciences and Engineering Council of Canada (NSERC) grants 137135 and 386537 to SJ and JBC. The authors also acknowledge the support of FRQNT-Regroupment Strategique-Centre SEVE Grant RS-144 to both SJ and JBC. Abstract Plant growth-promoting bacteria (PGB) induce positive effects in plants, for instance, increased growth and reduced abiotic stresses susceptibility. The mechanisms by which these bacteria impact the host plant are numerous, diverse and often specific. Here, we studied the agronomical, molecular and biochemical effects of the endophytic PGB Bacillus subtilis B26 on the full life cycle of Brachypodium distachyon Bd21, an established model species for functional genomics in cereal crops and temperate grasses. Inoculation of Brachypodium with B. subtilis strain B26 increased root and shoot weights, accelerated growth rate and seed yield as compared to control plants. B. subtilis strain B26 efficiently colonized the plant and was recovered from roots, stems and blades as well as seeds of Brachypodium, indicating that the bacterium is able to migrate, spread systemically inside the plant, establish itself in the aerial plant tissues and organs, and is vertically transmitted to seeds. The presence of B. subtilis strain B26 in the seed led to systemic colonization of the next generation of Brachypodium plants. Inoculated Brachypodium seedlings and mature plants exposed to acute and chronic drought stress minimized the phenotypic effect of drought compared to plants not harbouring the bacterium. Protection from the inhibitory effects of drought by the bacterium was linked to upregulation of the drought-response genes, DREB2B-like, DHN3-like and LEA-14A-like and modulation of the DNA methylation genes, MET1B-like, CMT3-like and DRM2-like, that regulate the process. Additionally, total soluble sugars and starch contents increased in stressed inoculated plants, a biochemical indication of drought tolerance. In conclusion, we show a single inoculation of Brachypodium with a PGB affected the whole growth cycle of the plant, accelerating its growth rates, shortening its vegetative period, and alleviating drought stress effects. These effects are relevant to grasses and cereal crops. Competing Interests: The authors have declared that no competing interest exist. PLOS ONE | DOI:10.1371/journal.pone.0130456 June 23, 2015 1 / 23 Brachypodium as Model Plant for Endophyte Interaction Introduction Plant-growth promoting bacteria (PGB) are mainly soil and rhizosphere-derived organisms that are able to colonize plant roots and positively influence plant growth or reduce disease [1]. Several strains of Bacillus species, representing typical PGB have been widely studied and applied as commercialized products for efficient control of disease [2]. Bacillus spp. stimulate plant growth, increase yield and reduce pathogen infection without conferring pathogenicity [1]. The proposed mechanisms for plant growth promotion include increased nutrient availability, synthesizing plant hormones and production of volatiles [3–5]. Considerable progress has been made in understanding the mechanisms underlying Bacillus-mediated tolerance to biotic stress [6–8] however, information on Bacillus strains mitigating abiotic stress symptoms is limited [9,10] and the mechanisms underlying abiotic tolerance are largely elusive because most of the studies focus on evaluating plant growth promoting effects [11]. It has been demonstrated that a range of bacterial endophytes, the majority of which are derived from the rhizosphere, colonize the plant’s interior and many of them have been reported to improve plant growth [12]. Following rhizosphere establishment, endophytes may colonize various plant organs [12–14]. Bacillus species, considered as root colonizing rhizosphere competent bacteria are often also found as colonizers of internal tissues of plants [14,15]. Reports on the endophytic colonization of Bacillus subtilis are few focusing on the internal colonization of roots [16,17] and leaves of young seedlings [18] grown for a short period of time. However, no reports exist in which internal colonization, establishment and spread of B. subtilis were followed in vegetative and reproductive plant growth stages. We previously reported on a strain of B. subtilis B26, which was isolated from leaf blades and seeds of the bioenergy crop switchgrass (Panicum virgatum L.), and demonstrated that it is a growth enhancer of four-week-old switchgrass seedlings, as well as its ability to migrate from the roots to aerial parts of the seedlings [19], strongly suggesting that it behaves as a competent endophyte [12]. B. subtilis B26 culture filtrate contains several well-characterized lipopeptide toxins and phytohormones [19]. These qualities suggest that the endophytic ability of this strain is a biological requirement for survival in nature and has strong potential as bio-inoculant for biomass enhancement of bioenergy crops and boosting the plant’s defence against abiotic stress such as drought stress. In this study, we aim to investigate whether the internal colonization of B. subtilis endophytic strain B26 might modulate gene expression in plants, and the genes so expressed provide clues as to the effects of B26 in plants, and trigger the plant defence mechanisms to enhance resistance against abiotic stress. (...truncated)