Abstract
A valuable strategy to improve crop yield consists in the use of plant growth-promoting rhizobacteria (PGPRs). However, the influence of PGPR colonization on plant physiology is largely unknown. PGPR Burkholderia phytofirmans strain PsJN (Bp PsJN) colonized only Arabidopsis thaliana roots after seed or soil inoculation. Foliar bacteria were detected only after leaf infiltration. Since, different bacterial times of presence and/or locations in host plant could lead to different plant physiological responses, photosynthesis, and metabolite profiles in A. thaliana leaves were thus investigated following leaf, root, or seed inoculation with Bp PsJN. Only Bp PsJN leaf colonization transiently decreased cyclic electron transport and effective quantum yield of photosystem I (PSI), and prevented a decrease in net photosynthesis and stomatal opening compared to the corresponding control. Metabolomic analysis revealed that soluble sugars, amino acids or their derivatives accumulated differently in all Bp PsJN-inoculated plants. Octanoic acid accumulated only in case of inoculated plants. Modifications in vitamin, organic acid such as tricarboxylic acid intermediates, and hormone amounts were dependent on bacterial time of presence and location. Additionally, a larger array of amino acids and hormones (auxin, cytokinin, abscisic acid) were modified by seed inoculation with Bp PsJN. Our work thereby provides evidence that relative short-term inoculation with Bp PsJN altered physiological status of A. thaliana leaves, whereas long-term bacterization triggered modifications on a larger set of metabolites. Our data highlighted the changes displayed during this plant–microbe interaction to trigger physiological and metabolic responses that could explain the increase in plant growth or stress tolerance conferred by the presence of Bp PsJN.
Highlights
We previously showed that Bp Burkholderia phytofirmans strain PsJN (PsJN) bacterization of seeds or by soil irrigation led to root endophytic colonization of A. thaliana plants (0.4 × 104 or 1.7 × 104 cfu g−1 FW, respectively), and that no bacteria were detected inside and on surface of leaves (Su et al, 2015)
Burkholderia phytofirmans strain PsJN (Bp PsJN) leaf infiltration was employed to test whether physiological modifications induced by the presence of bacteria could be dependent on its location
We considered that A. thaliana leaf infiltration with Bp PsJN could lead to a response at local and short-term levels, whereas root or seed inoculation with Bp PsJN results in a response at distal and short- or long-term levels, respectively
Summary
Nonpathogenic microorganisms, such as PGPR, promote plant growth and improve plant resistance against biotic or abiotic stress (Agrawal et al, 2015; Yadav et al, 2015). Plant growth promotion and improvement of nutrient element acquisition induced by PGPRs could be the direct results of enhanced photosynthesis due to increased chlorophyll a and/or b contents and/or a better PS II activity (Esitken et al, 2003; Zhang et al, 2008; Cohen et al, 2015). Bacillus subtilis GB03 increased Arabidopsis thaliana chlorophyll contents, chloroplast number, and photosynthetic capacity through production of volatile organic compounds (Zhang et al, 2008). Carbohydrates produced via photosynthesis supply carbon and energy and act as signal molecules involved in plant growth, development, and responses to stresses (Rolland et al, 2002), bacterial endophytes could exert their beneficial effects on plant growth and health by modifying plant photosynthetic activities and carbohydrate partitioning (Bolton, 2009; Van Hulten et al, 2010)
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