Rhizobacterial volatile emissions regulate auxin homeostasis and cell expansion in Arabidopsis

Abstract

Certain plant growth-promoting rhizobacteria (PGPR), in the absence of physical contact with a plant stimulate growth via volatile organic compound (VOC) emissions, through largely unknown mechanisms. To probe how PGPR VOCs trigger growth in plants, RNA transcript levels of Arabidopsis seedlings exposed to Bacillus subtilus (strain GB03) were examined using oligonucleotide microarrays. In screening over 26,000 protein-coded transcripts, a group of approximately 600 differentially expressed genes related to cell wall modifications, primary and secondary metabolism, stress responses, hormone regulation and other expressed proteins were identified. Transcriptional and histochemical data indicate that VOCs from the PGPR strain GB03 trigger growth promotion in Arabidopsis by regulating auxin homeostasis. Specifically, gene expression for auxin synthesis was up regulated in aerial regions of GB03-exposed plants; auxin accumulation decreased in leaves and increased in roots with GB03 exposure as revealed in a transgenic DR5::GUS Arabidopsis line, suggesting activation of basipetal auxin transport. Application of the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) restricted auxin accumulation to sites of synthesis thereby preventing GB03-mediated decreases in shoot auxin levels as well as thwarting GB03-mediated growth promotion. In addition, microarray data revealed coordinated regulation of cell wall loosening enzymes that implicated cell expansion with GB03 exposure, which was confirmed by comparative cytological measurements. The discovery that bacterial VOCs, devoid of auxin or other known plant hormones regulate auxin homeostasis and cell expansion provides a new paradigm as to how rhizobacteria promote plant growth.

Sustained growth promotion in Arabidopsis with long-term exposure to the beneficial soil bacteriumBacillus subtilis(GB03)

“…A bouquet of over 25 bacterial volatile odors has been identified that trigger differential expression of approximately 600 transcripts related to cell wall modifications, primary and secondary metabolism, stress responses, hormone regulation and other expressed proteins. 9,12,13 These biologically active bacterial volatiles trigger plant growth promotion by a tissue specific re-distribution of endogenous auxin. 13 GB03 volatiles also augment Arabidopsis photosynthetic capacity by increasing photosynthetic efficiency and chlorophyll content.…”

“…9,12,13 These biologically active bacterial volatiles trigger plant growth promotion by a tissue specific re-distribution of endogenous auxin. 13 GB03 volatiles also augment Arabidopsis photosynthetic capacity by increasing photosynthetic efficiency and chlorophyll content.…”

“…15 With salt stress, GB03 volatiles increase salt tolerance by concurrently down and upregulated expression of the sodium transporter HKT1 in roots and shoots, respectively, resulting in lower Na + accumulation throughout the plant compared with controls unexposed to GB03 volatiles. 16 While utilizing Petri-dish grown Arabidopsis seedlings has proven to be an effective model system to mechanistically probe early growth responses activated by volatiles from beneficial bacteria, 13,14,16 an examination of long-term growth promotion by PGPR volatiles requires expanded growth conditions. Here we examine how Arabidopsis development and reproductive success is affected by uninterrupted long-term exposure to GB03 volatiles when plants are grown in unconfined Magenta boxes.…”

Do volatile compounds produced by Fusarium oxysporum and Verticillium dahliae affect stress tolerance in plants?

“…The involvement of auxin in plant stress tolerance has been well established (Fu and Wang 2011; Pieterse et al 2012; Naseem et al 2015; Forni et al 2017). Based on previous studies showing the critical role of auxin signalling in controlling plant responses to VCs produced by various microbes (Zhang et al 2007; Splivallo et al 2009; Bailly et al 2014; Bitas et al 2015; Garnica-Vergara et al 2016), we investigated how VCs produced by two V. dahliae strains affect the growth of A. thaliana mutants defective in auxin influx ( aux1-7 ) or response ( tir1-1 and axr1-3 ) in the presence of 100 mM NaCl. These mutants were chosen because their growth responses to VCs produced by F. oxysporum and V. dahliae were defective (Bitas et al 2015; Li et al under revision).…”

Role of Plant Growth Promoting Rhizobacteria in Accumulation of Heavy Metal in Metal Contaminated Soil

“…The realization of this technique depends on the ability of plants to survive under heavy metal stress with high biomass production. The phytoremediation efficiency can be enhanced by increasing the heavy metal solubility and mobilization in the soil [4] and enhanced plant biomass by the help of plant growth promoting bacteria. This can be accomplished by emerging the relationship of hyper accumulator plants with plant growth promoting rhizobacteria (PGPR) like Bacillus and Pseudomonas having the ability for plant growth in adverse conditions.…”