Abstract
ABSTRACTPlant growth-promoting rhizobacteria (PGPR) are a functionally diverse group of microbes having immense potential as biostimulants and biopesticides. We isolated four PGPR (designated n, L, K, and Y) that confer growth-promoting effects on Arabidopsis thaliana. The present study describes the detailed polyphasic characterization of these PGPR. Classical methods of bacterial identification and biochemical test kits (API20E, API20NE, API ZYM, and API 50CH) revealed their metabolic versatility. All rhizobacterial isolates were positive for 1-aminocyclopropane-1-carboxylate (ACC) deaminase (ACCD) and indole acetic acid production and phosphorous solubilization. PCR analysis confirmed the presence of the nifH gene in strains n, L, and Y, showing their N2-fixation potential. In vitro dual culture methods and bacterial infestation in planta demonstrated that strains n and L exerted antagonistic effects on Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea 191 and provided protection to Arabidopsis plants against both phytopathogens. Short- or long-term bacterial treatment revealed significant changes in transcript levels of genes annotated to stress response and hormone metabolism in A. thaliana. In particular, the expression of stress-responsive genes in A. thaliana showed an upregulation under salinity stress. MAP kinase 6 (MPK6) was involved in the growth promotion induced by the four bacterial strains. Furthermore, these strains caused a significant increase in root dry weight of maize seedlings under gnotobiotic conditions. We conclude that the four rhizobacteria are good candidates as biofertilizers for enhancing growth of maize, among which strains n and L showed marked plant growth-promoting attributes and the potential to be exploited as functional biostimulants and biopesticides for sustainable agriculture.IMPORTANCE There are pressing needs to reduce the use of agrochemicals, and PGPR are receiving increasing interest in plant growth promotion and disease protection. This study follows up our previous report that the four newly isolated rhizobacteria promote the growth of Arabidopsis thaliana. We test the hypothesis that they have multiple PGP traits and that they can be used as biofertilizers and biopesticides. In vitro assays indicated that these four strains have various PGP properties related to nutrient availability, stress resistance, and/or pest organism antagonism. They significantly influenced the transcript levels of genes involved in stress response and hormone metabolism in A. thaliana. MPK6 is indispensable to the growth stimulation effects. Strains n and L protected A. thaliana seedlings against phytopathogens. Three strains significantly increased maize growth in vitro. In summary, introducing these four strains onto plant roots provides a benefit to the plants. This is the first study regarding the potential mechanism(s) applied by Mucilaginibacter sp. as biostimulants.
Highlights
Plant growth-promoting rhizobacteria (PGPR) are a functionally diverse group of microbes having immense potential as biostimulants and biopesticides
The four selected strains were thoroughly examined for several traits that are often associated with biocontrol and plant growth promotion
Since the buffering capacity of soil could limit the ability of bacteria to solubilize soil P, phosphate agar buffered with 100 mM Tris-HCl to mimic the buffering capacity of alkaline vertisols was chosen to ascertain the efficacy of PSB
Summary
Plant growth-promoting rhizobacteria (PGPR) are a functionally diverse group of microbes having immense potential as biostimulants and biopesticides. In vitro assays indicated that these four strains have various PGP properties related to nutrient availability, stress resistance, and/or pest organism antagonism They significantly influenced the transcript levels of genes involved in stress response and hormone metabolism in A. thaliana. Root-colonizing beneficial bacteria, referred to as plant growth-promoting rhizobacteria (PGPR), are known to include species of Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligen, Arthobacter, Burkholderia, Bacillus, and Serratia, which have been shown to assist plant growth and to control plant diseases [2, 3], either directly or indirectly, through nitrogen fixation [4], antagonism to phytopathogens [5], improving plant responses to abiotic stressors [6], alteration/production of phytohormones, and soil nutrient mobilization [7]. To cope with salt stress, stressed plants activate various mechanisms through conserved signal transduction pathways [18], resulting in the production and accumulation of diverse functional components, such as osmolytes (i.e., proline and glycine betaine) [19] and nonenzymatic (i.e., phenolics, flavonoids, and glutathione) and enzymatic (i.e., peroxidase, catalase, and the enzymes involved in the ascorbate-glutathione cycle) [20, 21] antioxidants, all of which mitigate the oxidative damage caused by high salinity [18]
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