Plant growth-promoting rhizobacteria (PGPR) may contribute to sustainable crop production by improving plant growth and/or plant tolerance to abiotic stresses. Soil salinity, which limits the productivity of crop plants, is one of the major concerns of modern agriculture, especially in countries heavily affected by climate change as Vietnam. Currently, only a few reports have studied local PGPR isolated in Vietnam, particular Pseudomonas. Therefore, our study aimed to isolate and identify a region-specific Pseudomonas strain and evaluate the effects of this strain on germination, growth promotion and gene expression of Arabidopsis thaliana under salt stress. The Pseudomonas named PS01 was isolated from maize rhizosphere collected in Ben Tre province, Vietnam. This strain was identified as a member of the Pseudomonas putida subclade. Pseudomonas PS01 could improve the germination rate of Arabidopsis seeds in 150 mM NaCl. A. thaliana plants inoculated with Pseudomonas PS01 survived under salt stress conditions up to 225 mM NaCl, while all non-inoculated plants were dead above 200 mM NaCl. The transcriptional levels of genes related to stress tolerance showed that only LOX2 was up-regulated, while APX2 and GLYI7 were down-regulated in inoculated plants in comparison to the non-inoculated controls. In turn, RD29A and RD29B did not show any significant changes in their expression profiles.

The objectives of this study were to evaluate the plant growth promoting effects on Arabidopsis by Pseudomonas sp. strains associated with rhizosphere of crop plants grown in Mekong Delta, Vietnam. Out of all the screened isolates, Pseudomonas PS01 isolated from maize rhizosphere showed the most prominent plant growth promoting effects on Arabidopsis and maize (Zea mays). We also found that PS01 altered root system architecture (RSA). The full genome of PS01 was resolved using high-throughput sequencing. Phylogenetic analysis identified PS01 as a member of the Pseudomonas putida subclade, which is closely related to Pseudomonas taiwanensis. PS01 genome size is 5.3 Mb, assembled in 71 scaffolds comprising of 4820 putative coding sequence. PS01 encodes genes for the indole-3-acetic acid (IAA), acetoin and 2,3-butanediol biosynthesis pathways. PS01 promoted the growth of Arabidopsis and altered the root system architecture by inhibiting primary root elongation and promoting lateral root and root hair formation. By employing gene expression analysis, genetic screening and pharmacological approaches, we suggested that the plant-growth promoting effects of PS01 and the alteration of RSA might be independent of bacterial auxin and could be caused by a combination of different diffusible compounds and volatile organic compounds (VOCs). Taken together, our results suggest that PS01 is a potential candidate to be used as bio-fertilizer agent for enhancing plant growth.

Ascorbic acid (AsA) and glutathione (GSH) are considered important factors to protect plants against abiotic stress. To investigate whether altered endogenous GSH and AsA affect seed germination, plant performance and the abiotic stress tolerance, GSH deficient mutant cad2-1 and AsA-deficient mutants (vtc2-4 and vtc5-2) were phenotypically characterized for their seed germination, shoot growth, photosynthetic activity and root architecture under abiotic stresses. The cad2-1, vtc2-4 and vtc5-2 mutants showed a decrease in osmotic and salt stress tolerance, in sensitivity to ABA during seed germination, and in plant performance under severe abiotic stresses. GSH deficiency in the cad2-1 plants affected plant growth and root development in plants exposed to strong drought, oxidative and heavy metal stress conditions. Plants with lower GSH did not show an increased sensitivity to strong salt stress (100 mM NaCl). In contrast, the mutants with lower AsA enhanced salt stress tolerance in the long-term exposures to strong salt stress since they showed larger leaf areas, longer primary roots and more lateral root numbers. Limitations on AsA or GSH synthesis had no effect on photosynthesis in plants exposed to long-term strong salt or drought stresses, whereas they effected on photosynthesis of mutants exposed to CdCl₂. Taken together, the current study suggests that AsA and GSH are important for seed germination, root architecture, shoot growth and plant performance in response to different abiotic stresses, and their functions are dependent on the stress-inducing agents and the stress levels.