Efficacy of Indole Acetic Acid and Exopolysaccharides-Producing Bacillus safensis Strain FN13 for Inducing Cd-Stress Tolerance and Plant Growth Promotion in Brassica juncea (L.)

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Abstract

Untreated wastewater used for irrigating crops is the major source of toxic heavy metals and other pollutants in soils. These heavy metals affect plant growth and deteriorate the quality of edible parts of growing plants. Phytohormone (IAA) and exopolysaccharides (EPS) producing plant growth-promoting rhizobacteria can reduce the toxicity of metals by stabilizing them in soil. The present experiment was conducted to evaluate the IAA and EPS-producing rhizobacterial strains for improving growth, physiology, and antioxidant activity of Brassica juncea (L.) under Cd-stress. Results showed that Cd-stress significantly decreased the growth and physiological parameters of mustard plants. Inoculation with Cd-tolerant, IAA and EPS-producing rhizobacterial strains, however, significantly retrieved the inhibitory effects of Cd-stress on mustard growth, and physiology by up regulating antioxidant enzyme activities. Higher Cd accumulation and proline content was observed in the roots and shoot tissues upon Cd-stress in mustard plants while reduced proline and Cd accumulation was recorded upon rhizobacterial strains inoculation. Maximum decrease in proline contents (12.4%) and Cd concentration in root (26.9%) and shoot (29%) in comparison to control plants was observed due to inoculation with Bacillus safensis strain FN13. The activity of antioxidant enzymes was increased due to Cd-stress; however, the inoculation with Cd-tolerant, IAA-producing rhizobacterial strains showed a non-significant impact in the case of the activity of superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) in Brassica juncea (L.) plants under Cd-stress. Overall, Bacillus safensis strain FN13 was the most effective strain in improving the Brassica juncea (L.) growth and physiology under Cd-stress. It can be concluded, as the strain FN13 is a potential phytostabilizing biofertilizer for heavy metal contaminated soils, that it can be recommended to induce Cd-stress tolerance in crop plants.

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Shoots, roots, and seeds of corn (Zea mays L., cv. Michigan 500), oats (Avena sativa L., cv. Au Sable), and peas (Pisum sativum L., cv. Wando) were analyzed for their superoxide dismutase content using a photochemical assay system consisting of methionine, riboflavin, and p-nitro blue tetrazolium. The enzyme is present in the shoots, roots, and seeds of the three species. On a dry weight basis, shoots contain more enzyme than roots. In seeds, the enzyme is present in both the embryo and the storage tissue. Electrophoresis indicated a total of 10 distinct forms of the enzyme. Corn contained seven of these forms and oats three. Peas contained one of the corn and two of the oat enzymes. Nine of the enzyme activities were eliminated with cyanide treatment suggesting that they may be cupro-zinc enzymes, whereas one was cyanide-resistant and may be a manganese enzyme. Some of the leaf superoxide dismutases were found primarily in mitochondria or chloroplasts. Peroxidases at high concentrations interfere with the assay. In test tube assays of crude extracts from seedlings, the interference was negligible. On gels, however, peroxidases may account for two of the 10 superoxide dismutase forms.

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Indole-3-acetic acid in microbial and microorganism-plant signaling.

Stijn Spaepen, Jos Vanderleyden, Roseline Remans (2007)

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.