Effect of zinc-resistant Lysinibacillus species inoculation on growth, physiological properties, and zinc uptake in maize (Zea mays L.).

Soil contamination by heavy metals is one of the major abiotic stresses that cause retarded plant growth and low productivity. Among the heavy metals, excessive accumulations of zinc (Zn) cause toxicity to plants. The toxicity caused by Zn could be managed by application of Zn-tolerant plant growth-promoting (PGP) bacteria. In this study, five Zn-tolerant bacteria (100-400 mg-1 Zn resistant) were selected and identified as Lysinibacillus spp. based on 16S rRNA gene sequencing. The PGP properties of the Lysinibacillus spp. showed the production of indole acetic acid (60.0-84.0 µg/ml) and siderophore, as well as solubilization of potassium. Furthermore, the isolates were evaluated under greenhouse condition with 2 g kg-1 Zn stress and without Zn stress along with control on Zea mays. The results showed that Lysinibacillus spp. coated seeds enhanced plant growth attributes and biomass yield in both conditions compared with control plants. The enhancement of root growth ranged from 49.2 to 148.6% and shoot length from 83.3 to 111.7% under Zn-stressed soils. Also, the inoculated seedlings substantially enhanced chlorophyll a and b, proline, total phenol, and ascorbic acid. The uptake of Zn by maize root ranged from 31.5 to 210.0% compared with control plants. Therefore, this study suggested that the tested Zn-tolerant Lysinibacillus spp. may be used for cultivation of Z. mays in Zn-contaminated agricultural lands.

Effect of copper-resistant Stenotrophomonas maltophilia on maize (Zea mays) growth, physiological properties, and copper accumulation: potential for phytoremediation into biofortification.

In this study, Cu-tolerant PGP bacteria were isolated from the contaminated soils of Tapi (Surat, Gujarat, India). From a set of 118 bacteria isolated from the contaminated soil, the isolate RBTS7 was found to be efficient in tolerating 0.3 g (w/v) Cu. The isolate was identified as Stenotrophomonas maltophilia, based on biochemical and 16S rRNA gene sequencing. Further, the isolate was also found to produce indole acetic acid (140 µg/ml) and siderophore, and solubilize potassium. Inoculation study was carried out in the presence and absence of Cu in the greenhouse. The results revealed that S. maltophilia enhanced plant growth and biomasses compared to control. In addition to plant growth attributes, the isolate also enhanced chlorophyll a and b (434.1 and 496.7%) contents and antioxidant properties such as proline (168.2%), total phenolic compounds (33.5%), and ascorbic acid oxidase (62.3%) compared to control with Cu and without Cu. Inoculation of S. maltophilia + Cu enhanced the uptake of Cu in maize root (77.4%) and stem (112.0%) compared to Cu-stressed control. The results clearly indicated the inoculation of S. maltophilia reduced the toxicity of Cu and in turn enhanced the plant growth and mobilization of Cu to the plant parts.

Phytoextraction of iron from contaminated soils by inoculation of iron-tolerant plant growth-promoting bacteria in Brassica juncea L. Czern.

Iron (Fe) is one of the essential micronutrients for all living organisms. Despite its abundance in most of the contaminated soil, it is usually in unavailable forms. The unavailable form of Fe could be mobilized to plants by the use of microorganisms. This study was carried out to show that the Fe-contaminated field soils could be used to accumulate Fe in the plant parts using bacterial inoculation. For this, from a set of bacterial isolates, four Fe-tolerant bacteria were selected and identified based on 16S rRNA gene sequencing. The Fe-tolerant bacteria belonged to the genus Bacillus toyonensis (MG430287), Rhodococcus hoagii (MG432495), Lysinibacillus mangiferihumi (MG432492), and Lysinibacillus fusiformis (MG430290). Screening of plant growth-promoting properties of these isolates revealed that all isolates were able to produce indole acetic acid (50.0-84.0 µg/ml), siderophore, and potassium solubilization (except R. hoagii). Pot assay using Fe-contaminated ((8.07-8.35 g kg-1) soils River Directorate of India) revealed that Fe-tolerant bacteria enhanced the growth of Brassica juncea and its biomass. Besides the improved plant growth, the inoculated plants also showed an overall percentage increase in the uptake of iron in root, stem, and leaf (57.91-128.31%) compared with uninoculated plants. In addition to enhanced plant growth attributes, the isolates also improved the total chlorophyll content and antioxidant properties such as total phenol, proline, and ascorbic acid oxidase. Thus, the results clearly indicated that these isolates could be used as a bioinoculant to improve the sequestration of Fe from the contaminated soils and alleviation of Fe stress in plants.