A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments.

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Maize seed endophytic bacteria: dominance of antagonistic, lytic enzyme-producing Bacillus spp.

Beneficial aspects of endophytic microorganisms have motivated researchers to explore plant endophytic world. The present study was aimed to isolate and characterize the seed-borne endophytic bacteria from diverse maize genotypes. Eighty maize seed endophytic bacteria (MSEB), isolated from 30 maize genotypes, were characterized using polyphasic approach. The dendrograms and phylogenetic tree generated on the basis of ARDRA analysis and metabolic profiling of endophytic bacteria revealed genotypic and biochemical diversity among MSEB. The 16S rDNA sequence analysis revealed Bacillus as the most dominant encountered genus affiliated with Phylum Firmicutes. Few isolates belonged to genus Staphylococcus, whereas one isolate was identified as Corynebacterium sp. under Phylum Actinobacteria. Majority of the MSEB isolates exhibited antagonism against phytopathogenic fungi, production of ammonia, and secretion of lytic enzymes; some isolates also exhibited indole acetic acid production, the traits of which can be helpful in endophytic establishment and advantageous to the host plant. Besides, many MSEB exhibited tolerance to salinity (10%), osmotic stress (40% PEG6000), and temperature (60 °C), indicating their possible application under stress conditions. Endophytic nature of the selected MSEB isolates was confirmed by tracking their presence in shoots, leaves, and roots of the host seedlings with the help of biochemical marker (rifampicin resistance). Thus, the MSEB identified in the present study can be explored as potential bioinputs for improving plant growth and productivity under stressed conditions, besides helping in understanding the plant-endophyte interactions.