Study on cellular changes and potential endotrophy of wheat roots due to colonization of Chromium reducing bacteria

Chromium is a highly toxic metal for all living organisms. Industrial use of chromium has resulted in serious widespread pollution. Biological treatment (bioremediation) has proven to be a cost-effective option for cleanup of metal-contaminated sites. Several bacteria and plant species are able to tolerate high levels of chromium compounds that can be used for cleanup. An experiment was designed to study the colonization behavior of two indigenous Cr(VI)-reducing bacterial strain Pseudomonas aeruginosa Rb-1 and Ochrobactrum intermedium Rb-2 that were grown in wheat system amended with and without Cr(VI). Hydroponically grown wheat seedlings were co-inoculated with bacterial cultures to study the root colonization potential by fluorescent and electron microscopy. Bacterial inoculation caused significant increase in the growth of seedlings under Cr(VI) stress. Fluorescent microscopy showed good colonization potential of both bacterial strains with roots of inoculated seedlings. Electron micrographs revealed that Rb-1 tended to accumulate in the form of clusters, while Rb-2 preferred to be attach in groups of two or three cells to the root surface of inoculated seedlings. Chromium stress led to the elongation of bacterial rods along with uneven cell surface due to wrapping of cells in mucilaginous material. Cr(VI) stress also resulted in the damaging of plant root surface. Hence, few cells of Rb-2 entered the damaged root cortex cells and appeared as endophytes. Excessive production of fibrillar material by both bacteria under chromium stress could clearly be observed. Both strains displayed auxin production and Cr(VI) reduction ability, showing promise for bioremediation purposes.