Abstract
Some plant growth promoting bacteria (PGPB) are enigmatic in enhancing plant growth in the face of increased metal accumulation in plants. Since most PGPB colonize the plant root epidermis, we hypothesized that PGPB confer tolerance to metals through changes in speciation at the root epidermis. We employed a novel combination of fluorophore-based confocal laser scanning microscopic imaging and synchrotron based microscopic X-ray fluorescence mapping with X-ray absorption spectroscopy to characterize bacterial localization, zinc (Zn) distribution and speciation in the roots of Brassica juncea grown in Zn contaminated media (400 mg kg−1 Zn) with the endophytic Pseudomonas brassicacearum and rhizospheric Rhizobium leguminosarum. PGPB enhanced epidermal Zn sequestration relative to PGBP-free controls while the extent of endophytic accumulation depended on the colonization mode of each PGBP. Increased root accumulation of Zn and increased tolerance to Zn was associated predominantly with R. leguminosarum and was likely due to the coordination of Zn with cysteine-rich peptides in the root endodermis, suggesting enhanced synthesis of phytochelatins or glutathione. Our mechanistic model of enhanced Zn accumulation and detoxification in plants inoculated with R. leguminosarum has particular relevance to PGPB enhanced phytoremediation of soils contaminated through mining and oxidation of sulphur-bearing Zn minerals or engineered nanomaterials such as ZnS.
Highlights
Zinc (Zn) is a constituent of many enzymes and proteins that sustain life, and is an essential trace metal for optimum growth and development in plants (Broadley et al, 2007)
We focused on the early stages of growth, having demonstrated in our previous study (Adediran et al, 2015) that the effects of these plant growth promoting bacteria (PGPB) were manifested in the first 2 weeks, allowing us to focus on the role of the different bacteria in metal speciation changes in these early stages
It is similar to the roll towel test (Ma et al, 2011) and the plant growth promotion assay on filter paper developed by Glick et al (1995) and modified by Belimov et al (2001) which has been used to assess root growth promotion in bacteria inoculated on B. juncea plants exposed to cadmium toxicity (Belimov et al, 2005) and in Orychophragmus violaceus plants exposed to zinc (He et al, 2010)
Summary
Zinc (Zn) is a constituent of many enzymes and proteins that sustain life, and is an essential trace metal for optimum growth and development in plants (Broadley et al, 2007). It is phytotoxic at elevated concentrations, with plants exhibiting stunted root and leaf chlorosis (Frerot et al, 2005). Zinc toxicity in plants is one of the most extensive microelement phytotoxicities (Chaney, 1993). The vacuole has been shown to be the main plant organelle responsible for storing toxic compounds in plant
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