Expression of SidD gene and physiological characterization of the rhizosphere plant growth-promoting yeasts.

There is increasing evidence that rhizosphere microbes contribute to the stress mitigation process, but the mechanisms of this plant-microbe interaction are not yet understood. Siderophores-producing microorganisms have been considered important for enhancing metal tolerance in plants. In this study, rhizosphere yeasts were isolated from wheat (Triticum aestivum L.) and examined for siderophores production and heavy metal resistance. Out of thirty-five isolates, only eight yeast strains showed heavy metal-resistance and plant-growth promotion properties. The highest inorganic phosphate-solubilization was shown by Trichosporon ovoides IFM 63839 (2.98 mg ml-1) and Saccharomyces cerevisiae FI25-1F (2.54 mg ml-1). Two strains, namely YEAST-6 and YEAST-16 showed high siderophore production and heavy metal-resistance, were investigated for sidD gene expression under different levels of Cd2+ and Pb2+ toxicity stress. The heavy metal-resistant yeast strains were characterized and identified based on the phenotypic characteristics and their 18S rRNA genes sequence. SidD gene expression was induced by yeasts growing under iron-limiting conditions and excess of other heavy metal, suggesting that expression of sidD gene increases in the presence of 600-800 μM heavy metal but under iron limitation. Extensive studies of the microbe-plant micronutrient interactions will enrich our understanding of the rhizosphere role in the terms of plant growth promotion.