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 ( 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 IFM 63839 (2.98 mg ml) and FI25-1F (2.54 mg ml). 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 Cd and Pb 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.