Role of Root Exudates in Cadmium Accumulation of a Low-Cadmium-Accumulating Tobacco Line (Nicotiana tabacum L.)

Abstract

Root exudates are tightly linked with cadmium (Cd) uptake by the root and thus affect plant Cd accumulation. A hydroponic experiment was carried out to explore the role of root exudates in Cd accumulation of a low-Cd-accumulating tobacco line (RG11) compared with a high-Cd- accumulating tobacco line (Yuyan5). Greater secretion of organic acids and amino acids by the roots was induced by an exogenous Cd addition in the two tobacco lines. The concentration of organic acid secreted by RG11 was only 51.1~61.0% of that secreted by Yuyan5. RG11 roots secreted more oxalic acid and acetic acid and less tartaric acid, formic acid, malic acid, lactic acid, and succinic acid than Yuyan5 under Cd stress. Oxalic acid accounted for 26.8~28.8% of the total organic acids, being the most common component among the detected organic acids, and was significantly negatively correlated with Cd accumulation in RG11. Propionic acid was only detected in the root exudates of RG11 under Cd stress. Lactic acid was positively linked with Cd accumulation in Yuyan5, being less accumulated in RG11. Similarly, RG11 secreted more amino acids than Yuyan5 under Cd stress. Aspartic acid, serine, and cysteine appeared in RG11 when it was exposed to Cd. Lysine was the most secreted amino acid in RG11 under Cd stress. RG11 roots secreted less lysine, histidine, and valine, but more phenylalanine and methionine than Yuyan5 under Cd stress. The results show that organic acids and amino acids in root exudates play a key role in Cd uptake by the root, and this contribution varied with cultivar/genotype. However, further research is still needed to explore the mechanisms underlying low Cd translocation to the leaf, which may be the key contribution of low Cd accumulation in RG11 to the security of tobacco leaf.