Phytotoxicity of aluminum on root growth and indole-3-acetic acid accumulation and transport in alfalfa roots

Aluminum (Al) toxicity in acid soils is a major constraint on crop production. The objective of this study of alfalfa (Medicago sativa L.) was to determine whether Al-induced inhibition of root growth in alfalfa (M. sativa L.) is related to Al distribution in different root tissues, changes in endogenous level of indole-3-acetic acid (IAA) in root tips, and the expression of key genes in IAA metabolism and translocation. Roots of alfalfa were exposed to 100μM Al³⁺ in half-strength Hoagland's nutrient solution. The inhibitory effects of Al on root elongation was more pronounced than on root and shoot biomass accumulation. Lumogallion, an Al specific stain, was used to monitor tissue locations of Al in the root. Lumogallion-Al was mainly detected in the root cap, epidermis, and stele of Al-treated roots. In the meristematic region of the root tip, Al accumulated mainly in the cell wall, intracellular membrane system and center of the nucleus. The similar distribution of Al ions in the root with that of auxin revealed that Al in roots may affect IAA levels. High performance liquid chromatography analysis demonstrated that IAA levels increased in the base of the root and decreased in the root tips treated with 100μM Al³⁺ for 3d compared to that of the control (without Al). Reverse transcription and quantitative PCR showed that the expressions of three genes, auxin transporter-like protein, auxin efflux carrier component, and cationic peroxidase, were significantly higher in Al-stressed alfalfa roots than in the control while the expression of auxin conjugate hydrolase was significantly lower. These results suggested that Al-inhibition of root elongation could be associated with Al accumulation in apoplast and membrane system, and alteration of IAA transport in the root.