Aluminum toxicity in alfalfa (Medicago sativa) is alleviated by exogenous foliar IAA inducing reduction of Al accumulation in cell wall

- The most amount of Al in the CW was accumulated in hemicellulose 1 in the two alfalfa varieties.
- IAA reduced Al accumulation in cell wall by decreasing Al content in pectin through reducing pectin content and PME activity in root tips.
- Exogenous application of IAA increase H+ secretion to rhizosphere and balance H+ level between internal and external cells.

The objective of this study was to investigate the effect of IAA on Al distribution in plant and cell and its mechanism in alfalfa. Seedlings of two alfalfa varieties, WL525 (Al-resistant) and WL440 (Al-sensitive), were exposed to 0 (−Al) or 100 μM L−1 AlCl3, and were foliar sprayed with water or 6 mg L−1 IAA during 3-d treatment. The results showed that Al contents in shoots, root tips and cell wall (CW) were significantly lower in WL525 than in WL440. WL440 had higher Al accumulation in HC1, and WL525 had higher Al accumulation in pectin compared to each other. Exogenous application of IAA significantly reduced Al contents in root tips, CW and pectin fractions, and decreased pectin methylesterase activity (PME) and Al3+ adsorption under Al3+ condition. In addition, exogenous application of IAA alleviated the Al3+-induced damage of cell structure and arrangement in root tips, increased plasma membrane H+-ATPase activity and acidification of rhizosphere. These results indicate that IAA-induced reduction of Al accumulation in cell wall can be associated with decreases in Al3+ binding ability in pectin by increasing H+ secretion to rhizosphere and balancing H+ level between internal and external cells.