Silicon relieves aluminum-induced inhibition of cell elongation in rice root apex by reducing the deposition of aluminum in the cell wall

Background and aims Silicon (Si) is known to alleviate aluminum (Al) toxicity in a number of plant species; however, the mechanisms of Si-triggered Al detoxification have not been elucidated, especially in rice (Oryza sativa). We investigated the interactions between Si and Al in root and the role of cell wall polysaccharides in rice resistance to Al. Methods Rice seedlings were grown in 0.5 mM CaCl2 solution with or without 50 mu M Al and 1.0 mM H4SiO4 for 24 h. The cell expansion and oxidative injury of root were measured by histochemical analyses; Al accumulation and distribution in root apices were determined by inductively coupled plasma mass spectrometry (ICP-MS); the degree of pectin methylesterification was analyzed by immunofluorescence, and the expression of genes (e.g. OsPME) related to Al resistance was measured by quantitative real-time PCR. Results Si relieved the inhibition of cell elongation and oxidative injury caused by Al and significantly reduced Al content (by about 41 %) in cell wall (CW). Meanwhile, Si inhibited pectin methylesterase (PME) activity and its gene expression, thereby increasing the degree of pectin methylesterification, and reduced the content of hemicellulose, which was the main binding site for Al in CW, under Al stress. Furthermore, Si decreased the non-exchangeable Al fraction of CW in root, but did not change the cation exchange capacity of CW. Conclusions Si reduces the deposition of Al in the cell wall of rice root apex by decreasing both the degree of pectin demethylesterification and the content of hemicellulose, thereby alleviating Al stress.

Automated summary

This study revealed that silicon can alleviate aluminum-induced inhibition of cell elongation and oxidative damage in rice roots, reducing aluminum content in the cell wall. Silicon also affects pectin methylesterase activity and gene expression, thus increasing pectin methylesterification and reducing hemicellulose content in the cell wall. Overall, silicon plays a crucial role in decreasing aluminum accumulation in the root cell wall, thus mitigating aluminum stress in rice.