Abstract:

Aims Border cells and their secretory mucilage can form a layer over the surface of younger parts of roots to help form a “rhizosheath” protecting the root tip from biotic and abiotic stresses. Our objective was to investigate the ecological role of mucilage secretion strategies in response to Al exposure and absence.

Methods Soybean (Glycine max) ‘Zheqiu No. 2’ and ‘Zhechun No. 3’ were used in this study. Seedlings were aeroponically cultured in 0.2 mmol·L ^-1CaCl₂ solution, pH 4.5, containing 0, 100, 200, 300 or 400 µmol·L ^-1AlCl₃.Half of the seedlings were harvested after 24 h Al exposure and the other half continued to culture in the absence of Al³⁺. We measured percent viability of border cells, mucilage thickness and relative root elongation.

Important findings Percent viability of root border cells decreased with increasing external Al³⁺concentrations in both cultivars. Percent viability of ‘Zheqiu No. 2’ border cells decreased slightly after a 3-12 h treatment with 100, 200, or 400 µmol·L ^-1Al³⁺and then slightly increased with treatment time, whereas the same change in ‘Zhechun No. 3’ seedlings was noted at Al ³⁺concentrations of 300 and 400 µmol·L ^-1. Al³⁺induced a thicker mucilage layer around border cells of both cultivars, with a maximum of 17 µm after 24 h at 400 µmol·L ^-1Al³⁺. ‘Zheqiu No. 2’ border cells developed a thicker mucilage layer within 3-6 h in response to 100 and 200 µmol·L ^-1Al³⁺, while ‘Zhechun No. 3’ cells exhibited the same change after 12 h to 300 µmol·L ^-1Al³⁺. Relative root elongation of ‘Zheqiu No. 2’ exposed to 400 µmol·L ^-1Al³⁺was significantly higher than to 100-300 µmol·L ^-1Al³⁺treatment, while ‘Zhechun No. 3’ exhibited reduced relative root elongation after exposure to 100-400 µmol·L ^-1Al³⁺. With Al³⁺recovery treatment, ‘Zheqiu No. 2’ border cells developed a thinner layer of mucilage in the absence of Al ³⁺compared with Al³⁺exposure, while ‘Zhechun No. 3’ border cells continued to secrete a conspicuous layer of mucilage. Rapid induction of mucilage production exposure to Al ³⁺stress and abrupt halt by border cells is a specific response to Al³⁺stress and recovery for Al-resistant soybean cultivar.

Key words: soybean, aluminum toxicity, root borer cells, mucilage