Response of rice (Oryza sativa) with root surface iron plaque under aluminium stress

Abstract

Background and aims: Rice (Oryza sativa) is an aquatic plant with a characteristic of forming iron plaque on its root surfaces. It is considered to be the most Al-tolerant species among the cereal crops. The objective of this study was to determine the effects of root surface iron plaque on Al translocation, accumulation and the change of physiological responses under Al stress in rice in the presence of iron plaque.

Methods: The japonica variety rice, Koshihikari, was used in this study and was grown hydroponically in a growth chamber. Iron plaque was induced by exposing the rice roots to 30 mg L(-1) ferrous iron either as Fe(II)-EDTA in nutrient solution (6 d, Method I) or as FeSO(4) in water solution (12 h, Method II). Organic acid in root exudates was retained in the anion-exchange resin and eluted with 2 m HCl, then analysed by high-performance liquid chromatography (HPLC) after proper pre-treatment. Fe and Al in iron plaque were extracted with DCB (dithionite-citrate-bicarbonate) solution.

Key results and conclusions: Both methods (I and II) could induce the formation of iron plaque on rice root surfaces. The amounts of DCB-extractable Fe and Al on root surfaces were much higher in the presence of iron plaque than in the absence of iron plaque. Al contents in root tips were significantly decreased with iron plaque; translocation of Al from roots to shoots was significantly reduced with iron plaque. Al-induced secretion of citrate was observed and iron plaque could greatly depress this citrate secretion. These results suggested that iron plaque on rice root surfaces can be a sink to sequester Al onto the root surfaces and Fe ions can pre-saturate Al-binding sites in root tips, which protects the rice root tips from suffering Al stress to a certain extent.

Fig. 1.

Scanning electron micrographs (A, B and C) and energy-dispersive X-ray analysis (EDAX) (D, E and F) of precipitates formed on rice (Koshihikari) roots in hydroponic culture supplemented with iron or aluminium. (A) Rice was grown in Kimura B nutrient solution (pH 4·5) containing 30 mg L⁻¹Fe(II)-EDTA for 6 d (Method I). (B) Rice was pre-treated with a solution (pH 4·5) containing 30·0 mg L⁻¹ FeSO₄ for 12 h (Method II). (C) Rice was not pre-treated with iron but was exposed for 24 h to 0·5 mm CaCl₂ solution (pH 4·5) containing 50 μm AlCl₃. (D), (E) and (F) correspond to (A), (B) and (C), respectively. These micrographs are representative of the general morphology and image.

Fig. 2.

Fe (A) and Al (B) concentrations in dithionite–citrate–bicarbonate (DCB) extracts. Two-week-old seedlings were pre-treated on the root surfaces with 30 mg L⁻¹ Fe(II)-EDTA [the control (–Fe) was1·12 mg L⁻¹ Fe(II)-EDTA] in Kimura B nutrient solution (pH 4·5) for 6 d (Method I) or 3-week-old seedlings were pre-treated with water solution (pH 4·5) containing 30·0 mg L⁻¹ FeSO₄ for 12 h (Method II). Error bars represent ± s.e. (n = 3).

Fig. 3.

Aluminium (Al) content in the root apex of rice (Koshihikari). When the seedlings [pre-treated with Fe(II)-EDTA] were harvested, root apices (1 cm) were excised and extracted with 2 m HCl. The Al concentration was detected by ICP-AES. Error bars represent ± s.e. (n = 3).

Fig. 4.

Effect of iron plaque on the distribution of aluminium (Al) in whole roots and shoots. After extraction with DCB, the whole root and shoot were dried and digested with concentrated HNO₃. Error bars represent ± s.e. (n = 3).

Fig. 5.

Effect of iron plaque on the release of citrate from rice (Koshihikari) under aluminium (Al) stress. Fe(II)-EDTA (30·0 mg L⁻¹) was used to pre-treat the root surfaces for 6 d, and then the seedlings were exposed for 24 h to 0·5 mm CaCl₂ solution (pH 4·5) containing 50 μM AlCl₃, and root exudate was collected. Error bars represent ± s.e. (n = 3).

Fig. 6.

Iron (Fe) content in the root apex of rice (Koshihikari). When the seedlings were harvested, root apices (1 cm) were excised and extracted with 2 m HCl. The iron concentration was detected by ICP-AES. Error bars represent ± s.e. (n = 3).