Root phosphate exudation and pH shift in the rhizosphere are not responsible for aluminum resistance in rice

Abstract

A series of hydroponic experiments and an agar culture experiment were carried out to investigate aluminum (Al) accumulation and translocation in two rice (Oryza sativa L.) cultivars (Kasalath and Koshihikari) that differ in Al resistance. Al-resistance mechanisms, including Pi exudation under Al stress and pH shifts in the rhizosphere, were also studied. Al content in rice shoots was 41 mg kg -1 on average and did not differ between the two cultivars, which demonstrated that the rice cultivars were not Al accumulators. The majority of Al (95-97%) accumulated in roots. Al content in roots in the resistant cultivar (Koshihikari) was lower than that in the sensitive cultivar (Kasalath), which indicated that Al-exclusion mechanisms were mainly acting in rice. However, the rate of Pi exudation from the whole root or root tips was very low in both cultivars and was not significantly influenced by Al exposure, and thus seemed not to be the main Alresistance mechanism. On the other hand, experiments with pH-buffered solution and color changes following culture in agar medium containing bromocresol purple revealed that the Al-induced pH increase could not explain the high Al resistance of rice. In addition, the Al content in shoots of Koshihikari was lower after the formation of iron plaque on the root surface, whereas that of Kasalath was not lower. These results suggested that rice roots cell wall components or root surfaces such as iron plaque, rather than pH changes and/or root exudates including organic acids and phosphate, play important roles in Al resistance in rice.

Proteomics of aluminum tolerance in plants

“…It was suggested that exclusion mechanism rather than internal detoxification might be the reason of tolerance in Koshihikari cultivar. However, neither the amount of the secreted citrate, which was induced by Al treatment, nor the phosphate (Pi) exudation and pH shift in the rhizosphere was significantly different among them . This indicates that the OA secretion cannot fully explain the high Al tolerance in rice.…”

Aluminium–phosphorus interactions in plants growing on acid soils: does phosphorus always alleviate aluminium toxicity?

“…However, in some cases it is difficult to apply this method, e.g. when using plants with taproots, or the complicated equipment might limit its applicability, because the size of root chambers might differ for different plants and growth periods with different root volumes. Method V : long‐term AlP alternate treatment 12, 25, 26. This method can also successfully avoid AlP interaction in the solution, thus maintaining P and Al at exact levels.…”

“…Method V : long‐term AlP alternate treatment 12, 25, 26. This method can also successfully avoid AlP interaction in the solution, thus maintaining P and Al at exact levels.…”

Coadaptation of Plants to Multiple Stresses in Acidic Soils

“…Several studies showed that Fe 2+ and Fe 3+ increased the toxic effect of Al on suspensioncultured tobacco cells (Ono et al, 1995;Yamamoto et al, 1997;Ikegawa et al, 1998;Yamaguchi et al, 1999). However, recent studies have demonstrated that the formation of iron plaques on the root surface of rice plants decreased Al accumulation in root apices and protected the plant against Al toxicity Chen and Shen, 2008;Cai et al, 2012). These results suggested that Al toxicity can be aggravated by Fe.…”