Indole-3-acetic acid (IAA) has been found to be involved in plant resistance to various types of environmental stress. Aluminum (Al) toxicity, as one of the most important environmental stress in acid soils, is coped by most plants through the efflux of organic acids via anion channel. This study aims to evaluate the effect of IAA on efflux of malic acid from wheat (Triticum aestivum L.) under Al stress. Hydroponic experiments were performed by wheat ET8 (Al-tolerant). The efflux of malic acid was investigated under different treatments. Results showed that Al treatments increased the accumulation of endogenous IAA, but decreased the activity of IAA oxidase in a dose-dependent manner. A good correlation between all the data of malic acid efflux rate and endogenous IAA content was obtained (R 2 = 0.9859**). IAA treatment alone had no effect on the efflux of malic acid. But compared to Al (50 μM) treatment, the efflux of malic acid increased significantly under the co-treatment of IAA (50 μM) and Al (50 μM). In split-root experiments, the root with half of it being treated with Al (CK/Al), the other part (CK) showed significantly higher malic acid efflux rate and endogenous IAA content in root apexes, compared with the root without such treatment (CK/CK). The Al-induced malic acid efflux decreased under the treatments of IAA transport inhibitor N-1-napthyl-phtalamic acid (NPA) (or 2,3,5-triiodobenzoic acid, TIBA). These above results suggested the possible involvement of IAA in the stimulation of malic acid efflux under Al stress. In addition, anion channel inhibitor treatment experiment showed that IAA (50 μM) relieved the inhibiting effect of 5 μM anthracene-9-carboxylic acid (A9C) (or niflumic acid, NIF) on malic acid efflux induced by Al (50 μM), compared to the cotreatment of Al (50 μM) and 5 μM anion channel inhibitor A9C (or NIF) it is thus speculated that the anion channel might have been activated when IAA was involved in malic acid efflux. This study showed that IAA was involved in aluminuminduced efflux of malic acid from wheat.

Aluminum (Al)-tolerance of different cultivars shows considerable differences. Elevation of rhizosphere pH is an external Al-resistant mechanism of plants. To elucidate the correlation between Al tolerance and the capacity of plants to modify the rhizosphere pH at different Al-tolerant levels, a comparative study on the wheat (Triticum aestivum L.) cultivars ET8 (Al-tolerant) and ES8 (Al-sensitive) was performed. Rhizosphere pH of ET8 was much higher than that of ES8 under the same treatment, significant correlations were obtained among all the data of rhizosphere pH and relative root elongation (R² = 0.9209**), or Al content in root apex (R²= 0.9321**), which indicated that Al tolerance may be related to pH changes in the rhizosphere. The elevation of rhizosphere pH was inhibited by H⁺-ATPase specific inhibitor DCCD (dicylcohexylcarbodiimide, 25 mmol). Relative PM (plasma membrane) H⁺-ATPase activity of ET8 was significantly higher than that of ES8 under the same treatment. Significant correlation between all the data of relative PM H⁺-ATPase activity and rhizosphere pH (R² = 0.8319**) were obtained. Taken together, these results suggest that PM H⁺-ATPase was involved in regulating rhizosphere pH. Under Al stress, the Al-tolerant line showed a stronger capacity of up-regulating rhizosphere pH by PM H+-ATPase than the Al-sensitive line, which may explain the observed differences in Al tolerance between the two wheat cultivars.