The trivalent form (Al3*) of aluminum (Al),an abundant metal in the Earths crust,is highly toxic to plant cells when released into soils under acidic conditions.Because approximately 50％ of the worlds arable soils are acidic,Al toxicity poses a big threat to crop production worldwide.Physiologically,Al toxicity inhibits root growth,leading to poor ion and water uptake by roots.Over long-term evolution,many plants have developed certain strategies to tolerate Al toxicity.One well-characterized mechanism in several species such as wheat,sorghum,and maize involves root exudation of organic acids such as citrate and malate that bind Al to form nontoxic complexes and prevent its entrance into cells.