Aluminum toxicity stress
Aluminum (Al) is the most abundant metal in the earthfs crust, but its ionic form (Al3+) is toxic to all living cells. Low concentrations of ionic Al rapidly inhibit root elongation by targeting multiple cellular sites, including the cell walls and plasma membranes, and various cellular processes, such as signal transduction pathways and Ca homeostasis. Consequently, aluminum inhibits uptake of water and nutrients and increases the plantfs sensitivity to various stresses, especially drought. Aluminum toxicity has been recognized as a major factor limiting crop production in acid soils, which comprise approximately 30-40% of the worldfs arable soils.
However, Tsome plant species and cultivars have evolved mechanisms to cope with Al toxicity both externally and internally. Al-tolerant cultivars of barley secrete citrate from the roots, which is mediated by an Al-activated citrate transporter HvAACT1, localized at the epidermal cells (Plant Cell Physiol. 48:1081-1091, 2007). In rice, a bacterial-type ABC transporter STAR1/2 is involved in the Al tolerance (Plant Cell, 21: 655-667, 2009)
On the other hand, some species such as buckwheat, hydrangea, are able to accumulate Al at high concentration without showing toxicity. The internal detoxification is achieved by sequestration of Al into vacuoles and complexation with organic acid anions (Nature, 390: 569-570, 1997, Plant Physiol. 113:1033-1039, 1997)