Fighting Fire with Fire: Plants Tolerate Acid Soils by Releasing Organic Acids

Plants have evolved a number of different mechanisms for dealing with toxic metals in the environment, and these can involve both avoidance (exclusion of the metal from a plant tissue/organ) or true tolerance, which involves chelation and/or sequestration of the metal in an internal compartment. The best characterized mechanism of plant metal tolerance is associated with crop aluminum (Al) tolerance. Al toxicity is a worldwide problem that arises when soil pH values drop to 5 or below; in these acidic soils rhizotoxic forms of Al are solubilized into the soil solution, damaging roots and resulting in reduced water and nutrient uptake. The considerable genetic variability for aluminum (Al) tolerance within many crop plant species has been utilized by plant breeders for a number of years to enhance Al tolerance. But beyond this, genetic variability in Al tolerance has been an excellent experimental resource that is being mined by researchers to elucidate the molecular basis for this trait. Because of the agronomic importance of Al toxicity, research on the identification of crop Al tolerance genes has attracted significant interest from a number of laboratories around the world. This talk will focus on a major Al tolerance mechanism which involves Al exclusion from the root tip mediated by Al activation of specialized transporters that release organic acids into the rhizosphere, where they chelate and prevent Al from entering the root. To date, the Al tolerance genes that have been identified are from two different families of membrane transporters mediating this root organic acid efflux. For both types of transporters, Al-inducible regulation of transporter gene expression plays an important role in differential Al tolerance. It is likely that differences in protein structure and function also play a role in differential tolerance, although to date there is no data supporting this. The identification of genes conferring Al tolerance now provides the necessary molecular tools to more effectively address a worldwide agronomic problem that is only exceeded by drought stress with regards to abiotic limitations to crop production.