A molecular, genetic and physiological analysis of plant aluminum tolerance (abstract)

Aluminum (Al) toxicity is an important agronomic trait, limiting crop production on acid soils that comprise up to 50% of the world's potentially arable lands. A significant genetic variation in Al tolerance exists in both crop plants and Arabidopsis. The exploitation of this genetic variation to breed crops with increased Al tolerance has been a productive and active area of research, however, the underlying molecular, genetic and physiological bases are still not well understood. Only very recently was the first Al tolerance gene, ALMT1, isolated in wheat and shown to be a novel Al-activated malate transporter. Work in our laboratory has focused on using integrated genomic (gene and protein expression profiling), molecular genetic and physiological approaches to identify novel Al tolerance genes and the physiological mechanisms they control in the cereal crops maize and sorghum, and also in arabidopsis. In sorghum we had previously shown that Al tolerance is the result of a single locus, Alt/sub SB/ which maps to the top of sorghum chromosome 3 in a region totally distinct from where the major Al tolerance maps in wheat and other related members of the Triticeae. Very recently, we have used map-based cloning techniques in sorghum to clone Alt/sub SB/ and have found it is a novel Al tolerance gene. Here we will present a molecular characterization of the Alt/sub SB/ gene and also the physiological mechanism of sorghum Al tolerance it controls. In arabidopsis, we have previously shown that Al tolerance is a quantitative trait and have identified two major Al tolerance QTL on chromosomes 1 and 5. These genes function to confer tolerance via Al via activated root malate release. We found that a member of the arabidopsis gene family that is a close homolog to wheat ALMT1 maps near the largest tolerance QTL on chromosome 1 and have also found this gene encodes the Al-activated malate transport involved in arabidopsis Al tolerance. However, we have clear molecular genetic and functional evidence that this gene is not responsible for the nearby QTL and can not explain the variation in Al tolerance in our Col x Ler RIL mapping population. Fine scale mapping of this QTL indicates it actually could harbor two different Al tolerance genes, which we are in the process of identifying. We are speculating that one or both of the proteins encoded by these genes interacts with the Al-activated malate transporter encoded by the nearby ALMT1 homolog. In maize, where Al tolerance is a complex trait, we have identified 5 QTL that explain 60% of the variation in Al tolerance in the IBM population of maize RIL. We also have shown recently that mechanisms of maize Al tolerance in addition to Al-activated root citrate release must be operating, and thus we hypothesize that the multiple Al tolerance genes in maize encode multiple physiological mechanisms. We are in the process of constructing near isogenic lines for each of the 5 QTL and these will be an excellent resource for identifying the additional maize tolerance mechanisms as well as isolating the tolerance genes responsible for these mechanisms via a combined genomic and molecular genetic approach. Furthermore, we are in the process of determining if any of the maize Al tolerance QTL are encoded by homologos of wheat ALMT1, sorghum Alt/sub SB/, or the arabidopsis Al tolerance genes we are close to identifying. (author)