Identification and QTL Mapping of Aluminum Tolerance in Tetraploid Alfalfa.

Alfalfa (Medicago sativa L.) is one of the most important forage legumes worldwide and its productivity is limited by aluminum (Al) toxicity in acid soils. Limited variation for Al tolerance exists within tetraploid Medicago sativa germplasm and no commercially available Al-tolerant alfalfa cultivar has been developed. Al tolerance in alfalfa was identified in a diploid Medicago sativa subs. caerulea accession and has been successfully integrated at the tetraploid level in the Al-tolerant genotype Altet-4. The objectives of this study are to identify and confirm quantitative trait loci (QTL) for Al tolerance in tetraploid alfalfa. Two populations of at least 190 individuals each were developed from crosses between Altet-4 (Al-tolerant) and the Al-susceptible genotypes 95-608 derived from CUF-101, and NECS141, a semi-dormant breeding line developed in Iowa. We have established three laboratory and greenhouse screening protocols using callus, plants in culture, and plants in soil to assess Al tolerance of clonally propagated material from the mapping populations.  The absolute growth and ratio of callus, aboveground biomass and roots (biomass, length and branching) from Al-containing media and Al-free media will be used as quantitative data to determine Al tolerance.  Genetic linkage maps were constructed using EST-SSRs and markers developed from candidate genes associated with Al tolerance.  Interval mapping was used to identify Al tolerance QTL and these were compared among the different screening methods.  The long term goal of this research is to utilize marker-assisted selection to accelerate the development of alfalfa cultivars with improved productivity in Al-toxic soils.