Identification and characterization of aluminum tolerance loci in Arabidopsis (Landsberg erecta x Columbia) by quantitative trait locus mapping. A physiologically simple but genetically complex trait

Abstract

Aluminum (Al) toxicity, which is caused by the solubilization of Al3+ in acid soils resulting in inhibition of root growth and nutrient/water acquisition, is a serious limitation to crop production, because up to one-half of the world's potentially arable land is acidic. To date, however, no Al tolerance genes have yet been cloned. The physiological mechanisms of tolerance are somewhat better understood; the major documented mechanism involves the Al-activated release of Al-binding organic acids from the root tip, preventing uptake into the primary site of toxicity. In this study, a quantitative trait loci analysis of Al tolerance in Arabidopsis was conducted, which also correlated Al tolerance quantitative trait locus (QTL) with physiological mechanisms of tolerance. The analysis identified two major loci, which explain approximately 40% of the variance in Al tolerance observed among recombinant inbred lines derived from Landsberg erecta (sensitive) and Columbia (tolerant). We characterized the mechanism by which tolerance is achieved, and we found that the two QTL cosegregate with an Al-activated release of malate from Arabidopsis roots. Although only two of the QTL have been identified, malate release explains nearly all (95%) of the variation in Al tolerance in this population. Al tolerance in Landsberg erecta x Columbia is more complex genetically than physiologically, in that a number of genes underlie a single physiological mechanism involving root malate release. These findings have set the stage for the subsequent cloning of the genes responsible for the Al tolerance QTL, and a genomics-based cloning strategy and initial progress on this are also discussed.

Figure 1.

Distribution of mean root lengths among RIL in the Ler × Col population. A, Day six. Mean root lengths for each RIL are organized into bins (0.5-mm increment) to show the distribution within the population. Control-treated plants are shown with vertical gray bars, whereas Al-treated plants are shown with vertical black bars. Average root lengths for Ler (L) and Col (C), including 1 sd above and below the mean, are given as horizontal bars. B, Day eight.

Figure 2.

Distribution of RRG estimates between RIL in the Ler × Col population. A, RRG variable. Root growth inhibition was estimated using RRG for d 6: RL_Al6/RL_C6; RRG is a scaled variable and is a measure of RRG. RRG values are expressed as percentage values and organized into bins (5% increments) to show the distribution within the population. RRG values are shown for the Ler (L) and Col (C) parental varieties. B, D₆ variable. Root growth inhibition was measured by D₆ = RL_C6 - RL_Al6. Also shown are D₆ values for Ler (L) and Col (C).

Figure 3.

Likelihood ratio (LR) plots for significant Al tolerance QTL. LR values for three traits plotted against a linear representation of the Arabidopsis genome. A, RL_Al6. B, RL_Al8. C, D₆.

Figure 4.

Representative capillary electrophoresis traces for hydroponic media solutions. A, Representative complete electropherogram for root exudates collected from Col seedlings grown under control conditions. B, Col, control sample for time window 3.0 to 3.6 min, which brackets the elution of citrate (first peak) and malate/phosphate (second peak). C, Col, Al treated, for 3.0- to 3.6-min elution period. D, Ler, control, for 3.0- to 3.6-min elution period. E, Ler, Al treated, for 3.0- to 3.6-min elution period.

Figure 5.

Root organic ligand exudation by six Arabidopsis genotypes. A, Root malate exudation. Mean malate release (picomoles of malate per plant per 2 d) from hydroponically grown Arabidopsis in the presence (black) and absence (gray) of Al. Vertical error bars depict se. Six varieties were tested: ecotypes Col and Ler and the four QTL classes that derive from two QTLs (CC, CL, LC, and LL). B, Root citrate exudation. C, Root phosphate exudation.

Figure 6.

Correlation analysis of root malate exudation in Al-treated plants versus root growth in Al-treated plants. Error bars depict se for both root length and malate release.