Aluminum Tolerance

Contributed by Perry Gustafson (pgus@missouri.edu)

References

1. How Do Crop Plants Tolerate Acid Soils? Mechanisms of Aluminum Tolerance and Phosphorous Efficiency. Kochian LV, Hoekenga OA, Piñeros MA. In: Annual Review of Plant Biology, 2004, 55:459-493

Acid soils significantly limit crop production worldwide because approximately 50% of the world's potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.

2. Molecular mapping of a gene responsible for Al-activated secretion of citrate in barley. Ma JF, Nagao S, Sato K, Ito H, Furukawa J, Takeda K. In: Journal of Experimental Botany, 2004, 55(401):1335-1341

Aluminium (Al) toxicity is an important limitation to barley (Hordeum vulgare L.) on acid soil. Al-resistant cultivars of barley detoxify Al externally by secreting citrate from the roots. To link the genetics and physiology of Al resistance in barley, genes controlling Al resistance and Al-activated secretion of citrate were mapped. An analysis of Al-induced root growth inhibition from 100 F₂ seedlings derived from an Al-resistant cultivar (Murasakimochi) and an Al-sensitive cultivar (Morex) showed that a gene associated with Al resistance is localized on chromosome 4H, tightly linked to microsatellite marker Bmag353. Quantitative trait locus (QTL) analysis from 59 F₄ seedlings derived from an F₃ plant heterozygous at the region of Al resistance on chromosome 4H showed that a gene responsible for the Al-activated secretion of citrate was also tightly linked to microsatellite marker Bmag353. This QTL explained more than 50% of the phenotypic variation in citrate secretion in this population. These results indicate that the gene controlling

3. Genetic and Physical Characterization of Chromosome 4DL in Wheat Rodriguez Milla MA, Gustafson JP. In: Genome, 2001, 44:883-892

The long arm of chromosome 4D in wheat (Triticum aestivum L.) has been shown in previous studies to harbor genes of agronomic importance. A major dominant gene conferring Aluminum (Al) tolerance (Alt2 in 'Chinese Spring' and AltBH in 'BH 1146'), and the Kna1 locus controlling the K+/Na+ discrimination in saline environments have been mapped to this chromosome arm. However, accurate information on the genetic and physical location of markers related to any of these genes is not available and would be useful for map-based cloning and marker-assisted plant breeding. In the present study, using a population of 91 recombinant inbred lines segregating for Al tolerance, we provide a more extensive genetic linkage map of the chromosome arm 4DL based on RFLP, SSR, and AFLP markers, delimiting the AltBH gene to a 5.9-cM interval between markers Xgdm125 and Xpsr914. In addition, utilizing a set of wheat deletion lines for chromosome arm 4DL, the AltBH gene was physically mapped to the distal region of the chromosome, between deletion breakpoints 0.70 and 0.86, where the kilobase/centimorgan ratio is assumed to be low, making the map-based cloning of the gene a more realistic goal. The polymorphism rates in chromosome arm 4DL for the different types of markers used were extremely low, as confirmed by the physical mapping of AFLPs. Finally, analysis of 1 Mb of contiguous sequence of Arabidopsis chromosome 5 flanking the gene homologous to the BCD1230 clone (a cosegregating marker in our population coding for a ribulose-5-phosphate-3-epimerase gene), revealed a previously identified region of stress-related and disease-resistance genes. This could explain the collinearity observed in comparative mapping studies among different species and the low level of polymorphism detected in the chromosome arm 4DL in hexaploid wheat.

4. AFLP Markers Tightly Linked to the Aluminum-Tolerance Gene Alt3 in Rye (Secale cereale L.) Miftahudin, Scoles GJ, Gustafson JP. In: Theoretical and Applied Genetics, 2002, 104:626-631

Rye (Secale cereale L.) is considered to be the most aluminum (Al)-tolerant species among the Triticeae. It has been suggested that aluminum tolerance in rye is controlled by three major genes (Alt genes) located on rye chromosome arms 3RL, 4RL, and 6RS, respectively. Screening of an F₆ rye recombinant inbred line (RIL) population derived from the cross between an Al-tolerant rye (M39A-1–6) and an Al-sensitive rye (M77A-1) showed that a single gene controls aluminum tolerance in the population analyzed. In order to identify molecular markers tightly linked to the gene, we used a combination of amplified fragment length polymorphism (AFLP) and bulked segregant analysis techniques to evaluate the F₆ rye RIL population. We analyzed approximately 22,500 selectively amplified DNA fragments using 204 primer combinations and identified three AFLP markers tightly linked to the Alt gene. Two of these markers flanked the Alt locus at distance of 0.4 and 0.7 cM. Chromosomal localization using cloned AFLP and a restriction fragment length polymorphism (RFLP) marker indicated that the gene was on the long arm of rye chromosome 4R. The RFLP marker (BCD1230) cosegregated with the Alt gene. Since the gene is on chromosome 4R, the gene was designated as Alt3. These markers are being used as a starting point in the construction of a high resolution map of the Alt3 region in rye.

5. Development of PCR-based codominant markers flanking the Alt3 gene in rye . Miftahudin, Scoles GJ, Gustafson JP. In: Genome, 2004, 47: 231–238

Aluminum (Al) toxicity is considered to be a major problem for crop growth and production on acid soils. The ability of crops to overcome Al toxicity varies among crop species and cultivars. Rye (Secale cereale L.) is the most Al-tolerant species among the Triticeae. Our previous study showed that Al tolerance in a rye F₆ recombinant inbred line (RIL) population was controlled by a single gene designated as the aluminum tolerance (Alt3) gene on chromosome 4RL. Based on the DNA sequence of a rice (Oryza sativa L.) BAC clone suspected to be syntenic to the Alt3 gene region, we developed two PCR-based codominant markers flanking the gene. These two markers, a sequence- tagged site (STS) marker and a cleaved amplified polymorphic sequence (CAPS) marker, each flanked the Alt3 gene at an approximate distance of 0.4 cM and can be used to facilitate high-resolution mapping of the gene. The markers might also be used for marker-assisted selection in rye or wheat (Triticum aestivum L.) breeding programs to obtain Al-tolerant lines and (or) cultivars.

6. Linkage of RFLP markers to an aluminum tolerance gene in wheat. Riede CR, Anderson JA. In: Crop Science, 1996, 36:905-909.

More than 20% of the world's agricultural land may contain acid soil. Aluminum toxicity becomes a serious growth limiting factor in many plant species when the soil pH is lower than 5.5. Wheat (Triticum aestivum L.) genotypes differ widely in tolerance to excess Al, and breeding of more tolerant genotypes is a priority in many affected regions. Our objectives were to confirm the number of genes for Al tolerance and determine their chromosomal arm location in the Brazilian wheat cv. BH 1146 and to identify DNA markers linked to Al tolerance gene(s). A population of recombinant inbred lines, developed by single-seed descent from the cross of BH 1146 x 'Anahuac', was screened for Al tolerance with two methods in hydroponic nutrient solution. Both methods showed a bimodal distribution for Al tolerance, consistent with single gene inheritance. Eighty-three low-copy DNA clones were used to screen for polymorphism between the parents. Clone bcd1230 detected a restriction fragment that is located 1.1 centiMorgans from this gene on Chromosome 4DL and explained 85% of the phenotypic variation in Al tolerance. We propose the symbol Alt-_BH to designate the major gene for Al tolerance located on 4DL. This RFLP marker can aid the transfer of this gene into sensitive germplasms.