Contributed by Perry Gustafson (email@example.com)
Approximately 50% of the arable soils in the world are acidic, with a pH of 5.5 or lower. Besides this natural occurrence of soil acidity, the extensive use of ammonia fertilizers cause further soil acidification. One of the more important limitations of acid soils are the toxic levels of aluminum they can have. The earlier negative effect of Al is the inhibition of root growth, compromising water and minerals uptake, it can also negatively affect the cytoeskeleton, different signal transduction pathways, and induce an increase in the levels of oxygen reactive species. Even when the primary target is the root, at a later stage Al can also affect symplastic targets.
During evolution plants have developed strategies to cope with high levels of Al. These strategies can be divided in two groups: first, mechanisms to exclude Al from the root apex and second, mechanisms to tolerate higher Al levels in the roots and shoots symplasm. So far, most of the research on Al tolerance has focused on root mechanisms: Al exclusion based on Al-activated organic acid (OA) exudation and internal detoxification of symplastic Al via complexation with organic ligands, mainly organic acids (1).
The proposed model of Al tolerance by exclusion postulates that organic acids are continuously released to the first few millimeters of the root apex in response to an increase in Al levels. Once the organic acid concentrations at the surface of the root apex reach levels high enough they would be able to chelate and detoxify a significant fraction of the rhizosphere Al, preventing Al entry into the root.
In wheat, the organic acid involved in this mechanism is malate, while in other cereals would be citrate or a mixture of citrate and malate or citrate and oxalate.
The genetics of Al tolerance in wheat has been well characterized and lines with good levels of tolerance were developed. Crosses between Al-tolerant cultivars and Al-sensitive varieties showed that Al tolerance segregates as a single, dominant locus. However, the segregation patterns of other crosses suggest that two loci are responsible for tolerance. One Al tolerance locus, called AltBH or Alt2, has been mapped to the long arm of chromosome 4D. The existence of other loci can be inferred from the study of varieties that contain chromosomal deletions and have diminished tolerance to Al stress. However, the importance of these additional non-4D loci (5AS, 7AS) has been difficult to evaluate because genetic experiments have yet to confirm these lci actually contain Al tolerance genes (3, 5).
Rye (Secale cereale), the most Al-tolerant species among the Triticeae, has several Al tolerance loci. Like wheat, a locus with a major effect was detected on the long arm of chromosome 4, called Alt3. This locus has a tight linkage with markers linked to AltBH in wheat, suggesting that they are orthologous loci (4,5).
Barley (Hordeum vulgare) also contains a major Al tolerance locus, Alp, on the long arm of chromosome 4 and is linked to the same markers of AltBH in wheat. This locus overlaps with a QTL for citrate secretion (2).
Al tolerance in rice (Oryza sativa L.) appears to be associated with quantitative trait loci (QTL). QTLs for Al tolerance in rice have been mapped to both chromosomes 1 and 3. Rice chromosome 3 and homoeologous group 4 of the Triticeae have a strong syntenic relationship and the two Al tolerance gene regions may share common sequences. Miftahudin et al. (2) were able to use genomic information from rice to generate molecular markers for Al tolerance that could be potentially useful in other cereals.
Miftahudin et al.(5) developed PCR markers for the Al tolerance gene (Alt3) of rye, that could be used in wheat. For marker assisted selection in wheat other RFLP and SSR markers are also available. For experimental details, visit the methods section.
Crop improvement programs in Brazil and the United States have lead to the development of excellent cultivars, spring wheat BH 1146 and winter wheat Atlas 66, respectively. BH1146 is the source of AltBH, considered one of the most effective alleles for aluminium tolerance in wheat. Molecular markers are available to facilitate the transfer of the AltBH chromosome segment (see Methods section). The Alt3 allele from rye is currently being transferred to BH1146 to compare its effect with the AltBH allele (P. Gustafson unpublished).
1. How Do Crop Plants Tolerate Acid Soils? Mechanisms of Aluminum Tolerance and Phosphorous Efficiency. Kochian LV, Hoekenga OA, Piņeros MA. In: Annual Review of Plant Biology, 2004, 55:459-493 [abstract]
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 [abstract]
3. Genetic and Physical Characterization of Chromosome 4DL in Wheat Rodriguez Milla MA, Gustafson JP. In: Genome, 2001, 44:883-892 [abstract]
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 [abstract]
5. Development of PCR-based codominant markers flanking the Alt3 gene in rye. Miftahudin, Scoles GJ, Gustafson JP. In: Genome, 2004, 47: 231-238 [abstract]
6. Linkage of RFLP markers to an aluminum tolerance gene in wheat. Riede CR, Anderson JA. In: Crop Science, 1996, 36:905-909. [abstract]