Flavonoid-type phenolics can possibly detoxify Al inside plant cells. Kidd et al. [77] found that phenolics including catechol and quercetin were released in maize treated with Al and Si, and the release was dependent on Al concentration. However, due to a lack of efficient methodologies, our understanding of internal mechanisms of Al tolerance in plants is still fragmentary. Genetic markers are useful tools to reveal Al tolerance mechanisms in higher plants following their detection by inheritance studies and identification

of relevant genes or loci. During the last two decades, molecular markers based on DNA sequence variations were widely used to study Al tolerance. By detecting molecular markers, the gene or trait could be easily identified and traced [78]. Based on the techniques used, molecular markers could be classified as PCR-based PD0332991 concentration or hybridization-based [79]. DArT (Diversity Arrays Technology) and RFLP (restriction fragment length polymorphism) are hybridization-based markers, whereas AFLP (amplified fragment length polymorphism), RAPD (randomly amplified of polymorphic DNA), SSR (simple sequence repeat) check details and SNP (single

nucleotide polymorphism) are based on polymerase chain reaction (PCR) techniques. PCR-based markers are preferred and widely used as they are highly efficient, use less DNA, are less labor intensive and amenable to automation and avoidance of autoradiography [80]. The use of molecular markers in Al-tolerance studies includes Al-tolerance gene/loci identification and molecular mapping as well as MAS. One RFLP marker bcd1230, co-segregating with a major gene for Al tolerance, on wheat chromosome 4DL, explained 85% of the phenotypic variation in Al tolerance [81]. Using an F2 population derived from barley varieties Dayton and Harlan, three RFLP markers, Xbcd1117, Xwg464 and Xcdo1395, were closely linked to Alp on chromosome 4H [82]. The authors pointed out that Al tolerance in barley was controlled by a single gene that could be an ortholog of AltBH on wheat chromosome

4D. Five AFLP markers, AMAL1, AMAL2, AMAL3, AMAL4 and AMAL5, were closely linked to, and flanked Alt3 on the long arm of chromosome 4R [83]. After screening 35 Al-tolerant wheat landrace accessions using ten AFLP primer combinations, Stodart et al. [84] found that these accessions had diverse Thalidomide genetic background and were therefore valuable germplasms for Al tolerance breeding. RAPD marker OPS14705 was linked to the Alt3 locus in rye. A SCAR marker ScOPS14705 derived from a RAPD marker, was further shown to be linked to Alt3 locus [85]. Ma et al. [86] reported SSR markers Xwmc331 and Xgdm125 flanking the ALMT locus and they indicated that these markers could be used for MAS in breeding Al-tolerant wheat cultivars. In barley, several SSR markers, Bmag353, HVM68 and Bmac310, were closely linked with an Al tolerance gene [87] and [88]. Wang et al.