A comparative mapping study found extensive synteny or co-linearity among the genomes of rice, wheat, barley, rye, oat, maize and sorghum (Devos and Gale, 2000).
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Genetic control of Al-tolerance in cereals is mainly associated with genes that control protein families linked to membrane transport (Table1.3). In a diverse range of wheat genotypes, a major aluminium tolerance gene at AltBH in wheat, TaALMT1, Triticum aestivum Aluminium activated Malate Transporter1, encodes for an Al-activated plasma membrane protein that allows for the efflux of malate from root apices upon exposure to Al (Sasaki et al., 2004; Raman et al., 2005b). This gene was mapped to Chromosome 4DL using ‘Chinese spring’ deletion lines and its absence resulted in the loss of Al-tolerance and malate exudation (Raman et al., 2005b). Another mechanism of Al-tolerance was found in Brazilian wheat cultivars that involves the efflux of citrate from root apices. The controlling gene, which resides on Chromosome 4BL, has been identified (Ryan et al., 2009). These authors indicated that the citrate efflux is controlled by a single gene, which could explain 50% of the phenotypic variation in citrate efflux. However, unlike the TaALMT1, this gene belongs to a gene encoding a multidrug and toxic compound extrusion protein and was designated as TaMATE1 (Ryan et al., 2009).
In barley, Echart et al. (2002) found that an F2 generation analysed with haematoxylin staining followed the Mendel’s segregation ratio of 3:1 for Al toxicity tolerant to sensitive plants, revealing the fact that the trait is controlled by single dominant gene at the Alp locus, which is located on the long arm of Chromosome 4H. This locus is associated with the Al-induced efflux of citrate from the root apices of tolerant barley encoded by a multidrug and toxic compound extrusion (HvMATE) protein (Wang et al., 2007). Quantitative trait loci that explained 50% of the phenotypic variation werealso associated with the same chromosomal location (Ma et al., 2004). Similarly, Raman et al. (2005a) identified QTLs for root elongation under aluminium stress on 3H, 4H, 5H and 6H chromosomal locations.
In rye, four independent loci, Alt1, Alt2, Alt3 and Alt4, located on chromosome arms 6RS, 3RS, 4RL and 7RS, are known to confer tolerance to Al-toxicity (Matos et al., 2007). Specifically, the Alt4 locus contained cluster of genes homologous to the Al- activated malate transporter (TaALMT1) (Collins et al., 2008). Tolerant and sensitive rye genotypes contained five and two genes of the clusters at the locus, respectively.
Out of these, two ScALMT1-M39.2 and one ScALMT1-M77 genes were highly expressed in the root tip (Collins et al., 2008). Silva-Navas et al. (2012) subsequently located a gene coding for a multidrug and toxic compound extrusion protein family
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(ScMATE), an aluminium-activated citrate transporter, at the same chromosomal location, the 7RS chromosome arm, 25 cM away from the ScALMT1.
Magalhaes et al. (2007) identified a gene encoding for a member of the multidrug and toxic compound extrusion family (SbMATE), which is the responsible gene for the major sorghum aluminium tolerance locus, AltSb. They also suggested that polymorphisms in the regulatory regions of AltSb are likely to contribute to large allelic effects, acting to increase AltSb expression in the root apices of tolerant genotypes.
Earlier, Caniato et al. (2007) suggested the possibility of the presence of additive or co-dominant effects of different loci that would explain the occurrence of transgressive segregation observed in some tolerant lines.
In maize, an Al-activated efflux of citrate from roots is well characterized and is the most important mechanism of Al tolerance (Renato and Paulo, 1997). The responsible gene is a member of the multidrug and toxin extrusion family (Maron et al., 2008 ).
Maron et al. (2010) identified two members of the MATE family, ZmMATE1 and ZmMATE2, which co-localized with two independent Al-tolerance QTLs. The authors clearly showed the association of ZmMATE1 with up-regulation of citrate release at the root tips of tolerant varieties upon exposure to Al. In their most recent study, the authors indicated that a higher copy number of the gene encoding for ZmMATE1 was responsible for quantitative tolerance to Al toxicity (Maron et al., 2013). ZmNrat1, a maize homolog to the rice OsNrat1, described below, was also found to have a role in maize Al tolerance (Guimaraes et al., 2014).
In rice, several QTLs have been identified that contribute to phenotypic variations for Al-tolerance (Ma et al., 2002b; Nguyen et al., 2002). A recent study indicated that multiple genes regulated by the Al Resistance Transcription Factor1 (ART1) control Al-tolerance (Yamaji et al., 2009). ART1 is a C2H2-type zinc-finger transcription factor and is found in the nuclei of all root cells (Yamaji et al., 2009). Among the multiple genes regulated by ART1 and associated with internal and external detoxification, STAR1 and STAR2 (Huang et al., 2009); Nrat1 (Xia et al., 2010); OsFRDL4 (OsMATE) (Yokosho et al., 2011); OsALS1 (Huang et al., 2012) and Oryza sativa Magnesium Transporter1 (OsMGT1) (Chen et al., 2012) have been characterized. STAR1 and STAR2 encode for a bacterial-type ATP binding cassette (ABC) transporter complex that transports UDP–glucose (Huang et al., 2009). The authors suggested that UDP- glucose (a glycoside derived compound) is released from vesicles into the apoplast by
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exocytosis and that it modifies the cell walls to mask the binding sites for aluminium, resulting in aluminium tolerance in rice. Both genes are expressed mainly in the roots and are specifically induced by Al. Disruption of either genes results in increased sensitivity to Al-toxicity (Huang et al., 2009).
Nrat1 belongs to the Nramp (natural resistance-associated macrophage protein) family and is a plasma membrane-localized transporter for trivalent Al (Xia et al., 2010). OsALS1 encodes for a half-size ABC transporter that is a member of the TAP (transporter associated with antigen processing) sub-group (Huang et al., 2012).
OsALS1 is localized to the tonoplast of root cells and is responsible for sequestration of Al into vacuoles, which is required for internal detoxification of Al in rice (Huang et al., 2012). OsALS1 and Nrat1 operate cooperatively in that Al transported by Nrat1 is sequestered by OsALS1 (Huang et al., 2012).
OsFRDL4 that encodes for a citrate transporter and is homologous to SbMATE of sorghum, sharing a 70% identity at the amino acid level (Yokosho et al., 2011).
Knockout of this gene results in decreased citrate secretion and increased Al sensitivity. However, the contribution of the OsFRDL4 gene in overall Al-tolerance of rice is relatively small (Yokosho et al.)
OsMGT1 is a plasma-membrane localized transporter for Mg in rice and its expression is specifically boosted by Al (Chen et al., 2012). Up-regulation of this transporter gene is required for conferring Al tolerance by increasing Mg uptake into the cells. Knockout of OsMGT1 resulted in increased sensitivity to Al in both solution and soil culture. It is hypothesised that Mg prevents Al binding to enzymes and other cellular components and enables its detoxification (Chen et al., 2012).