Genetic Dissection of Natural Variation for Aluminium Tolerance in Common Wheat

Acid soils limit wheat production worldwide mainly due to aluminium (Al³⁺) toxicity. In order to meet food shortages and improve yield on acid soil, identification and dissection the genetic variation for Al³⁺ tolerance in natural wheat germplasm is crucial. We screened over 1500 lines representing A. tauschii, T. monococcum, T. dicoccoides, T. zhukovskyi, T. timopheevii, T. turgidum and T. urartu, for Al³⁺ tolerance using nutrient solution culture and identified a range of genetic variation for this trait. Hexaploid genotypes belonging to T aestivum, and its morphological groups such as spelta, macha, vavilovii, compactum and sphaerococcum exhibited the highest level of Al³⁺ tolerance as compared to diploid and tetraploid species.  Three accessions of T. turgidum (durum); 202 (AUS), 249 (AUS) and 267 (AUS), originated from Iran, Turkey and Spain were moderately Al³⁺-resistant.  Linkage analysis of common wheat biparental populations and genome-wide association analysis of 1055 diverse wheat accessions representing both modern cultivars and traditional landraces revealed that several unlinked loci control Al³⁺ tolerance. Physiological studies revealed that Al³⁺ activated malate and citrate efflux from root apices are the predominant mechanisms for Al³⁺ tolerance. We have mapped loci for efflux of both organic anions in biparental populations to corresponding Al3+ tolerance loci on chromosomes 4B (citrate) and 4D (malate). Linkage analysis showed that TaALMT1 (aluminium activated malate transporter) is associated with Al³⁺ tolerance via malate efflux, and TaMATE (multi-drug and toxic compound extrusion) is associated with Al³⁺ tolerance via efflux of citrate. At least seven different variants of the upstream sequence of TaALMT1 were identified in wheat germplasm, which suggests that intensive selection pressure acted at this locus. Haplotyping of TALMT1 alleles showed that Al³⁺ tolerance donor sources have multiple origins; one of them is from Aegilops tauschii where as others may have derived from A/B genome(s) progenitors.