A conditional mutation in a wheat (Triticum aestivum L.) gene regulating root morphology

KEY MESSAGE

Characterisation and genetic mapping of a key gene defining root morphology in bread wheat. Root morphology is central to plants for the efficient uptake up of soil water and mineral nutrients. Here we describe a conditional mutant of hexaploid wheat (Triticum aestivum L.) that when grown in soil with high Ca2+ develops a larger rhizosheath accompanied with shorter roots than the wild type. In wheat, rhizosheath size is a reliable surrogate for root hair length and this was verified in the mutant which possessed longer root hairs than the wild type when grown in high Ca2+ soil. We named the mutant Stumpy and showed it to be due to a single semi-dominant mutation. The short root phenotype at high Ca2+ was due to reduced cellular elongation which might also explain the long root hair phenotype. Analysis of root cell walls showed that the polysaccharide composition of Stumpy roots is remodelled when grown at non-permissive (high) Ca2+ concentrations. The mutation mapped to chromosome 7B and sequencing of the 7B chromosomes in both wild type and Stumpy identified a candidate gene underlying the Stumpy mutation. As part of the process to determine whether the candidate gene was causative, we identified wheat lines in a Cadenza TILLING population with large rhizosheaths but accompanied with normal root length. This finding illustrates the potential of manipulating the gene to disconnect root length from root hair length as a means of developing wheat lines with improved efficiency of nutrient and water uptake. The Stumpy mutant will be valuable for understanding the mechanisms that regulate root morphology in wheat.

Introgression of genes from bread wheat enhances the aluminium tolerance of durum wheat

The aluminium tolerance of durum wheat was markedly enhanced by introgression of TaALMT1 and TaMATE1B from bread wheat. In contrast to bread wheat, TaMATE1B conferred greater aluminium tolerance than TaALMT1. Durum wheat (tetraploid AABB, Triticum turgidum) is a species that grows poorly on acid soils due to its sensitivity of Al(3+). By contrast, bread wheat (hexaploid AABBDD, T. aestivum) shows a large variation in Al(3+) tolerance which can be attributed to a major gene (TaALMT1) located on chromosome 4D as well as to other genes of minor effect such as TaMATE1B. Genotypic variation for Al(3+) tolerance in durum germplasm is small and the introgression of genes from bread wheat is one option for enhancing the ability of durum wheat to grow on acid soils. Introgression of a large fragment of the 4D chromosome previously increased the Al(3+) tolerance of durum wheat demonstrating the viability of transferring the TaALMT1 gene to durum wheat to increase its Al(3+) tolerance. Here, we used a ph1 (pairing homoeologous) mutant of durum wheat to introgress a small fragment of the 4D chromosome harboring the TaALMT1 gene. The size of the 4D chromosomal fragment introgressed into durum wheat was estimated by markers, fluorescence in situ hybridisation and real-time quantitative PCR. In a parallel strategy, we introgressed TaMATE1B from bread wheat into durum wheat using conventional crosses. Both genes separately increased the Al(3+) tolerance of durum wheat in both hydroponics and soil cultures. In contrast to bread wheat, the TaMATE1B gene was more effective than TaALMT1 in increasing the Al(3+) tolerance of durum wheat grown on acid soil.

The genetics of rhizosheath size in a multiparent mapping population of wheat

Rhizosheaths comprise soil that adheres to plant roots and, in some species, are indicative of root hair length. In this study, the genetics of rhizosheath size in wheat was investigated by screening the progeny of multiparent advanced generation intercrosses (MAGIC). Two MAGIC populations were screened for rhizosheath size using a high throughput method. One MAGIC population was developed from intercrosses between four parents (4-way) and the other from intercrosses between eight parents (8-way). Transgressive segregation for rhizosheath size was observed in both the 4-way and 8-way MAGIC populations. A quantitative trait loci (QTL) analysis of the 4-way population identified six major loci located on chromosomes 2B, 4D, 5A, 5B, 6A, and 7A together accounting for 42% of the variation in rhizosheath size. Rhizosheath size was strongly correlated with root hair length and was robust across different soil types in the absence of chemical constraints. Rhizosheath size in the MAGIC populations was a reliable surrogate for root hair length and, therefore, the QTL identified probably control root hair elongation. Members of the basic helix-loop-helix family of transcription factors have previously been identified to regulate root hair length in Arabidopsis and rice. Since several wheat members of the basic helix-loop-helix family of genes are located within or near the QTL, these genes are candidates for controlling the long root hair trait. The QTL for rhizosheath size identified in this study provides the opportunity to implement marker-assisted selection to increase root hair length for improved phosphate acquisition in wheat.

Can citrate efflux from roots improve phosphorus uptake by plants? Testing the hypothesis with near-isogenic lines of wheat

Phosphorus (P) deficiency in some plant species triggers the release of organic anions such as citrate and malate from roots. These anions are widely suggested to enhance the availability of phosphate for plant uptake by mobilizing sparingly-soluble forms in the soil. Carazinho is an old wheat (Triticum aestivum) cultivar from Brazil, which secretes citrate constitutively from its root apices, and here we show that it also produces relatively more biomass on soils with low P availability than two recent Australian cultivars that lack citrate efflux. To test whether citrate efflux explains this phenotype, we generated two sets of near-isogenic lines that differ in citrate efflux and compared their biomass production in different soil types and with different P treatments in glasshouse experiments and field trials. Citrate efflux improved relative biomass production in two of six glasshouse trials but only at the lowest P treatments where growth was most severely limited by P availability. Furthermore, citrate efflux provided no consistent advantage for biomass production or yield in multiple field trials. Theoretical modeling indicates that the effectiveness of citrate efflux in mobilizing soil P is greater as the volume of soil into which it diffuses increases. As efflux from these wheat plants is restricted to the root apices, the potential for citrate to mobilize sufficient P to increase shoot biomass may be limited. We conclude that Carazinho has other attributes that contribute to its comparatively good performance in low-P soils.

Transposon-mediated alteration of TaMATE1B expression in wheat confers constitutive citrate efflux from root apices

The TaMATE1B gene (for multidrug and toxic compound extrusion) from wheat (Triticum aestivum) was isolated and shown to encode a citrate transporter that is located on the plasma membrane. TaMATE1B expression in roots was induced by iron deficiency but not by phosphorus deficiency or aluminum treatment. The coding region of TaMATE1B was identical in a genotype showing citrate efflux from root apices (cv Carazinho) to one that lacked citrate efflux (cv Egret). However, sequence upstream of the coding region differed between these two genotypes in two ways. The first difference was a single-nucleotide polymorphism located approximately 2 kb upstream from the start codon in cv Egret. The second difference was an 11.1-kb transposon-like element located 25 bp upstream of the start codon in cv Carazinho that was absent from cv Egret. The influence of these polymorphisms on TaMATE1B expression was investigated using fusions to green fluorescent protein expressed in transgenic lines of rice (Oryza sativa). Fluorescence measurements in roots of rice indicated that 1.5- and 2.3-kb regions upstream of TaMATE1B in cv Carazinho (which incorporated 3' regions of the transposon-like element) generated 20-fold greater expression in the apical 1 mm of root compared with the native promoter in cv Egret. By contrast, fluorescence in more mature tissues was similar in both cultivars. The presence of the single-nucleotide polymorphism alone consistently generated 2-fold greater fluorescence than the cv Egret promoter. We conclude that the transposon-like element in cv Carazinho extends TaMATE1B expression to the root apex, where it confers citrate efflux and enhanced aluminum tolerance.