SEMINAR: Plant Biology Research Seminar

Soil salinity impedes crop production in 30% of irrigated land area and 7% of dryland agriculture (FAO, 2008). Members of the High Affinity K+ Transporter (HKT) gene family have been implicated in Na+ transport in taxonomically diverse species making these genes potential targets in crop improvement for salt tolerance. However, the bread wheat orthologs of most of these genes were not known. This study improved our understanding of HKT gene organisation and function in bread wheat.

A member of the TaHKT2;1 family was previously identified as a Na+ transporter with a possible role in root Na+ uptake. The full-length cDNA sequence of this member was used as a basis to query the International Wheat Genome Survey Sequence and to characterize the homeologous gene family. Six TaHKT2;1 genes were characterized including four functional and two pseudogenes on chromosomes 7A, 7B and 7D. The homeologous members were variable for protein domains for cation specificity and in cis-regulatory elements for salt response in gene promoters. The functional genes were expressed under low and high NaCl conditions in roots and leaf sheaths, but were down regulated in leaf blades. Aneuploid lines null for each gene showed that the individual genes were not involved in controlling Na+ net uptake from the external medium into roots but may have a role in ‘excluding’ Na+ from leaves or possibly involved in maintaining K+ status in the plant, being newly proposed role for group II HKT genes in bread wheat.

Knowledge on rice HKT genes and the whole genome sequence was used in comparative gene analysis to identify remaining orthologous wheat group II HKT genes. A distinct gene family, TaHKT2;2, was isolated with a single copy of the gene on each homeologous chromosome arm 7AL, 7BL and 7DL. The proteins encoded by members of the TaHKT2;2 gene family revealed more than 93% amino acid sequence identity but ≤52% amino acid identity compared to the proteins encoded by the TaHKT2;1 family. Similar to orthologous rice genes on chromosome 6L, TaHKT2;1 and TaHKT2;2 genes were located approximately 3 kb apart on wheat chromosomes 7AL, 7BL and 7DL, forming a static syntenic block in the two species. Most members of both TaHKT2;1 and TaHKT2;2 showed widespread expression and maintenance of transcript levels in tissues of plants in control and 200 mM NaCl conditions. However, 2-3 fold differences for TaHKT2;1 7DL-1 and TaHKT2;2 7AL-1 may indicate their involvement in controlling Na+ accumulation in a tissue specific manner. The chromosomal region on 7AL containing TaHKT2;1 7AL-1 co-located with a QTL for shoot Na+ concentration and yield in some saline environments. Evidence indicated that neither TaHKT2;2 nor TaHKT2;1 were associated with primary root Na+ uptake but TaHKT2;1 may be associated with trait variation for Na+ ‘exclusion’ from leaves and yield in some, but not all, saline environments. Phylogenetic analysis of these gene families in wheat and related species revealed a single relatively recent gene duplication event in an ancestral species resulting in two distinct phylogenetic lineages in the group II HKT genes, identifying two subfamilies I and II. The proteins were highly conserved in the structural core and the main functional domains but variable between subfamilies in the cytoplasmic and external domain. A subset of codons in external loops of subfamily I proteins showed signatures of episodic adaptive evolution identifying these sites as targets of selection and potentially important in molecular function.

This study identified two phylogenetically distinct group II HKT families with differences in copy number, gene sequences and encoded proteins. TaHKT2;1 was involved in potentially redundant functions for root-to-shoot Na+ transport influencing apparent Na+ ‘exclusion’. The HKT genes did not appear to influence root Na+ concentration. These findings revealed a broader understanding of HKT genes in bread wheat which possibly through a coordinated network influence Na+ trafficking within the plants.