More genes for a higher fitness under salt stress: identification of genes whose expressional differences, based on differences in ploidy, lead to a better salt tolerance.
Plant Genetics and Genomics
Plant Cultivation, Plant Nutrition, Agricultural Technology
Final Report Abstract
2013 Chao et al. reported a fitness benefit for neo-tetraploid A. thaliana accessions under salt stress. This was the first time a potential evolutionary benefit of whole genome duplication events could be documented. Several of these events occurred during land plant evolution but although improved tolerance to various abiotic stressors had previously been reported a positive impact of WGD on fitness had so far never been shown. Additionally Chao et al. reported, that neo-tetraploids accumulate higher potassium levels in shoots. In grafting experiments it was established that this phenotype was driven by the karyotype of the root. The molecular mechanisms by which neotetraploid achieve greater fitness or enhanced potassium uptake are as of yet completely unknown. Aim of the DFG project was to investigate molecular changes resulting from whole genome duplication (WGD). Focus was to be placed on potassium homeostasis and enhanced fitness under Nastress. To this end RNAseq analysis was performed which allowed a comparison of gene expression in tetraploids and diploids under control conditions and salt stress. A difference in the expression of HAK5 (high affinity K-transporter 5) was discovered in roots. HAK5 is known to be involved in uptake of potassium from the external medium. Tetraploids showed a higher expression of HAK5. As such the gene was identified as a candidate likely to explain potassium accumulation differences between diploids and tetraploids. HAK5 mutants were subjected to WGD by colchicine treatment. For this purpose a protocol for the generation of neo-tetraploid lines was generated and optimised for high throughput. Subsequently ionomics analysis of tetraploid and diploid HAK5 mutants and wild type plants showed HAK5 not to be involved in improved potassium uptake in neo-tetraploid Arabidopsis thaliana plants under high external K supply. The effect of HAK5 on the fitness of neo-tetraploids under salt stress is still under investigation as well as other factors which may contribute to enhanced fitness such as ABA responses. Expression comparison between diploids and tetraploids revealed a fare higher number of genes differentially regulated under control conditions than under Na-stress. This surprising observation hints at a pre-adaption of tetraploids to Na-stress. Gene regulation becomes more similar for the two karyotypes once the stress is applied. Among the genes which show a stronger expression in tetraploids under control conditions, genes classified as “responsive to ABA” are significantly enriched. This observation indicates a difference in the ABA response in tetraploids. In line with that, previous studies observed an ABA phenotype resulting from WGD. In summary the DFG project enabled the molecular analysis of WGD events. It discovered expression differences for the high affinity K uptake transporter HAK5 but showed that it does not contribute to enhanced potassium accumulation in neo-tetraploids under high external K. The effect of HAK5 in neo-tetraploids under low external K is still under investigation. The project also reviled that tetraploids may be preadapted to abiotic stress, possibly through an altered ABA signalling, and highlighted candidate genes whose expression may influence this different behaviour. As such it lead to a gain of knowledge not only in potassium homeostasis but also contributed to the understanding of land plant evolution.
