Comparison Between A Tetraploid Turnip And Its Diploid Progenitor For Their Adaptation To Salinity And Cadmium Stress And The Underlying Molecular Mechanism

Polyploidy is pursued in plant breeding programs due mainly to its ability to yield larger vegetative or reproductive organs. It could also be used to improve plant adaptation to saline or heavy metal contaminated soils because of the redundant genes implicated in stress tolerance, the status of the genome methylation degree and the pattern of gene expression at transcriptional level. However, little information is available about the ability of an autopolyploid to survive in stressed condition in comparison with its diploid progenitor. In this study, a tetraploid turnip (cv. Ai-Jiao-Huang 4X) and its diploid progenitor (cv. Ai-Jiao-Huang 2X) were compared for their tolerance to salinity stress and cadmium toxicity by investigating a group of morphological and physiological parameters. Methylation sensitive amplification polymorphism (MSAP) and cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis were employed to compare the degree of genome methylation and identify the genes induced or repressed under salinity and cadmium stresses. The major results are listed as the following.1. The tetraploid turnip exhibited better adaptation to a highly concentrated salt medium (200 mM), as evidenced by a less affected germination rate and a healthier morphological appearance at the seedling stage. Furthermore, an extension of salinity stress up to a certain period of time at the 5-7 leaf stage shows differences between the tetraploid turnip and its diploid progenitor. The former had a higher K+/Na+ ratio in the roots, higher glutathione concentration and antioxidant activities in the leaves, and smaller reductions in photosynthetic capacity in terms of leaf chlorophyll content. On the other hand, the tetraploid turnip had a better tolerance to the cadmium treatment, reflected by higher germination rate, root length at seedling stage. An extension of cadmium toxicity at 5-7 leaf stage resulted in a less obvious impairment of chlorophyll, less alternation of antioxidants'activity and higher concentration of K+ and Ca2+ ions in shoots, but lower ratio of Na+/K+ in tetraploid, but higher concentration of Na+ in shoots and less availability of K+ and Ca2+ in both roots of the diploid. 2. Salinity stress and cadmium toxicity gave rise the significant changes of genome DNA methylation degree, as reflected by the MSAP analysis, which identified a total of 14382 bands generated by 56 primer pairs. A among a total of 2924 methylaiton bands,1731 were full the methylaiton type and 1193 were the half methylation type. The total methylaton ratio of the diploid in the control, salinity, and cadmium stressed conditions were 20.3,18.8,22.7; whereas the total methylaton ratio of the tetraploid in the control, salinity and cadmium stressed conditions were 20.3,18.8,22.7. The total methylaton level declined in both the diploid and tetraploid after salt treatment, but increased after cadmium treatment. In diploid the methylaton level decreased by 1.5 point, whereas in the tetraploid it decreased by 2.5 point after salt treatment. The full methylation ratio in diploid was 2.2 points higher than in tetraploid, while half methylation ratio in tetraploid was 1.6 points higher than in the diploid.3. By using 96 primer pair combinations, about 2648,2650,2703,2730,2551 and 2580 cDNA fragments were generated respectively under the following treatments (1) tetraploid without stress; (2) diploid without stress; (3) tetraploid under salinity stress; (4) diploid under salinity stress; (5) tetraploid with cadmium toxicity; (6) diploid with cadmium toxicity. Salinity stress and cadmium toxicity led to 36 and 40 TDFs, respectively, which represented the functional genes that can be classified in to 3 or 4 categories, namely, transcriptional factors, stress responsive genes, genes relating to signal transduction and so on. Among the TDFs caused by salinity stress,14 and 7 were repressed in the diploid and tetraploid, respectively; 5 and 10 were induced in the diploid and tetraploid, respectively. On the other hand, among the TDFs resulted from cadmium toxicity, 7 and 16 were repressed in the diploid and tetraploid, respectively; and 12 and 5 were induced in the diploid and tetraploid, respectively.Taken together, the tetraploid turnip had a better adaptation in the salinity and cadmium stressed environments. The MSAP and cDNA-AFLP analysis revealed the underlying genetic mechanisms that cause the morphological and physiological differences. Our work is a contribution to the advancement of our understanding on the mechanism that an autopolyploid turnip deals with adverse environment.