Autotetraploidization in Ziziphus jujuba Mill. var. spinosa enhances salt tolerance conferred by active, diverse stress responses

Chinese jujube (Ziziphus jujuba Mill.), native to China, is an important deciduous fruit tree. This species is mainly propagated by grafting, with Z. jujuba var. spinosa widely used as the rootstock because of its resistance to adverse conditions such as saline environments. In a previous study, we found that autotetraploid jujube is phenotypically superior to the corresponding diploid, furthermore, the differences in salt tolerance between the two plant materials and the underlying molecular mechanism were still unclear. To clarify this issue, we investigated the leaf and shoot morphology and physiology of diploid and autotetraploid plants treated with NaCl for 0, 2, 6, 10, and 14 days. We also carried out transcriptome profiling after 0, 1, and 2 days of salinity treatment. Comparisons of diploid and autotetraploid plants under salinity conditions in morphology and eight physiological traits indicating that the autotetraploid has superior salt tolerance. Many differentially expressed genes related to salt stress response in diploid and autotetraploid Z. jujuba were assigned to functional categories such as detoxification, stimulus response, catalytic enzyme activity, antioxidant enzyme activity, glutathione metabolism, phenylpropanoid biosynthesis, flavonoid biosynthesis, peroxisome, plant hormone signaling, and ABC transport. These genes constituted a complex regulatory network for maintaining osmosis and intracellular ion homeostasis and resisting oxidative stress. In addition, several genes encoding MYB, AP2/ERF-ERF, WRKY, GRAS, and C2H2 as well as hub proteins, including aquaporins, heat stress transcription factors, chaperone, cytochrome P450, and kinases, were differentially expressed under salt stress. The number, function, and fold changes of these genes differentially expressed under salinity treatment differed between the diploid and its autotetraploid, with more active, diverse strategies taking place in the autotetraploid to adapt to long-term salt stress. Finally, we found that there were more up-regulated DEGs and greater fold change related to osmotic adjustment in autotetraploid than in diploid which could further explain the enhanced salt stress tolerance in the autotetraploid. Our findings contribute to an understanding of the genetic basis of salt tolerance in autotetraploid jujube, which can guide future tolerance breeding of jujube cultivars and their rootstocks.