Studies On Responses Mechanism Of Ramie(Boehmeria Nivea (L.) Gaud.) At Seedling Stage Under Low Temperature Stress

Ramie is a kind of important fiber crop in China. Low temperature stress may affect the growth and development of ramie, as well as the yield and fiber quality of ramie. The studies on response mechanism of ramie at the seedling stage under low temperature stress were performed including morphological characters, physiology and biochemistry and gene expression for the first time. We hope that the studies would be helpful for screening cold resistance ramie germplasm and breeding cold resistance ramie varieties. The results are as follows.1. The responses of seedling ramie morphological characters and leaves anatomical structure under low temeprature stressZhongzhu No.1 was studied under low temperature. The results showed that the leaf was the sensitive organ under low temperature, and its phenotype changed very obviously. The seedlings got damage very seriously or even death under 0℃ stress. The mortality of ramie plants would rise with the increase of days of 0℃ stress. The immature leaves were more sensitive than mature leaves. The leaf thickness increased so as to resist the low temperature stress, but 0℃ stress made leaves thinner than the control because of the leaves dehydration. The anatomical structure of leaves showed that the layers of palisade tissue, inclusions, ice cluster and vascular tissue increased in leaf mesophyll. The changes of mature leaves CTR, SR and the ratio between palisade and sponge tissue thickness showed significant differences with different temperature stress. The results were comparable with different time stress except for SR. For immature leaves, the changes of the ratio between palisade and sponge tissue thickness showed significant differences with different temperature or time, but the changes of CTR and SR both showed no significant differences.2. The responses of physiology and biochemistry of seedling ramie under low temperature stressEight ramie varities were used to study the responses of physiology and biochemistry of seedlings under low temperature stress. We found that the ability to resist low temperature for Qingmazhu and Wanxian Douma was the strongest, and the ability for Japanese Zhuma No.8 and Yihuangjia Ma was the weakest according to ramie mortality under 0℃ stress. Furthermore, we studied 6 biochemical and physiological indices including POD activity, CAT activity, SOD activity, MDA content, soluble protein content and chlorophyll content under low temperature stress. The changes of POD activity, CAT activity, SOD activity, MDA content and chlorophyll content except for soluble protein content showed significant differences with different low temperature stress. The change trends of 6 physiological and biochemical indices were different. CAT and SOD activity were higher than the control. The changes of POD activity, SOD activity, MDA content and soluble protein showed significant differences with diffrent time stress. POD activity, SOD activity, MDA content and chlorophyll content increased with the increase of the stress time, and they reached the peak after 3 days stress, however, CAT activity declined with longer time stress and soluble protein content was the highest after 2 days stress. The POD activity, CAT activity, SOD activity, soluble protein content and chlorophyll content except MDA content showed significant differences among the selected 8 varities. The results also showed that the ability to resist low temperature can be evaluated by POD activity and SOD activity content as they were similar to the evaluation by seedling ramie mortality under 0℃ stress.3. The responses of seedling ramie gene expression under low temperature stressIn this study, high-throughput de novo transcriptomic were performed on seedling ramie leaves with and without low temperature stress. A total of 133,222 unigenes were identified with an average length of 855.7bp (base pair), among which we obtained 3791 differentially expressed genes between the low temperature stress and the control. Among 3791 expressed genes,8 genes were validated which were transcription factors (6 MYB Family,2 NAC Family). The results show that MYB and NAC transcription factors played important roles in the process of the transcription regulation of gene expression in ramie under low temperature stress. The differential expressed unigenes were assigned to gene ontology (GO) and eukaryotic orthologous groups (KOG), The study results showed that the differential expressed genes mainly aggregated organelle, macromolecular comple, organelle part and membrane, and the main functions were about structural, molecule activity, transporter activity and catalytic activity. In the studies, the differential expressed unigenes also were performed GO and KEGG enrichment analysis. The study results showed that low temperature stress stimulated ramie gene response and signal transduction, induced regulator of cool acclimation, influenced the activities of enzymes and photosynthesis, and changed the secretion of cytokines in ramie. Furthermore, the membrane was damaged under low temperature stress, the biochemical reactions in membrane also were influenced, and thus.ramie cells were damaged...