Integrating Transcriptomics To Analyse The Process Of Astragalus Seed Germination And Endogenous Hormones

The process of seed germination means that plant will start a new life cycle, during which seeds underwent drastic morphological changes. A tiny seed transforms into a normal seedling, implying energetic metabolism and complex regulation of gene expression. Most medicinal plants were grown under incomplete domestication or without artificial' domestication, maintaining their wild characters which led to inconsistent physiological dormancy and irregular germination. The roots of Astragalus membranacens var. mongholicus and Astragalus membranaceus are important Chinese herbal medicines in great demand, which mainly rely on artificial cultivation at present. However, low germination potential limited their large-scale standard cultivation for the last decades. We investigated the physiological and metabolic changes during the germination of A. var. mongholicus and A. membranaceus. Meanwhile, we combined transcriptomics (Illumina 2000) to investigate the gene regulation of gibberellin, abscisic acid and zeatin in Astragalus seed germination. The results of present study are as follows:1. After cultivating under normal conditions for 6 days, the germination rate of A. membranaceus var. mongnolicus was 28% lower than A. membranaceus. The germination rate of A. membranaceus var. mongholicus and A. membranaceus seeds was increased by 3.5 and 1 times respectively by soaking for 10 min in concentrated sulfuric acid, without affecting the seed vigor. During the germination, fresh weight continued to increase, while dry weight decreased in later germination stage.2. The content of total protein and fatty acids reduced in the process of Astragalus seed germination, while the soluble sugar content increased first and then decreased. Flavonoids accumulated within the 48 h germination. Highest contents of gibberellin. abscisic acid and salicylic acid were detected after 6 hours, which significantly decreased after 12 hours. There was no obvious change in the content of zeatin.3. Transcriptomics demonstrated that the most expressed genes were mainly related to gene expression, nutrition and energy metabolism. The differential gene expression analysis of A. membranaceus var. mongholicus germination revealed that only 5.0% and 3.1% of the unigenes remained up or down regulated during the 48 h of cultivation. For A. membranaceus, the percentage of up or down regulated unigenes was 4.2%and 2.8%, respectively. In the same germination period, up or down regulated unigenes were 11.1% and 14.5% of the total unigenes of two kinds of Astragalus.4. As for the phytohormone related gene expressions during the germination, such as gibberellin, abscisic acid and zeatin, we found that the most expressed genes were associated with their signaling pathways, which were 161,256 and 96 unigenes, respectively. The majority of differentially expressed genes of gibberellin were involved in the regulation of KAO, GA20OX, GA3OX and GA2OX enzyme activities. For abscisic acid, the differentially expressed genes were concentrated in the regulation of BG1 enzyme activities. Moreover, zeatin inactivation pathway was found animated in the germination.