Evaluation Of Fall Dormancy Rating Among Different Alfalfa (Medicago Sativa L.) Cultivars And Effect Mechanisms Of Fall Dormancy On Winter Survival Rate

Fall dormancy(FD) is an adaptive trait that defines alfalfa( Medicago sativa L.) adaptation to environmental changes in late autumn. As a key basis of winter survival and regeneration for alfalfa, FD is strongly associated with spring regrowth, forage yield and cold hardiness. Recently, FD has been recognized as one of the main basis of alfafa cultivation and planting regionalization. In this study,alfalfa cultivars from domestic and abroad were selected as the trial material and 11 standard FD alfalfa cultivars were selected as the control material. According to the method by Teuber(1998), FD rating and an average score(ASI) of winter survival were evaluated, respectively. The objective of this study was to examine the effect of FD rating on alfalfa productivity, root traits and cold hardiness related gene expression, to screen the differential expression gene at the transcriptome level by the next generation sequencing technology, and to explore the expression mode of differential gene and metabolic pathways.Results will improve knowledge on the molecular mechanism of FD in alfalfa. The main results were as follows:1. In China, certified alfalfa varieties basically belong to fall dormancy type with higher fall dormancy ability. For example, fall dormancy rates(FDR) of Gongnong No.2, Gongnong No.3, Hailaer alfalfa, and a series of Longmu alfalfa varieties were 1, and the FDR of Gannong No.1 was 2. There was no significant effect of FDR on alfalfa production performance during the first harvest in the second year. Although there was little or no correlation between plant height and FDR during the growing stage in summer, the stem elongation strategy of alfalfa with different FDR was notably different. There was significant correlation between internode length and internode number and FDR or fall dormancy categories. Internode length of alfalfa varieties with higher FDR was longer, but internode number tended to decrease as the FDR was increasing.2. Through correlation analysis of FDR, an average score(ASI) and winter survival rate(WSR) of 46 alfalfal varieties, we found that alfalfa FDR was significantly positively correlated with ASI, but was notably negatively correlated with WSR. That is to say, more stronger alfalfa fall dormancy ability was,more stronger alfalfa winter survival ability was, and vice versa. Root traits of alfalfa with different FDR showed differential responses characteristics to cold stress in winter. Alfalfa enhanced its winter survival rate mainly through increasing collar diameter and lateral root number, and did not depend on the absolute size of other root phenotypic traits.3. During the cold acclimation, with the increasing of alfalfa FDR, chlorophyll content and net photosynthetic rate showed a decreasing trend after the initial increasing, and the soluble sugar content of leaf tended to decline. Furthermore, the amino acid content in crown, taproot and lateral root of standard alfalfa varieties in winter had consistent changing rule with FDR. When FDR of alfalfa was increasing, all amino acid content performed notably increasing trends. The expression level of cas18,vsp and corF from alfalfa crown, taproot and lateral root showed significant increasing trend.4. In this study, we sequenced two standard varieties of alfalfa(dormant and non-dormant) rootsusing the Illumina NextSeq 500 platform. The de novo transcriptome assembly generated 23,470 Unigenes in the alfalfa root transcriptome. After assembly, 16,190 Unigenes were annotated to one or more gene ontology(GO) terms. A total of 7,696 Unigenes were successfully mapped to 321 Kyoto Encyclopedia of Genes and Genomes pathways(KEGG) and 5 KEGG biochemical pathways. 18,617 Unigenes were annotated against the eggNOG database with 25 functional categories. Gene expression between dormant and non-dormant alfalfa were performed, and we indentified 2,326 differentially expressed genes potentially related to fall dormancy, including 1,308 up-regulated genes and 1,018down-regulated genes. Furthermore, 1,721 differentially expressed genes were obtained GO annotation.In addition, differentially expressed genes were significantly enriched in energy metabolism,biosynthesis of other secondary metabolites, carbohydrate metabolism and Amino acid metabolism processes. We also identified 1,497 transcription factors(TFs) and 7,005 simple sequence repeats(SSRs).