The Molecular Regulatory Mechanisms Of Mucilage Traits In Artemisia Sphaerocephala Seed

Artemisia sphaerocephala,a semi-shrub belonging to the Artemisia genus of the Compositae family,is one of very important windproof and sand-fixing plants on the flowing and semi-fixed sand dunes in the arid desert area of northwest China,and it has characteristics of drought resistance,wind erosion resistance,sand resistance and sand fixation.In addition,A.sphaerocephala seeds are rich in mucilage which is a polysaccharide with significant physiological,ecological,and economic value.However,the molecular regulatory mechanism of mucilage formation remains unclear.For this research,seven different developmental stagesof A.sphaerocephala seeds(10,20,30,40,50,60,70 days after flowering)were employed as samples.Through determining the length,width,weight and mucilage content of A.sphaerocephala seeds to analyze the accumulation pattern of seed mucilage at different developmental stages.Meanwhile the transcriptome sequencings were performed.Moreover,the gene co-expression network was established by using weighted gene co-expression network analysis(WGCNA),and the pathway of seed mucilage polysaccharide biosynthesis was established.The purpose is to further explore the molecular regulation mechanism of mucilage formation in A.sphaerocephala seeds.The main findings are as follows:(1)The length,width,weight and mucilage content of A.sphaerocephala seeds increased continuously during 10-60 days after flowering and decreased in 70 days after flowering.The rapid accumulation period of seed mucilage was 10-30 days after flowering.The mucilage content increased from 0 to 49.42 % of the dry seed weight during 10-30 days after flowering,but there was no significant increase during 30-70 days after flowering.The mucilage content of mature seeds was 39.81%,among which glucose(34.79%),xylose(17.17%),galactose(13.86%),arabinose(11.14%)and mannose(10.50%)were the main monosaccharide components.(2)Through transcriptome sequencing data,1,297 genes related to mucilage formation were screened.The gene co-expression network was successfully constructed,and the key module was identified in the network.Among the top 10 pathways of gene number,8 pathways belong to Carbohydrate metabolism and Glycan biosynthesis and metabolism,which indicated that they were the main pathways of regulating mucilage formation in A.sphaerocephala seed.(3)The molecular regulatory network of mucilage formation in A.sphaerocephala seeds was preliminarily revealed.Transcription factors played a core role in the regulatory process.21 key regulatory genes for mucilage formation were identified,including one KNOTTED ILIKE HOMEODOMAIN 7(KNAT7)gene,one TRANSPARENT TESTA GLABRA 1(TTG1)gene,17 genes of NAC REGULATED SEED MORPHOLOGY2 / NAM(NARS2/NAM)gene families and 2 genes of APETALA2(AP2)gene families.In S1 ~ S3,AsNAM had the highest expression level with a peak value which is 802.35;in S4 ~ S7,its expression level decreased significantly,and the expression level of S4 was only the 7.55% of S1.The expression trend of AsAP2 was similar to that of AsNAM,and its expression level of S4 is only the 11.22% of S1.In S6 and S7,the expression level of AsTTG1 increased,and it was 2.01 and 1.53 times of S5 respectively.The up-regulation of AsTTG1 promoted that expression level of AsKNAT7 was up-regulated to 2.26 and 1.51 times of S5.The above results showed that AsNAM and AsAP2 jointly promote mucilage formation during the 10-30 days after flowering;during 30-70 days after flowering,the promotion by AsNAM and AsAP2 was weaken,and AsTTG1 possiblly facilitated the up-regulation of AsKNAT7,thereby inhibiting mucilage formation.Together,these two aspects led to no further increases jointly in mucilage from 30-70 days after flowering.(4)The mucilage biosynthes pathway of A.sphaerocephala seed was established,and the molecular regulatory mechanisms of mucilage biosynthesis in seeds were revealed.Uridine diphosphate glucose pyrophosphorylase(UGPase)and Phosphoglucomutase(PGM),the upstream genes of UDP-glucose(UDP-Glc),were highly expressed.And their peak expression level was 290.28 and 617.08 respectively.UDP-glucose-4-epimerase(GALE),UDP-glucose 6-dehydrogenase(UGDH)and UDP-glucose 4,6-dehydratase(RHM),the downstream genes of UDP-Glc,only had the peak expression level with 281.22,182.62,and 125.01 respectively.These indicated that highly expressed pgm and UGPase,and minimally expressed GALE,UGDH,and RHM led to the accumulation of UDP-Glc.UDP-apiose/xylose synthase(AXS),the upstream genes of UDP-xylose(UDP-Xyl),had the peak expression level with 447.38.UDP-arabinose 4-epimerase(UXE),the downstream genes of UDP-Xyl,only had the peak expression level with 152.56.This indicated that highly expressed AXS and minimally expressed UXE led to the accumulation of UDP-Xyl.This study might assist with elucidating the molecular regulatory mechanisms of the seed mucilage traits,and it could establish a foundation for the application of mucilage associated genes in genetic breeding.