Preliminary Study On The Formation Mechanism Of "Hedgehog Head" Of Downy Mildew In Setaria Italica

Downy mildew is an important disease in foxtail millet,‘hedgehog head’ and ‘hairline’ are two phenotypes that directly affect the quality and yield of foxtail millet.In order to explore the cause of the symptoms of ‘hedgehog head’ and ‘hairline’ caused by the interaction between Sclerospora graminicola and foxtail millet,this study used the early maturing variety Xiaomi as the experimental material to observe the pathogenesis of foxtail millet downy mildew.The activity of cell wall degrading enzyme and the expression of related genes in the leaves of the same disease stage were determined,and the reason of the host leaves cracking was explored.And explored the interaction mechanism between the pathogen and the host through the analysis of the mixed transcriptome of Sclerospora graminicola and foxtail millet;The main results of this study are as follows:1.Fluorescence microscopy was used to observe the infection of Sclerospora graminicola in different infection stages.Firstly,through parasitizing between the vascular bundles of the host,with the development of the deformed lobules of the ‘hedgehog head’ of the host extending to the growth point,during the expansion,it will expand from the vascular bundles to the mesophyll cells between the vascular bundles,and finally to the whole ‘hedgehog head’ deformed lobules.The parasitic strategy of Sclerospora graminicola is as follows: vascular bundle parasitism,with the growth point expanding,at the same time,the cell gap expanding and differentiation of the haustorium which has a strong ability to penetrate the cell wall to absorb nutrients,so as to supply the whole leaflet with parasitism.2.The development process of the sexual oospores of Sclerospora graminicola was observed by means of microscope,scanning electron microscope and transmission electron microscope.It was found that at the primary stage,the cell wall of Sclerospora graminicola was monolayer,and the inner part was mainly composed of mitochondrion,nucleus and dense electron layer,usually ellipsoid.With the development of oospores,the inner part gradually differentiated into fingerprint vesicles,producing more white liposomes,The nuclear structure can be observed obviously,the outer wall of which will form a gradually thickened outer wall,and the individual oospore will gradually become round.In the process of development,it can be observed that the spherical substance of the host cell is absorbed into the oospore;A large number of white liposomes are accumulated in the oospore which gradually enters the dormancy stage,and the outer wall is formed and thick;the outer wall of the oospore in the dormancy stage is thick,the individual is large,and the diameter can reach 65 μm at most;3.Through the measurement of the activity of cell wall degrading enzyme: The activity of cellulose complex enzyme continued to rise in five stages,and the maximum value of enzyme activity was 2977.64U/g.The β-glucosidase in cellulase was significantly higher than that of the control group,reaching the peak value in the fourth stage;the activity value of pectin complex enzyme was higher than that of the control group,and the activity value of polygalacturonase was higher than that of the control group,reaching the peak value in the third stage,with the activity value of 433.81 U/g;the difference between the treatment group and the control group was small;the difference between the treatment group and the control group was small;the β-1,3-glucanase activity value of the treatment group was higher than the control group as a whole,and reached the peak value of 1420.65 U/g in the third stage;4.Through the transcriptome analysis of interaction in the formation of ‘hedgehog head’ and ‘hairline’:it was found that 109 genes related to hormone were identified.At the early stage of ‘hedgehog head’development,10 salicy acid and 16 jasmonic acid were identified to be involved in the regulation,which may be related to the host response to the infection of Sclerospora graminicola.In addition,the host responded to the stress of the pathogen mainly through chitin metabolism,flavonoid metabolism,phenylpropane metabolism,toxin metabolism and trehalose metabolism in the middle and late stage;the main metabolic pathways of the pathogen in the process of infection were starch and sucrose metabolism,glyceride metabolism,fatty acid biosynthesis,fructose and mannose metabolism;cellulase and pectinase were mainly secreted to participate in the invasion The expression level of cellulose β-glucosidase related genes was higher in infection and expansion.A total of 7 genes encoding β-glucosidase were identified to play an important role in the formation of ‘hedgehog head’;5.Metabonomics analysis showed that the main hormone metabolites identified by differential metabolites were acetylcholine,adrenaline,serotonin,glutamic acid,L-aspartic acid and L-glutamic acid,and the carbohydrate metabolites were significantly enriched in D-(+)-maltose and α-lactose,which were considered as fatty amide and flavonoid metabolites.The main metabolic pathway of the later stage of hedgehog head was tryptophan metabolism Histidine metabolism,A total of 414 metabolites were identified to participate in the formation of hedgehog head.Mannitol and sorbitol were identified as monosaccharide metabolites.D-(+)-maltose and α-lactose were the main oligosaccharide metabolites.