Mechanism Of Pod Shattering In Soybean(Glycine Max L.Merr.)

Pod shattering or pod dehiscence refers to the opening and releasing the seeds along the ventral and dorsal suture in the fully mature pods before harvest.Pod shattering is a concurrent process with complex physical and physio-biochemical reactions.The pod ventral suture and its anatomy are closely related to pod shattering.Loss of seeds from mature vegetable soybean(Glycine max L.Merr.)pods is one of the most outstanding disadvantages,which restrains the development of vegetable soybean industry.To better understand the phenotypic syndrome of pod shattering characteristics and pod shattering mechanismin soybean,two-year field experiment was conducted for phenotypic identification and pod shattering classification with 94 grain soybean cultivars,30 vegetable soybean cultivars and 16 small-grain soybean cultivars.Based on the two-year field experiment,five dehiscent vegetable soybean cultivars and five indehiscent grain soybean cultivars were selected for further analysis.The differences in pod ventral suture anatomy using micromorphological observation methods were explored,and pod wall nutrient content,cell wall hydrolase,endogenous hormone content and transcriptomics differences of ventral suture during beginning seed(R5),full seed(R6)and full maturity(R8)grow stages were compared.The main results are as follows:(1)Of the 94 grain soybean cultivars,83% are very resistant and resistant to pod shattering;among 30 vegetable soybean cultivars,93% are very susceptible to pod shattering,while the small-grain soybean cultivars are intermediate to pod shattering.(2)The phenotypic syndrome of pod shattering in dehiscent vegetable soybeans is characterized by an umbrella-shaped and open plant type.They have in general shorter plant height,shorter pod height at bottom,more productive branch,less number of nodes on main stem with more round leaf and white flower,longer pod length,wider pod width,larger pod area,more 1-seeded pod and 2-seeded pod,larger 100-seed weight,and non-yellow seeds.(3)The soluble total sugar,sucrose and moisture content in pod walls of dehiscent vegetable soybean are higher than those of grain soybean at beginning seed(R5)and full seed(R6)stages,but significantly lower than that of grain soybean at fully maturity(R8)stages.(4)The transfer rate of sucrose in pod walls from full seed(R6)to fully maturity(R8)stage in dehiscent vegetable soybean cultivars is 24.9% higher than that of grain soybean.The higher neutral detergent fiber(NDF)content and poor water retention in pod walls result in greater tension caused by dehydration in vegetable soybean cultivars.The nitrogen(N),iron(Fe),magnesium(Mg),manganese(Mn)nutrients content in pod walls are likely to affect pod shattering in dehiscent vegetable soybean.(5)The dehiscent vegetable soybean cultivars have a larger vascular bundle area(VBA)in the cross sections of ventral sutures,which is 1.3 times larger than that of indehiscent grain soybean.What‘s more,the dehiscent vegetable soybean cultivars also have a larger bundle cap area with even and closely arranged cells,and less interstitial substance.A significantly shorter and straight route from the top of fiber cap cells to the connecting point of the two valves(RFCV)of the pod ventral suture in dehiscent vegetable soybean cultivars is observed.The length of RFCV indehiscent vegetable soybean cultivars is 29.4% shorter than that of indehiscent grain soybean.The shorter RFCV weakens the adhesion between two valves and is an important factor triggering vegetable soybean pod shattering.(6)The cellulase,pectinase and polygalacturonase activities in ventral suture of dehiscent vegetable soybean cultivars are higher than those of indehiscent grain soybean cultivars.The synergistic effects of auxin(IAA)and gibberellin(GA3)in the pod ventral suture promote the occurrence of pod shattering in vegetable soybean cultivars.(7)Transcriptome analysis of pod ventral suture in dehiscent vegetable soybean cultivars and indehiscent grain soybean cultivars showed that there are 2520 differentially expressed genes,with 1641 up-regulated and 879 down-regulated genes.The GO annotation and KEGG metabolic pathway enrichment analysis of these differentially expressed genes found that genes associated with glycosyltransferase and chalcone synthase of flavonoid metabolism and digestive proteins related of phenylpropanoid biosynthetic are extremely enriched.12 genes encoding auxin(IAA)all showed down regulation in vegetable soybean cultivars,2 genes encoding endoglucanase and 2 genes encoding pectinase showed up regulation in vegetable soybean cultivars.