The Study On The Infection Of Soybean Rhizobium And The Formation Of Soybean Root Nodules

Soybean(Glycine max(L.) Merrill) is one kind of leguminous plants and has the high efficient nitrogen fixation ability.Soybean is widely cultivated around the world.The nitrogen fixation ability of soybean was formed by rhizobium infecting root nodule,which can provide abudant plant protein for the daily life of human beings.Therefore,the cultivation of soybean received extensive attention and soybean has become Chinese important food crops.In this study,Dongnong 46 was taken as experimental material,the development processes of soybean nodule and rhizobia have been intensive researched through analyzing paraffin section,semi-thin section and ultrathin section,by the transmission electron microscopy(TEM)and scanning electron microscopy(SEM)technology.The structures of soybean nodule and rhizobia were observed by using optical microscopy in the microstructure level,and the ultrastructure level.The structures of soybean nodule and rhizobia were observed by TEM and SE as well.This research was deepened on the previous studies,and the characteristics of the ultrastructure of soybean nodule and rhizobia were studied more comprehensively and systematically than before.Then we can understand more about the characteristics of nitrogen fixation during development and the mechanism of soybean nitrogen fixation.This study may provide theoretical support for increasing soybean yield.The research results showed that:(1)The rhizobia of the symbiotic nitrogen fixation nodule with soybean root infected the host cell by forming the infection thread,which became a nodules with cortex for external and central tissue for internal.The central tissue was the infection zone,which was divided into the infection cell and the non-infection cell.But the volume of the infection cells was much larger than the volume of the non-infection cells.(2)The infection line broke the cell walls of the host cell though releasing degradable substances of cell wall,rhizobia entered into the host cell with the infection line,completed the infection process to cells.In the early stages of infection,the rhizobium was coated by complete peribacteroid membrane,which contains one or several rhizobium in each peribacteroid membrane.(3)In the early stage of nodule formation,part of the rhizobia inside the infection line has begun to divide and proliferate.There were some polyphosphate particles(PP)existed in rhizobia,which the content is less and mostly located around rhizobia nucleoid.Along with the proliferation of rhizobium,the polyphosphate particles divided and the content increased.With the line wall of infected host cells degrading,rhizobia was released and come into the host cell.In the early stage of nodule development,few polyphosphate particles were produced.With the proliferating of rhizobia,polyphosphate were increasingly accumulated.In a certain period of proliferation,PHB particles were produced,and the polyphosphate particles in rhizobia were decreased with the increasing of PHB particles and the nitrogen fixation ability of rhizobia gradually strengthened.(4)In the mature stage of root nodule,almost all of the host cells were rhizobia,and there were a number of poly-?-hydroxybutyrate particles exist in rhizobia.Both the volume and the number of infected cells reached the maximum number and much greater than that of non-infected cells.There were more deformed mitochondria in the host cells at mature stage,and the number of rough endoplasmic reticulum and ribosomes increased,and almost all of the starch granules disappeared.At this time,the nodules in the host cells were small and more,the structure was clear and the membrane was completed,and the nitrogen fixation ability of the nodules became the strongest.(5)In the degenerating stage nodular,firstly,the cell nucleus and nucleolus was degenerated,nucleolus was disintegrated and cell nucleus was serious deformed,and nuclear membrane was destructed.Incomplete structure of the nuclear membranes was observed by transmission electron microscop,which had no obvious boundaries.With a large number of ribosomes free endoplasmic reticulum monolayer structure increased,a large number of free ribosomes were appeared.During the degenerating stage,nodules had a vesicle structure in the host cell,and presented near the endoplasmic reticulum of cell wall and rhizobia.At the end of the degenerating stage,the number of rhizobia in nodules decreased gradually,the host cells became empty and the organelles disappearing and the poly-?-hydroxybutyrate particles decreasing with the reducing number of rhizobia.