Keywords: 
• forest protection /
• Casuarina /
• Frankia /
• infection /
• nodule development

Abstract: Symbiotic nitrogen-fixing Frankia strains are well known to promote the growth of Casuarina trees in coastal area. The studies on their infection process are very important to utilize them as bioagents. The infection process of two symbiotic Frankia strains，isolated from the root nodules of Casuarina equisetifolia in Zhejiang Province，was studied using parallel semi-thin and ultra-thin sections that were periodically analyzed with light and electron microscopy. Results showed that Frankia penetrated root hair cells and infection proceeded intracellularly in the root cortex. This stimulated infected cells which started producing abundant irregular dense electron deposits. Frankia，packaged by deposits，infected continuously，forming infection threadsthat penetrated adjacent cortex cells and ultimately approached nodules. Chronologically， nodule development began when Frankia infected root hair cells 6 d after inoculation. At 12 d，after being activated by endogenous hormones，pericycle cells underwent vigorous mitosis giving rise to lobe primordium. At 15 d，when a portion of parenchyma cells were infected by Frankia，the primordium transformed into pre-nodules. Finally，24 d following inoculation，after forming of the apophysis from the cortex，pre-nodules developed into new nodules and finally mature nodules. Thus，Frankia infection process consisted of the following processes：Frankia infection-infection thread-formation ofinterfacial matrix-penetration of cell wall-propagation-vesicles development-spores producing；and the development process for the nodule was primordium-pre-nodule-mature nodule.［Ch，9 fig. 20 ref.］