Penicillium nodositatum infects the roots of alder trees and induces the formation of structures called myconodules, which are similar to young actinorhizae. Root infection of Alnus incana by P. nodositatum as well as myconodule development were studied by light and electron microscopy and observations were compared with those described for the infection by Frankia spp. We have established an obvious homology between the early steps of the infection caused by both microorganisms. The presence of the fungus near the roots induces deformation of root hairs. The infection site is probably localized in a folding of a deformed hair. As soon as hyphae penetrate into the hair, they become enclosed in a polysaccharide matrix. Initially, P. nodositatum colonizes a region near the infected root hair that may correspond to a slightly developed prenodule. Then a nodular primordium is initiated at some distance from the initial contact and the new nodular cortex is invaded by the fungus. The zone of infection is limited to the cortical cells by a barrier of tannins. Myconodules remain small and unilobed and have an outer morphology similar to that of an incompatible Frankia nodule. Key words: Alnus, myconodule formation, Penicillium, root hair infection.
SUMMARYPenicillium nodositatum Valla penetrates Alnus incana (L.) Moench roots through deformed root hairs. It then colonizes the cortical cells and induces the formation of myconodules which have structures similar to those of young actinorhizas. The ultrastructure of infected cortical cells has been compared with that of cells infected by pathogenic or symbiotic fungi as well as with that of cells infected by Frankia. Our observations show that an infected host cell shows few visible signs of resistance to the fungal invasion. As soon as P. nodositatum enters the cell, it is surrounded by a wall-like pecto-cellulosic matrix produced by the host cell. The host plasmalemma, invaginated around the endophyte, keeps its integrity as it does in symbiotic associations. At the beginning of the infection, the host cytoplasm and the fungus are intact but when the cell is completely invaded, the host cytoplasm becomes disorganized and subsequently the fungus also senesces, as in roots of Ericaceae infected by an endomycorrhizal fungus or as in older tissues of actinorhizas. P. nodositatum could survive the dead host cell by using its own reserves. Uninfected neighbouring host cells remain intact. P. nodositatum can be considered to behave as a neutral microsymbiont of A. incana, in a way similar to that of a compatible but ineffective Frankia strain.
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