Root Hair Infection in Actinomycete-Induced Root Nodule Initiation in Casuarina, Myrica, and Comptonia

Abstract

The infection process leading to the development of root nodules of Comptonia peregrina, Casuarina cunning hamiana, Myrica gale, and M cerifera was studied by light and electron microscopy. Deformed growth of root hairs was observed as early as 24 h after seedlings grown aeroponically or hydroponically were inoculated with suspensions of crushed nodules or cultures of the actinomycetous endophyte of Comptonia The extent of root hair deformation showed a positive correlation with the number of nodules which subsequently developed The essential features of infection in each of these species were very similar. The actinomycete entered a deformed root hair of the host in a region of folding of the cell wall A convoluted elaboration of the root hair wall which occurred at this presumptive penetration site was continuous with the more evenly deposited capsule of the endophytic actinomycete An associated feature of this wall deposition was thickening of the cell wall of the infected root hair and the adjacent prenodular cells. The actinomycete encapsulation was thickest at the presumed site of penetration and thinner in later stages of endophytic growth away from this site. These observations suggest a period of initial disequilibrium caused by the infection, followed by more harmonious symbiotic growth The observation of a morphologically and cytologically similar root hair infection process in these three genera indicates that root hair infection involves a specific and orderly interaction which represents the common mode of invasion in the initiation of actinomycete-induced root nodules