Infection threads from Rhizobium on Medicago (Dan Gage)
Frankia infect host plants by root hair infection (RHI) (Alnus, Casuarina, Comptonia, Myrica; (Berry, 1983; Callaham, 1979; Torrey, 1976), or by direct intercellular penetration (IP) of root epidermis cells and cortex (Ceanothus, Elaeagnus, Shepherdia; (Liu, 1991; Miller, 1985; Racette, 1989).
RHI is preceded by root hair branching and curling. Cell division is stimulated in the hypodermis and cortex leading to the formation of a "prenodule". Frankia penetrates and passes into the cortex of the root. The nodule develops like a lateral root. Nodule lobe primordia arises in the pericycle, endodermis, or cortex during prenodule development. Frankia enters the developing nodule lobe primordia cells to form the infected nodule.
During IP infection (Elaeagnus, Shepherdia and Ceanothus) root hairs are either not present or are not infected. Frankia becomes encapsulated in a pectic capsule extending through the intercellular spaces.
Some strains can infect both Elaeagnus and Myrica, or both Shepherdia and Gymnostoma. They enter via the route dictated by the host (Miller, 1986; Racette, 1989).
As Frankia hyphae enter the root hair they become encapsulated with plant cell wall-like material and remain outside the host plasmalemma. Encapsulation surrounds hyphae during invasion and vesicles in the mature nodule (Lalonde et al., 1975, 1977). The encapsulation apparently contains pectin in Alnus (Lalonde, 1975), and also contains cellulose, hemicellulose (xylans). At least for Alnus and Casuarina, the capsule can be envisioned as a tube consisting of plant cell wall polymers (Berg, 1990). In Ceanothus the capsule has a high amount of polygalacturonans and appears to be distinct from the cell wall (Liu et al, 1991).