Figure 4.46 Rhizobial and mycorrhizal colonization. a | Flavonoids released by the legume root signal to rhizobia in the rhizosphere, which in turn produce Nod factors that are recognized by the plant. Nod factor perception activates the symbiosis signalling pathway, leading to calcium oscillations, initially in epidermal cells but later also in cortical cells preceding their colonisation (See Figure 3.21). Rhizobia gain entry by infection root hair cells that grow around the bacteria attached at the root surface, trapping the bacteria inside a root hair curl. Infection threads are invaginations of the plant cell that are initiated at the site of root hair curls and allow invasion of the rhizobia into the root tissue. The nucleus relocates to the site of infection, and an alignment of ER and cytoskeleton, known as the pre-infection thread, predicts the path of the infection thread. Nodules initiate below the site of bacterial infection and form by de novo initiation of a nodule meristem in the root cortex. The infection threads grow towards the emergent nodules and ramify within the nodule tissue. In some cases, the rhizobia remain inside the infection threads, but more often, the bacteria are released into membrane-bound compartments inside the cells of the nodule, where the bacteria can differentiate into a nitrogen-fixing state. b | Strigolactone release by the plant root signals to arbuscular mycorrhizal fungi (AMF) in the rhizosphere. Perception of strigolactones promotes spore germination and hyphal branching. AMF produce mycorrhizal factors (Myc factors), including lipochitooligosaccharide (LCOs) and, possibly, signals that activate the symbiosis signalling pathway in the root, leading to calcium oscillations. AMF invasion involves an infection peg from the hyphopodium that allows fungal hyphal growth into the root epidermal cell. The route of hyphal invasion in the plant cell is predicted by a pre-penetration apparatus, which is a clustering of ER and cytoskeleton in a zone of the cell below the first point of fungal contact. The fungus colonizes the plant root cortex through intercellular hyphal growth. Arbuscules are formed in inner root cortical cells from the intercellular hyphae. (Reproduced by permission from Macmillan Publishers Ltd from G.E.D. Oldroyd, Nature Rev Microbiol 11: 252-632, 2013. Part b image is modified by permission from M. Parniske Nature Rev Microbiol 6: 736-775, 2008)