Fig. 1

Infection thread formation. a A rhizobium cell (arrow) makes contact close to the growing tip of a root hair (rh). b Nod factors produced by the rhizobia trigger the tip of the root hair to curl around the bacterial cell, creating an enclosed pocket known as the infection chamber. c The infection chamber expands inwards as the bacteria begin to divide; exocytotic activity (yellow) at the root hair tip shifts to surround the chamber, secreting a matrix (cyan) into the infection chamber. d The infection chamber expands further as bacteria continue to divide, and the infection thread initiates by an inverted tip-growth from a localised weakening of the chamber wall. e The infection thread elongates through the root hair, preceded by the nucleus which maintains cytoplasmic streaming to the infection thread tip. f The infection thread fuses with the basal wall of the root hair, and bacteria are released into the intracellular space. g Localised areas of adjacent cortical cells are weakened and expand inwards to form new infection threads. h Cortical infection threads continue to elongate, often branching; and reforming in adjacent cells to carry rhizobia further into the developing nodule. Infection thread formation is varied within and between species, and the specific variants shown here—such as the ‘shepherds crook’ root hair curl, and the formation of two cortical threads from the junction of three cells—were chosen to best illustrate the main stages of formation. rh root hair, cc cortical cell