Figure 1.

Internal and external immune responses of S. oryzae bacteriocytes. The internal immune response (solid arrows) consists of the intracellular activity of ColA on endosymbionts. Endosymbiont density is sensed by a still unknown mechanism, and ColA is produced accordingly, which ensures bacterial intracellular seclusion. The external immune response (dotted arrows) consists of the induction of AMP coding gene expression, including colA expression, after recognition of exogenous bacteria. This response is likely to involve an IMD-like pathway. In case of an infection, both responses coexist and do not seem to interfere with each other.