Flavan-containing cells delimit Frankia-infected compartments in Casuarinaglauca nodules

We investigated the involvement of polyphenols in the Casuarina glauca-Fran kia symbiosis. Histological analysis revealed a cell-specific accumulation of phenolics in C. glauca nodule lobes, creating a compartmentation in the cortex. Histochemical and biochemical analyses indicated that these phenoli c compounds belong to the flavan class of flavonoids. We show that the same compounds were synthesized in nodules and uninfected roots. However, the a mount of each flavan was dramatically increased in nodules compared with un infected roots. The use of in situ hybridization established that chalcone synthase transcripts accumulate in flavan-containing cells at the apex of t he nodule lobe. Our findings are discussed in view of the possible role of flavans in plant-microbe interactions.