Regulation of nodulation in Discaria trinervis (Rhamnaceae)-Frankia symbiosis

Nodulation in Discaria trinervis (Hook. et Arn.) Reiche was mainly located around the position of the taproot tip at the moment of inoculation with Fr ankia. Nodule distribution, but not final level of nodulation, was affected by the inoculum dose and the culture age of Frankia. Taproot inoculation r esulted in distal suppression of nodulation of the growing root as early as 3 days after inoculation that is, before the first nodules could be detect ed. Systemic inhibition in split root systems was maximal, but not complete , for a delay of 20 days between inoculations on both sides. Reinoculation of 9.5-week-old nodulated D. trinervis plants did not cause further nodulat ion. Nevertheless, nodule excision, with or without new inoculation, allowe d the plant to develop new nodules not only at the infectible region of the young developing root bur also in the region of prior existing nodules, wh ere we observed arrested nodules at an early developmental stage. We conclu de that root nodulation in D. trinervis might be controlled by two differen t pathways that operate through inhibition of infection and nodule developm ent, One pathway is activated immediately after the first stages of root ce ll division are induced because of root inoculation with Frankia. The inhib ition becomes systemic and is widespread in the root system before host cel l invasion is carried out at the infection sites. The second pathway requir es the presence of mature and N-2-fixing nodules.