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

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Abstract
Nodulation in Discaria trinervis (Hook. et Arn.) Reiche was mainly located around the position of the taproot tip at the moment of inoculation with Frankia. Nodule distribution, but not final level of nodulation, was affected by the inoculum dose and the culture age of Frankia. Taproot inoculation resulted in distal suppression of nodulation of the growing root as early as 3 days after inoculation, that is, before the first nodules could be detected. 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 nodulation. Nevertheless, nodule excision, with or without new inoculation, allowed the plant to develop new nodules not only at the infectible region of the young developing root but also in the region of prior existing nodules, where we observed arrested nodules at an early developmental stage. We conclude that root nodulation in D. trinervis might be controlled by two different pathways that operate through inhibition of infection and nodule development. One pathway is activated immediately after the first stages of root cell division are induced because of root inoculation with Frankia. The inhibition becomes systemic and is widespread in the root system before host cell invasion is carried out at the infection sites. The second pathway requires the presence of mature and N2-fixing nodules.Key words: actinorhiza, autoregulation, Discaria trinervis, feedback inhibition, Frankia, intercellular infection.