Regulation of nodulation in the absence of N2 is different in actinorhizal plants with different infection pathways

WALL LG; VALVERDE C; HUSS-DANELL K

JOURNAL OF EXPERIMENTAL BOTANY

OXFORD UNIV PRESS

Año: 2003 p. 1253 - 1258

0022-0957

Root nodulation in actinorhizal plants, like Discaria trinervis and Alnus incana, is subjected to feedback regulatory mechanisms that control infection by Frankia and nodule development. Nodule pattern in the root system is controlled by an autoregulatory process that is induced soon after inoculation with Frankia. Final number of nodules, as well as nodule biomass in relation to plant biomass, are both modulated by a second mechanism which seems to be related to the N status of the plant. Mature nodules are in part involved in the latter process, since nodule excision from the root system release the inhibition of infection and nodule development. To study the effect of N2 fixation in this process, nodulated D. trinervis and A. incana plants were incubated under a N2-free atmosphere . Discaria trinervis is an intercellularly infected species while A. incana is infected intracellularly, via root hairs. Both symbioses responded with an increment in nodule biomass, but with different strategies.  Discaria trinervis increased biomass of existing nodules   without significant development of new nodules, while in A. incana nodule biomass increased due to development of nodules from new infections , but also from release of arrested infections. It appears that in D. trinervis nodules there is an additional source for inhibition of new infections and nodule development  that is independent of N2 fixation and nitrogen assimilation. We propose that the  intercellular Frankia filaments commonly present in the D. trinervis nodule apex, is the origin for the autoregulatory signals that sustain the blockage of initiated nodule primordia and prevent new root from infections. When turning to A. incana plants, it seems likely this signal related to early autoregulation of nodulation in A. incana seedlings, is no longer present in mature nodules. Thus, actinorhizal symbioses belonging to relatively distant phylogenetic groups and displaying different infection pathways, show different feedback regulatory processes that control root nodulation by Frankia.