The actinorhizal symbiosis (Review written by invitation of the editor Ann Hirsch, UCLA, USA)

WALL LG

JOURNAL OF PLANT GROWTH REGULATION

SPRINGER

Año: 2000 p. 167 - 182

0721-7595

The term actinorhiza refers to the actinomycete nature of the microsymbiont, the filamentous bacteria Frankia, and to the root location of the nitrogen fixing nodules. Actinorhizal plants are classified in four subclasses, eight Families and 25 genera comprising more than 220 species. The host diversity allows their use for different purposes. Although being related to lateral root origin, many differentially expressed genes have been detected in actinorhizal nodules supporting the idea of the uniqueness of this new organ. Two pathways for root infection by compatible Frankia and nodule development exits: root hair infection or intercellular penetration. Morphological and anatomical features of actinorhizal nodules reinforce their molecular phylogeny. Four clades of actinorhizal plants can be defined. Meanwhile Frankia molecular phylogeny separates three major groups. Although the phylogenies of the symbionts are not full congruent, a close relationship exists between the plants and bacterial groups. A model for actinorhizal specificity is proposed that include different levels or degrees of specificity, from full compatible interactions to incompatible ones. In between of both situations, compatible but delayed or limited interactions are discussed. Actinorhizal plants show feed back regulation of symbiosis. At least two different and consecutive signals lead to a mechanism which control root nodulation by opening or closing the window of susceptibility for infection, and inhibiting infection and nodule development in the growing root, despite their different infection mechanism. The need for at least two molecular recognition steps for actinorhizal symbioses is finally discussed.