Nitrogen Fixation as a Stress-Avoidance Strategy Among Actinorhizal (Non-Legume) Trees and Shrubs

Summary Actinorhizal species of trees and shrubs are a diverse group of plants that may survive in poor soils by virtue of their associations with the actinomycete Frankia. These species include several important woody plants that are well suited for horticultural use in temperate climates. The symbiosis between Frankia and actinorhizal species shows some similarity to symbioses between rhizobia and woody legumes, and a common ancestor has been proposed for the predisposition to root-nodule symbiosis. Despite their probable common origin, important differences exist between actinorhizal and leguminous symbioses; characteristics of the microsymbiont, nodule architecture, and mechanisms controlling oxygen relations of the nodule are among the ways the two systems differ. If nitrogen fixation is sustained under unfavorable conditions, woody plants that associate with nitrogen-fixing organisms may show enhanced tolerance of environmental stress; species of plants capable of nitrogen-fixing symbioses are known to have comparatively strong resistance to invasion by pathogens. Expansion of the capacity to form nitrogen-fixing symbioses to novel species is a goal of those concerned with the economic and ecological impact of chemical fertilizers. Small in-roads have been made in this regard, but much remains to be discovered about introducing nitrogen fixation to additional species. Herein we review biological aspects of actinorhizal symbioses; consider the horticultural potential of temperate, woody species that form these symbioses; and discuss how nitrogen-fixing symbiosis may impact the stress resistance and use of actinorhizal species as horticultural crops.