New perspectives on nodule nitrogen assimilation in actinorhizal symbioses

Abstract

Nitrogen-fixing root nodules are plant organs specialised for symbiotic transfer of nitrogen and carbon between microsymbiont and host. The organisation of nitrogen assimilation, storage and transport processes is partitioned at the subcellular and tissue levels, in distinctive patterns depending on the symbiotic partners. In this review, recent advances in understanding of actinorhizal nodule nitrogen assimilation are presented. New findings indicate that Frankia within nodules of Datisca glomerata (Presl.) Baill. carries out both primary nitrogen assimilation and biosynthesis of arginine, rather than exporting ammonium. Arginine is a typical storage form of nitrogen in plant tissues, but is a novel nitrogen carrier molecule in root nodule symbioses. Thus Frankia within D. glomerata nodules exhibits considerable metabolic independence. Furthermore, nitrogen reassimilation is likely to take place in the host in the uninfected nodule cortical cells of this root nodule symbiosis, before amino acid export to host sink tissues via the xylem. The role of an augmented pericycle in carbon and nitrogen exchange in root nodules deserves further attention in actinorhizal symbiosis, and further highlights the importance of a comprehensive, structure–function approach to understanding function in root nodules. Moreover, the multiple patterns of compartmentalisation in relation to nitrogen flux within root nodules demonstrate the diversity of possible functional interactions between host and microsymbiont that have evolved in the nitrogen-fixing clade.

Do plants need nitrate? The mechanisms by which nitrogen form affects plants

“…Generally, NH 4 + produced via N 2 fixation is transported from the rhizobial bacteroids and Frankia endophytes to the plant root cells where it is assimilated into amino acids via the GS/GOGAT pathway (Betti et al ., ; Seabra et al ., ). However, evidence is strong that Frankia carries out both N 2 fixation and the biosynthesis of arginine within the nodules of Datisca glomerata (Berry et al ., ). It has also been suggested that alanine is a major transport compound from the bacteroids in soybean, possibly through the action of alanine dehydrogenase (Waters et al ., ), but this has not been confirmed (White et al ., ; Mulley et al ., ).…”

Aspects of nitrogen-fixingActinobacteria, in particular free-living and symbioticFrankia

“…Actinorhizal plants fall into three phylogenetically related groups: Fagales (Betulaceae, Casuarinaceae, Myricaceae), Cucurbitales (Datiscaceae, Coriariaceae) and Rosales (Rosaceae, Elaeagnaceae, Rhamnaceae) (Berry et al, 2011). Together with the legumes (Fabales), actinorhizal plants form a single nitrogen-fixing clade, suggesting a common evolutionary origin for the underlying capacity to evolve root nodule symbiosis (Soltis et al, 1995).…”

“…The actinorhizal plants that become infected by Gram-positive Frankia sp. are members of three phylogenetically related groups: Fagales (Betulaceae, Casuarinaceae, Myricaceae), Cucurbitales (Datiscaceae, Coriariaceae) and Rosales (Rosaceae, Elaeagnaceae, Rhamnaceae) (Berry et al, 2011). Many actinorhizal plants are pioneers of disturbed sites and early successional sites and therefore have ecologically important impacts on late successions.…”

Asparagine: an amide of particular distinction in the regulation of symbiotic nitrogen fixation of legumes

“…Data derived fromBerry et al (2011) and expressed as % total amino acids.Critical Reviews in Biotechnology Downloaded from informahealthcare.com by Library of Health Sci-Univ of Il on 10/19/12For personal use only.…”