Progress on research on actinorhizal plants

In recent years, our understanding of the plant side of actinorhizal symbioses has evolved rapidly. No homologues of the common nod genes from rhizobia were found in the three Frankia genomes published so far, which suggested that Nod factor-like molecules would not be used in the infection of actinorhizal plants by Frankia. However, work on chimeric transgenic plants indicated that Frankia Nod factor equivalents signal via the same transduction pathway as rhizobial Nod factors. The role of auxin in actinorhizal nodule formation differs from that in legume nodulation. Great progress has been made in the analysis of pathogenesis-related and stress-related gene expression in nodules. Research on nodule physiology has shown the structural and metabolic diversity of actinorhizal nodules from different phylogenetic branches. The onset of large-scale nodule transcriptome analysis in different actinorhizal systems will provide access to more information on the symbiosis and its evolution.

Phenotypic characterization of sym8 and sym9, two genes conditioning non-nodulation in Pisum sativum 'Sparkle'

Two non-nodulating and non-allelic mutants of P. sativum (L.) 'Sparkle', R25 (sym8) and R72 (sym9), were obtained by γ-radiation. Non-nodulation in each mutant was conditioned by a single recessive gene. Growth analysis indicated that if the mutants were provided with adequate fixed nitrogen, they both had the same growth rate as their nodulating parent, 'Sparkle'. Reciprocal grafts between mutants and parent plants indicated that non-nodulation was controlled by the root genotype. Both mutants induced the common nod genes in rhizobia, and had either higher or similar numbers of rhizobia attached to their roots as had 'Sparkle'. Microscopic studies indicated that root hairs of R72 deformed after inoculation, whereas root hairs of R25 did not show such deformation. In both mutants hair curling, infection thread formation, and nodule meristem initiation did not occur. Key words: mutants, sym genes, nodulation stages, symbiosis.