Assessment of Polygala paniculata (Polygalaceae) characteristics for evolutionary studies of legume–rhizobia symbiosis


Root nodule (RN) symbiosis is a mutualistic interaction observed between nitrogen-fixing soil bacteria and nodulating plants, which are scattered in only four orders of angiosperms called nitrogen-fixing clade. Most of legumes engage in RN symbiosis with rhizobia. Molecular genetic analyses with legumes and non-leguminous nodulating plants revealed that RN symbiosis utilizes early signalling components that are required for symbiosis with arbuscular mycorrhizal (AM) fungi. However detailed evolutionary processes are still largely unknown. Comparative analyses with non-nodulating species phylogenetically related to legumes could be better strategies to study the evolution of RN symbiosis in legumes. Polygala paniculata is a non-leguminous species that belongs to a family different from legumes but that is classified into the same order, Fabales. It has appropriate characteristics for cultivation in laboratories: small body size, high fertility and short lifecycles. Therefore, we further assessed whether this species is suitable as a model species for comparative studies with legumes. We first validated that the plant we obtained in Palau was truly P. paniculata by molecular phylogenetic analysis using rbcL sequences. The estimated genome size of this species was less than those of two model legumes, Lotus japonicus and Medicago truncatula. We determined conditions for cultivation in vitro and for hairy root formation from P. paniculata seedlings. It would facilitate to investigate gene functions in this species. The ability of P. paniculata to interact with AM fungi was confirmed by inoculation with Rhizophagus irregularis, suggesting the presence of early signalling factors that might be involved in RN symbiosis. Unexpectedly, branching of root hairs was observed when inoculated with Mesorhizobium loti broad host range strain NZP2037, indicating that P. paniculata has the biological potential to respond to rhizobia. We propose that P. paniculata is used as a model plant for the evolutionary study of RN symbiosis.

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The seed samples were permitted to export by Palau Government with the certificate (No. ROP-018-2014). Authors thank Ms. Sachiko Tanaka (NIBB) for preliminary experiments and kind support on germination condition, seedling growth and seed harvesting. We thank Mr. Naoki Morooka and model plant research facility at NIBB for the analyses of genome size using CyFlow SL. This study was financially supported by Grant-in-Aid for Scientific Research on Innovative Areas (16H01248 to HF and 16H06279 to WI) of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and Grant-in-Aid for Scientific Research (C) (16K08149 to TS and 17K07509 to SA) of Japan Society for the Promotion of Science.

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Correspondence to Masayoshi Kawaguchi.

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Tokumoto, Y., Hashimoto, K., Soyano, T. et al. Assessment of Polygala paniculata (Polygalaceae) characteristics for evolutionary studies of legume–rhizobia symbiosis. J Plant Res 133, 109–122 (2020) doi:10.1007/s10265-019-01159-x

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  • Fabales
  • Genome size
  • Hairy root
  • Polygala paniculata
  • rbcL
  • RN symbiosis
  • Root hair response