Silencing of the chalcone synthase gene in Casuarina glaucahighlights the important role of flavonoids during nodulation

摘要

Nitrogen-fixing root nodulation is confined to four plant orders, including > 14 000 Leguminosae,one nonlegume genus Parasponia and c. 200 actinorhizal species that form symbioseswith rhizobia and Frankia bacterial species, respectively. Flavonoids have been identified asplant signals and developmental regulators for nodulation in legumes and have long beenhypothesized to play a critical role during actinorhizal nodulation. However, direct evidence oftheir involvement in actinorhizal symbiosis is lacking. Here, we used RNA interference to silence chalcone synthase, which is involved in the firstcommitted step of the flavonoid biosynthetic pathway, in the actinorhizal tropical treeCasuarina glauca. Transformed flavonoid-deficient hairy roots were generated and used tostudy flavonoid accumulation and further nodulation. Knockdown of chalcone synthase expression reduced the level of specific flavonoids andresulted in severely impaired nodulation. Nodule formation was rescued by supplementingthe plants with naringenin, which is an upstream intermediate in flavonoid biosynthesis. Our results provide, for the first time, direct evidence of an important role for flavonoidsduring the early stages of actinorhizal nodulation.