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Published on April 13, 2007; 10.1104/pp.106.095018

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Received December 19, 2006
Accepted April 2, 2007

Flavone Synthases from Medicago truncatula are Flavanone-2-hydroxylases and are Important for Nodulation

Juan Zhang , Senthil Subramanian , Yansheng Zhang , and Oliver Yu *

Donald Danforth Plant Science Center, St. Louis, Missouri 63132

* Corresponding author; email: oyu{at}danforthcenter.org.

Flavones are important co-pigments found in the flowers of many higher plants and play a variety of roles in plant adaptation to stress. In Medicago species, flavones also act as signal molecules during symbiotic interaction with the diazotropic bacterium Sinorhizobium meliloti. They are the most potent nod gene inducers found in root exudates. However, flavone synthase (FNS II), the key enzyme responsible for flavone biosynthesis has not been characterized in Medicago species. We cloned two FNS II genes from M. truncatula using known FNS II sequences from other species and named them MtFNSII-1 and MtFNSII-2. Functional assays in yeast suggested that the catalytic mechanisms of both cytochrome P450 monooxygenases were similar to the other known legume FNS II from Glycyrrhiza echinata. MtFNSII converted flavanones to 2-hydroxyflavanones instead of flavones whereas FNS II from the non-legume Gerbera hybrida, converted flavanones to flavones directly. The two MtFNSII genes had distinct tissue-specific expression patterns. MtFNSII-1 was highly expressed in roots and seeds whereas MtFNSII-2 was highly expressed in flowers and siliques. In addition, MtFNSII-2 was inducible by S. meliloti and methyl jasmonate treatment, whereas MtFNSII-1 was not. Histochemical staining of transgenic hairy roots carrying the promoter-reporter constructs indicated that the MtFNSII-2 induction was tissue-specific, mostly localized to vascular tissues and root hairs. RNA interference-mediated suppression of MtFNSII genes resulted in flavone depleted roots and led to significantly reduced nodulation when inoculated with S. meliloti. Our results provide genetic evidence supporting that flavones are important for nodulation in M. truncatula.




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