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Diarylheptanoids are secondary metabolites of plants that comprise a C 6 –C 7 –C 6 scaffold. They can be broadly classified into linear-type and cyclic-type diarylheptanoids based on their chemical structures. Actinorhizal trees, such as Casuarina , Alnus , and Myrica , which form nodule symbiosis with actinomycetes Frankia , produce cyclic diarylheptanoids (CDHs); in Alnus sieboldiana Matsum. in particular, we have reported that the addition of CDHs leads to an increase in the number of nodules. However, the information available on the biosynthesis of CDHs is scarce. A greater number of plants CDHs (including those isolated from actinorhizal trees) with a saturated heptane chain have been isolated compared with linear, non-cyclic diarylheptanoids. To identify the genes involved in the synthesis of these compounds, genes with significant sequence similarity to existing plant double-bond reductases were screened in A. sieboldiana. This report describes the isolation and characterization of two A. sieboldiana double-bond reductases (AsDBR1 and AsDBR2) that catalyze the NADPH-dependent reduction of bisdemethoxycurcumin and curcumin. The optimum pH for the two enzymes was 5.0. The apparent K m values for bisdemethoxycurcumin and NADPH were 4.24 and 3.53 μM in the case of AsDBR1, and 2.55 and 2.13 μM for AsDBR2. The k cat value was 9.4-fold higher for AsDBR1 vs. AsDBR2 when using the bisdemethoxycurcumin substrate. Interestingly, the two AsDBRs failed to reduce the phenylpropanoid monomer. [Display omitted] • Two double-bond reductases (AsDBR1 and AsDBR2) were isolated from Alnus sieboldiana. • Two reductases recognized α,β-unsaturated diarylheptanoids as specific substrates. • There was a 9.4-fold difference in turnover number (k cat) between the two reductases. • AsDBRs are the first diarylheptanoid C–C double-bond reductases reported in plants. [ABSTRACT FROM AUTHOR]

The nitrogen-fixing actinomycete Frankia coexists with actinorhizal plants via nodules and supplies nitrogen compounds to the plants. Although communication has been suggested to exist through chemical substances in this nodule symbiosis, the details underlying this mechanism remain elusive. The biphenyl-type diarylheptanoids (BP-CDHs), alnusonol, and alnusdione, previously isolated from the actinorhizal plant A. sieboldiana branch wood, are secondary metabolites that accumulate in a limited number of plant species. However, since relatively widely distributed in actinorhizal plants, we investigated whether adding A. sieboldiana root extracts and these BP-CDHs could affect plant seedlings inoculated with Frankia. The results showed that the addition of root extract or alnusonol significantly increased the number of nodules and lobes more than two times compared with that upon Frankia supplementation only. We also proved that the extracted components of this plant affected nodule symbiosis. Finally, we confirmed through LC–MS that the root extract component contained BP-CDH, alnusonol. The above-described results indicate that BP-CDHs, at leaset alnusonol, might function as signal compounds from the plant side of the actinorhizal symbiosis between A. sieboldiana and Frankia. [Display omitted] • The coexistence of biphenyl-type diarylheptanoids with Frankia to promoted nodule formation in seedlings of A. sieboldiana. • LC-MS analysis of methanol-extracts of A. sieboliana roots revealed the presence of alnusonol, a BP-CDH. • These results suggests that BP-CDH may function as a signaling compound from the plant side in the actinomycete symbiosis between A. sieboldiana and Frankia. [ABSTRACT FROM AUTHOR]