Overexpression of Terpenoid Biosynthesis Genes From Garden Sage (Salvia officinalis) Modulates Rhizobia Interaction and Nodulation in Soybean

In legumes, many endogenous and environmental factors affect root nodule formation through several key genes, and the regulation details of the nodulation signaling pathway are yet to be fully understood. This study investigated the potential roles of terpenoids and terpene biosynthesis genes on root nodule formation in Glycine max. We characterized six terpenoid synthesis genes from Salvia officinalis by overexpressing SoTPS6, SoNEOD, SoLINS, SoSABS, SoGPS, and SoCINS in soybean hairy roots and evaluating root growth and nodulation, and the expression of strigolactone (SL) biosynthesis and early nodulation genes. Interestingly, overexpression of some of the terpenoid and terpene genes increased nodule numbers, nodule and root fresh weight, and root length, while others inhibited these phenotypes. These results suggest the potential effects of terpenoids and terpene synthesis genes on soybean root growth and nodulation. This study provides novel insights into epistatic interactions between terpenoids, root development, and nodulation in soybean root biology and open new avenues for soybean research.

FrankiaDiversity in Host Plant Root Nodules Is Independent of Abundance or Relative Diversity ofFrankiaPopulations in Corresponding Rhizosphere Soils

ABSTRACTActinorhizal plants form nitrogen-fixing root nodules in symbiosis with soil-dwelling actinobacteria within the genusFrankia, and specificFrankiataxonomic clusters nodulate plants in corresponding host infection groups. In same-soil microcosms, we observed that some host species were nodulated (Alnus glutinosa,Alnus cordata,Shepherdia argentea,Casuarina equisetifolia) while others were not (Alnus viridis,Hippophaë rhamnoides). Nodule populations were represented by eight different sequences ofnifHgene fragments. Two of these sequences characterized frankiae inS. argenteanodules, and three others characterized frankiae inA. glutinosanodules. Frankiae inA. cordatanodules were represented by five sequences, one of which was also found in nodules fromA. glutinosaandC. equisetifolia, while another was detected in nodules fromA. glutinosa. Quantitative PCR assays showed that vegetation generally increased the abundance of frankiae in soil, independently of the target gene (i.e.,nifHor the 23S rRNA gene). Targeted Illumina sequencing ofFrankia-specificnifHgene fragments detected 24 unique sequences from rhizosphere soils, 4 of which were also found in nodules, while the remaining 4 sequences in nodules were not found in soils. Seven of the 24 sequences from soils represented >90% of the reads obtained in most samples; the 2 most abundant sequences from soils were not found in root nodules, and only 2 of the sequences from soils were detected in nodules. These results demonstrate large differences between detectableFrankiapopulations in soil and those in root nodules, suggesting that root nodule formation is not a function of the abundance or relative diversity of specificFrankiapopulations in soils.IMPORTANCEThe nitrogen-fixing actinobacteriumFrankiaforms root nodules on actinorhizal plants, with members of specificFrankiataxonomic clusters nodulating plants in corresponding host infection groups. We assessedFrankiadiversity in root nodules of different host plant species, and we related specific populations to the abundance and relative distribution of indigenous frankiae in rhizosphere soils. Large differences were observed between detectableFrankiapopulations in soil and those in root nodules, suggesting that root nodule formation is not a function of the abundance or relative diversity of specificFrankiapopulations in soils but rather results from plants potentially selecting frankiae from the soil for root nodule formation. These data also highlight the necessity of using a combination of different assessment tools so as to adequately address methodological constraints that could produce contradictory data sets.