Studies of thick-leaf Yerba Santa have shown the presence of secondary metabolites useful in human health; sterubin and eriodictyol are produced by the plant. During May 2022, we sampled tissue and soil from a stand of Eriodictyon crassifolium leaf and fruit following a drought winter in the Gold Creek Preserve, Angeles National Forest. cDNA libraries were sequenced at BGIA; metagenomic sequencing was conducted by BGIA using DNBseq. DNA from rootzone soil samples was sent to JMU for sequencing using the Illumina MiSeq platform. The bioinformatics pipeline for RNAseq consisted of QC, de novo assembly, evaluation, quantification, and annotation. Our metabarcoding analysis used DNA Subway Purple Line, implementing QIIME and DADA2. Our metagenomic data analysis used Nephele BioBakery and analyzed the data with MicrobiomeDB and STAMP.

The BUSCO assessment indicated that the E. crassifolium transcriptome assembly was 94.88% complete. Discontiguous megaBLAST results revealed secondary metabolite candidate genes in related species that have antioxidant functions. The transcript for 4-coumarate ligase was 80.45% similar to Arnebia euchroma. The candidate gene for Trans-cinnamate-4-monooxygenase was 82.07% similar to A. euchroma. Delta-8-fatty acid desaturase was 77.59% similar to Borago officinalis. The annotations included antioxidant-related transcripts that were previously uncharacterized in Boraginaceae, Namaceae, or Eriodictyon. Among these were Chalcone synthase (92.031% similar to Vitis), Tryptophan synthase beta chain 2 (94.551% similar to Camptotheca), and Shikimate-O-hydroxycinnamoyl transferase (87.356% similar to Nicotiana).

In Yerba Santa, there was an overlap between highly expressed genes involved in flavonoid and aromatic compound production and rhizosphere bacteria with genes for producing similar compounds. In the rootzone, there were increased Pedobacter and Pseudomonas reads identified within the 16S metabarcoding reads. According to the STAMP output, in the Yerba Santa rootzone, L-Tryptophan biosynthetic gene counts were elevated in the rootzone bacteria, along with Flavin biosynthesis I genes. Further studies should consider the cross-validation of elevated secondary metabolite genes with reference sequences from closely related species.