Genome-wide transcriptional and metabolic responses of Eschscholzia californica to salt stress

Eschscholzia californica, as an abiotic stress-tolerant ornamental plant, has long been studied for phylogenetic analysis and biosynthesis of benzylisoquinoline alkaloids. However, little is known about its environmental adaptation mechanisms. In this study, we performed comprehensive investigations on responses of E. californica to salt stress, one of serious threats to modern agriculture. The effects of 250 mM NaCl solution treatment on plant growth and several stress response related indices were investigated at 0d, 7d, and 10d, representing non-stressed (CK), mildly-stressed (St0), and severely-stressed (St1), respectively. RNA-seq analysis revealed salt-stress responsive and differentially expressed genes (DEGs) enriched in photosynthesis-related pathways, phenylpropanoid biosynthesis, ɑ-Linolenic acid metabolism, isoquinoline alkaloid biosynthesis, benzoxazinoid biosynthesis, etc. With the aggravation of salt stress, pathways of the tryptophan metabolism, plant hormone signal transduction, ɑ-Linolenic acid metabolism, and isoquinoline alkaloid biosynthesis, etc., were more significantly enriched. We also identified DEGs involved in intracellular ion homeostasis, osmotic regulation, oxidative stress and detoxification, plant hormone biosynthesis and signaling, as well as those encoding transcription factors. Furthermore, Comparison of metabolites in CK and St1 showed that the differentially accumulated metabolites under salt stress were mainly alkaloids (23.6%), phenolic acids (16.5%), lipids (15.2%), organic acids (8.1%), amino acids and their derivatives (7.6%), flavonoids (6.5%), etc. Further joint analysis of transcriptome and metabolome data revealed key pathways in responding to salt stress. The jasmonic acid biosynthesis and signaling and isoquinoline alkaloid biosynthesis were particularly noteworthy due to their extremely significant up-regulation by salt stress and unclear or debatable roles in regulating salt stress tolerance. The DEGs in the two pathways, such as AOX, LOX, and JAZs, as well as NCS1, 6OMT, TNMT, and BBEs, etc., are therefore valuable for uncovering the functional roles of two pathways in salt stress tolerance of E. californica, and future breeding salt-tolerant crops.