Transcriptional and functional characterization in the terpenoid precursor pathway of the early land plant <i>Physcomitrium patens</i>

Isoprenoids comprise the largest group of plant specialized metabolites. 1-deoxy-D-xylulose-5-phosphate synthase (DXS) is one of the major rate-limiting enzymes in their biosynthesis. The DXS family expanded structurally and functionally during evolution and is believed to have significantly contributed to metabolic complexity and diversity in plants. This family has not yet been studied in Physcomitrium patens or other bryophytes. Here, we assessed the degree of evolutionary expansion in the DXS family in bryophytes and, more specifically, in P. patens using phylogenetic analysis. Transcriptome profiling was applied to investigate tissue-specific, developmental, and environmental responses, such as salt stress, in the DXS family. Moreover, the effect of salt stress on terpenoid biosynthesis was monitored through metabolomics. The phylogenetic analysis of DXS revealed that a structural expansion occurred in bryophytes, but not in P. patens. Functional complementation assay revealed functional activity in all four copies. Comparative transcriptomics showed tissue- and condition-specific divergence in the expression profiles of DXS copies and demonstrated specific stress responses for PpDXS1D, particularly to salt stress. These findings coincide with increased flux in the pathway towards downstream metabolites under salt stress. Additionally, co-expression network analysis revealed significant differences between the co-expressed genes of the DXS copies and illustrated enrichment of stress-responsive genes in the PpDXS1D network. These results suggest that the DXS family in P. patens is conserved but undergoes differential transcriptional regulation, which might allow P. patens to fine-tune DXS levels under different conditions, such as abiotic stress.