3.5 Iridoids biosynthesis
Iridoids of S. tetraptera comprise loganin, Sinoswertiamatin,
sweroside, and gentiopicroside. We measured their concentrations in
seven tissues. The content of each iridoid in the seven tissues showed
similar variation trends to those reported previously (Yue Liu et al.,
2017). Four iridoids had the highest content in CL flowers out of the
seven tissues examined, and their contents in these flowers were all
obviously higher than those in leaves from the branch on which the CL
flowers were growing (termed the ”CL leaf”) (Figure 5a, Table S27). The
regulatory pathway of the biosynthesis of iridoids has been reported to
consist of the downstream seco-iridoids pathway and the upstream
2-C-methyl-D-erythritol 4- phosphate (MEP) and mevalonate (MVA) pathways
(Kellner, Kim, Clavijo, Hamilton, Childs, Vaillancourt, Cepela,
Habermann, Steuernagel, Clissold, Mclay, et al., 2015; Yue Liu et al.,
2017; Vranová, Coman, & Gruissem, 2013). We identified a total of 56
candidate genes for these two biosynthesis pathways and assessed their
expression levels in different tissues for subsequent analyses (Table
S28-29). As the primary supplier of intermediate products in the
biosynthesis of iridoids, the MEP pathway acts mainly in leaves (Oudin,
Courtois, Rideau, & Clastre, 2007; Vranová et al., 2013). However,
these iridoid products are always concentrated in the flowers ofSwertia mussotti (Yue Liu et al., 2017) and S. tetraptera .
Therefore, the intermediate products of these iridoids may be
transported from leaves to flowers in S. tetraptera .
We reconstructed a weighted gene co-expression network for iridoid
biosynthesis pathways based on intersections of DEGs containing 8067
genes (618 TFs and 7449 structural genes) for the seven tissues.
A total of 12 modules were
clustered, and module 8 (blue) was indeed related to leaves, while
module 9 (turquoise) was linked to flowers (Figure 5b, Figure S23-24).
Many of the previously identified 56 candidate genes for biosynthesis of
iridoids were clustered into these two modules. The leaf-related module
contained most genes belonging to the MEP pathway
(Figure 5b). Most of them showed
higher expression levels in leaves than in flowers (Figure 5c, Figure
S25-26). However, the candidate genes in the MVA pathway were mainly
clustered in the flower-related module and highly expressed in flowers
(Figure 5b,5c, Figure S25-26). Similar patterns were also revealed from
the correlation analysis between co-expression network modules and the
measured iridoid contents (Figure S27). These different clustered
networks and differentially expressed genes in the two tissues may
suggest the potential genetic basis for the distinct function between
leaves and flowers for iridoid production and transport. Furthermore, we
found that the genes from the seco-iridoids pathway were distributed in
both modules. Of these, two SLS genes were clustered as the hub
genes (Figure 5b). They may play an essential role in transporting
intermediate products during iridoid biosynthesis between different
tissues in S. tetraptera .