GA signaling involved in the biosynthesis of tanshinones and
SAs
GA is a diterpene phytohormone that modulates growth and development
throughout the whole life cycle of the plant (Sun, 2011). A study has
shown that GA could promote the accumulation of tanshinones in the
wild-type hairy roots of S. miltiorrhiza (Bai et al. ,
2017), but its regulatory mechanisms are poorly understood. We found the
SmGRAS5 was the most sensitive genes to GA response of the five SmGRASs
genes in our previous study (Bai et al. , 2017). We speculated
that it might play a major role in SmGRASs in the GA signaling. As a key
regulator of GA signaling, GRAS has been classified into 13 different
subgroups in Arabidopsis (Zhang et al ., 2018). Our
phylogenetic analysis showed SmGRAS5 belonged to SCL3 subfamily, which
is involved in the regulation of Arabidopsis root cell
elongation, GA homeostasis and signaling (Heo et al. , 2011).
AtSCL3 inhibited GA biosynthesis and antagonized with DELLA in
maintaining GA homeostasis by feedback regulating upstream GA
biosynthesis genes, as well as in modulating downstream GA responses by
direct protein-protein interaction (Zhang et al. , 2011). DELLA
protein interacted with AtMYB12 activating expression of flavonol
biosynthetic genes (Tan et al. , 2019). The tissue-specific
expression of GRAS genes often indicates their functional roles
in development. It has been reported that diterpenoid tanshinones not
only accumulated but also biosynthesized in the roots periderm ofS. miltiorrhiza (Xu et al. , 2015). We found thatSmGRAS5 highly expressed in the periderm and co-localized with
tanshinones, which indicated that SmGRAS5 might be involved in the
regulation of tanshinones biosynthesis. Overexpression of SmGRAS5significantly improved tanshinones accumulation through upregulating the
expressions of biosynthetic genes, while in the SmGRAS5 AE lines
the tanshinones accumulation was decreased. Notably, the promotion of
SmGRAS5 to the tanshinones biosynthesis is the most significant in all
of the SmGRASs, which we are currently studying. So we speculated that
it might play a vital role in the regulation of tanshinones
biosynthesis. Considering the correlation between tanshinones and GA
biosynthetic pathways, we found that the expressions of the most of GA
biosynthetic pathway genes were downregulated, which resulted in the
decrease of GA content in the SmGRAS5 OE lines. Similarly,
overexpressingHaGRASL also reduced the metabolic flow of GAs inArabidopsis (Fambrini et al. , 2015). SilencingSlGRAS26 inhibited the GA biosynthetic pathway, promoted the GA
inactivation pathway, and resulted in GA deficiency in tomato (Zhouet al. , 2018). SlGRAS24 could downregulate GA biosynthetic
genes, disrupt GA homeostasis and participate in a series of
developmental processes through modulating GA and auxin signaling (Huanget al. , 2017). We speculated that SmGRAS5 might regulate the
metabolic flow of diterpenoid GA and tanshinones biosynthesis. In
addition, the GA application could promote the tanshinones biosynthesis
in SmGRAS5 OE lines as well as control lines. Therefore, these
results implied that SmGRAS5 could be induced by GA and might catalyze
the precursor GGPP to synthesize more tanshinones. Moreover,
transcriptome data also revealed that SmGRAS5 could regulate many
secondary metabolites biosynthesis. It could inhibit the GA biosynthetic
and signaling pathway at the same time, and induce tanshinones
biosynthetic pathway through regulating biosynthetic genes.