## 3 Discussion
### AtMYB2 Promotes Tolerance to Salt Stress by Improving Antioxidant System Function
The aforementioned findings suggest that AtMYB2 boosts the activity of ROS-scavenging enzymes while reducing damage to the lipid structure of plant cell membranes under salt stress, thereby promoting tolerance to salt stress in _S. multiorrhiza_ by improving antioxidant system function. This once again demonstrated the important role of _AtMYB2_ in plant response to salt stress, which involves responding to the damage caused by salt stress through various different pathways. It can be seen that _AtMYB2_ plays an important role in plant response to high-salinity environmental stress. Consequently, studying and using it well is conducive to the development, protection, and utilization of plants in harsh environments. For _Salvia multiorrhiza_, _AtMYB2_ is a good candidate gene for improving salt tolerance, which has positive significance for molecular breeding to improve the quality of _Salvia multiorrhiza_. AtMYB2 Promotes Tanshinones and Salvianic Acid Content by Upregulation the Expression of Biosynthesis Genes  
Secondary metabolites often accumulate in specific tissues [41]. For instance, artemisinin biosynthesis and accumulation occur in glandular trichomes, while gossypol primarily accumulates in pigment glands in cotton [42]. In the case of _S. multiorrhiza_, roots serve as the main site for the accumulation of tanshinones and phenolic acids. In _Arabidopsis_, a number of genes have been proven to regulate the content of tanshinones and salvianic acid. _AtMYC2_ positively contributes to the accumulation of both tanshinones and phenolic acids. In the most favorable transgenic line, _AtMYC2-3_, the tanshinone yield reached 14.06 mg/g dw, marking a remarkable 5.45-fold increase compared to the control line. Similarly, phenolic acid production was enhanced to 95.9 mg/g dw, representing a significant 3.3-fold increase compared to the control [43]. Moreover, ectopic expression of _AtWRKY18_, _AtWRKY40_, and _AtMYC2_ resulted in increased production of abietane diterpenes, such as carnosic acid and salvipisone, in hairy roots of _Salvia sclarea_. These transcription factors were capable of coordinating the regulation of several genes involved in the MEP-derived terpene pathway [44]. In addition, _AtMYB75_ (_AtPAP1_) [19] and _AtMYB12_ [20] are both positive regulators of phenolic acid biosynthesis. In this study, we demonstrated that _AtMYB2_ can simultaneously promote the content of tanshinones and phenolic acid in both hairy roots and plants by upregulating the expression of biosynthesis genes. TAT (tyrosine aminotransferase) is the initial key enzyme in the tyrosine-derived pathway, playing a crucial role in the development and metabolic processes of plants [45]. According to the results of qRT-PCR, the expression levels of _SmTAT1_ in _AtMYB2-L6_ transgenic hairy roots (Figure S4) and _OE-AtMYB2-L7_ transgenic plants (Figure 6) reached 3.19 times and 3.10 times that of WT, respectively. It was reported that the overexpression of C4H triggers the activation of two parallel pathways for RA biosynthesis, resulting in the accumulation of a variety of downstream metabolites [46]. In this study, the expression levels of _SmC4H1_ and _SmC4H2_ were significantly increased in _AtMYB2_ transgenic plants (Figure 6). As the key enzyme genes in the phenolic acid synthetic pathway, the increase in the expression of these genes plays an important positive role in the accumulation of phenolic acid content. Actually, the SalB content increased by 38.5% (40.5 mg/g dw) in _OE-AtMYB2-L5_ transgenic plants when compared with that in the WT (Figure 5H). In recent years, genes such as _SmGGPPS_, _SmDXR_, _SmCPS1_, _SmKSL1_, and _SmCYP76 AH1_, which play important roles in tanshinone metabolism pathway, have been characterized from _S. militorhiza_[47].