Medicinal Plant Research 2024, Vol.14, No.4, 223-233 http://hortherbpublisher.com/index.php/mpr 226 accumulation of ganoderic acid is mediated by increased cytosolic Ca2+ levels and the expression of HSPs (Zhang et al., 2016). Water stress, on the other hand, enhances ganoderic acid production through increased ROS levels and NOX activity (Zhu et al., 2022). While the provided data does not extensively cover the impact of stress on polysaccharides, it is known that environmental stress can influence the overall metabolic profile of G. lucidum, potentially affecting the production of other bioactive compounds like polysaccharides and triterpenoids (Sharma et al., 2019; Swallah et al., 2023). 4 Molecular Mechanisms Regulating Secondary Metabolite Pathways 4.1 Gene expression changes in response to stress Environmental stress significantly impacts the gene expression profiles in Ganoderma lucidum, leading to alterations in secondary metabolite pathways. For instance, nitrogen limitation induces the expression of the transcription factor GCN4, which in turn promotes the synthesis of ganoderic acid (GA) by activating genes involved in antioxidant enzyme biosynthesis, such as glutathione reductase, glutathione S-transferase, and catalase (Lian et al., 2021). Similarly, heat stress activates the AMP-activated protein kinase (AMPK)/Sucrose-nonfermenting serine-threonine protein kinase 1 (Snf1), which mediates metabolic rearrangement to cope with oxidative stress and influences GA biosynthesis (Hu et al., 2019). Water stress also triggers changes in gene expression, notably increasing the levels of reactive oxygen species (ROS) and GA content through the modulation of NADPH oxidase (NOX) activity and aquaporin (GlAQP) expression (Zhu et al., 2022). 4.2 Key transcription factors and signaling pathways involved Several key transcription factors and signaling pathways are involved in regulating secondary metabolite pathways in G. lucidum. The MADS-box transcription factor GlMADS1 has been shown to negatively regulate GA and flavonoid accumulation, as silencing this gene enhances the content of these secondary metabolites (Meng et al., 2020). Additionally, the general control non-derepressible 4 (GCN4) transcription factor plays a crucial role under nitrogen limitation conditions by binding to the promoter region of the mitochondrial pyruvate carrier (GlMPC) and activating its expression, thereby regulating the tricarboxylic acid (TCA) cycle and GA biosynthesis (Wang et al., 2023) (Figure 2). Furthermore, transcriptome and metabolome analyses have identified homeobox transcription factors and velvet family proteins as significant regulators of GA biosynthesis during the development of G. lucidum(Meng et al., 2022b). Figure 2 Mitochondrial pyruvate transport was significantly increased under nitrogen limitation conditions (Adopted from Wang et al., 2023) Image caption: (A) Images of mycelial growth in WT, Si-control, GCN4-silenced, and GlMPC1/2-silenced strains under 3 mM and 60 mM Asn. (B) Relative inhibition rates under 3 mM and 60 mM Asn in WT, Si-control, GCN4-silenced, and GlMPC1/2-silenced strains. The growth inhibition rate in each strain was calculated as follows: [diameter (60 mM Asn) - diameter (3 mM Asn)]/diameter (60 mM Asn)]. Data are presented as the mean ± SD (n = 3). Statistical significance is represented by different letters corresponding to P< 0.05 based on Tukey’s multiple range test (Adopted from Wang et al., 2023)
RkJQdWJsaXNoZXIy MjQ4ODYzNA==