miR5298b regulated taxol biosynthesis by acting on TcNPR3, resulting in an alleviation of the strong inhibition of the TcNPR3-TcTGA6 complex in Taxus chinensis

TcMYB73, a salicylic acid-responsive R2R3-MYB transcription factor, positively regulates paclitaxel biosynthesis in Taxus chinensis in direct and indirect ways

BACKGROUND

Paclitaxel (Taxol) is an invaluable secondary metabolite extracted from Taxus species, wildly utilized in cancer therapeutics. Salicylic acid (SA), an important phytohormone, substantially elevates paclitaxel accumulation in Taxus cell suspension cultures. However, the molecular mechanisms governing SA-induced modulation of paclitaxel biosynthesis remain poorly elucidated. Our previous studies identified TcMYB73, an SA-responsive R2R3-MYB transcription factor (TF), which demonstrates a robust positive correlation with paclitaxel biosynthesis, implying its orchestrating role in this metabolic pathway.

RESULTS

Expression pattern analysis revealed that TcMYB73 displays predominant expression in lateral roots. Both overexpression and RNA interference (RNAi) of TcMYB73 demonstrated its regulatory function in modulating key paclitaxel biosynthetic genes, including taxadiene synthase (TASY), 10-deacetylbaccatin III-10-O-acetyltransferase (DBAT), and 3'-N-debenzoyl-2'-deoxytaxol-N-benzoyltransferase (DBTNBT). Transient TcMYB73 overexpression in Taxus chinensis (T. chinensis) needles induced 2.38-, 2.87-, and 1.79-fold increases in 10-DAB, baccatin III, and paclitaxel accumulation, respectively, compared to controls. Additionally, yeast one-hybrid (Y1H), Electrophoretic Mobility Shift Assay (EMSA), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR), and dual-luciferase (Dual-LUC) assays verified that TcMYB73 directly binds to MYB recognition elements in the T10OH promoter, enhancing its transcription. Furthermore, TcWRKY33, a transcriptional activator of DBAT, functions as a positive regulator mediating SA signaling within the paclitaxel biosynthetic pathway. Subsequent investigations validated that TcMYB73 upregulates DBAT expression via direct transcriptional activation of TcWRKY33. Collectively, these results demonstrate that TcMYB73 transduces SA signals to T10OH and TcWRKY33, coordinately regulating paclitaxel biosynthesis through dual mechanisms: direct activation of biosynthetic genes and indirect modulation of upstream regulators.

CONCLUSIONS

Our results indicated that the SA-responsive R2R3-MYB TF, TcMYB73 transcriptionally governs paclitaxel biosynthesis in T. chinensis through direct activation the expression of the T10OH gene, and activating TcWRKY33 expression, thereby modulating DBAT expression. This study provides mechanistic insights into the role of TcMYB73 in mediating SA-induced transcriptional regulation of paclitaxel biosynthesis in Taxus species.

Role of an endodermis-specific miR858b-MYB1L module in the regulation of Taxol biosynthesis in Taxus mairei

Taxol, a chemotherapeutic agent widely used for treating various cancers, is extracted from the stems of Taxus mairei. However, current knowledge regarding the effects of stem tissue and age on Taxol accumulation is limited. We employed matrix-assisted laser desorption/ionization mass spectrometry to visualize taxoids in stem section sections of varying ages from T. mairei. Laser capture microdissection integrated with data-dependent acquisition-MS/MS analysis identified that several Taxol biosynthesis pathway-related enzymes were predominantly produced in the endodermis, elucidating the molecular mechanisms underlying endodermis-specific Taxol accumulation. We identified an endodermis-specific MYB1-like (MYB1L) protein and proposed a potential function for the miR858-MYB1L module in regulating secondary metabolic pathways. DNA affinity purification sequencing analysis produced 92 506 target peaks for MYB1L. Motif enrichment analysis identified several de novo motifs, providing new insights into MYB recognition sites. Four target peaks of MYB1L were identified within the promoter sequences of Taxol synthesis genes, including TBT, DBTNBT, T13OH, and BAPT, and were confirmed using electrophoretic mobility shift assays. Dual-luciferase assays showed that MYB1L significantly activated the expression of TBT and BAPT. Our data indicate that the miR858b-MYB1L module plays a crucial role in the transcriptional regulation of Taxol biosynthesis by up-regulating the expression of TBT and BAPT genes in the endodermis.

Post-genomic illumination of paclitaxel biosynthesis

Paclitaxel rapidly became one of the most effective anticancer drugs. However, the production of paclitaxel is hindered by substantial challenges, particularly considering the significant quantities of drug required and the inherently low concentration of paclitaxel and its intermediates in plants. Paclitaxel is currently produced in a so-called semi-synthesis in which baccatin III is extracted from Taxus species and chemically converted to paclitaxel. Despite the fact that many of the intermediates of paclitaxel biosynthesis are yet to be experimentally determined, a set of recent papers-facilitated by the sequencing and assembly of three Taxus genomes-has uncovered the minimal gene sets for both baccatin III and paclitaxel biosynthesis. Here we summarize the key milestones towards our understanding of paclitaxel biosynthesis and highlight recent advancements made possible by genome-level analysis of potential key genes involved. We argue that these studies will ultimately pave the way towards the elucidation of the entire paclitaxel biosynthetic pathway and facilitate the industrial production of paclitaxel via synthetic biology approaches. However, several major challenges lie ahead before we can fully tap into the amazing curative potential that taxanes provide.

Fungal invasion-induced accumulation of salicylic acid promotes anthocyanin biosynthesis through MdNPR1-MdTGA2.2 module in apple fruits

In the field, necrosis area induced by pathogens is usually surrounded by a red circle in apple fruits. However, the underlying molecular mechanism of this phenomenon remains unclear. In this study, we demonstrated that accumulated salicylic acid (SA) induced by fungal infection promoted anthocyanin biosynthesis through MdNPR1-MdTGA2.2 module in apple (Malus domestica). Inoculating apple fruits with Valsa mali or Botryosphaeria dothidea induced a red circle surrounding the necrosis area, which mimicked the phenotype observed in the field. The red circle accumulated a high level of anthocyanins, which was positively correlated with SA accumulation stimulated by fungal invasion. Further analysis showed that SA promoted anthocyanin biosynthesis in a dose-dependent manner in both apple calli and fruits. We next demonstrated that MdNPR1, a master regulator of SA signaling, positively regulated anthocyanin biosynthesis in both apple and Arabidopsis. Moreover, MdNPR1 functioned as a co-activator to interact with and enhance the transactivation activity of MdTGA2.2, which could directly bind to the promoters of anthocyanin biosynthetic and regulatory genes to promote their transcription. Suppressing expression of either MdNPR1 or MdTGA2.2 inhibited coloration of apple fruits, while overexpressing either of them significantly promoted fruit coloration. Finally, we revealed that silencing either MdNPR1 or MdTGA2.2 in apple fruits repressed SA-induced fruit coloration. Therefore, our data determined that fungal-induced SA promoted anthocyanin biosynthesis through MdNPR1-MdTGA2.2 module, resulting in a red circle surrounding the necrosis area in apple fruits.

A comprehensive review of TGA transcription factors in plant growth, stress responses, and beyond

TGA transcription factors (TFs), belonging to the D clade of the basic region leucine zipper (bZIP) family, exhibit a specific ability to recognize and bind to regulatory elements with TGACG as the core recognition sequence, enabling the regulation of target gene expression and participation in various biological regulatory processes. In plant growth and development, TGA TFs influence organ traits and phenotypes, including initial root length and flowering time. They also play a vital role in responding to abiotic stresses like salt, drought, and cadmium exposure. Additionally, TGA TFs are involved in defending against potential biological stresses, such as fungal bacterial diseases and nematodes. Notably, TGA TFs are sensitive to the oxidative-reductive state within plants and participate in pathways that aid in the elimination of reactive oxygen species (ROS) generated during stressful conditions. TGA TFs also participate in multiple phytohormonal signaling pathways (ABA, SA, etc.). This review thoroughly examines the roles of TGA TFs in plant growth, development, and stress response. It also provides detailed insights into the mechanisms underlying their involvement in physiological and pathological processes, and their participation in plant hormone signaling. This multifaceted exploration distinguishes this review from others, offering a comprehensive understanding of TGA TFs.