GhMYC2 activates cytochrome P450 gene CYP71BE79 to regulate gossypol biosynthesis in cotton

Identification of transcription factor genes responsive to MeJA and characterization of a LaMYC2 transcription factor positively regulates lycorine biosynthesis in Lycoris aurea

Jasmonates (JAs) are among the main phytohormones, regulating plant growth and development, stress responses, and secondary metabolism. As the major regulator of the JA signaling pathway, MYC2 also plays an important role in plant secondary metabolite synthesis and accumulation. In this study, we performed a comparative transcriptome analysis of Lycoris aurea seedlings subjected to methyl jasmonate (MeJA) at different treatment times. A total of 31,193 differentially expressed genes (DEGs) were identified by RNA sequencing. Among them, 732 differentially expressed transcription factors (TFs) comprising 51 TF families were characterized. The most abundant TF family was WRKY proteins (80), followed by AP2/ERF-EFR (67), MYB (59), bHLH (52), and NAC protein (49) families. Subsequently, by calculating the Pearson's correlation coefficient (PCC) between the expression level of TF DEGs and the lycorine contents, 41 potential TF genes (|PCC| >0.8) involved in lycorine accumulation were identified, including 36 positive regulators and 5 negative regulators. Moreover, a MeJA-inducible MYC2 gene (namely LaMYC2) was cloned on the basis of transcriptome sequencing. Bioinformatic analyses revealed that LaMYC2 proteins contain the bHLH-MYC_N domain and bHLH-AtAIB_like motif. LaMYC2 protein is localized in the cell nucleus, and can partly rescue the MYC2 mutant in Arabidopsis thaliana. LaMYC2 protein could interact with most LaJAZs (especially LaJAZ3 and LaJAZ4) identified previously. Transient overexpression of LaMYC2 increased lycorine contents in L. aurea petals, which might be associated with the activation of the transcript levels of tyrosine decarboxylase (TYDC) and phenylalanine ammonia lyase (PAL) genes. By isolating the 887-bp-length promoter fragment upstream of the start codon (ATG) of LaTYDC, we found several different types of E-box motifs (CANNTG) in the promoter of LaTYDC. Further study demonstrated that LaMYC2 was indeed able to bind the E-box (CACATG) present in the LaTYDC promoter, verifying that the pathway genes involved in lycorine biosynthesis could be regulated by LaMYC2, and that LaMYC2 has positive roles in the regulation of lycorine biosynthesis. These findings demonstrate that LaMYC2 is a positive regulator of lycorine biosynthesis and may facilitate further functional research of the LaMYC2 gene, especially its potential regulatory roles in Amaryllidaceae alkaloid accumulation in L. aurea.

The dual role of GoPGF reveals that the pigment glands are synthetic sites of gossypol in aerial parts of cotton

Gossypol and the related terpenoids are stored in the pigment gland to protect cotton plants from biotic stresses, but little is known about the synthetic sites of these metabolites. Here, we showed that GoPGF, a key gene regulating gland formation, was expressed in gland cells and roots. The chromatin immunoprecipitation sequencing (ChIP-seq) analysis demonstrated that GoPGF targets GhJUB1 to regulate gland morphogenesis. RNA-sequencing (RNA-seq) showed high accumulation of gossypol biosynthetic genes in gland cells. Moreover, integrated analysis of the ChIP-seq and RNA-seq data revealed that GoPGF binds to the promoter of several gossypol biosynthetic genes. The cotton callus overexpressing GoPGF had dramatically increased the gossypol levels, indicating that GoPGF can directly activate the biosynthesis of gossypol. In addition, the gopgf mutant analysis revealed the existence of both GoPGF-dependent and -independent regulation of gossypol production in cotton roots. Our study revealed that the pigment glands are synthetic sites of gossypol in aerial parts of cotton and that GoPGF plays a dual role in regulating gland morphogenesis and gossypol biosynthesis. The study provides new insights for exploring the complex relationship between glands and the metabolites they store in cotton and other plant species.

GhMYC1374 regulates the cotton defense response to cotton aphids by mediating the production of flavonoids and free gossypol

Myelocytomatosis (MYC) transcription factors (TFs) in plants are well-known regulators of plant defense against herbivores. However, the role and mechanism of MYC TFs in cotton (Gossypium hirsutum L.) defense against cotton aphids (Aphis gossypii Glover) remain still elusive. Herein, on the basis of aphid-induced cotton transcriptome analysis, GhMYC1374, a cotton MYC2-like TF that was highly induced by cotton aphid attack, has been identified that confers cotton aphid resistance in cotton. GhMYC1374 was an intranuclear transcription factor with three domains: bHLH-MYC_N, RBR and bHLH_AtAIB_like. GhMYC1374 was induced under cotton aphid feeding, exogenous methyl jasmonate (MeJA) and salicylic acid (SA) treatments. GhMYC1374 transient overexpression in cotton plants enhanced cotton aphid-resistance, while GhMYC1374 silence through VIGS (virus induced gene silencing) decreased cotton aphid-resistance. GhMYC1374 transient overexpression of in cotton plants activated the phenylpropane pathway and promoted the synthesis of flavonoids, and resistance to thus enhanced the cotton resistance against aphids. In contrast, GhMYC1374 silence inhibited the biosynthesis of flavonoids. In addition, GhMYC1374 also positively activated the expression of the biosynthetic genes of free gossypol, leading to the high content of free gossypol. Taken together, our results suggest that GhMYC1374 is involved in the cotton defense response against cotton aphids by regulating the biosynthesis of flavonoids and free gossypol.

MYC2: A Master Switch for Plant Physiological Processes and Specialized Metabolite Synthesis

The jasmonic acid (JA) signaling pathway plays important roles in plant defenses, development, and the synthesis of specialized metabolites synthesis. Transcription factor MYC2 is a major regulator of the JA signaling pathway and is involved in the regulation of plant physiological processes and specialized metabolite synthesis. Based on our understanding of the mechanism underlying the regulation of specialized metabolite synthesis in plants by the transcription factor MYC2, the use of synthetic biology approaches to design MYC2-driven chassis cells for the synthesis of specialized metabolites with high medicinal value, such as paclitaxel, vincristine, and artemisinin, seems to be a promising strategy. In this review, the regulatory role of MYC2 in JA signal transduction of plants to biotic and abiotic stresses, plant growth, development and specialized metabolite synthesis is described in detail, which will provide valuable reference for the use of MYC2 molecular switches to regulate plant specialized metabolite biosynthesis.