Overexpression of TcCHS Increases Pyrethrin Content When Using a Genotype-Independent Transformation System in Pyrethrum (Tanacetum cinerariifolium)

A comprehensive review of in planta stable transformation strategies

Plant transformation remains a major bottleneck to the improvement of plant science, both on fundamental and practical levels. The recalcitrant nature of most commercial and minor crops to genetic transformation slows scientific progress for a large range of crops that are essential for food security on a global scale. Over the years, novel stable transformation strategies loosely grouped under the term "in planta" have been proposed and validated in a large number of model (e.g. Arabidopsis and rice), major (e.g. wheat and soybean) and minor (e.g. chickpea and lablab bean) species. The in planta approach is revolutionary as it is considered genotype-independent, technically simple (i.e. devoid of or with minimal tissue culture steps), affordable, and easy to implement in a broad range of experimental settings. In this article, we reviewed and categorized over 300 research articles, patents, theses, and videos demonstrating the applicability of different in planta transformation strategies in 105 different genera across 139 plant species. To support this review process, we propose a classification system for the in planta techniques based on five categories and a new nomenclature for more than 30 different in planta techniques. In complement to this, we clarified some grey areas regarding the in planta conceptual framework and provided insights regarding the past, current, and future scientific impacts of these techniques. To support the diffusion of this concept across the community, this review article will serve as an introductory point for an online compendium about in planta transformation strategies that will be available to all scientists. By expanding our knowledge about in planta transformation, we can find innovative approaches to unlock the full potential of plants, support the growth of scientific knowledge, and stimulate an equitable development of plant research in all countries and institutions.

TcMYB8, a R3-MYB Transcription Factor, Positively Regulates Pyrethrin Biosynthesis in Tanacetum cinerariifolium

Pyrethrins are a mixture of terpenes, with insecticidal properties, that accumulate in the aboveground parts of the pyrethrum (Tanacetum cinerariifolium). Numerous studies have been published on the positive role of MYB transcription factors (TFs) in terpenoid biosynthesis; however, the role of MYB TFs in pyrethrin biosynthesis remains unknown. Here, we report the isolation and characterization of a T. cinerariifolium MYB gene encoding a R3-MYB protein, TcMYB8, containing a large number of hormone-responsive elements in its promoter. The expression of the TcMYB8 gene showed a downward trend during the development stage of flowers and leaves, and was induced by methyl jasmonate (MeJA), salicylic acid (SA), and abscisic acid (ABA). Transient overexpression of TcMYB8 enhanced the expression of key enzyme-encoding genes, TcCHS and TcGLIP, and increased the content of pyrethrins. By contrast, transient silencing of TcMYB8 decreased pyrethrin contents and downregulated TcCHS and TcGLIP expression. Further analysis indicated that TcMYB8 directly binds to cis-elements in proTcCHS and proTcGLIP to activate their expression, thus regulating pyrethrin biosynthesis. Together, these results highlight the potential application of TcMYB8 for improving the T. cinerariifolium germplasm, and provide insight into the pyrethrin biosynthesis regulation network.

TcMYC2 regulates Pyrethrin biosynthesis in Tanacetum cinerariifolium

Pyrethrins constitute a class of terpene derivatives with high insecticidal activity and are mainly synthesized in the capitula of the horticulturally important plant, Tanacetum cinerariifolium. Treatment of T. cinerariifolium with methyl jasmonate (MeJA) in the field induces pyrethrin biosynthesis, but the mechanism linking MeJA with pyrethrin biosynthesis remains unclear. In this study, we explored the transcription factors involved in regulating MeJA-induced pyrethrin biosynthesis. A single spray application of MeJA to T. cinerariifolium leaves rapidly upregulated the expression of most known pyrethrin biosynthesis genes and subsequently increased the total pyrethrin content in the leaf. A continuous 2-week MeJA treatment resulted in enhanced pyrethrin content and increased trichome density. TcMYC2, a key gene in jasmonate signaling, was screened at the transcriptome after MeJA treatment. TcMYC2 positively regulated expression of the pyrethrin biosynthesis genes TcCHS, TcAOC, and TcGLIP by directly binding to E-box/G-box motifs in the promoters. The stable overexpression of TcMYC2 in T. cinerariifolium hairy roots significantly increased the expression of TcAOC and TcGLIP. Further transient overexpression and viral-induced gene-silencing experiments demonstrated that TcMYC2 positively promoted pyrethrin biosynthesis. Collectively, the results reveal a novel molecular mechanism for MeJA-induced pyrethrin biosynthesis in T. cinerariifolium involving TcMYC2.