Glandular Trichomes and Isoprenoid Biosynthesis

Plant glandular trichomes are epidermal differentiations that are dedicated to the production of specialized metabolites, which constitute a first line of defense against pathogens and herbivores. The secretions of these metabolic factories are chemically very diverse, including of terpenoid, fatty acid, or phenylpropanoid origins. They find uses in various industrial areas, for example as pharmaceutical, flavor, or fragrance ingredients or as insecticides. Recent progress in the elucidation of biosynthesis pathways for these compounds has opened up novel opportunities for metabolic engineering in microorganisms as well as in plants.

Transcription activator‐like effectors (TALEs) can be programmed to bind specific DNA sequences. This property was used to construct libraries of synthetic TALE‐activated promoters (STAPs), which drive varying levels of gene expression. After a brief description of these promoters, we explore how these STAPs can be used for various applications in plant synthetic biology, in particular for the coordinated expression of multiple genes for metabolic engineering and in the design and implementation of gene regulatory networks.

Environmental stress is one of the major factors reducing crop productivity. Due to the oncoming climate changes, the effects of drought and high light on plants play an increasing role in modern agriculture. These changes are accompanied with a progressing contamination of soils with heavy metals. Independent of their nature, environmental alterations result in development of oxidative stress, i.e. increase of reactive oxygen species (ROS) contents, and metabolic adjustment, i.e. accumulation of soluble primary metabolites (amino acids and sugars). However, a simultaneous increase of ROS and sugar concentrations ultimately results in protein glycation, i.e. non-enzymatic interaction of reducing sugars or their degradation products (α-dicarbonyls) with proteins. The eventually resulting advanced glycation end-products (AGEs) are known to be toxic and pro-inflammatory in mammals. Recently, their presence was unambiguously demonstrated in vivo in stressed Arabidopsis thaliana plants. Currently, information on protein targets, modification sites therein, mediators and mechanisms of plant glycation are being intensively studied. In this chapter, we comprehensively review the methodological approaches for plant glycation research and discuss potential mechanisms of AGE formation under stress conditions. On the basis of these patterns and additional in vitro experiments, the pathways and mechanisms of plant glycation can be proposed.

Plant secondary metabolites, also termed specialized plant metabolites, currently comprise more than 200 000 natural products that are all based on a few biosynthetic pathways and key primary metabolites. Some pathways like flavonoid and terpenoid biosynthesis are universally distributed in the plant kingdom, whereas others like alkaloid or cyanogenic glycoside biosynthesis are restricted to a limited set of taxa. Diversification is achieved by an array of mechanisms at the genetic and enzymatic level including gene duplications, substrate promiscuity of enzymes, cell‐specific regulatory systems, together with modularity and combinatorial aspects. Specialized metabolites reflect adaptations to a specific environment. The observed diversity illustrates the heterogeneity and multitude of ecological habitats and niches that plants have colonized so far and constitutes a reservoir of potential new metabolites that may provide adaptive advantage in the face of environmental changes. The code that connects the observed chemical diversity to this ecological diversity is largely unknown. One way to apprehend this diversity is to realize its tremendous plasticity and evolutionary potential. This chapter presents an overview of the most widespread and popular secondary metabolites, which provide a definite advantage to adapt to or to colonize a particular environment, making the boundary between the “primary” and the “secondary” old fashioned and blurry.

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Many plant species have evolved specialized organs dedicated to the production of a restricted number of secondary metabolites. These organs have secretory tissues which can lead to very significant accumulations of products, in the range of mg per g of fresh weight. These natural cell factories are therefore interesting targets for metabolic engineering. Plant glandular trichomes in particular have attracted interest because of the relative ease to isolate them and to analyse the compounds they produce because they are secreted onto the leaf surface. Depending on the species, trichomes can produce a variety of metabolites. Terpenoids however are particularly well represented and have been used by humans in a variety of industries, including as aroma, fragrance and pharmaceutical ingredients. Tobacco trichomes produce diterpenoids in large amounts and were therefore chosen as a model system for engineering the biosynthesis of this important class of compounds. We present here our strategy and first results, which bode well for the future of glandular trichomes as engineered natural cellular factories.