Abstract
Glucosinolates (GLSs) are a well-defined group of specialized metabolites, and like any other plant specialized metabolites, their presence does not directly affect the plant survival in terms of growth and development. However, specialized metabolites are essential to combat environmental stresses, such as pathogens and herbivores. GLSs naturally occur in many pungent plants in the order of Brassicales. To date, more than 200 different GLS structures have been characterized and their distribution differs from species to species. GLSs co-exist with classical and atypical myrosinases, which can hydrolyze GLS into an unstable aglycone thiohydroximate-O-sulfonate, which rearranges to produce different degradation products. GLSs, myrosinases, myrosinase interacting proteins, and GLS degradation products constitute the GLS-myrosinase (GM) system (“mustard oil bomb”). This review discusses the cellular and subcellular organization of the GM system, its chemodiversity, and functions in different cell types. Although there are many studies on the functions of GLSs and/or myrosinases at the tissue and whole plant levels, very few studies have focused on different single cell types. Single cell type studies will help to reveal specific functions that are missed at the tissue and organismal level. This review aims to highlight (1) recent progress in cellular and subcellular compartmentation of GLSs, myrosinases, and myrosinase interacting proteins; (2) molecular and biochemical diversity of GLSs and myrosinases; and (3) myrosinase interaction with its interacting proteins, and how it regulates the degradation of GLSs and thus the biological functions (e.g., plant defense against pathogens). Future prospects may include targeted approaches for engineering/breeding of plants and crops in the cell type-specific manner toward enhanced plant defense and nutrition.
Highlights
One of the most extensively studied classes of anti-herbivore chemical defenses in plants is glucosinolates (GLSs), a group of sulfur-rich, amino acid-derived metabolites combining a β-d-glucopyranose residue linked via a sulfur atom to an N-hydroxyimino sulfate ester, which are plant-derived natural products (Halkier and Gershenzon, 2006; Halkier, 2016).Chemodiversity of Glucosinolate-Myrosinase SystemGLSs are widely distributed in the order Brassicales, which includes vegetables, spice plants supplying condiments, and reference species, Arabidopsis thaliana (Fahey et al, 2001; Reichelt et al, 2002)
Upon insect feeding or mechanical disruption, GLSs are hydrolyzed by myrosinases into unstable thiohydroximateO-sulfonates, which rearrange to form different hydrolytic products such as isothiocyanates (ITCs), nitriles, and other by-products depending on the nature of the GLS side chain and the reaction conditions, such as iron, pH, and presence of myrosinase interacting proteins (Chen and Andreasson, 2001; Wittstock et al, 2016a)
TGGs are known to be present in all A. thaliana organs and were reported in A. thaliana and B. napus phloem parenchyma as well as in guard cells (Andréasson et al, 2001; Thangstad et al, 2004)
Summary
One of the most extensively studied classes of anti-herbivore chemical defenses in plants is glucosinolates (GLSs), a group of sulfur-rich, amino acid-derived metabolites combining a β-d-glucopyranose residue linked via a sulfur atom to an N-hydroxyimino sulfate ester, which are plant-derived natural products (Halkier and Gershenzon, 2006; Halkier, 2016). All the genes in the GLS biosynthetic pathways have been identified, and it is somewhat known where GLSs are stored (Koroleva et al, 2000; Andréasson et al, 2001), but it has remained elusive where GLSs are produced at the subcellular, cellular, and tissue levels (Rask et al, 2000; Nintemann et al, 2017) Neither is it clear about the cellular and subcellular compartmentation of different myrosinases and their interacting proteins, which include myrosinase-binding proteins (MBPs), myrosinaseassociated proteins (MyAPs), and different specifier proteins. These products and the amounts used are of synthetic origin and abundance It is not known what degradation products are produced and how much in vivo, which GLSs and myrosinases [TGGs and/or Penetration 2 (PEN2)] are involved, and how protein interactions regulate the GLS breakdown in guard cells.
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