Chemodiversity of the Glucosinolate-Myrosinase System at the Single Cell Type Resolution

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.

Identification and Characterization of Three Epithiospecifier Protein Isoforms in Brassica oleracea

“…GLS accumulate in vacuoles of sulfurrich S-cells that are found in close proximity of myrosin cells and guard cells, which both store myrosinases in their vacuoles (Shirakawa and Hara-Nishimura, 2018). In contrast to this, AtESP is localized in the cytosol (Burow et al, 2007;Chhajed et al, 2019). Measurements of the in vivo organelle pH revealed an acidification gradient from the cytosol and endoplasmatic reticulum, with pH around pH 7, to lytic vacuoles ranging from pH 5-6 (Martinière et al, 2013;Shen et al, 2013).…”

Atypical Myrosinase as a Mediator of Glucosinolate Functions in Plants

“…Given that the aforementioned experiments were performed under non-invasive conditions, it is conceivable that GLS breakdown by the Myr II myrosinases may be less dependent on tissue damage. Hence, not only EE-type but also classical myrosinases could regulate subcellular GLS–myrosinase systems in addition to the so-called “mustard oil bomb.” Our understanding in this regard will be ameliorateded by single-cell-level analysis of the specialized compartments in different cell types, as reviewed by Chen et al (2019) in this issue.…”

“…In addition, the mechanisms underlying the degradation of total GLS amount independent of TGG1 and TGG2 during early developmental stages remain to be clarified (Barth and Jander, 2006). Given that up-regulation of particular BGLUs has yet to be observed under these conditions, we should consider the post-translational regulation of myrosinase activities with regard to myrosinase-associated proteins or small molecule elicitors such as ascorbate (Wittstock et al, 2016a; Bhat and Vyas, 2019; Chen et al, 2019). Under non-disruptive conditions, the physiological functions of myrosinases, including TGGs, are probably controlled more strictly and dynamically than expected till now.…”

“…Knowledge on the chemical diversity embedded in the GLS metabolism and on the molecular mechanisms underlying the GLS–myrosinase system is frequently updated. For example, energetical investigations have been made of GLS biosynthetic genes (Halkier, 2016; Barco and Clay, 2019), end products directed by specifier proteins (Wittstock et al, 2016a), tissue localization via GLS transporters (Jørgensen et al, 2015; Halkier, 2016), differences in GLS contents among species and accessions (Kliebenstein and Cacho, 2016), and proteins that interact with myrosinases to regulate their activity and stability (Bhat and Vyas, 2019; Chen et al, 2019). However, most of the insights that have been gained regarding the molecular basis and physiological importance of GLS breakdown are based on the intercellular and tissue damage-dependent GLS–myrosinase system.…”

Genomic Origin and Diversification of the Glucosinolate MAM Locus

“…Glucosinolates (GSL) are a diverse class of amino-acid derived sulfur containing metabolites characteristic of plants of the order Brassicales (Rodman et al, 1998;Borpatragohain et al, 2016;Kliebenstein and Cacho, 2016;Olsen et al, 2016;Chhajed et al, 2019;Blazevic et al, 2020). When the plant experiences physical damage, such as chewing by herbivores, compartments of the cell rupture and release myrosinase enzymes that hydrolyze the GSLs to create an isothiocyanate anion, damaging the attacker (Rodman et al, 1998).…”