Localization of the glucosinolate biosynthetic enzymes reveals distinct spatial patterns for the biosynthesis of indole and aliphatic glucosinolates

Abstract

Glucosinolates constitute the primary defense metabolites in Arabidopsis thaliana (Arabidopsis). Indole and aliphatic glucosinolates, biosynthesized from tryptophan and methionine, respectively, are known to serve distinct biological functions. Although all genes in the biosynthetic pathways are identified, and it is known where glucosinolates are stored, it has remained elusive where glucosinolates are produced at the cellular and tissue level. To understand how the spatial organization of the different glucosinolate biosynthetic pathways contributes to their distinct biological functions, we investigated the localization of enzymes of the pathways under constitutive conditions and, for indole glucosinolates, also under induced conditions, by analyzing the spatial distribution of several fluorophore-tagged enzymes at the whole plant and the cellular level. We show that key steps in the biosynthesis of the different types of glucosinolates are localized in distinct cells in separate as well as overlapping vascular tissues. The presence of glucosinolate biosynthetic enzymes in parenchyma cells of the vasculature may assign new defense-related functions to these cell types. The knowledge gained in this study is an important prerequisite for understanding the orchestration of chemical defenses from site of synthesis to site of storage and potential (re)mobilization upon attack.

Leaf Endoplasmic Reticulum Bodies Identified in Arabidopsis Rosette Leaves Are Involved in Defense against Herbivory

“…Although it is not known whether these cells accumulate glucosinolates, this distribution of L-ER bodies in leaves may be an efficient defense against herbivores. The key enzymes involved in biosynthesis of indole and aliphatic glucosinolates are localized in vascular cells (Nintemann et al, 2018), suggesting high concentration of glucosinolates in vascular tissue. L-ER bodies in the epidermal cells covering the midrib may efficiently protect vascular tissue from herbivores that feed on the leaf sap.…”

Chemical Priming by Isothiocyanates Protects Against Intoxication by Products of the Mustard Oil Bomb

“…The myrosinase-glucosinolate system is a powerful defense system of Brassicales plants. S-cells in the vasculature of leaves and along leaf margins can accumulate up to 130 mM glucosinolates (Nintemann et al, 2018) which in contact with myrosinases can potentially produce local concentrations of ITCs that are at least 2 orders of magnitude higher than the toxic concentrations for microorganisms (Dufour et al, 2015) and plant cells (Andersson et al, 2015). To ensure safe storage of the ITC precursors, myrosinases are separated from glucosinolates in different specialized cells.…”

The Phenylpropanoid Case – It Is Transport That Matters

“…Such a separation of the intermediates in different cell types might help to avoid undesired secondary modifications (e.g., acetylation or methylation) of the alkaloids ( Weid et al, 2004 ). Likewise, in Arabidopsis , the key steps of the glucosinolate biosynthesis are localized in distinct cells, in order to minimize the risk of self-toxication ( Fuchs et al, 2016 ; Nintemann et al, 2018 ). In Medicago enzymes like PAL and IFS represent enzymatic crossroads in the central phenylpropanoid pathway and isoflavonoid biosynthesis, respectively.…”