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

Abstract

In Brassicaceae, tissue damage triggers the mustard oil bomb i.e., activates the degradation of glucosinolates by myrosinases leading to a rapid accumulation of isothiocyanates at the site of damage. Isothiocyanates are reactive electrophilic species (RES) known to covalently bind to thiols in proteins and glutathione, a process that is not only toxic to herbivores and microbes but can also cause cell death of healthy plant tissues. Previously, it has been shown that subtoxic isothiocyanate concentrations can induce transcriptional reprogramming in intact plant cells. Glutathione depletion by RES leading to breakdown of the redox potential has been proposed as a central and common RES signal transduction mechanism. Using transcriptome analyses, we show that after exposure of Arabidopsis seedlings (grown in liquid culture) to subtoxic concentrations of sulforaphane hundreds of genes were regulated without depletion of the cellular glutathione pool. Heat shock genes were among the most highly up-regulated genes and this response was found to be dependent on the canonical heat shock factors A1 (HSFA1). HSFA1-deficient plants were more sensitive to isothiocyanates than wild type plants. Moreover, pretreatment of Arabidopsis seedlings with subtoxic concentrations of isothiocyanates increased resistance against exposure to toxic levels of isothiocyanates and, hence, may reduce the autotoxicity of the mustard oil bomb by inducing cell protection mechanisms.

The isothiocyanate sulforaphane induces respiratory burst oxidase homologue D‐dependent reactive oxygen species production and regulates expression of stress response genes

“…In this way, Andersson et al (2015) show that in plants treated with SFN, the concentration of GSH is considerably reduced, although no reference is made to its precise mode of action. On the other hand, Ferber et al (2020) show that SFN increased SFN‐GSH adduct but without reduction of cellular GSH pool. In animal cell lines, SFN rapidly ingresses to cell, form an adduct with GSH and later, most SFN shifts to form adducts with proteins (Mi et al, 2007, 2011; Nakamura et al, 2018).…”

“…This late response triggered by SFN could be related to the internalization kinetics of the molecule. Interestingly, Ferber et al (2020) shows that it takes about 2–3 h for 100‐μM SFN to completely ingress into plant cells. This timing of entry of SFN to cells correlate to maximum ROS production for same SFN concentration in our experimental conditions.…”

“…However, it is not known exactly the molecular mechanisms of how SFN exerts its effect in plants. Few evidences showed that SFN alter the plant cellular glutathione content leading to a redox imbalance, although this effect is dependent on the concentration used and the application method (Andersson et al, 2015; Ferber et al, 2020). Previous experiments performed in our lab on tomato cell suspensions indicated that SFN induces ROS production (Figure S1).…”

“…Among the glucosinolate breakdown products, isothiocyanates possess the highest chemical reactivity (Pastorczyk & Bednarek, 2016). In Arabidopsis , around 40 biologically inactive glucosinolates have been identified in several accessions and the most abundant is glucoraphanin that upon myrosinase action produce is the isothiocyanate sulforaphane (SFN) (Ferber et al, 2020, and reviewed in Burow & Halkier, 2017). Glucosinolates are stored at high levels (>130 mM) in specific cells called S‐cells localized in the phloem cap along the vasculature and along the leaf margins (Koroleva et al, 2010).…”

“…In plants the signaling pathway of SFN is not known in detail. It has been reported that SFN causes a decrease in the cellular content of glutathione and as a consequence of increase in the redox potential (Andersson et al, 2015; Ferber et al, 2020). Likewise, SFN produces covalent modifications in histone H3, causing the unpacking of chromatin and the activation of the expression of the defense genes WRKY6 and PDF1.2 (Schillheim et al, 2017).…”