Elucidating the Role of Transport Processes in Leaf Glucosinolate Distribution

Abstract

In Arabidopsis (Arabidopsis thaliana), a strategy to defend its leaves against herbivores is to accumulate glucosinolates along the midrib and at the margin. Although it is generally assumed that glucosinolates are synthesized along the vasculature in an Arabidopsis leaf, thereby suggesting that the margin accumulation is established through transport, little is known about these transport processes. Here, we show through leaf apoplastic fluid analysis and glucosinolate feeding experiments that two glucosinolate transporters, GTR1 and GTR2, essential for long-distance transport of glucosinolates in Arabidopsis, also play key roles in glucosinolate allocation within a mature leaf by effectively importing apoplastically localized glucosinolates into appropriate cells. Detection of glucosinolates in root xylem sap unambiguously shows that this transport route is involved in root-to-shoot glucosinolate allocation. Detailed leaf dissections show that in the absence of GTR1 and GTR2 transport activity, glucosinolates accumulate predominantly in leaf margins and leaf tips. Furthermore, we show that glucosinolates accumulate in the leaf abaxial epidermis in a GTR-independent manner. Based on our results, we propose a model for how glucosinolates accumulate in the leaf margin and epidermis, which includes symplasmic movement through plasmodesmata, coupled with the activity of putative vacuolar glucosinolate importers in these peripheral cell layers.

Transport of defense compounds from source to sink: lessons learned from glucosinolates

“…Thus, accumulation of glucosinolates in these cells (which do not de novo synthesize glucosinolates) suggests that transport processes are essential for distribution of glucosinolates to the defensive outer perimeter tissues. Detailed analyses of glucosinolate content within the leaf (i.e., in dissected leaf parts, stripped epidermal layers, apoplastic fluids, and guttation drops) showed that atgtr1 atgtr2 mutants overaccumulate glucosinolates in the leaf margin and in the abaxial epidermis compared with wild type [49]. This was unexpected because we thought (intuitively and in hindsight naively) that, without transporters, the glucosinolates would not be moved.…”

“…Interestingly, behavioral assays showed that 4MTB is a stronger oviposition cue than 4MSB for Plutella xylostella and Pieris rapae at concentrations naturally found in the epidermal cell layers [42]. This suggests that 4MTB has a special role in plant-insect interactions and that the distribution patterns of specific glucosinolates are important for plant defense and fitness [42,49].…”

“…Additionally, AtGTR1 localizes to mesophyll cells, which suggests an additional role for AtGTR1 in import into symplastic domains [19]. Intercellular glucosinolate transport via plasmodesmata along a concentration gradient imposed by vacuolar import processes at the leaf margins has been proposed [49]. Import and export of glucosinolates across the tonoplast and export across the plasma membrane occurs by yet unknown transport processes; for example by vesicular endo-and exocytosis or transporter-mediated import or export (black arrows).…”

“…Accordingly, the overaccumulation of long-chained, aliphatic glucosinolates in rosette leaves of ungrafted atgtr1 atgtr2 mutants can now be explained by an inability to Overview of Arabidopsis glucosinolate transport processes. In leaves, glucosinolates are suggested to move either through the phloem for long-distance transport to other organs or through symplasmic domains towards leaf edges [49]. AtGTR1 and AtGTR2 localize to phloem companion cells and have a role in phloem loading [21].…”

Evaluation of Seasonal Variations in the Glucosinolate Content in Leaves and Roots of Four European Horseradish (Armoracia rusticana) Landraces

“…Phloemmediated transport of GLS may coordinate de novo biosynthesis and promote the use of GLS as defence compounds in various organs. Recent studies demonstrated the presence of GLS in the root xylem sap of Arabidopsis, which indicates that a transport pathway is involved in root-to-shoot GLS allocation [Madsen et al, 2014;Jørgensen et al, 2015]. In contrast, Li et al [1999] did not report correlations between the GLS content in the leaves, stems and roots of Brassica napus, and they concluded that both synthesis and accumulation of GLS are controlled by plant tissues.…”