Guard Cell- and Phloem Idioblast-Specific Expression of Thioglucoside Glucohydrolase 1 (Myrosinase) in Arabidopsis

Abstract

Thioglucoside glucohydrolase 1 (TGG1) is one of two known functional myrosinase enzymes in Arabidopsis. The enzyme catalyzes the hydrolysis of glucosinolates into compounds that are toxic to various microbes and herbivores. Transgenic Arabidopsis plants carrying ␤-glucuronidase and green fluorescent protein reporter genes fused to 0.5 or 2.5 kb of the TGG1 promoter region were used to study spatial promoter activity. Promoter activity was found to be highly specific and restricted to guard cells and distinct cells of the phloem. No promoter activity was detected in the root or seed. All guard cells show promoter activity. Positive phloem cells are distributed in a discontinuous pattern and occur more frequent in young tissues. Immunocytochemical localization of myrosinase in transverse and longitudinal sections of embedded material show that the TGG1 promoter activity reflects the position of the myrosinase enzyme. In the flower stalk, the myrosinase-containing phloem cells are located between phloem sieve elements and glucosinolate-rich S cells. Our results suggest a cellular separation of myrosinase enzyme and glucosinolate substrate, and that myrosinase is contained in distinct cells. We discuss the potential advantages of locating defense and communication systems to only a few specific cell types.Arabidopsis ecotype Columbia has been shown to contain 23 different glucosinolates (Hogge et al., 1988). Myrosinase (EC 3.2.3.1), also known as ␤-thioglucoside glucohydrolase, catalyzes the hydrolysis of glucosinolates into Glc and an unstable intermediate that undergoes nonenzymatic rearrangement to form sulfate and isothiocyanates, thiocyanates, nitriles, epithioalkanes, or elementary sulfur dependent on the concentration of H ϩ , metal ions, epithiospecifier protein, and/or other cofactors (Bones and Rossiter, 1996; Foo et al., 2000). The complexity of the myrosinase-glucosinolate system suggests a diverse and multifunctional role in the cruciferous plants.Glucosinolates are a diverse group of sulfurcontaining glycosides that may serve as a sink for nitrogen and sulfur, and the hydrolysis products may have important roles in the defense of the plant against microorganisms and insects (Bones and Rossiter, 1996; Rask et al., 2000). Previous studies have shown that myrosinase is located in idioblasts named myrosin cells, which have been found in several species of Brassicacea, including oilseed rape (Brassica napus), white mustard (Sinapis alba), cauliflower (B. oleracea), and Chinese cabbage (B. campestris; Bones and Iversen, 1985; Thangstad et al., 1990 Thangstad et al., , 1991 Bones et al., 1991; Hö glund et al., 1991 Hö glund et al., , 1992 Geshi et al., 1998). Myrosin cells occur as scattered cells in radicles, stems, leaves, petioles, seeds, and seedlings (Bones and Rossiter, 1996; Rask et al., 2000). The myrosinase enzyme has been localized to myrosin cells by immunocytochemical methods (Bones et al., 1991; Thangstad et al., 1990 Thangstad et al., , 1991 Hö glund et al., 1991; Geshi et al., 1998), a...

Functional Differentiation of Brassica napus Guard Cells and Mesophyll Cells Revealed by Comparative Proteomics

“…Interestingly the myrosinase system including myrosinases, myrosinase-binding proteins, and myrosinase-associate proteins generally did not show high expression in B. napus GC. This is in strong contrast to what was found in Arabidopsis (52,53). The functional significance of the difference is not known.…”

The Potassium-Dependent Transcriptome of Arabidopsis Reveals a Prominent Role of Jasmonic Acid in Nutrient Signaling

“…The counterpart to the synthesis of glucosinolates is their specific degradation by thioglucoside glucohydrolase also known as myrosinases. None of the three myrosinase genes characterized to date (TGG1, At5g26000; TGG2, At5g25980; pseudogene TGG3, At5g48375; Husebye et al, 2002) changed expression in our experiments. However, two myrosinase-like genes (BGL1, At1g52400 and At5g28510) were strongly up-regulated during K 1 starvation (average fold change of 8.6 and 7.6, respectively) and down-regulated upon K 1 resupply.…”

Defence mechanisms of Brassicaceae: implications for plant-insect interactions and potential for integrated pest management. A review

“…Myrosin cells exist as scattered cells in stems, leaves, seeds, seedlings, petioles and roots. Brassica plants contain the enzyme myrosinase (β-thioglucoside glucohydrolase, thioglucosidase, EC 3.2.3.147 (formerly EC 3.2.3.1) (Bones and Slupphaug, 1989;Bones, 1990;Bones andRossiter, 1996, 2006), which is thought to be exclusively present in myrosin cells (Thangstad et al, 1990Bones et al, 1991;Höglund et al, 1991;Husebye et al, 2002;Thangstad et al, 2004;Kissen et al, 2009). In brassicas, myrosinases can be divided into three different gene families; the MA, MB and MC families (Xue et al, 1992;Chadchawan et al, 1993;Lenman et al, 1993;Thangstad et al, 1993;Falk et al, 1995).…”

“…Glucosinolates and myrosinases are spatially segregated (Kelly et al, 1998;Koroleva et al, 2000;Husebye et al, 2002), but insect herbivory or tissue damage bring them together, which facilitates glucosinolate hydrolysis into thiocyanates, isothiocyanates, nitriles, oxazolidine-2-thiones and epithionitriles, depending upon pH and other conditions (Fig. 4) (Pivnick et al, 1992;Bones andRossiter, 1996, 2006;Wittstock and Halkier, 2002).…”