LOCALIZATION AND TYPE OF CELLS INVOLVED IN THE EMISSION OF VOCs

In the last decades, the discovery of genes encoding enzymes catalyzing the VOCs’ formation (Chapter 8; Dudareva, Pichersky, and Gershenzon 2004) enabled not only to improve our understanding of the regulation of VOCs production but also to discover the sites of their biosynthesis within the plant tissue. It has been shown that the biosynthesis of VOCs in flowers occurs almost exclusively in epidermal cells, which are in closest proximity to the atmosphere (Dudareva et al. 1996; Scalliet et al. 2006). Indeed, snapdragon 5-adenosyl-l-methionine:benzoic acid carboxyl methyltransferase (BAMT) that catalyzes methylbenzoate formation (Kolosova et al. 2001) and rose orcinol O-methyltransferase responsible for 3,5-dimethoxytoluene production (Scalliet et al. 2006), both display uniform distribution along the petal cross sections with highest expression in the cells of adaxial epidermal cell layer, and lower levels in the cell of abaxial epidermis. Interestingly, the adaxial epidermal cell layer of some flowers has a unique conical-shape presumably to enhance emission (Kolosova et al. 2001; Bergougnoux et al. 2007).

In leaves, and stems, epidermal cells can develop into specialized structures, leaf-hairs or tri- chomes, that are involved in the biosynthesis, storage and release of VOCs (McCaskill, Gershenzon, and Croteau 1992; Turner, Gershenzon, and Croteau 2000; Schilmiller, Last, and Pichersky 2008). Glandular trichomes on aerial organs are widespread in the plant kingdom and can be found in 30% of all vascular plants (Glas et al. 2012). These structures are biosynthetically active and produce volatiles of all major classes, but the blends are often dominated by terpenoids and phenylpropanoids (Dai et al. 2010). Despite the fact that trichomes accumulate high levels of VOCs, they may not actively emit volatiles but release their content upon mechanical disruption (Iijima et al. 2004).

Vegetative VOC production is not limited to trichomes and can also occur in the inner leaf tissues. Indeed, in gray poplar (Populus x canescens) expression of isoprene synthase, catalyzing isoprene formation was shown to be highest in palisade parenchyma and significantly decreasing within the spongy mesophyll (Cinege et al. 2009). While VOCs produced in leaves could be released via stomata (Niinemets and Reichstein 2003; Hiive et al. 2007), until now', there is no direct or conclusive evidence supporting this hypothesis. Regardless of stomata involvement, at the subcellular level, vegetative VOCs still have to move from the site of their biosynthesis through the cytosol, the PM, and a hydrophilic layer of the cell wall to be released either to the intercellular air spaces connected to stomata or exit the cell through the cuticle.

Volatiles produced in roots are emitted to rhizosphere as part of belowground defense mechanism (Rasmann et al. 2005; Robert et al. 2012). Analysis of VOCs production revealed that it occurs in many types of root cells, but not in the root tip. Expression of 1,8-cineole synthase, for example, was found primarily in the epidermis, cortex, and stele of mature primary and lateral roots, but not in the root meristem or the elongation zone (Chen et al. 2004). In addition, roots of many plant species have a unique type of cells, border cells, which form a boundary between the root and the rhizosphere (Baetz and Martinoia 2014) and were shown to accumulate and secrete specialized metabolites, including VOCs (e.g., hexanal) (Watson et al. 2015).

 
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