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  Section: Molecular Biology of Plant Pathways » Pathways for the Synthesis of Polyesters in Plants: Cutin,
  Suberin, and Polyhydroxyalkanoates
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Plants synthesize several classes of hydrophobic biopolyesters. Cutin and suberin, two complex lipid-based polyesters, are unique to the plant kingdom. Cutin is the main part of the cuticle (representing 40–80% of the cuticle) and evolved circa 400 million years ago when vascular plants established themselves on dry land and needed a barrier to protect themselves from water loss and various environmental aggressions. Although the structures of cutin and suberin are related, being primarily composed of esterified fatty acid derivatives, several features distinguish them. Notably, cutin forms a continuous layer covering the epidermal cell layer of all aerial portions of the plant, while the deposition of suberin is more diversified, encompassing both roots and aerial organs. Several reviews have been published in the past years on the structure and biochemistry of cutin and suberin (Bernards, 2002; Heredia, 2003; Kolattukudy, 2001; Nawrath, 2002). This review will particularly focus on recent insights on the complex structure and composition of cutin and suberin, as well as report on the advances that have been made to understand their biosynthesis.

The third type of polyester naturally found in plants is polyhydroxybutyrate (PHB), a polymer of 3-hydroxybutyric acid and a member of the family of polyhydroxyalkanoates (PHAs). Although the literature on PHA is primarily focused on the high-molecular-weight polyester produced in bacteria as a carbon reserve that has thermoplastic properties, a low-molecular-weight PHB is also produced in prokaryotes and eukaryotes (Reusch, 1999). This low-molecular-weight PHB, referred to as cPHB, is found in membranes associated with polyphosphate and has been detected in very small quantities in a wide spectrum of organisms, including bacteria, yeast, plants, and animal tissues (Reusch, 1999). The biochemical pathway of
cPHB has not been identified and its physiological role remains uncertain, although the polyphosphate/PHB complex has been found to have ion channel properties (Reusch, 1999, 2002). In view of the paucity of information on cPHB associated with plants, this chapter will focus on the synthesis of the highmolecular- weight PHA, hereafter simply referred to as PHA, which has been produced in transgenic plants as a source of renewable and environment-friendly plastics.

Despite the interesting properties of PHAs as biodegradable thermoplastics and elastomers, use of these bacterial polyesters as substitutes for petroleumderived plastics is limited by the expenses related to bacterial fermentation, making bacterial PHA substantially more expensive than petroleum-based polymers, such as polypropylene. It is in this context that agriculture has been regarded as a promising alternative for the production of PHAs on a large scale and at low cost (Poirier, 1999; Poirier et al., 1995a). Transgenic plants producing different types of PHAs have now been demonstrated in several species and will be described in this chapter. Synthesis of PHA in crops fits into a larger concept of using plants as vectors for the renewable and sustainable synthesis of carbon building blocks that are presently largely provided by the petrochemical industry.

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