Introduction
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.