Future Perspectives
During the past years, research on cutin and suberin demonstrated that the structure
of these polyesters is complex and well organized. Some progress has been made
to identify genes and proteins involved in cutin and suberin biosynthesis. Although
mutant phenotypes indicating defects in cutin formation have been identified,
no means to identify mutants in suberin formation have yet been found. However,
even when mutants have been found, more work is required to link the genes
identified by mutation to the exact functions of the proteins and a deeper understanding
of the formation of the polyesters. In
Arabidopsis, many resources are
available that might contribute to the understanding of cutin and suberin biosynthesis
in the future. In addition, analysis of cutin monomer composition have been
shown to be feasible in
Arabidopsis, an important progress for assigning functions to
proteins (Bonaventure
et al., 2004; Franke
et al., 2005; Xiao
et al., 2004).
Meanwhile, cutin and suberin began to attract attention as biological polymers
(Kolattukudy, 2001). Studies were undertaken to increase our knowledge of
the physical properties of cutin and suberin (Cordeiro
et al., 1998; Heredia,
2003). A detailed review on the physical properties of the cutin of tomato
has been published (Heredia, 2003). Results will be briefly summarized here. Cutin is an amorphous and insoluble polymer with a molecular spacing of
0.4–0.5 nm between the polymer chains, having very low water sorption and
permeability. The specific heat of cutin is higher than that of other polymers of
the cell wall, possibly playing a role in thermoregulation of the plant.
Most of the
water diffuses as single molecules through the cuticle and not through pores
(Riederer and Schreiber, 2001). These water molecules may act as a plasticizer,
contributing to the molecular flexibility of the polymer(s) resulting in a viscoelastic
polymer network. In this context, it may be important to consider that foliar
application of chemicals may change the permeability of the polymer, possibly
affecting problems related to a too rigid cutin polymer, such as cuticle cracking of
fruits (Aloni
et al., 1998).
More research will be needed until the synthesis of these natural polyesters
is understood well enough to consider engineering them to either improve
their properties
in situ in order to make plants more stress resistant, that is, reduce
the cracking of the cuticle of fruits, or use them in industrial applications (Aloni
et al., 1998).
Until then, the natural polymers may be used for some industrial applications.
For example, cuticular material containing 40–80% cutin occurs in large quantities
as a valuable by-product in the waste of fruit processing. Refractory to most
treatments, cutin may be recovered from waste by physical, chemical, and
biological processes, and the monomers released by hydrolysis could be polymerized
for various applications, for example, as either lubricants or for biomedical
applications.
Cork, the bark of
Quercus suber, contains up to 50% suberin, besides 22% lignin,
20% carbohydrates, and some additional extractable components. This
polymer has already been commercially exploited for centuries. The excellent
insulation property for polar liquids gives cork its special importance in the
wine industry as stoppers. Cork also insulates against sound and heat and is
used in insulation boards. Over 280,000 tons of raw material are used per year,
from which about 20–30% is left as waste in the form of cork dust, which
could potentially be useful in other applications. Recently, cork has also been
tested in ink as well as a base for the synthesis of polyurethane (Cordeiro
et al., 1997, 2000). Cork extracts are also recognized as having antimutagenic effects
(Krizkova
et al., 1999).
