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).
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