Polymers from suberin monomers

It has been reported that the major aliphatic monomers of suberin polymers from several tree barks are fatty acids with both a>- and mid-chain oxygenation (192). These monomers include 9,10-epoxy-18-hydroxyoctadecanoic acid and 9,10,18-trihydroxyoctadecanoic acid, both of which are common components of cutin polymers (232). For example, 9,10-epoxy-18-hydroxyoctadecanoic acid was reported to be the major monomer (31%) of Quercus ilex bark suberin and... 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

The other hpid polymer, suberin, is a heteropolymer, consisting of an aliphatic polyester associated with cross-linked polyaromatics and embedded waxes. Upon transesterification of suberin, the monomers released include C16-C28 m-hydroxy fatty acids and C16-C26 ct, -dicarboxyhc acids, the latter of which are diagnostic monomers, unsubstituted very-long-chain fatty acids (VLCFAs C>i8) and alcohols, glycerol and ferulate. Usually the major components of suberin are -hydroxy derivatives of palmitic and/or oleic acids, but in some cases oo-hydroxy C220 also is a dominant component [37]. Dicarboxylic FAs derived from further oxidation of the -hydroxy-FAs are also found in suberin. 

Lignin, suberin, vegetable oUs, tannins, natural monomers like terpenes, and monomers derived from sugars are typically natural precursors for biobased industrial polymers. Glycerol and ethanol also play a potential role as future precursors to monomers (17). 

The time course of deposition of aromatic monomers into the phenolic portion of suberin in wound-healing slices of Solanum tuberosum tubers was determined using the amount of / -hydroxybenzaldehyde and vanillin generated by alkaline nitrobenzene oxidation as a measure of aromatic deposition (85). The deposition of such phenolics into the polymer exhibited a lag period of about three days after wounding followed by several days when the deposition of phenolics increased rapidly, and subsequently the process ceased. Exogenous L-[U- C]-phenylalanine and [U- Clcinnamic acid were incorporated into the insoluble polymeric material by wound-healing slices of S. tuberosum (85). Nitrobenzene oxidation of the polymeric material derived from labeled cinnamic acid released labeled /7-hydroxybenzaldehyde and vanillin. The time course of incorporation of phenolics into the suberin polymer correlated with the time course for the deposition of aliphatic monomers into the polymer, the deposition of suberin-associated waxes into the periderm layer and the development of diffusion resistance (Fig. 6.4.12) (86). 

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