Cutin, a biopolyester composed of hydroxy and epoxy fatty acids, is the barrier between the aerial parts of higher plants and their environment. Suberin a polymer containing aromatics and polyesters, functions as a barrier in underground parts, wound surfaces, and a variety of internal organs. The composition and probable structure of these polymers are discussed. The biosynthesis of the hydroxy, epoxy, and dicarboxylic acids of the polyesters from the common cellular fatty acids is elucidated. An extracellular enzyme transfers the hydroxy and epoxyacyl moieties from their coenzyme A derivatives to the growing polyester. The enzymes acting in the biodegradation of the polyesters have been isolated from fungi, pollen, and mammals and characterized. The function and possible practical implications of these polyester barriers are briefly discussed.

https://science.sciencemag.org/content/sci/208/4447/990.full.pdf

Biopolyester Membranes of Plants: Cutin and Suberin

Most plant-pathogenic fungi gain access into their host by penetration of unwounded tissue Some pathogens such as rusts invade the host via stomata (Hoch and Staples, 1987 and Chapter 2), whereas others penetrate the intact leaf surface without the requirement of natural openings (Aist, 1976; Emmett, 1975; Kunoh, 1984) The latter type of direct penetration encounters the plant cuticle, a noncellular hydrophobic structure covering the layer of epidermal cells The cuticle thus serves as the first surface barrier that the pathogen has to breach There is little evidence for the mere physical strength of the plant cuticle as a major factor in plant defense against pathogens In some cases, the thickness of plant cuticles has been correlated with an increased passive resistance against fungal attack This correlation, however, appears to be an exception rather than the rule (Martin, 1964) Furthermore, the cuticle has not been considered to play a major role in the active defense mechanisms of disease-resistant cultivars There is good evidence that the cuticle is penetrated by the attacking pathogen before the sequential steps of disease development are halted by the active defense reactions of the challenged plant Recent examples for this lack of cuticle involvement in cultivar resistance are the host—pathogen interactions of Venturia ivaequalis—apple (Valsangiacomo and Gessler, 1988) or Phytophthora infestans—potato (Gees and Hohl, 1987) The breaching of the cuticle can also be accomplished in many interactions of pathogens with nonhost plants (Heath, 1987)

The Plant Cuticle

Polyesters occur in higher plants as the structural component of the cuticle that covers the aerial parts of plants. This insoluble polymer, called cutin, attached to the epidermal cell walls is composed of interesterified hydroxy and hydroxy epoxy fatty acids. The most common chief monomers are 10, 16-dihydroxy C16 acid, 18-hydroxy-9, 10 epoxy C18 acid, and 9, 10, 18-trihydroxy C18 acid. These monomers are produced in the epidermal cells by ω hydroxylation, in-chain hydroxylation, epoxidation catalyzed by P450-type mixed function oxidase, and epoxide hydration. The monomer acyl groups are transferred to hydroxyl groups in the growing polymer at the extracellular location. The other type of polyester found in the plants is suberin, a polymeric material deposited in the cell walls of a layer or two of cells when a plant needs to erect a barrier as a result of physical or biological stress from the environment, or during development. Suberin is composed of aromatic domains derived from cinnamic acid, and aliphatic polyester domains derived from C16 and C18 cellular fatty acids and their elongation products. The polyesters can be hydrolyzed by pancreatic lipase and cutinase, a polyesterase produced by bacteria and fungi. Catalysis by cutinase involves the active serine catalytic triad. The major function of the polyester in plants is as a protective barrier against physical, chemical, and biological factors in the environment, including pathogens. Transcriptional regulation of cutinase gene in fungal pathogens is being elucidated at a molecular level. The polyesters present in agricultural waste may be used to produce high value polymers, and genetic engineering might be used to produce large quantities of such polymers in plants.

Polyesters in Higher Plants

Synthetic plastics are pivotal in our current lifestyle and therefore, its accumulation is a major concern for environment and human health. Petroleum-derived (petro-)polymers such as polyethylene (PE), polyethylene terephthalate (PET), polyurethane (PU), polystyrene (PS), polypropylene (PP), and polyvinyl chloride (PVC) are extremely recalcitrant to natural biodegradation pathways. Some microorganisms with the ability to degrade petro-polymers under in vitro conditions have been isolated and characterized. In some cases, the enzymes expressed by these microbes have been cloned and sequenced. The rate of polymer biodegradation depends on several factors including chemical structures, molecular weights, and degrees of crystallinity. Polymers are large molecules having both regular crystals (crystalline region) and irregular groups (amorphous region), where the latter provides polymers with flexibility. Highly crystalline polymers like polyethylene (95%), are rigid with a low capacity to resist impacts. PET-based plastics possess a high degree of crystallinity (30-50%), which is one of the principal reasons for their low rate of microbial degradation, which is projected to take more than 50 years for complete degraded in the natural environment, and hundreds of years if discarded into the oceans, due to their lower temperature and oxygen availability. The enzymatic degradation occurs in two stages: adsorption of enzymes on the polymer surface, followed by hydro-peroxidation/hydrolysis of the bonds. The sources of plastic-degrading enzymes can be found in microorganisms from various environments as well as digestive intestine of some invertebrates. Microbial and enzymatic degradation of waste petro-plastics is a promising strategy for depolymerization of waste petro-plastics into polymer monomers for recycling, or to covert waste plastics into higher value bioproducts, such as biodegradable polymers via mineralization. The objective of this review is to outline the advances made in the microbial degradation of synthetic plastics and, overview the enzymes involved in biodegradation.

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Microbial and Enzymatic Degradation of Synthetic Plastics

Publisher Summary The polymers are embedded in or associated with a complex mixture of relatively nonpolar lipids that are collectively called wax because of the similarity of their physical properties to those of the honeycomb material. In most plants, wax can be found on the surface and the crystalline structure of this wax is a rather unique characteristic of each species. The most widespread site of occurrence of wax is the cuticle, and the abundance of production of cuticular waxes by some aerial parts of plants allows easy removal of the most familiar and widely utilized plant waxes such as carnauba wax. Internal organs usually contain little wax, except in rare plants such as jojoba in which large amounts of wax esters are stored as the major energy reserve. This chapter presents a methodology that is useful for examining waxes and polymerized lipids. It discusses the types of compounds found in plant waxes and in lipid-derived polymers and explains their biosynthesis, degradation, and possible functions.

Cutin, suberin, and waxes

The properties of the homogeneous cutinase I, cutinase II, and the nonspecific esterase isolated from the extracellular fluid of cutin-grown Fusarium solani F. pisi (R.E. Purdy and P.E. Kolattukudy (1975), Biochemistry, preceding paper in this issue) were investigated. Using tritiated apple cutin as substrate, the two cutinases showed similar substrate concentration dependence, protein concentration dependence, time course profiles, and pH dependence profiles with optimum near 10.0. Using unlabeled cutin, the rate of dihydroxyhexadecanoic acid release from apple fruit cutin by cutinase I was determined to be 4.4 mumol per min per mg. The cutinases hydrolyzed methyl hexadecanoate, cyclohexyl hexadecanoate, and to a much lesser extent hexadecyl hexadecanoate but not 9-hexadecanoyloxyheptadecane, cholesteryl hexadecanoate, or hexadecyl cinnamate. The extent of hydrolysis of these model substrates by cutinase I was at least three times that by cutinase II. The nonspecific esterase hydrolyzed all of the above esters except hexadecyl cinnamate, and did so to a much greater extent than did the cutinases. None of the enzymes hydrolyzed alpha- or beta-glucosides of p-nitrophenol. p-Nitrophenyl esters of fatty acids from C2 through C18 were used as substrates and V's and Kms were determined...

Kolattukudy Pe

Papers

Hydrolysis of plant cuticle by plant pathogens. Properties of cutinase I, cutinase II, and a nonspecific esterase isolated from Fusarium solani pisi.