Suberin degradation

Suberized cell walls are some of the last components of tree barks that remain after being buried in soil for years [135]. Fungi can penetrate suberized walls as 

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When polyester-hydrolyzing activity was isolated using synthetic polyesters such as polycaprolactone, and the enzyme was examined in detail, it was found that it was a cutinase that was responsible for the hydrolysis [113]. Similarly, the polyester domains of suberin were found to be degraded by cutinase. Cutinase is a polyesterase, and similar enzymes may be widely distributed and can degrade a variety of natural and synthetic polyesters. Microbial polyhydroxy-alkanoic acids that are attracting increasing attention as biodegradable polyesters can be hydrolyzed by bacterial polyesterases that share some common features with cutinases [114] and this area is covered in another chapter [115]. 

In certain instances, however, factors other than the cell wall polymers of the phellem may be important in the protection provided by the secondary surface. Rosellinia desmazieresii inoculated in a food base onto the underground stems of a resistant Salix repens hybrid (5. x Friesiana) exhibited greatly reduced epiphytic growth and cord formation compared with inoculations onto susceptible S. repens itself. Attempted penetration was not observed on the resistant hybrid (30). This behaviour suggests that diffusible chemical inhibitors at the stem surface may be important in resistance to this pathogen, which has a demonstrated ability to degrade suberin and penetrate the surface periderm (30). 

The only way for microbes to enter a healthy plant is via the stomata or at sites of injury, inflicted by herbivory, wind, or other accidents. At the site of wounding, plants often accumulate suberin, lignin, callose, gums, or other resinous substances which close off the respective areas (4.17). In addition, antimicrobial agents are produced such as lysozyme and chitinase, lytic enzymes stored in the vacuole which can degrade bacterial and fungal cell walls, protease inhibitors which can inhibit microbial proteases, or secondary metabolites with antimicrobial activity. 

Based on detailed analyses of the chemical nature of SOM, Hatcher and Spiker (1988) have extended this humification model to include other resistant biopolymers, including plant cutin and suberin, and microbial melanins and paraffinic macromolecules. During decomposition, these biopolymers are selectively preserved and modified to become part of what can be operationally defined as humin (acid and alkali insoluble component of humus) (Hatcher and Spiker, 1988 Rice, 2001). The humin becomes progressively enriched in acidic groups leading to the formation of first humic acids and then fulvic acids, which under this degradative scheme of SOM formation would be regarded as the most humified of humic substances (Stevenson, 1994). 

B. Degradation of Cutin and Suberin 1. Bacterial and Mammalian Degradation... 

Cutin and suberin are embedded in a matrix of wax. They are degraded by exoenzymes with esterase activity (cutinases). After the decay of plants most of the cutin and suberin is hydrolyzed by soil bacteria and fungi. Degradation of both types of compounds also plays a role in the penetration of plants by phyto-pathogenic fungi (E 5.4). 

Picea abies TEM, SEM Fungal degradation of bark lamellar suberin in CaC204 crystal cells 343, 349, 482... 

Degradation of suberin was demonstrated by chemical analysis in the case of attack on the periderm of Rubus idaeus by Thomasiniana theobaldi (155). After enzymatic attack by larvae-released enzymes, the periderm was shown to have 52% fewer aliphatic suberin components, and hemicellulose and cellulose were decreased by 28% and 54%, respectively. The cell wall degradation caused by this insect attack was thought to provide subsequent access for fungal infection. 

Since suberin contains both aliphatic and aromatic domains, attempts were made to detect enzymes that degrade either domain. For the aromatic domains, S. tuberosum wound-periderm suberin, biosynthetically labeled by incorporation of [ H]cinnamate into wound-healing S. tuberosum slices, was used, and the release of soluble radioactivity was used as a measure of such activity. To measure the activity of the enzyme that hydrolytically releases the esterified aliphatic acids, either S, tuberosum periderm suberin, biosynthetically labeled with [ H]oleic acid, or radioactive cutin was used as the substrate. The extracellular fluid from... 

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