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

I. Spore Attachment and Invasion.- 1. Adhesion of Fungi to the Plant Surface: Prerequisite for Pathogenesis.- 2. Signaling for Infection Structure Formation in Fungi.- 3. The Plant Cell Wall as a Barrier to Fungal Invasion.- 4. Rust Basidiospore Germlings and Disease Initiation.- 5. Attachment of Mycopathogens to Cuticle: The Initial Event of Mycoses in Arthropod Hosts.- 6. The Fate of Fungal Spores in the Insect Gut.- 7. Candida Blastospore Adhesion, Association, and Invasion of the Gastrointestinal Tract of Vertebrates.- 8. Infectious Propagules of Dermatophytes.- II. Fungal Spore Products and Pathogenesis.- 9. Melanin Biosynthesis: Prerequisite for Successful Invasion of the Plant Host by Appressoria of Colletotrichum and Pyricularia.- 10. The Plant Cuticle: A Barrier to Be Overcome by Fungal Plant Pathogens.- 11. Appearance of Pathogen-Related Proteins in Plant Hosts: Relationships between Compatible and Incompatible Interactions.- 12. The Role of Cuticle-Degrading Enzymes in Fungal Pathogenesis in Insects.- 13. Potential for Penetration of Passive Barriers to Fungal Invasion in Humans.- 14. Dihydroxynaphthalene (DHN) Melanin and Its Relationship with Virulence in the Early Stages of Phaeohyphomycosis.- III. Host Response to Early Fungal Invasion.- 15. Invasion of Plants by Powdery Mildew Fungi, and Cellular Mechanisms of Resistance.- 16. Induced Systemic Resistance in Plants.- 17. The Plant Membrane and Its Response to Disease.- 18. The Fungal Spore: Reservoir of Allergens.- 19. Conidia of Coccidioides immitis: Their Significance in Disease Initiation.- 20. Cell-Mediated Host Response to Fungal Aggression.- 21. Suppression of Phagocytic Cell Responses by Conidia and Conidial Products of Aspergillus fumigatus.- IV. Molecular Aspects of Disease Initiation.- 22. Molecular Approaches to the Analysis of Pathogenicity Genes from Fungi Causing Plant Disease.- 23. Current Status of the Molecular Basis of Candida Pathogenicity.- Taxonomic Index.

The Fungal spore and disease initiation in plants and animals

The aerial surfaces of plants are covered with a wax layer that is primarily a waterproof barrier but that also provides protection against environmental stresses. The ubiquitous presence of cuticular wax is testimony to its essential function. Genetic and environmental factors influence wax quantity and composition, which suggests that it is an actively regulated process. The basic biochemistry of wax production has been elucidated over the past three decades; however, we still know very little about its regulation. This review presents a discussion along with new perspectives on the regulatory aspects of wax biosynthesis. Among the topics discussed are the partitioning of fatty acid precursors into wax biosynthesis and the elongation of fatty acids with particular emphasis on the nature of the acyl primer, and the role of ATP in fatty acid elongation. The recent cloning of wax biosynthetic genes and the transport of wax to plant surfaces are also discussed.

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Biochemistry and molecular biology of wax production in plants

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

Differential deposition of suberin phenolic and aliphatic domains and their roles in resistance to infection during potato tuber (Solanum tuberosum L.) wound-healing

Publisher Summary This chapter discusses lipid-derived defensive polymers and waxes and their role in plant-microbe interaction. Elongation of fatty acids by cell-free preparations from epidermal cells, where alkanes are known to be generated, has been demonstrated. Thus, microsomal preparations generate > C 20 acids from acyl-CoA using malonyl-CoA and NADPH as substrates. Although long acids with chain lengths approaching those of the alkanes can be generated by some of the cell-free preparations, chain length distribution of the products generated in vitro does not often correspond to that of the alkanes. As the epidermis generates many classes of lipids, each with its own characteristic chain length distribution, it is likely that different chain-elongating enzyme systems are involved in their synthesis. The different cell-free preparations so far studied contain more than one elongating system. The results obtained with the crude solubilized preparations supported the previous conclusions that multiple elongating systems are present in the membrane preparations.

P.E. Kolattukudy

Papers

Lipid-Derived Defensive Polymers and Waxes and Their Role in Plant–Microbe Interaction