Agriculture Reference
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they are ubiquitously produced in many plant-pathogen interactions. Penetration of
plant surfaces might be supported by these enzymes.
Essential to an understanding of cuticle and CWDEs is the knowledge of the
complex structure of these surface barriers. The first barrier to be breached is the
plant cuticle which covers the epidermal cells. It consists of two lipid polymers,
which may occur in any ratio, embedded into wax. The dominant structural
component is the lipid polyester cutin, a polar cross-linked polymer containing
predominantly C16 and C18 fatty acids. It is readily solubilized by alkaline
hydrolysis. A very insoluble residue remains after saponification - the poly-
methylene polymer cutan. For detailed reviews, see Jeffree (1996) and Riederer
and Schreiber (2001).
After breaching the host cuticle, the next barrier to the invading fungus is the cell
wall. In mature plant cells, the cell wall mainly consists of two layers: the primary
and secondary cell wall. The structural complexity of the primary cell wall is
described in Carpita and Gibeaut (1993) and Carpita et al. (1996).
The major polysaccharides of the walls of many flowering plants have been
described. In most flowering plants the cell wall consists of chains of ß-1,4-linked
glucose interwoven with a xyloglucan polymer embedded in a matrix of pectin. In
contrast, the cellulose microfibrils of primary cell walls of the Poaceae contain
chains of ß-1,4-xylose, instead of xyloglucan, which are connected by arabinose and
less frequently by glucoronic acid. Both types of primary cell wall are associated
with protein components. The major structural protein is extensin, a hydroxyprolin-
rich glycoprotein (Showalter, 1993).
Cuticle degrading enzymes . The diversity of the physical structure and chemical
composition varies not only in different plant species. Even within the same species,
it depends on the type of tissue, environmental conditions and the age of the plant.
Cuticle penetration could be achieved by degradation of the cutin polymer by
cutinases which are serine esterases belonging to the
-ß hydrolase fold class of
lipases (Longhi and Cambillau, 1999). Numerous plant pathogenic fungi produce
cutinases but the role of these enzymes in pathogenicity is disputed. In some cases
the fungi only penetrate the cuticle and grow between the cuticle and cell wall.
Rhynchosporium secalis (the causal agent of barley leaf scald) and Venturia
inaequalis (causal agent of apple scab) develop such subcuticular mycelia.
The first cutinase studied in plant pathogens was that of the pea pathogen
Fusarium solani f.sp. pisi ( Nectria haematococca ). Kolattukudy and co-workers
demonstrated the secretion of this enzyme during penetration of the host cuticle
(Shaykh et al. , 1977). Inhibitors of cutinase as well as antibodies to it prevented
fungal infection on intact host surfaces but no effect has been found on wounded
cuticle (Maiti and Kollatukudy, 1979; Köller et al. , 1982). Furthermore, insertion of
a cutinase gene derived from F. solani f.sp. pisi to the cutinase-deficient wound
parasite Mycosphaerella spp. enabled the transformants to infect intact surfaces of
papaya fruits (Dickman et al. , 1989).
However, gene disruption studies performed with F. solani (Stahl and Schäfer,
1994; Stahl et al. , 1994), Magnaporthe grisea (Sweigard et al. , 1992) and Botrytis
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