Appressorium Formation and Maturation

Appressorium Formation and Maturation Once appressoria are formed, they adhere tightly to the leaf surface. Subsequently, appressoria secrete extracellular enzymes, generate physical indicates that the maturation of appressoria and their specific biochemical activity are intimately associated with genetic control of the initial development of appressoria.

The production of penetration hyphae by appressoria, or directly from germ tubes, is not well understood at the genetic level. Production of the penetration peg requires the localization of actin to the hyphal tip and rapid biosynthesis of the cell wall as the hypha grows through the cuticle and the layers of the epidermal cell walls. Production of penetration hyphae appears to be regulated by a MAP kinase pathway.

Recognition between Host and Pathogen

Recognition between Host and Pathogen It is still unclear how pathogens recognize their hosts and vice versa. It is assumed that when a pathogen comes in contact with a host cell, an early event takes place that triggers a fairly rapid response in each organism that either allows or impedes further growth of the pathogen and development of disease.

The nature of the “early event” is not known with certainty in any host–parasite combination, but it may be one of many biochemical substances, structures, and pathways. These may include specific host signal compounds or structures, or specific pathogen elicitor molecules, and either of them may induce specific actions or formation of specific products by the other organism .

Germination of Spores and Seeds

Germination of Spores and Seeds Almost all pathogens in their vegetative state are capable of initiating infection immediately. Fungal spores and seeds of parasitic higher plants, however, must first germinate. Spores germinate by producing a typical mycelium that infects and grows into host plants or they produce a short germ tube that produces a specialized infectious structure, the haustorium.

2. Penetration

2. Penetration The initial invasion of a host by a pathogen. Many fungi and bacteria enter plants through stomata, and some enter through hydathodes, nectarthodes, and lenticels.

A. Direct Penetration through Intact Plant Surfaces

A. Direct Penetration through Intact Plant Surfaces Direct penetration through intact plant surfaces is probably the most common type of penetration by fungi, oomycetes, and nematodes and the only type of penetration by parasitic higher plants. None of the other pathogens can enter plants by direct penetration.

Of the fungi that penetrate their host plants directly, the hemibiotrophic, i.e., nonobligate parasitic ones, do so through a fine hypha produced directly by the spore or mycelium obligately parasitic ones do so through a penetration peg produced by an appressorium. The fine hypha or appressorium is formed at the point of contact of the germ tube or mycelium with a plant surface. The fine hypha grows toward the plant surface and pierces the cuticle and the cell wall through mechanical force and enzymatic softening of the cell wall substances. Most fungi, however, form an appressorium at the end of the germ tube, with the appressorium usually being bulbous or cylindrical with a flat surface in contact with the surface of the host plant. Then, a penetration peg grows from the flat surface of the appressorium toward the host and pierces the cuticle and the cell wall.

Direct penetration

Direct penetration

Parasitic higher plants also form an appressorium and penetration peg at the point of contact of the radi

Parasitic higher plants also form an appressorium and penetration peg at the point of contact of the radicle with the host plant, and penetration is similar to that in fungi. Direct penetration in nematodes is accomplished by repeated back-and-forth thrusts of their stylets. Such thrusts finally create a small opening in the cell wall; the nematode then inserts its stylet into the cell or the entire nematode enters the cell (Fig. 2-12).