General Features Of Pathogenesis

Once fungi were recognized as a pathogen, it was understood that disease is a state of altered metabolism of cells and tissues resulting from a reciprocal interaction of a parasite and host, each of which is individually influenced by the environment.

Environment

The example of a biotrophic fungus (for example, a rust fungus) can be used to understand what is conveyed by the above diagram and to point to some generic features of pathogenesis. After arriving on the leaf surface, the urediospore germinates if free water is available and the weather is mild. For infection to occur, the germ tube must find a portal of entry. The urediospore germ tube always penetrates through a natural opening in the host. However, contrary to earlier notions that held to the random encounter of germ tube with stomata or a volatile chemical emanating from the tube, the host itself contributes to fungal penetration by directing the growth of the germ tube toward itself. When the germ tube encounters a stomatal pore, a bulbous structure termed the appressorium is formed from the tip of the germ tube that is appressed to the plant surface (Figure 4.3). From the underside of the appressorium, a short infection peg is formed which forces entry into the leaf (Maheshwari et al., 1967a, b). The infection peg enlarges into a substomatal vesicle from which infection hypha develop that ramify intercellularly and form specialized structures called haustorium that penetrate the plant cell and through which nutrients from the living cells of the host are absorbed. The morphological transition from a germ tube into infective hyphae that occurs in response to thigmotropic stimulus is crucial for the initiation of parasitic

Urediospore Appressorium

Phytopathogens Fungi Stomata

Epidermis

Haustorium mother cell

Mesophyll cell of leaf

Haustorium

Figure 4.3 Diagram of infection structures of a rust fungus (Puccinia) formed on leaf. The leaf is shown in vertical section.

Urediospore Appressorium

Epidermis

Haustorium mother cell

Mesophyll cell of leaf

Haustorium

Figure 4.3 Diagram of infection structures of a rust fungus (Puccinia) formed on leaf. The leaf is shown in vertical section.

mode of life. Once an interface between the host and fungus is established, signaling mechanisms begin to operate either by initiating defense reactions that limit the colonization of tissue by fungal mycelium or the fungal mycelium overcoming these reactions and acting as a metabolic sink into which photosynthates (sugars and other host metabolites) from uninfected parts are redirected at the cost of their normal translocation pattern toward developing grains, fruits or storage organs. The formation of pustules containing spores is a morphological disruption of host tissue. The respiratory rate of the infected plant tissue is increased while photosynthesis is reduced, resulting in the grains or fruits being shrivelled or not being formed at all. The plant, in common parlance, is diseased.

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