They excrete into the plant not toxins that kill cells, but suppressor substances, or effectors , suppressing the course of immune reactions. In the results of this cell, the plants remain alive, but helpless, unable to repel the parasite attacking them, and the fungus acquires the ability to feed on living cells. Of course, even in this case the infected cells die, but by the time of their death the hyphae of the fungus have already advanced and will infect new ones. Thus, when the plant is infected with biotrophic fungi, in contrast to necrotrophic fungi, the spread of the parasite in the tissue does not lag behind the necrosis, but outstrips it.
The nutrition of the contents of living cells caused the parasites to change the ways of attack in such a way that the cells of the infected plant remain viable for as long as possible. And for this, as it was said, one can not damage the cytoplasmic membrane (plasmalemma) surrounding the protoplast. Some biotrophic fungi do not infect cells at all, their hyphae spread inside the plants (endophytio) across the intercellular spaces and, with the help of their metabolites, cause plants to excrete into the intercellular spaces nutrients that the parasite uses for itself. But most biotrophs secrete from the tip of the hypha, which contacts the cell wall of the plant, a few enzymes that locally destroy the cell wall, and through the formed hole of the hypha reaches the plasmalemma, which does not destroy, but gently presses into the cell. A membrane bag is formed in which the expanded end of the hypha is located. Since the portion of the plasmalemma surrounding the haustorium is not covered by the wall, a signal is sent to the nucleus of the infected cell to fix the breach, and a stream of vesicles rushes to the site of infection, containing sugars, which, instead of going to the construction of the wall, are intercepted by a haustorium using them as food (Figure 1.4).
Major diseases caused by biotrophic fungi:
- plaques , in which the surface of the leaves and/or stems is covered with a white or gray-violet coating formed by the mycelium and spores of the parasite. Often the leaves are as if sprinkled with flour - this disease is called powdery mildew ;
- pustules are formed due to the fact that the plants forming inside (eidophytic) are sporiferous and the spores exert pressure on the cuticle (and some fungi also release enzymes - depolymerases), rupture and expose spore-staining spots (Figure 1.5).
Fig. 1.4. Haustoria of the rust fungus in the host plant cell :
hrnc - the parent cell of the haustorium; h - Haustoria; ehm - exohaustorial membrane (continuation of host cell plasmalemma)
Fig. 1.5 . Stem rust Puccinia graminis :
photo of pustule with urediniosporami
Depending on the color of the spores, the spots may be red-brown ( rust ) or white (rust white) ';
- bunt : some fungi form dense accumulations of mycelium in the infected tissues of plants, which decompose into separate cells covered with thick dark-colored walls. The weight of the spores on the surface of the infected organs gives them the appearance of charred fireheads. The head is most often developed on the generative organs of plants (ears, panicles), but is also found on leaves and stems (Figure 1.6);
Fig. 1.6. Head mushrooms of cereals:
left to right - oat bunt, hard and dusty wheatgrass
tumors , galls : many biotrophic fungi release into plants substances that cause not the death of their cells, but on the contrary, growth, resulting in a tumor inside which is a parasite (Figure B.1). The increase in cell size is called hyperplasia , and the increase in the size of the plant's organ is hypertrophy.
A huge number of mushrooms - plant parasites - has a mixed type of food. Initially, they behave like biotrophs, grow in intercellular space and are able even to form haustoria, but after the death of plant cells - they occupy them and develop as necrothrophy.
The most common disease symptom caused by hemibiotrophs is spotting , in which local dark or light spots are sometimes formed on infected organs, sometimes surrounded by a red rim due to accumulation of phenolic pigments as a response to infection. On thick, watered tissues, for example, beans of beans or peas, melon or pumpkin fruits, spots can be pressed into the fetal tissue and contain on the surface mucus into which the fungal spores are submerged. This type of disease is called anthracnose.
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