Pathological Changes in Structure

Pathological Changes in Structure Sequence of changes in infected living tissue. The response of living cells to infection has been observed in potato plants attacked byPhytophthora infestanscause of leaf blight. Potato cultivars that have the R gene are hypersensitive when inoculated with compatible races. If the cut surface of a leaf stalk or tuber is inoculated with zoospores, penetration will occur within 15 minutes and structures containing the infection will spread into the host cells within the next 20 minutes. These events occurred in a similar manner in sensitive and resistant combinations and on surfaces that were inoculated immediately after cutting and that had been previously stored for 5 hours or more in a humid chamber. However, if tissues that have undergone storage are then inoculated with an incompatible race, the cells will be killed very quickly: 30% die within 20 minutes and 80% within 40 minutes after penetration. In contrast, if a fresh cut surface was then inoculated with an incompatible race, cell death was greatly slowed, with only about half of the penetrated cells dying 8 hours after inoculation. For penetration by compatible races, both on the networkstored previously or freshly, these cells remain alive for at least 2 days while the fungus branches in the tissue.Cell death that occurs rapidly in inoculation of tissue that has undergone storage involves a certain sequence of events. In the early stages of penetration, there is a marked increase in the number of cytoplasmic threads, the cell nucleus moves towards the place where penetration occurs. This event is followed by what appears to be cytoplasmic granules in very fast Brownian motion. At this time the cell can be said to be still alive, because it absorbs neutral-red (dye) and undergoes plasmolysis. Cell death is characterized by cytoplasmic granulation and the appearance of more particles or granules in Brownian motion. Meanwhile, the contents of the cells turn yellow and finally after the Brownian movement stops the color changes to brown. For several hours after cell death, fungal hyphae grow rapidly in both hypersensitive tissue and those that are sensitive to cells that have not been killed.Cell death after penetration by ras-race P. infestansincompatible ones can also be greatly slowed by treating the tissues with metabolic inhibitors such as sodium acid. Tomiyama (1971) argued that similar metabolic paralysis caused by cutting could slow down the cell death observed when inoculation was carried out on fresh (freshly cut) tissue. In this hypothesis, hypersensitive reactions are influenced by the level of host metabolic activity.Infection of the hairy roots of grasses by zoosporesPhytium aphanidermatumalso causes very rapid cell death. As soon as the germ tube of the zoospore makes contact with the hair root, an appressorium is formed and within 20 minutes a penetration peg is formed. Once penetration occurs, cyclosis in the hair roots slows and then stops approximately 35 minutes after penetration. Five minutes later, the cytoplasm undergoes granulation and 20 minutes later severe plasmolysis occurs.The pathological response of living cells has also been studied by interference microscopy combined with histochemical methods. On shallots inoculated with Botrytis allii, cells adjacent to fungal hyphae showed a decrease in nuclear area by 28% and nuclear dry weight by 40%. Cells 2 cm apart showed a slight increase in these two traits. Results of shallot inoculation with Aspergillus nigershowed the same thing, only the increase in leaf area and core dry weight in cells within 2 cm of the hyphae was greater. The results obtained for this weak parasite were different from those obtained when observing rust-infected wheat. The nuclei in rust-infected cells increase in size and this is due to a doubling in RNA. In all the case studies carried out cell death was associated with the destruction of the nucleus and the loss of most of the DNA. Pathological Changes in Ultrastructure Even though it is possible to observe structures in full (detail) using an electron microscope, which would not be possible to observe using a light microscope, the interpretation of electron micrographs causes problems. Electron micrographs depicting dead cells, coated with slices and stained with heavy metals should be visually interpreted with caution. Because continuous and sequential observation of the same cell is not possible, the sequence of changes that occurduring infection must be deduced from indirect observations. Samples are generally taken from various times of inoculation, but the changes are rarely described synchronously to be able to discover or determine the sequence of events in an individual way. Another approach is based on the idea that if a change (X) can often be observed in the absence of a second change (Y) but Y can never be observed in the absence of X, then X precedes Y.