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Analysis of SCN invasion using Microscopy: Collaborator: Robert Yaklich The roles of individual resistance genes in the resistance responses to the soybean cyst nematode (SCN) are unknown. At the cellular level, several mechanisms have been proposed for resistance of soybean to SCN. One mechanism is formation and thickening of cell wall material to seal off the syncytium. Another proposed mechanism is early degeneration of the syncytium, while a third proposal is necrosis of cells that are immediately surrounding the juvenile nematode. Some of the morphological changes seen by light, transmission, and scanning electron microscopy of soybean roots induce by SCN invasion have been documented in the literature. Certainly, the area around the site of invasion is metabolically hyperactive. After invasion of a susceptible root, the nematode may feed for up to two months. During this time, a syncytium or giant feeding cell is formed from which the nematode feeds. A number of changes occurs to the cells. The nuclei and nucleoli hypertrophy, cytoplasmic organelles proliferate, the central cell vacuole is reduced or lost, the cell expands, and the cell wall becomes perforated. The perforations enlarge as the cell expands. Surrounding cells merge at the perforations to form a syncytium. In contrast, in the SCN resistant soybean cultivar 'Forrest' a necrotic layer surrounds the syncytium and the syncytium becomes necrotic. This is similar to the response noted in the resistant cultivar 'Peking', wherein wall depositions formed and there was an increase in lipid globules before necrosis. Syncytia were degenerating four days after inoculation with SCN. Thus, a major mechanism of resistance may be to wall off the area of nematode invasion with secondary wall to either starve the nematode or to allow toxic by-products to build up in the area. Another resistant cultivar 'Bedford' had a somewhat different response, in that the nuclei degraded, then the cytoplasm degraded. Interestingly, syncytia form in both resistant and sensitive cultivars. We will build upon this knowledge base to monitor changes seen in cells of soybean roots with different levels of sensitivity and resistance to different races of SCN. We hope to correlate specific cellular changes with differences in gene expression patterns obtained from microarray analysis. 
Fig. Anterior portion of nematode with stylet puncturing a soybean root cell. |