Histology of Sclerotium cepivorum infection of onion roots and the spatial relationships of pectinases in the infection process

Abstract

Sclerotium cepivorum (isolate Sc4) hyphae penetrated the epidermis and hypodermis of onion roots and grew into the cortex. Immediately following penetration only the cells through which S. cepivorum grew were lysed, but subsequently cells were killed and cell walls disintegrated ahead of the infection hyphae. Sclerotium cepivorum produced two polygalacturonases (PG) and two pectinesterases (PE) in culture. These isozymes were also found in infected onion root tissues and another PG and a PE were occasionally detected. Two isozymes of PG and three isozymes of PE diffused ahead of the infection hyphae. The spatial distribution of these enzymes was associated with cell death and cell wall degradation. The epidermis, hypodermis, endodermis and vascular tissues were more resistant to hydrolysis than the cortex, but only the endodermis and cells within it retained nuclei following hydrolysis of the surrounding cortical tissues. The cavity within the root cortex became ®lled with swollen, vacuolate S. cepivorum hyphae.

The process of antagonism of Sclerotium cepivorum in white rot affected onion roots by Trichoderma koningii

“…Infection of sterile onion seedlings (cv. Regular Creamgold) produced in vitro with pregerminated S. cepivorum (isolate Sc4) sclerotial inocula was performed as previously described (Metcalf & Wilson, 1999). To infect seedlings a 0·5‐cm cube of soil agar containing a germinated sclerotium was placed in a sterile Petri dish and an onion seedling added with the root tip in close proximity to the sclerotium (48–72 h at 20°C).…”

“…Some preliminary observations (10 replicate roots) were made by hand‐sectioning roots using a razor blade before mounting in 0·05% (w/v) Ruthenium Red (Sigma) for 2 min, then flooding with distilled water to destain. The majority of observations (a further 30 replicate roots) were Feulgen‐stained, embedded in paraffin, sectioned and examined (Metcalf & Wilson, 1999). The experiment was repeated using a further 16 roots which were used in experiments for localization of chitinolytic enzymes.…”

“…Mycoparasitic coiling is sometimes observed, and release of chitin oligomers from the host cell wall may occur (Benhamou & Chet, 1993). Following studies to develop a better understanding of S. cepivorum 's infection process (Metcalf & Wilson, 1999), this study was undertaken to determine the mechanism by which Tr5 impeded normal development of S. cepivorum infections within onion roots. The process of root colonization, antagonism, and production of chitinolytic enzymes by Tr5 during colonization of S. cepivorum‐ infected onion roots is reported in these experiments.…”

White rot (Sclerotium cepivorum Berk) - an aggressive pest of onion and garlic in Ethiopia: An overview

“…During the early stages of infection, cell death is limited to the cells penetrated by the hyphae of S. cepivorum; however, onion cell walls in the path of the leading infection hyphae often dissolve before the hyphal tips reach them. This was shown to be indicative of enzymic action by certain pectinases (Metcalfe and Wilson, 1999). Plant-to-plant spread of infection may occur in densely-sown or closely-spaced crops.…”

“…When this occurs sclerotia germinate producing hyphae that may grow 1 to 2 cm through the soil to infect the roots of onions. They penetrate the root epidermis producing appressoria and infection cushions and the hyphae enter between cell wall junctions (Metcalfe and Wilson, 1999). Then they invade the hypodermis and grow into the cortex of the roots.…”