Morphology, element composition, and response to acids and oxidizing agents of the haustorial neckband of Uromyces phaseoli var. vignae

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Abstract
In unstained sections of glutaraldehyde-fixed, nonsporulating colonies of the cowpea rust fungus Uromyces phaseoli var. vignae, the most common type of haustorial neckband comprised a single, electron-opaque, periodic acid sensitive ring encircling the neck of the haustorium. Energy-dispersive X-ray (EDX) analysis revealed such rings to be particularly rich in iron and phosphorus. Neckbands in similarly fixed sporulating colonies usually consisted of two rings. The ring towards the haustorial body was rich in iron, phosphorus, and calcium, with minor, variable amounts of manganese and potassium. Fe:P ratios were lower than those recorded for single-ring neckbands, apparently because of a relative increase in the amount of phosphorus. The iron-rich ring was removed by periodic acid treatment, in contrast to the second ring which was insensitive to periodic acid and was shown by EDX analysis to be rich in silicon. In stained sections, neckbands were associated with a differentiated region of the haustorial neck wall, and the circumscribed fungal cytoplasm, which was present before the neckband developed. Single-ring neckbands in unstained sections from a glutaraldehyde – osmium tetroxide fixed nonsporulating colony resembled those from tissue fixed in glutaraldehyde alone with respect to appearance, element composition, and sensitivity to periodic acid. In tissue fixed with or without osmium tetroxide, an increase in the electron translucency of the neckband was induced in unstained material by treating sections with acetic or hydrochloric acid, or by conventional staining with uranyl acetate and lead citrate. Such an effect was only rarely induced by hydrogen peroxide treatment. Acid-or stain-induced erosion of the neckband often was associated with the removal of osmiophilic material from the adjacent fungal and host cytoplasm. The results suggest that the neckband region of the haustorial neck represents an area of complex, and changeable, differentiation of fungus, plant, and extrahaustorial matrix.