The use of image analysis in studies of powdery mildew haustoria: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Horticultural Science with Honours at Lincoln University

Abstract

Fungal haustoria are specialized absorbing organs whose role in the interchange of assimilates has been studied extensively, especially in the powdery mildew-pea interaction. Little is known about the structure and arrangement of the haustorial body and lobes which presumably facilitate nutrient uptake by the fungus. The purpose of this study was to visualise the complex three-dimensional structure of the pea powdery mildew haustorium and to measure surface area - volume ratios of the haustorial body and lobes using image analysis techniques. Serial ultrathin (90 nm) and 0.5µm sections of epidermal cells of pea (Pisum sativum L.) infected with Erysiphe pisi DC. were viewed with a transmission electron microscope. The images were captured electronically or recorded on a video tape and transferred to a Magiscan II image analysis system. The images were enhanced by pointsetting, grey image processing, manual thresholding and binary image processing for three-dimensional reconstruction. Both methods of image capture were found suitable for the reconstruction of the lobe structure from ultrathin sections, but electron micrographs were preferable for reconstruction of ultrastructural detail. The perimeter of the haustorial body and lobes equalled or exceeded the perimeter of the extrahaustorial membrane. The perimeter/area ratio of the haustorial body formation and lobes was 1.4 - 2.5 times that of the extrahaustorial membrane. The perimeter and area measurements of three haustoria indicated the extent of lobe and possibly the age of the haustoria. Image analysis techniques were shown to be suitable for ultrastructural studies of fungal pathogens. The quality of the initial images must be the best possible to achieve objective, accurate and rapid assessment of specimens.