Ultrastructure of conidium development in Erysiphe pisi

Abstract

The ultrastructure of all vegetative and reproductive cell types throughout the cycle of conidium development in Erysiphe pisi has been examined using light and electron microscopy. The conidial apparatus comprises a conidiophore with, in succession above, a generative cell, and first- and second-stage conidial initials. At approximately 23 °C in a 16 h light: 8 h dark period, the onset of elongation and division of the generative cells, onset of elongation of first-stage conidia, and maturation and abscission of second-stage conidia were individually synchronized in one diurnal cycle. Synchrony was also maintained between 24 and 30 °C, and 12 h light: 12 h dark photoperiods but the generative cells elongated more slowly, achieving greater lengths than at lower temperatures. At 23 °C in continuous light, development was not synchronized. The ultrastructure of each cell was characteristic of its type and developmental phase. Glycogen became progressively abundant in maturing conidia and occurred in all other cells except hyphal apices and generative cells. In addition to the organelles usually found in Ascomycetes, two special endomembrane components have been found. One is a reticular vacuolar system which is expanded as a major vacuole at one end. The whole system encloses a region (vacuole-associated body) 1-2 μm across, rarely 8 μm, which is continuous with the main ground plasm via pores in the network. The vacuole-associated bodies always lacked ribosomes but sometimes contained glycogen. The other component is a compact endoplasmic reticulum organized as spheres 0.5–5 μm in diameter and composed of branching tubules which terminated in diverticulae and were sometimes interspersed with 8-nm filaments. Evidence for the functions of these organelles is discussed. The development and structure of the septa, of cytoplasmic components associated with conidial abscission, and of the walls of hyphae and conidia are discussed. All subaerial cell walls are covered with an osmiophilic cuticle with characteristic trilamellar structure and sometimes bearing granules and platelets. Evidence is presented that the cuticle is renewed following damage.