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Eukaryotic Cell, February 2007, p. 157-170, Vol. 6, No. 2
1535-9778/07/$08.00+0     doi:10.1128/EC.00247-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

High Viscosity and Anisotropy Characterize the Cytoplasm of Fungal Dormant Stress-Resistant Spores

J. Dijksterhuis,^1,2* J. Nijsse,^3, F. A. Hoekstra,³ and E. A. Golovina³

Applied and Industrial Mycology, CBS Fungal Biodiversity Centre, Uppsalalaan 8, 3584 CT, Utrecht,¹ Agrotechnology and Food Innovations and Wageningen Centre for Food Sciences (WCFS), P.O. Box 557, 6700 AN Wageningen,² Laboratory of Plant Physiology, Wageningen University, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands³

Received 1 August 2006/ Accepted 31 October 2006

Ascospores of the fungus Talaromyces macrosporus are dormant and extremely stress resistant, whereas fungal conidia—the main airborne vehicles of distribution—are not. Here, physical parameters of the cytoplasm of these types of spores were compared. Cytoplasmic viscosity and level of anisotropy as judged by spin probe studies (electron spin resonance) were extremely high in dormant ascospores and during early germination and decreased only partly after trehalose degradation and glucose efflux. Upon prosilition (ejection of the spore), these parameters fell sharply to values characteristic of vegetative cells. These changes occurred without major volume changes that suggest dramatic changes in cytoplasmic organization. Azide reversibly inhibited prosilition as well as the decline in cytoplasmic parameters. No organelle structures were observed in etched, cryoplaned specimens of ascospores by low-temperature scanning electron microscopy (LTSEM), confirming the high cytoplasmic viscosity. However, cell structures became visible upon prosilition, indicating reduced viscosity. The viscosity of fresh conidia of different Penicillium species was lower, namely, 3.5 to 4.8 cP, than that of ascospores, near 15 cP. In addition the level of anisotropic motion was markedly lower in these cells (h₀/h₊₁ = 1.16 versus 1.4). This was confirmed by LTSEM images showing cell structures. The decline of cytoplasmic viscosity in conidia during germination was linked with a gradual increase in cell volume. These data show that mechanisms of cytoplasm conservation during germination differ markedly between ascospores and conidia.

Published ahead of print on 10 November 2006.

Present address: Unilever R&D, P.O. Box 114, 3130 AC Vlaardingen, The Netherlands.