Induction of contour sensing in Aspergillus niger by stress and its relevance to fungal growth mechanics and hyphal tip structure

Thigmotropism (contour sensing) has been assigned an important role in both plant and human fungal pathogens. However, outside these systems, out-knowledge of the function of thigmotropism in fungal growth control is relatively poor. Furthermore, the effects of environmental stress on thigmotropic responses have received scant attention. To try to elucidate some of the mechanisms behind hyphal contour sensing in response to nutrient-poor environments, we have used micro-engineered substrates and several imaging techniques to investigate the thigmotropic reactions of the ubiquitous fungus Asperigllus niger. This organism not appear to demonstrate thigmotropic growth under normal conditions. Our results show, that A. niger undergoes significant morphological changes during growth on solid Substrates and demonstrate that the intensity of contour sensing varies depending on the area of the hyphal tip which initiates contact with the substrate. We propose that growth under nutrient-limited conditions triggers several factors that combine to increase thigmotropic sensitivity while conversely creating a 60 degrees are at the hyphal tip which is blind to topographical variations. This has important consequences for out-general understanding of the hyphal mode of growth in fungi as well as more specific aspects of hyphal tip development under stress. (c) 2006 Elsevier Inc. All rights reserved.