How to enter the state of dormancy? A suggestion by Trichoderma atroviride conidia

Abstract

It is generally accepted that conidia, propagules of filamentous fungi, exist in the state of dormancy. This state is defined mostly phenomenologically, e.g., by germination requirements. Its molecular characteristics are scarce and are concentrated on the water or osmolyte content, and/or respiration. However, a question of whether conidia are metabolic or ametabolic forms of life cannot be answered on the basis of available experimental data. In other words, are mature conidia open thermodynamic systems as are mycelia, or do they become closed upon the transition to the dormant state? In this article, we present observations which may help to define the transition of freshly formed conidia to the putative dormant forms using measurements of selected enzyme activities, 1H- and 13C-NMR and LC-MS-metabolomes, and 14C-bicarbonate or 45Ca2+ inward transport. We have found that Trichoderma atroviride and Aspergillus niger conidia arrest the 45Ca2+ uptake during the development stopping thereby the cyclic (i.e., bidirectional) Ca2+ flow existing in vegetative mycelia and conidia of T. atroviride across the cytoplasmic membrane. Furthermore, we have found that the activity of α-ketoglutarate dehydrogenase was rendered completely inactive after 3 weeks from the conidia formation unlike of other central carbon metabolism enzymes. This may explain the loss of conidial respiration. Finally, we found that conidia take up the H14CO3- and convert it into few stable compounds within 80 d of maturation, with minor quantitative differences in the extent of this process. The uptake of H13CO3- confirmed these observation and demonstrated the incorporation of H13CO3- even in the absence of exogenous substrates. These results suggest that T. atroviride conidia remain metabolically active during first ten weeks of maturation. Under these circumstances, their metabolism displays features similar to those of chemoautotrophic microorganisms. However, the Ca2+ homeostasis changed from the open to the closed thermodynamic state during the early period of conidial maturation. These results may be helpful for studying the conidial ageing and/or maturation, and for defining the conidial dormant state in biochemical terms.