Germination of conidia of Aspergillus niger is accompanied by major changes in RNA profiles

Abstract

The transcriptome of conidia of Aspergillus niger was analysed during the first 8 h of germination. Dormant conidia started to grow isotropically two h after inoculation in liquid medium. Isotropic growth changed to polarised growth after 6 h, which coincided with one round of mitosis. Dormant conidia contained transcripts from 4 626 genes. The number of genes with transcripts decreased to 3 557 after 2 h of germination, after which an increase was observed with 4 780 expressed genes 8 h after inoculation. The RNA composition of dormant conidia was substantially different than all the subsequent stages of germination. The correlation coefficient between the RNA profiles of 0 h and 8 h was 0.46. They were between 0.76–0.93 when profiles of 2, 4 and 6 h were compared with that of 8 h. Dormant conidia were characterised by high levels of transcripts of genes involved in the formation of protecting components such as trehalose, mannitol, protective proteins (e.g. heat shock proteins and catalase). Transcripts belonging to the Functional Gene Categories (FunCat) protein synthesis, cell cycle and DNA processing and respiration were over-represented in the up-regulated genes at 2 h, whereas metabolism and cell cycle and DNA processing were over-represented in the up-regulated genes at 4 h. At 6 h and 8 h no functional gene classes were over- or under-represented in the differentially expressed genes. Taken together, it is concluded that the transcriptome of conidia changes dramatically during the first two h and that initiation of protein synthesis and respiration are important during early stages of germination.

Structural Features of Sugars That Trigger or Support Conidial Germination in the Filamentous Fungus Aspergillus niger

“…Previous estimates of the D-trehalose content of A. niger spores show that approximately 5% of the dry weight is comprised of D-trehalose (18,31), which could possibly contribute toward the energy for the early stages of germination (19). Since the internal stores are limited in amount, a transition must take place in metabolism of the stores to the carbon source present in the conidial environment.…”

“…Previous studies have commonly used D-glucose to induce the germination of fungal conidia (10,31), and to date, no study has investigated the functionality of D-glucose analogues in triggering and supporting conidial germination. Furthermore, it is only the proportion of germination (percent germination) that is often reported, with the initial swelling of conidia often not being distinguished as a separate event (12).…”

“…The model of germination accepts that there is a series of morphological and biochemical changes that are associated with conidial development. These involve the initial increase in conidial size and the catabolism of internal storage compounds, followed by the formation of hyphae (15,18,31), all of which have been experimentally monitored in this study. From our data, we believe that this model can be extended to include the initial sensing of correctly shaped sugars that trigger the germination process.…”

“…However, we certainly cannot rule out the possibility that very low levels of sugars (experimentally undetectable) could penetrate the spore and trigger germination via an internal sensor. Signaling was not explored further here, although we and others (12,25,31,36) have reported changes in levels of relevant transcripts during conidial germination. Germination in the presence of D-glucose showed, for example, that transcript levels for RgsA and RasA were higher at 1 h than in 0-h conidia, whereas transcript levels for adenylate cyclase (An11g01520) and the cyclic AMP (cAMP)-dependent protein kinase regulatory (An16g03740) and catalytic (An02g04270) subunits were higher in the 0-h conidia.…”

Transcriptional Regulation and Responses in Filamentous Fungi Exposed to Lignocellulose

“…The germination of fungal condiospores starts with the breaking of dormancy, after which the conidiospore swells, takes up water and activates carbon storages. Subsequently, cell polarity is established and a germ tube is formed [61][62][63]. The breaking of dormancy is regulated by a 'germination trigger' or inducer, usually a sugar molecule, that results in mobilisation of internal energy stores such as trehalose and the swelling of the conidiospore and activation of metabolism.…”