Conidiation of Aspergillus niger in continuous culture

Summary Conidiation of Aspergillus niger was studied in carbon-limited and nitrogen-limited chemostat culture. Under citrate-limitation conidiation intensity varied inversely with dilution rate. Conidiophores were less complex than in aerial conidiation and at high dilution rates conidia occasionally developed from modified hyphal tips. Conidiation was difficult to achieve under glucose-limitation. At the low dilution rates that allowed limited conidiation steady state could not be maintained due to onset of autolysis. At higher dilution rates when steady state was readily obtained conidiation did not occur. The maximum yield constants under citrate-limitation and glucose-limitation were respectively 0.145 and 0.4 mg dry weight/mg substrate, while the relative specific maintenance values were 0.045 and 0.018 mg substrate/mg dry weight/h. Under ammonium-limitation with citrate as the carbon source there was no conidiation. When nitrate became the limiting nitrogen source conidiophore initiation occurred but biomass production was low and wash-out occurred at D=0.034 h^-1.     

Light stimulates conidiation of the entomopathogenic fungus Beauveria bassiana

Beauveria bassiana is a commercially important entomopathogenic fungus. Like other insect fungal pathogens, B. bassiana usually produces asexual reproductive bodies, conidia, for dispersal, transmission and infection of insects. Adequate mass-production of high quality conidia is crucial to development of an efficient B. bassiana insecticide. However, little is known about details of conidiation in this fungus in response to environmental signals, which limits understanding of the mechanism of conidiation and improvement in conidia production. Here, morphologenetic changes of B. bassiana under different light conditions are reported. When cultured in total darkness, B. bassiana hyphae can grow continuously with few reproductive structures differentiated, while illumination with white light resulted in prolific formation of conidiophores bearing abundant conidia, indicating that light could stimulate conidiation of B. bassiana. Among the single colour lights tested, blue light was the most effective to stimulating sporulation. Colonies became adapted for blue light stimulus only after hyphae had grown in total darkness for at least 96 h, whereas the photoadaptation obviously declined after 144 h. For the exposure time, 3 min of blue light pulse was enough to stimulate conidiation in the photoadapted mycelia, while prolonged light exposure over 3 min resulted in a decrease in conidia yield. Our results provided useful clues for understanding the mechanism of conidiation mediated by light in B. bassiana.     

Microcycle conidiation and the conidial properties in the entomopathogenic fungus Metarhizium acridum on agar medium

Conidiation of the entomopathogenic fungus Metarhizium acridum on agar media was investigated. M. acridum CQMa102 exhibits two different conidiation patterns on agar media: normal conidiation in which conidia are formed on extended hyphae and microcycle conidiation in which conidiation occurs directly after conidia germination. Microcycle conidiation resulted in a mass of conidia produced via budding by accelerated development at the inoculation site. The mean total conidial yield (conidiation at day 10) was 4–5-fold greater after microcycle conidiation than during normal conidiation. Insect pathology assays indicated that microcycle conidia produced on SYA agar were as effective as normal aerial conidia against the locust. Ultraviolet (UV)-resistance tests showed no significant differences between the two types of cell propagules. However, microcycle conidia were more heat resistant than normal aerial conidia, and accumulated higher levels of trehalose in response to heat induction compared to normal aerial conidia.     

Synchronous Conidiation of Piricularia oryzae by the Replacement Culture

For the purpose of studying the mechanism of conidiogenesis in Piricularia oryzae, methods were developed to separate the phase of mycelial growth from that of conidiation and also to evaluate them quantitatively.

When P. oryzae was grown on media with low carbon : nitrogen ratio and high nitrogen concentration, conidiation did not take place, in spite of its vegetative growth. Conidiation occurred in a very short period of time when the above mycelia were replaced on nutritionally poor media. Cellophane membrane was overlaid on solid medium and conidia were spread uniformly. Evenly grown mycelial mat which could be easily transferred onto the post-culture medium was obtained. As the preculture medium, MSA medium with carbon: nitrogen ratio of 6.3 and nitrogen concentration of 1.5 g/liter was used. The evenly grown mycelial mat was cut into small square mats of 1.44 cm² each and the small square mycelial mats were transferred onto the post-culture medium together with the cellophane membrane. The conidiation took place in the post-culture and the vegetative growth in the preculture and the conidiation in the post-culture could be observed separately and quantitatively.

Conidiation did not occur at all in the preculture and the degree of conidiation which took place in the post-culture varied according to the precultural conditions. This means that it is a certain state of physiological condition in the preculture which determines the degree of conidiation in the post-culture. The authers designated this state of physiological condition as the “latent activity of conidiation” (LAC). For the purpose of the quantitative estimation of this activity, we expressed LAC in terms of the degree of conidiation in the post-culture under a defined cultural condition. The LAC was subject to change very easily, declined rapidly and disappeared upon prolonged preculture. Only young mycelia showed to have this activity.

The influences of the precultural condition on the development of the LAC and vegetative growth were generally parallel. However, the LAC was generally more sensitive to the environmental condition than the vegetative growth, especially to the temperature change.

The conidia formed were uniform in size and had high rate of germination. Several strains of P. oryzae tested showed very similar behavior.     

Acremonium-like submerged conidiation inPaecilomyces nostocoides andP. lilacinus

Paecilomyces nostocoides, in which conidia of smaller or larger sizes appear in chains, was newly isolated from Japan. In addition to the typicalPaecilomyces-type conidiation, the Japanese isolate showed additionalAcremonium-like submerged conidiation in and/or on some agar media. The submerged conidiation was also observed in the ex-type strains, but not in the type specimens ofP. nostocoides. The same submerged conidiation was observed inP. lilacinus, the species most similar toP. nostocoides. The species definitions ofP. nostocoides andP. lilacinus were emended to include the submerged conidiation.Paecilomyces-type conidia were uninucleate in bothP. nostocoides andP. lilacinus. Paecilomyces nostocoides andP. lilacinus had the Q-10(H₂) ubiquinone system.     

基于转录组的冬虫夏草菌微循环产孢研究

为研究冬虫夏草菌（Ophiocordyceps sinensis）微循环产孢过程中的分子机理,采用高通量测序技术,对微循环产孢前后冬虫夏草菌的转录组进行研究。筛选获得差异表达基因6 902个。Gene Ontology（GO）功能聚类分析表明,差异表达基因主要参与分子功能、胞内运输、核糖核蛋白复合体等生物学通路。京都基因与基因组百科全书（Kyoto Encyclopedia of Genes and Genomes,KEGG)功能富集分析结果表明,差异表达基因参与了核糖体、嘧啶代谢、蛋白酶体、嘌呤代谢、甘氨酸代谢等过程。Mmc、Mcb、MaH1、MaPP5、MaAGA、Pyk等候选基因不同程度的参与了冬虫夏草菌微循环产孢过程,其中Mcb家族基因可能起到关键作用。通过qRT PCR验证差异表达基因,定量结果与转录组测序结果一致。本研究为进一步揭示冬虫夏草菌微循环产孢分子机制以及关键基因的调控提供了重要信息。     

A vacuolar glucoamylase,Sga1, participates in glycogen autophagy for proper asexual differentiation in Magnaporthe oryzae

Nutrient limitation acts as a trigger for the synthesis of glycogen, which serves as a carbon and energy reserve during starvation. Recently, we reported that an autophagy-deficient mutant (atg8Δ) shows severe reduction in aerial hyphal growth and conidiation in the rice-blast fungus Magnaporthe oryzae, and proposed that autophagy plays an important role in facilitating glycogen homeostasis to ensure proper asexual differentiation in Magnaporthe. Here, we identify and characterize a vacuolar glucoamylase function (Sga1) that hydrolyses glycogen to meet the energy requirements during asexual development in Magnaporthe. Loss of SGA1 resulted in significant reduction in conidiation compared to the wild-type Magnaporthe strain. More importantly, an sga1Δ atg8Δ double deletion mutant showed further reduction in conidiation compared to the atg8Δ mutant in Magnaporthe. Forced localization of GFP-Sga1 to the cytoplasm (through removal of the predicted signal peptide) led to increased conidiation in wild type and the sga1Δ, but more interestingly, significantly restored conidiation in the atg8Δ mutant. Our results indicate that autophagy and Sga1 act cooperatively in vacuolar glycogen breakdown, which is essential for conidia formation but dispensable for pathogenicity in Magnaporthe.     

Regulation of periodicity and intensity of photo-induced conidiation ofTrichoderma viride

After a short irradiation at 366 nm with 200 lx, the intensity of conidiation ofTrichoderma viride colonies grown in the dark increased for the first 10 s proportionally with time. The increase slowed down after 10 s—6 min of exposure and after 10–60 min of irradiation the conidiation intensity began to decrease. When photo-induced by daylight, the conidiation started at a high rate after 25 h and persisted even after 48 h. The conidiation had no circadian character and its periodicity depended on the periodicity of photo-induction. Its intensity was also influenced by the carbon sources used, a maximum being reached with glucose (1–2 %). Higher glucose concentrations inhibited conidiation but had no influence on growth of colonies.     

The relation between growth, conidiation and trehalase activity in Neurospora crassa

The extent to which trehalose is accumulated in the vegetative mycelium of strains of Neurospora crassa is significantly affected by conidiation. In heavily conidiating strains a rapid decrease in mycelial trehalose occurs following the initiation of conidiation. Meanwhile, trehalase activity in the vegetative mycelium of heavily conidiating strains increases rapidly following the initiation of conidiation, although apparently it is not directly caused by the sporulation process. High levels of both trehalase and trehalose appear concomitantly in the newly formed conidia.     

Light-independent conidiation in Trichoderma spp.: a novel approach to microcycle conidiation

Microcycle conidiation in Trichoderma hamatum and T. harzianum has been achieved in complete darkness for the first time. The time required for mass conidiation without intervening vegetative growth was decreased to 24 h instead of 6 to 7 days. The conidia produced by microcycle conidiation were viable and had pigmentation and antagonistic behaviour similar to those of the parental stock cultures.N. Khurana, R.K. Saxena, R. Gupta and R.C. Kuhad are with the Department of Microbiology, University of Delhi South Campus, Benito Juarez Road, New Delhi-110021, India.     

Vegetative growth, aging- and light-induced conidiation ofTrichoderma viride cultivated on different carbon sources

The growth and conidiation of the agedTrichoderma viride culture grown in the dark, and after an induction by a light pulse. was examined in the presence of selected mono-, di(tri)saccharides, amino acids and alcohols as sole carbon sources. Hexoses and disaccharides, but not pentoses and amino acids, promoted proportionally both growth and conidiation induced by aging or light. All compounds but pentoses promoted the conidiation in aged cultures and photoconidiation in a close correlation. Ethanol, glycerol and ethylene glycol supported both growth and conidiation but these processes were not supported equally. Conidia formation with hexoses and amino acids as sole carbon sources seems to be a function of growth promotion, rather than of growth restriction (starvation, stress, aging). With glucose as sole carbon source the conidiation was not triggered by nutrient limitation, nor by the accumulation of waste metabolites. The aging-induced conidiation can be considered to be triggered by the genetic program of the microorganism rather than by its nutrient status.     

Malonate Metabolism, Stimulation of Conidiation and Succinate Dehydrogenase Activity in Neurospora crassa

Malonate was studied for its effect on succinate dehydrogenase activity and conidiation. It Was found to stimulate the succinate dehydrogenase activity and also conidiation of Neurospora crassa. The efficiency of sucrose metabolization for cellular synthesis was improved in malonate supplemented cultures. High Concentration (0.5 M) had a distinct toxic effect on conidiation and economic efficiency. Teratological structures were observed at 0.5 M malonate concentration. A stimulation of the glyoxylate cycle is considered to be one of the factors responsible for the conidiogenic effect.     

Growth and conidiation ofTrichoderma viride are affected by non-steroidal antiinflammatory agents

Nonsteroid antiinflammatory agents (NSAIA's) (inhibitors of cyclooxygenase and lipoxygenase) of several structural series inhibited growth ofTrichoderma viride. The most potent growth inhibitors were indomethacin and its derivative repanidal which inhibited in the range of 0.1 mmol/L. The weakest inhibitors were acetylosalicylic acid and lysine salicylate which exerted only a weak effect at concentrations above 1 mmol/L. The inhibition of growth was accompanied by a stimulation of conidiation in the dark. A light pulse increased the efficiency of these drugs to stimulate the conidiation.Saccharomyces cerevisiae was as sensitive to NSAIA's asT. viride whileBotrytis cinerea was less sensitive. The results indicate that arachidonate metabolism may play a role in the growth of fungi and may participate also in the process of conidiation.     

Photoinduced conidiation inTrichoderma viride: A study with inhibitors

The influence of a number of inhibitors affecting respiration, oxidative phosphorylation, cAMP-phosphodiesterase and of the antioxidant 1,4-dithiothreitol on growth and photoinduced conidiation ofTrichoderma viride were investigated. In all cases, growth and conidiation were influenced to a different extent. Among the first group of compounds, antimycin A was the most potent inhibitor of conidiation while it influenced growth much less. A similar effect was obtained with 2,4-dinitrophenol and 1,4-dithiothreitol. On the other hand, 3-isobutyl-1-methylxanthine (inhibitor of phosphodiesterase) greatly stimulated the conidiation induced by light without affecting growth. It is concluded that the redox reactions represent a vital component of the differentiation pathway and that cAMP may play a regulatory role in this process.     

GENETIC CONTROL OF CONIDIATION IN ASPERGILLUS RUGULOSUS

The potential for the production of abundant conidia was suggested when an essentially non-conidial isolate of Aspergillus rugulosus, obtained from dune sand, was grown at 25 C rather than at the usual 37 C. Ultraviolet-light-induced conidial strains were obtained from the original non-conidial isolate. Evidence is presented which suggests that the production of conidia is under the control of a single nuclear gene. The data, with a single exception, suggest that induced conidial mutations represent alterations at the same locus. Crosses with isolates from various localities indicate that the genes controlling conidiation in these strains may be linked to or independent of the genes studied in the original dune-sand isolate. No evidence has been obtained which would suggest involvement of a diffusible substance in the control of conidiation.