Glutamic acid decarboxylase gene disruption reveals signalling pathway(s) governing complex morphogenic and metabolic events in Trichoderma atroviride

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

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.

A high-throughput RNA-Seq approach to elucidate the transcriptional response of Piriformospora indica to high salt stress

Piriformospora indica, a root endophytic fungus, augments plant nutrition and productivity as well as protects plants against pathogens and abiotic stresses. High salinity is a major problem faced by plants as well as by microbes. Until now, the precise mechanism of salt stress tolerance in P. indica has remained elusive. In this study, the transcriptomes of control and salt-treated (0.5 M NaCl) P. indica were sequenced via the RNA-seq approach. A total of 30,567 transcripts and 15,410 unigenes for P. indica were obtained from 7.3 Gb clean reads. Overall 661 differentially expressed genes (DEGs) between control and treated samples were retrieved. Gene ontology (GO) and EuKaryotic Orthologous Groups (KOG) enrichments revealed that DEGs were specifically involved in metabolic and molecular processes, such as "response to salt stress", "oxidoreductase activity", "ADP binding", "translation, ribosomal structure and biogenesis", "cytoskeleton", and others. The unigenes involved in "cell wall integrity", "sterol biosynthesis", and "oxidative stress" such as Rho-type GTPase, hydroxymethylglutaryl-CoA synthase, and thioredoxin peroxidase were up-regulated in P. indica subjected to salt stress. The salt-responsive DEGs have shown that they might have a potential role in salt stress regulation. Our study on the salt-responsive DEGs established a foundation for the elucidation of molecular mechanisms related to P. indica stress adaptation and a future reference for comparative functional genomics studies of biotechnologically important fungal species.

Disruption of GABA shunt affects Trichoderma atroviride response to nutritional and environmental stimuli

The fungus Trichoderma atroviride is a member of the genus Trichoderma to which belong many species known for high cellulase production, formation of various antibiotics, plant biocontrol and antagonistic activities against other fungi. Deletion of T. atroviride glutamate decarboxylase gene gad caused minor defects in germination, hyphal branching, slower growth and disruption of conidiation pattern. GABA can be used by fungi as a secondary carbon source and as a primary nitrogen source. We analyzed the effect of different nutrient compositions and environmental conditions (light and temperature) on growth and development of T. atroviride in strains defective in the functional GAD. The gad mutants grown on NH4NO3 as a sole carbon source grew slower and formed conidiation bands closer to each other which was clearly demonstrated during their cultivation in race tubes. The gad mutants exhibited slightly lower apical extension growth rate at the room temperature but their apical extension rate dropped significantly at 30 °C. Higher temperature had also inhibitory effect on gad mutant conidiation, whereas 30 °C seems optimal temperature for the parental strain. The optimal temperature for gad mutant conidiation was lower than in F534, about 25 °C.

Nutrient transport into germinating Trichoderma atroviride conidia and development of its driving force

The exit from dormancy and the start of growth should be preceded or at least accompanied by the uptake of nutrients. In this work we studied changes in the transport of several nutrients into Trichoderma atroviride conidia. Germination started with a short period of isodiametric growth (conidial swelling), followed by polarized growth (germ tube formation) after about 8 h at 26 °C. The onset of isodiametric growth required the presence of external both phosphate and nitrate. At the same time, an increased uptake of precursors of macromolecules and phospholipids ((14)C- or (3)H-labelled valine, uracil, N-acetylglucosamine and choline) occurred. A low uptake of these precursors was observed also in non-germinating conidia. Concomitantly, this uptake developed an increased sensitivity to the uncoupler 3,3',4',5-tetrachlorosalicylanilide. Expression and activity of H(+)-ATPase started after completing isodiametric growth, suggesting that the proton-motive force (PMF) generated by H(+)-ATPase may be an accelerator of nutrient uptake and metabolism. (14)C-valine uptake was also measured into a mutant with disrupted pma1 gene. This mutant did not form conidia. The mutant also exhibited uncoupler sensitivity of (14)C-valine uptake. These observations showed that a PMF must have been generated by a mechanism(s) other than the H(+)-ATPase activity in the WT before H(+)-ATPase expression and in mycelia with disrupted H(+)-ATPase.

Changes in metabolome and in enzyme activities during germination of Trichoderma atroviride conidia

The aim of this work was to study the metabolic changes during germination of Trichoderma atroviride conidia along with selected marker enzyme activities. The increase in proteinogenic amino acid concentrations together with the increase in glutamate dehydrogenase activity suggests a requirement for nitrogen metabolism. Even though the activities of tricarboxylic acid cycle enzymes also increased, detected organic acid pools did not change, which predisposes this pathway to energy production and supply of intermediates for further metabolism. The concentrations of many metabolites, including the main osmolytes mannitol and betaine, also increased during the formation of germ tubes. The activities of H(+)-ATPase and GDPase, the only marker enzymes that did not have detectable activity in non-germinated conidia, were shown with germ tubes.