Photoperception mutants in Trichoderma: mutants that sporulate in response to stress but not light

The naming of the blue UV photoreceptor "cryptochrome": From despised pun to ubiquitously found chromophore(s) controlling multiple functions

A triple-peaked UV-blue photoacceptor was first found in fungi and nicknamed cryptochrome due to its being cryptic and found then only in cryptogamous plants. It was subsequently discovered in higher plants and even later in algae, insects, fish, amphibians, and mammals including humans. Cryptochrome is the photoacceptor controlling a plethora of properties in all these systems.

Identification and evaluation of suitable reference genes for RT-qPCR analyses in Trichoderma atroviride under varying light conditions

BACKGROUND

Trichoderma atroviride is a competitive soil-borne mycoparasitic fungus with extensive applications as a biocontrol agent in plant protection. Despite its importance and application potential, reference genes for RT-qPCR analysis in T. atroviride have not been evaluated. Light exerts profound effects on physiology, such as growth, conidiation, secondary metabolism, and stress response in T. atroviride, as well as in other fungi. In this study, we aimed to address this gap by identifying stable reference genes for RT-qPCR experiments in T. atroviride under different light conditions, thereby enhancing accurate and reliable gene expression analysis in this model mycoparasite. We measured and compared candidate reference genes using commonly applied statistical algorithms.

RESULTS

Under cyclic light-dark cultivation conditions, tbp and rho were identified as the most stably expressed genes, while act1, fis1, btl, and sar1 were found to be the least stable. Similar stability rankings were obtained for cultures grown under complete darkness, with tef1 and vma1 emerging as the most stable genes and act1, rho, fis1, and btl as the least stable genes. Combining the data from both cultivation conditions, gapdh and vma1 were identified as the most stable reference genes, while sar1 and fis1 were the least stable. The selection of different reference genes had a significant impact on the calculation of relative gene expression, as demonstrated by the expression patterns of target genes pks4 and lox1.

CONCLUSION

The data emphasize the importance of validating reference genes for different cultivation conditions in fungi to ensure accurate interpretation of gene expression data.

Light in the Fungal World: From Photoreception to Gene Transcription and Beyond

Fungi see light of different colors by using photoreceptors such as the White Collar proteins and cryptochromes for blue light, opsins for green light, and phytochromes for red light. Light regulates fungal development, promotes the accumulation of protective pigments and proteins, and regulates tropic growth. The White Collar complex (WCC) is a photoreceptor and a transcription factor that is responsible for regulating transcription after exposure to blue light. In Neurospora crassa, light promotes the interaction of WCCs and their binding to the promoters to activate transcription. In Aspergillus nidulans, the WCC and the phytochrome interact to coordinate gene transcription and other responses, but the contribution of these photoreceptors to fungal photobiology varies across fungal species. Ultimately, the effect of light on fungal biology is the result of the coordinated transcriptional regulation and activation of signal transduction pathways.

Trichoderma: sensing the environment for survival and dispersal

Species belonging to the genus Trichoderma are free-living fungi common in soil and root ecosystems, and have a broad range of uses in industry and agricultural biotechnology. Some species of the genus are widely used biocontrol agents, and their success is in part due to mycoparasitism, a lifestyle in which one fungus is parasitic on another. In addition Trichoderma species have been found to elicit plant defence responses and to stimulate plant growth. In order to survive and spread, Trichoderma switches from vegetative to reproductive development, and has evolved with several sophisticated molecular mechanisms to this end. Asexual development (conidiation) is induced by light and mechanical injury, although the effects of these inducers are influenced by environmental conditions, such as nutrient status and pH. A current appreciation of the links between the molecular participants is presented in this review. The photoreceptor complex BLR-1/BLR-2, ENVOY, VELVET, and NADPH oxidases have been suggested as key participants in this process. In concert with these elements, conserved signalling pathways, such as those involving heterotrimeric G proteins, mitogen-activated protein kinases (MAPKs) and cAMP-dependent protein kinase A (cAMP-PKA) are involved in this molecular orchestration. Finally, recent comparative and functional genomics analyses allow a comparison of the machinery involved in conidiophore development in model systems with that present in Trichoderma and a model to be proposed for the key factors involved in the development of these structures.

Enhanced responsiveness and sensitivity to blue light by blr-2 overexpression in Trichoderma atroviride

Light is an environmental factor that regulates pivotal processes in living organisms, and appropriate perception is key to adaptation to the environment. Blue light activates asexual reproduction in Trichoderma atroviride through transcription factors BLR-1 and BLR-2 which regulate light-responsive genes. Here, we show that blr-2 expression is a limiting factor for photo-perception and photo-transduction. Overexpression of blr-2 resulted in increased photoconidiation and stronger expression of light-induced genes. In contrast, overexpression of blr-1 resulted in reduced photoconidiaton and weaker expression of light-induced genes. blr-2 overexpression caused a marked reduction of growth when the fungus was grown under defined photoperiods, including a period of strong sensitivity to light, followed by a period of insensitivity. Long periods of incubation under this condition permitted recovery of a rhythmic growth similar to that of the wild-type. In addition, blr-2 expression is apparently regulated at the post-transcriptional level through the BLR proteins and its expression level is BLR-1-dependent even in the dark. Finally, we demonstrated that blr-2 overexpression caused higher sensitivity to blue light and we therefore propose that the preformation of BLR-1/BLR-2 complexes is key to adequate light perception in T. atroviride.

Novel light-regulated genes in Trichoderma atroviride: a dissection by cDNA microarrays

The influence of light on living organisms is critical, not only because of its importance as the main source of energy for the biosphere, but also due to its capacity to induce changes in the behaviour and morphology of nearly all forms of life. The common soil fungus Trichoderma atroviride responds to blue light in a synchronized manner, in time and space, by forming a ring of green conidia at what had been the colony perimeter at the time of exposure (photoconidiation). A putative complex formed by the BLR-1 and BLR-2 proteins in T. atroviride appears to play an essential role as a sensor and transcriptional regulator in photoconidiation. Expression analyses using microarrays containing 1438 unigenes were carried out in order to identify early light response genes. It was found that 2.8 % of the genes were light responsive: 2 % induced and 0.8 % repressed. Expression analysis in blr deletion mutants allowed the demonstration of the occurrence of two types of light responses, a blr-independent response in addition to the expected blr-dependent one, as well as a new role of the BLR proteins in repression of transcription. Exposure of T. atroviride to continuous light helped to establish that the light-responsive genes are subject to photoadaptation. Finally, evidence is provided of red-light-regulated gene expression and a possible crosstalk between the blue and red light signalling pathways.

Use of Green Fluorescent Protein and Image Analysis to Quantify Proliferation of Trichoderma harzianum in Nonsterile Soil

ABSTRACT One drawback of traditional methods for fungal biomass measurement is the inability to distinguish biomass of an introduced fungus from that of the indigenous microbial community in nonsterile soil. We quantified biomass of a specific fungal biological control agent in nonsterile soil using epifluorescence microscopy and image analysis of green fluorescent protein (GFP)-expressing Trichoderma harzianum (ThzID1-M3). Numbers of colony forming units on a semiselective medium were compared with biomass estimates from image analysis, after ThzID1-M3 was incubated in soil that either remained moist (-0.05 MPa) for 14 to 21 days or remained moist for approximately 5 days and then was allowed to dry to <-3.0 MPa. Recovery of significant numbers of ThzID1-M3 propagules lagged approximately 3 days behind initiation of hyphal growth. Reductions in both colony counts and biomass were observed over time when soil was allowed to dry. However, in soil that remained moist, colony counts increased over a 14- to 21-day period even though biomass declined after approximately 3 to 5 days. Our results confirm that use of GFP, along with epifluorescence microscopy, is a useful tool to distinguish active hyphal biomass, the form of the fungus that is functional for biological control, from inactive propagules such as conidia or chlamydospores that are enumerated by plate counts.

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

The growth and conidiation of the aged Trichoderma 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.

Characterization of blue-light and developmental regulation of the photolyase gene phr1 in Trichoderma harzianum

Blue light and development regulate the expression of the phr1 gene of the filamentous fungus Trichoderma harzianum. The predicted product of phr1, the DNA repair enzyme photolyase, is likely to help protect Trichoderma, which grows in the soil as a mycoparasite or saprophyte, from damage upon emergence and exposure to ultraviolet-c. phr1 is transiently expressed in mycelium and conidiophores after illumination. phr1 mRNA also accumulates in conidiophores during development and spore maturation. As no other genes displaying rapid, direct light regulation have been described previously in this organism, we have characterized the fluence and time dependence of phr1 induction, and its relation to sporulation and photoreactivation. Induction is transient following a pulse, and, with slower decay, in continuous light. This implies that the photoreceptor, transducers or response are capable of adaptation. About two-fold more light is required to induce phr1 than conidiation, but this difference is modest, so both responses could use the same or similar chromophore. Adenosine 3':5'-cyclic monophosphate bypasses the requirement for light for sporulation, while atropine prevents sporulation even after photoinduction. Light regulation of phr1, however, is indifferent to both these effectors. Induction of photolyase expression behaves as a direct, rapid response to light, independent of the induction of sporulation. Indeed, illumination of mature spores increases their capacity for photoreactivation. Blue light seems to warn the organism against the harmful effects of short wave-lengths, inducing phr1 expression and sporulation by pathways that are, at least in part, distinct.

Rapid blue light regulation of a Trichoderma harzianum photolyase gene

Photolyases and blue light receptors belong to a superfamily of flavoproteins that make use of blue and UVA light either to catalyze DNA repair or to control development. We have isolated a DNA photolyase gene (phr1) from Trichoderma harzianum, a common soil fungus that is of interest as a biocontrol agent against soil-borne plant pathogens and as a model for the study of light-dependent development. The sequence of phr1 is similar to other Class I Type I eukaryotic photolyase genes. Low fluences of blue light rapidly induced phr1 expression both in vegetative mycelia, which lack photoprotective pigments, and, to a greater extent, in conidiophores. Thus, visible light induces the development of pigmented, resistant spores as well as the expression of phr1, perhaps announcing in this way the imminent exposure to the more damaging short wavelengths of sunlight. Light induction of phr1 in non-sporulating mutants shows that a complete sporulation pathway is not required for photoregulation. The light requirements for photoinduction of phr1 were not altered in dimY photoperception mutants. This suggests that photoinduction of sporulation and of photolyase expression is distinct in their photoreceptor system or in the transduction of the blue light signal.

Raffinose-induced mutanase production from Trichoderma harzianum

The enzyme alpha(1-->3),3-glucanohydrolase (referred to as mutanase) from the filamentous fungus Trichoderma harzianum OMZ 779 is capable of degrading the water-insoluble glucan in dental plaque. Previously, it was necessary to produce the glucan (referred to as mutan) in vitro for use as the sole carbon source and inducer of mutanase synthesis in fungal cultures. We report here that raffinose also induces the production of mutanase. The metabolism of raffinose differed from that of other sugars in metabolic end products and secreted protein profile. In addition to mutanase, we observed an approximately 15,000 M(r) protein that was also regulated by carbon source and by illumination conditions.

Induction of Trichoderma sporulation by nanosecond laser pulses: evidence against cryptochrome cycling

An important question in the study of photoreceptor action in morphogenesis is whether the chromophore is unidirectionally photobleached, or whether it is recycled, allowing each receptor molecule to be counted more than once. The common soil fungus Trichoderma harzianum grows vegetatively in the dark and sporulates in response to a pulse of blue or UV-A light. Colonies were grown at 26 degrees C, transferred to 3 degrees C, illuminated with non-saturating light, and then put back at 26 degrees C to sporulate. The fluence-response curves for photoinduction in the cold and at 26 degrees C were identical, indicating that there are no enzymatic transduction processes during irradiation. Regions of the perimeter of dark-grown colonies were given single pulses (maximum duration, 30 ns) at 355 nm with a neodymium laser. We obtained a complete fluence-response curve for the laser pulses, which agreed with data for irradiations in the second to minute range. Photoinduction at 3 degrees C, and validity of Bunsen-Roscoe reciprocity from nanoseconds to minutes, support the hypothesis that the inductive event is a simple first-order photobleaching reaction.

Mutants of Arabidopsis thaliana with altered phototropism

Thirty five strains of Arabidopsis thaliana (L.) Heynh. have been identified with altered phototropic responses to 450-nm light. Four of these mutants have been more thoroughly characterized. Strain JK224 shows normal gravitropism and "second positive" phototropism. However, while the amplitude for "first positive" phototropism is the same as that in the wild-type, the threshold and fluence for the maximum response in "first positive" phototropism are shifted to higher fluence by a factor of 20-30. This mutant may represent an alteration in the photoreceptor pigment for phototropism. Strain JK218 exhibits no curvature to light at any fluence from 1 μmol·m(-2) to 2700 μmol·m(-2), but shows normal gravitropism. Strain JK345 shows no "first positive" phototropism, and reduced gravitropism and "second positive" phototropism. Strain JK229 shows no measurable "first positive" phototropism, but normal gravitropism and "second positive" phototropism. Based on these data, it is suggested that: 1. gravitropism and phototropism contain at least one common element; 2. "first positive" and "second positive" phototropism contain at least one common element; and 3. "first positive" phototropism can be substantially altered without any apparent alteration of "second positive" phototropism.