Impact of Light on Differentiation and Virulence of Botrytis cinerea
Plant Genetics and Genomics
Final Report Abstract
Fungi, like other organisms, actively sense the environmental light conditions to drive adaptive responses, including protective mechanisms against the light-associated stresses, and to regulate development. Ecological niches are characterized by different light regimes, e.g. light is absent underground, and light spectra from the sunlight are changed underwater or under the canopy of foliage due to the absorption of distinct wavelengths by bacterial, algal and plant pigments. Because fungi have evolved to adapt to their habitats, the complexities of their ‘visual’ systems may vary significantly. Fungi that are pathogenic on plants experience a special light regime because the host always seeks the optimum light conditions for photosynthesis – and the pathogen must cope with this stressful environment. When the pathogen lives under the canopy and is indirectly exposed to sunlight, it is confronted with an altered light spectrum enriched for green and far-red light. In these fungi, the property to sense and to react to light may be important to become successful pathogens by affecting the infection process (interaction with the host) and/or their overall fitness (success in dispersal, survival or achieving genetic diversity via sexual recombination). Botrytis cinerea – the gray mold fungus – is an aggressive plant pathogen infecting aboveground plant parts. The Leotiomycete senses (near)-UV, blue, green, red and far-red light signals through at least eleven photoreceptors (PRs) to trigger a variety of responses, i.e. protection (pigments, osmolytes, enzymatic systems), morphogenesis (conidiation, apothecial development), entrainment of a circadian clock, and positive and negative tropisms of multicellular (conidiophores, apothecia) and unicellular structures (conidial germ tubes). The light signaling machinery appears more sophisticated than those of the current models in photobiology, i.e. of the saprophytic fungi A. nidulans (Eurotiomycete) and N. crassa (Sordariomycete) which predominantly respond to red and blue light, respectively. In that sense, ‘looking through the eyes’ of B. cinerea will expand the knowledge of fungal photobiology as well as the role of light in fungus-plant interactions. The goal of our research is to understand the fundamental mechanisms of light signaling in B. cinerea and how light is related with virulence and morphogenesis. Previous findings showed that both processes are often co-regulated. For instance, blind strains are often attenuated in virulence as known for the always conidia phenotype that is e.g. caused by point mutations in velvet. We aimed to investigate the role of the PRs as prerequisite for photoresponses (input) and light-responsive transcription factors (LTFs) as mediators of the output (gene expression, morphogenesis, virulence, stress response). Deletion and overexpressing strains of all PR- encoding genes have been generated and studied for the morphogenetic and virulence phenotypes, demonstrating – in accordance with the previous studies using different wavelengths – that blue light sensed via the transcriptional White collar complex (WCC) causes the proliferation of vegetative mycelia as both conidiation and sclerotial development are blocked. Thereby, the WCC positively regulates several developmental repressors including LTFs and other PRs. However, the nature of the (near-)UV light effect on conidiation is still puzzling as the two cryptochromes (CRYs) are not involved. Thus, B. cinerea and other UV-responsive fungi may either possess a yet unknown PR, or this process is triggered indirectly via sensing UV-induced damages. The number of red/far-red PRs (phytochromes, PHYs) is increased in the Leotiomycetes, and our studies revealed a new function of PHY2-like proteins similarly to plant PHYs: the integration of temperature signals to co-control morphogenesis by light and temperature. We propose that the green light-sensing opsins mediate the observed retardation of conidial germination and germ tube growth and that certain PRs mediate the positive and negative phototropic responses of the germ tubes to guide the right way to the host. These questions can be addressed in the future using the available single/multiple (color-blind) mutants. Gene expression studies revealed 30 LTF-encoding genes whose induction by light follows different kinetics. The dev LTFs – expressed during (onset of) conidiation – are candidates for developmental regulators. By this, we identified LTF2 (sufficient and essential) and LTF7 (sufficient but dispensable) as activators of conidiation in two independent pathways and suggest that other co-regulated LTFs act upstream or downstream of these crucial regulators. To date, ten LTFs have been functionally characterized; they either counteract LTF2/conidiation or are dispensable. As transcriptional responses are fast and can be easily monitored, they are suitable to elucidate the regulatory networks. Thus, we found that the stressactivated MAPK SAK1 as downstream target of histidine kinases including the PHYs is required for transduction of light and stress signals. Forward genetics approaches revealed chromatin modifiers as additional regulators of morphogenesis and virulence, suggesting that the chromatin landscape is decisive for proper photoresponses as well.
Publications
- (2016) DHN melanin biosynthesis in the plant pathogenic fungus Botrytis cinerea is based on two developmentally regulated key enzyme (PKS)-encoding genes. Mol Microbiol. 99:729-748
Schumacher J
(See online at https://doi.org/10.1111/mmi.13262) - (2016) Light governs asexual differentiation in the grey mould fungus Botrytis cinerea via the putative transcription factor BcLTF2. Environ Microbiol. 18:4068-4086
Cohrs KC, Simon A, Viaud M, Schumacher J
(See online at https://doi.org/10.1111/1462-2920.13431) - (2017) A new transformant selection system for the gray mold fungus Botrytis cinerea based on the expression of fenhexamid-insensitive ERG27 variants. Fungal Genet Biol. 100:42-51
Cohrs KC, Burbank J, Schumacher J
(See online at https://doi.org/10.1016/j.fgb.2017.02.001) - (2017) How light affects the life of Botrytis. Fungal Genet Biol. 106:26-41
Schumacher J
(See online at https://doi.org/10.1016/j.fgb.2017.06.002) - (2017) Regulation of conidiation in Botrytis cinerea requires the light-responsive transcriptional regulators BcLTF3 and BcREG1. Curr Genet, 63:931-949
Brandhoff B, Simon A, Dornieden A, Schumacher J
(See online at https://doi.org/10.1007/s00294-017-0692-9) - (2017) The two cryptochrome/photolyase family proteins fulfill distinct roles in DNA photorepair and regulation of conidiation in the gray mold fungus Botrytis cinerea. Appl Environ Microbiol. 83:e00812-17
Cohrs KC, Schumacher J
(See online at https://doi.org/10.1128/AEM.00812-17) - (2018) The putative H3K36 demethylase BcKDM1 affects virulence, stress responses and photomorphogenesis in Botrytis cinerea. Fungal Genet Biol. 123:14-24
Schumacher J, Studt L, Tudzynski P
(See online at https://doi.org/10.1016/j.fgb.2018.11.003)
