Light is an important influential factor of the fungal lifestyle that has long been known to affect sexual or asexual reproduction, growth, virulence, or secondary metabolism [1–6]. Serving as environmental stress-signal which provides a fungus with information about time or its current location [7, 8], light has received increasing attention as it appears to impact more metabolic levels than initially thought. Indeed, a recent study with Neurospora crassa highlighted that up to 31% of all expressed genes in this organism responded to light exposure [9]. This surprisingly high percentage underlines that the role of light in fungi is at best poorly understood [6, 10, 11], especially as only a handful of fungi, mainly N. crassa and Aspergillus nidulans, have been studied in detail. However, understanding fungal photoresponses is far beyond being of mere academic interest. Considering, for example, the abundance of plant-fungi interactions that range from fungal plant pathogens causing severe crop loss worldwide [12, 13] to endophytic fungi which may foster plant growth [14–17], the question of how light influences these organisms and their virulence is of high economic interest.
Fungi of the agriculturally and biotechnologically important genera Metarhizium and Beauveria exhibit versatile lifestyles as (rhizosphere competent) saprophytes, endophytes or entomopathogens [18–21]. Amongst other factors, these different ecological niches vary in their respective light conditions that range from absolute darkness (e.g. within the soil or the insect), to the green-dominated phyllosphere [3, 8, 22, 23] or to sun (and thus UV-) exposed surfaces. Thus, the assumption suggests itself that these fungi have evolved mechanisms to cope with (fast) changing light conditions including UV-stress. Indeed, although photoresponses in Metarhizium and Beauveria species are scarcely explored, the little available data support a thorough adaptation of these organisms to light. Several studies highlight that in these fungi light induces changes in growth, morphology, virulence, conidiation, germination as well as stress tolerance [24–32].
So far, the majority of light studies with fungi in general, but also with respect to Metarhizium and Beauveria species in particular, are performed on Petri dishes. This way of cultivation allows fungi to grow on surfaces and provides easy handling as well as the possibility to incubate several experimental series at once. As there is no standardised procedure for photobiological studies with fungi, the literature offers a wide range of methods for illuminating fungi grown on Petri dishes. These methods include colour filters with defined transmission spectra placed on the Petri dish lid [5, 25, 33], modified incubators with specific lamps or custom-made devices (e.g., [2, 4]), and commercial growth cabinets with integrated illumination (e.g., [4, 34]). Only one recent paper describes for the first time a targeted light incubation from below, thus enabling the LED light source to be replaced more easily [35].
Exploring fungal light responses is far from being trivial and demands a high level of experimental standardization. Considering the stress-signal-function of light [7], how Petri dish grown fungi are subjected to different light spectra may already impact the metabolic response and thus the quality of the produced data. For example, the choice of luminant (i.e. bulb versus LED) can make a considerable difference in heat development during the experiment, which can become problematic if not compensated, since some fungi such as Metarhizium brunneum are considerably temperature sensitive [36]. Unwanted light impulses even in the range of seconds (e.g., by opening the lid of an incubator system within a room without controlled light conditions) can already alter fungal metabolism [25, 30, 37] and thus distort the observed photoresponse. While it is evident that the choice of wavelength and the applied intensity play a key role in fungal light responses [11], also the full width at half maximum bandwidth (FWHM) of the illumination conditions is decisive: the light source might comprise significantly different wavelengths from another color range (albeit in low intensities) and may inadvertently trigger additional photoreceptors [8]. Unfortunately, the illumination conditions, which are the prerequisite for a meaningful interpretation and comparison of data, are not defined clearly in many studies [8], thus considerably hampering the progress in the field.
In the present work, an easy-to-use illumination system with a light-trap-equipped ventilation system for surface cultures was developed, which can be placed in climate chambers. This highly standardized irradiation system allows a variable application of the desired range of light intensities and an easy exchange of LEDs of different wavelengths. Using this device, we explored for the first time the photoresponse of two industrial strains of M. brunneum (MA 43, formerly M. anisopliae var. anisopliae BIPESCO 5/F52) and B. brongniartii (BIPESCO 2) which were grown as Petri dish cultures in dependence of different irradiation scenarios, including variations in wavelengths and illumination intensity. By using this approach, we were able to demonstrate significant changes in morphology, pigmentation, colony diameter, conidial production and metabolite formation for both fungi.