Phycomyces: Electrical Response to Light Stimuli

Abstract

Electrical signals have been detected in response to light excitation of the fungus Phycomyces blakesleeanus. These signals are related to the wavelength and intensity of the stimulus and the growth stage of the fungus. A relationship between the signals and the possible photoreceptor-pigment system is explored.The fungus Phycomyces blakesleeanus sporangiophore de-velops in stages designated as I through IVb. These growth stages are related to the elongation of the sporangiophore and to the development of the sporangium. The sporangiophore is a single cell that can grow to centimeters in length. The growth zone, from 0.1 to just beyond 2 mm below the sporangium, is indicated as the sensory region (6). Sensitivity of the sporangiophore to a variety of physical stimuli has been documented, and the most extensively studied response is the phototropic response (3). To date no specific photoreceptor structure has been identified and it is still inconclusive whether the photoreceptor molecule is a flavin or carotenoid (7, 10). Detection of electrical signals from the sporangiophore in response to light excitation has not been reported despite efforts to obtain such data. We have now detected electrical signals upon light excitation and would like to report these experimental results. MATERIALS AND METHODSThe experimental apparatus consisted of a two-chambered plastic well across which the sporangiophore was placed after it had been plucked and its base had been crushed. The sporangiophore made contact with the same solution in each chamber, 10 mm KCl, 12% sucrose. These chambers were connected to Ag-AgCl electrodes by agar-KCl bridges. The well (including the amplifier) was housed in a light-tight copper box, with a 2-cm hole and sliding cover to admit light. The One or more negative going spikes occurred on an average of 2.5 min after exposure to the microflash source. These spikes reached amplitudes of 8 to 20 mv and durations at about 0.5 sec (Fig. lb). They have been observed to occur only in stage I in response to the microflash and in response to monochromatic light of 420 nm with reduced amplitude. Using a series of Corning filter combinations with the microflash, 420 nm was seen to be effective in producing repeated spiking. No spiking could be observed above 537 nm, and spiking occurred only occasionally between 442 and 537 nm.As the sporangiophore matures through stages II to IVb, the complexity and type of the observed electrical response appears to change. Although the receptor potential is seen for all stages studied, the sporadic early receptor potential and the spike response is not seen in these later stages. This bears a relationship to the pigment system of Phycomyces. Orientation of the photopigment molecule is related to the early receptor potential observed in photoreceptors and plants (2,9). Experiments have shown that rod crystals, having a vertical orientation, can be observed in Phycomyces sporangiophore under polarized light (7, 10). These crystals appear to be aligned in the growth...

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“…e effect of blue and red light on pigment creation and metabolism, growth, sexual, asexual development, pigment generation, and tropism in fungus has been examined [7,15]. e concentration of secondary metabolites (GABA, red pigments, MONK, and citrinin) has also been found to vary depending on the color of light and the most resistant to sunlight [16].…”

“…So, many photoreceptors are present in the fungi, which receive and transduce the photon energy [5,6]. For the growth phase of the fungus, the wavelength and intensity of electrical signals are related to light excitation, and it has been detected in Phycomyces blakesleeanus fungus [7]. In A. nidulans, condition induction is due to the response to other wavelengths of different lights [8,9].…”

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