547 578 |m
Figure 11.4 Diagram of distribution of Pilobolus sporangia adhering to a glass plate interposed between a culture and light dispersed by a prism. Traced from Page (1962).
photoreceptor is most likely a carotenoid or a flavin. The adaptive response of Pilobolus to light ensures the dissemination of its spores and the survival of the fungus.
The fungus Phycomyces blakesleeanus (Zygomycotina) occurs in decaying organic matter but is easily grown in laboratory media that is supplemented with thiamin. Its sporangio-phore is a 10 to 15 cm long single cell (http://www.es.embnet.org/~genus/phycomyces.html), suggesting that its cell wall must be very rigid. The Nobel Laureate Max Delbrück was attracted to this fungus and left his research on phage. The sporangiophore is very sensitive to blue light (400 to 450 nm) with a threshold close to 10 photons per square micrometer, indicating that it has extremely sensitive photoreceptors. The observations suggest that the dosage of light required by the fungi is very small, allowing sporangiophores to grow out of the decaying organic matter.
When viewed from above, the sporangiophore grows counterclockwise toward the light. The blue light photoresponse indicates that the photoreceptor is a flavin or a carotenoid molecule. The bending response (Figure 11.5) is limited to the growing zone that extends from 0.1 to 2 mm below the bright yellow sporangium, darkening to nearly black (Bergman et al., 1969). From multiple exposure photographs of a sporangiophore, Dennison (1958) discovered that the sporangiophore elongates by spiral growth. To explain the phototropic oscillations of a sporangiophore about its stable equilibrium position, a three-layered wall structure was postulated. The photoreceptor is attached to a "receiving and adaptive shell" in the sporangium and twists but does not stretch. An inner wall (responsive structure) bends
and grows and a "reactive structure" passively follows the bend and growth of the "inner wall" (Shropshire, 1963). These three hypothetical wall layers have not yet been related to visible structures.
A sporangiophore placed horizontally bends upwards (negative geotropism). Mutants of Phycomyces were isolated that are photototropically abnormal. These mutants fall into seven complementation groups, indicating that the transduction of the information from the photoreceptor to the morphogenetic response mechanism involves a number of steps. The mutants, madA and madB, are defective in gravitropic and avoidance responses. It was hypothesized that the inputs from the gravity and chemical sensors feed into the tropism path. Many physiological processes have been shown to be affected by light, including the inhibitory effect of blue light on the upward bending of a horizontally placed sporangiophore and the growth away from solid barrier (Bergman et al., 1969).
In 1976 (http://www.es.embnet.org/~genus/phycomyces.htm), Delbrück said, "I feel that if I make a serious experimental research effort (necessarily a very strenuous exercise) it should be in Phycomyces. I am still convinced that Phycomyces is the most intelligent primitive eukaryote and as such capable of giving access to the problems that will be central in the biology of the next decades. If I drop it, it will die. If I push it, it may yet catch on as phage ... caught on. Since I invested 25 years in this venture I might as well continue. I do not expect to make great discoveries, but if I continue to do the spade work my successors may do so." The difficulties in genetic analysis of Phycomyces, for example, the failure of stably transforming it with exogenous DNA, have dampened interest in this fungus.
Light accelerates the development of fruit bodies of the coprophilic fungus Coprinus sterquilinus (Basidiomycotina). Primordia appear in 8 to 11 days under continuous illumination but none appear in darkness (Madelin, 1956). Sequential light exposures are necessary to initiate and program fruit body morphogenesis in Basidiomycotina. Additional periods of illumination are required for the development of the cap, blue (400 to 520 nm) to near ultraviolet (320 to 400 nm) being most effective.
Barnett and Lily (1950) reported that a strain of the fungus Choaneophora cucurbitum (Mucorales/Zygomycotiuna) formed conidia only in alternating light and darkness whereas another strain formed conidia in complete darkness. There are other examples of striking differences in response to light between members of the same family (Hawker, 1966). In Penicillium isariiforme and P. claviforme (Fungi Anamorphici), light triggers the development of aggregated conidiophores (coremium). In Aspergillus nidulans, conidiation is promoted by light of red wavelength and is reversed by far-red (Mooney and Yager, 1990). This is characteristic of the plant pigment phytochrome (a protein containing a chromophore, the light absorbing portion), which undergoes a reversible interconversion between the biologically inactive red-light-absorbing form (Pr) and the active far-red light absorbing form (Pfr) and controls behavior responses in plants such as seed germination, stem elongation and flowering. These fungi are attractive subjects for further experimentation.
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