Sporangiophore formation in Physarum plasmodia starts about 10 hr after photoinduction. It is characterized by the induction of two tubulins and of at least 15 major sporangiophore morphogenetic proteins. In vitro translation of extracted mRNA revealed that differential gene expression is based on a highly synchronous temporal program of loss of plasmodial and induction of sporulation-specific mRNA species. Using a cloned cDNA encoding part of a sporangiophore morphogenetic protein from Physarum as a probe it was found that the induction of the complementary mRNA activity is due to the induction of the mRNA itself. The results suggest that light induces, with a lag phase of about 10 hr, the transient activation of sporulation-specific genes.Numerous developmental processes in microorganisms and plants are controlled by light (1-3). The mechanisms of signal transduction and differential gene expression during photomorphogenesis have been studied mainly in plant systems. The appearance of distinct mRNA species after induction by red or UV light in barley and in parsley suspension culture, respectively, is well established (4-7). According to the current hypothesis, light-induced gene expression in these systems is controlled at the transcriptional level (reviewed in ref. 8).The influence of light on gene expression in fungi and slime molds has been examined mostly at the level of inhibitor studies and overall RNA synthesis (1). We have chosen light-induced fruiting-body formation (sporulation) in plasmodia of the acellular slime mold Physarum polycephalum as a simple model system in which to study the expression of specific genes during light-induced development of a nonphotosynthetic organism. Sporulation in P. polycephalum is most effectively induced by UV/blue and red light (9, 10). Sporangiophore formation starts about 11 hr after the beginning of a 3-hr illumination period and is complete after about 20 hr. It includes a presporangial mitosis, cytoplasmic cleavage, and melanization of the sporangiophore heads (11, 12). Sporangiophore formation is sensitive to inhibitors of RNA and protein synthesis (12) and is accompanied by the synthesis of nonplasmodial RNA as well as of some spore-specific proteins (13,14). An unambiguous identification of these proteins, however, has not yet been achieved and nothing is known about the regulation of their expression during fruiting-body formation.Here we report that light induces a highly synchronous temporal program of mRNA regulation. Two of the photoinduced proteins were identified as tubulins. The cDNA of an unidentified sporangiophore morphogenetic protein (SMP) was cloned and used to demonstrate the induction of the corresponding mRNA sequences. MATERIALS AND METHODSCulture Conditions and Induction of Sporulation. Microplasmodia of the white mutant strain LU897xLU898 (15) were cultured as described (16). Macroplasmodia were grown for 2 days and then starved (17). Induction was accomplished on the 4th day of starvation by a 3-hr illumination with fluorescent ...

Blue light induces sporulation ofPhysarum polycephalum macroplasmodia and reversibly inhibits spherulation (sclerotization) of microplasmodia. Illuminated microplasmodia have an abnormal appearance. The photobiological responses of the plasmodia appear to be unaffected by the absence of yellow pigment in the white mutant strain used. Illumination of microplasmodial suspensions with blue light (Am,, -465 nm) results also in an early effect on glucose metabolism: glucose consumption is reversibly inhibited. By using radioactive glucose it was shown that the main products formed are a water-insoluble glucan and the disaccharide trehalose. Inhibition of glucose consumption in the light results in decreased production of these two compounds. Illumination of microplasmodial suspensions also causes a reversible effect on the pH of the medium which is interpreted as a decreased production of a yet unidentified acid from glucose. The action spectrum of the light-induced pH response shows maxima near 390, 465, and 485 nm. It resembles the absorption spectrum of a flavoprotein and confirms the existence of a blue-light receptor in P. polycephalum microplasmodia.white mutant strain of P. polycephalum. Spectroscopic and chromatographic analyses of extracts of white plasmodia have revealed that, within the limit of detection (<<0.1%) the cells contain none of the typical yellow pigments present in the wild type (6). The light induction ofsporulation ofthe white plasmodia, however, appears to be completely unaffected by the absence ofyellow pigment.We assume that the photoreceptor responsible for the induction ofsporulation is present in microplasmodia as well as in macroplasmodia. Here we report on the existence of a blue-light receptor in microplasmodia of the white strain. This pigment cannot mediate the induction ofsporulation in microplasmodia but it inhibits spherulation, a process that can be regarded as the alternative pathway of differentiation. It is further shown that illumination with blue light has an early effect on glucose metabolism.Starving macroplasmodia of the diploid yellow-pigmented myxomycete Physarum polycephalum display two alternative pathways of differentiation. In the dark they undergo conversion to resistant encysted structures (sclerotia) which, upon addition ofnutrients, revert into plasmodia. In the light, irreversible differentiation into fruiting bodies (sporangia) is induced which is followed by meiosis and allows the initiation of a new life cycle. Germination of the spores liberates mononucleated haploid amoeba which generate a plasmodium by sexual fusion (for reviews see refs. 1-3).Microplasmodia of P. polycephalum that are cultured in suspension rather than in surface culture also differentiate into encysted structures (microsclerotia or spherules) when they are incubated under starvation conditions. Illumination of starving microplasmodia, however, does not result in the formation of spores, a process that apparently requires a dry environment.The action spectrum of the light indu...

The influence of blue light on protein synthesis in spherulating Physarum polycephalum microplasmodia was studied using two‐dimensional protein separation techniques. The starvation‐induced plasmodium‐spherule transition proceeds in the dark and is accompanied by the synthesis of 20 major differentiation‐specific proteins as revealed by in vivo labelling with [35S]methionine. Three of these proteins are identical with cell wall components with respect to their mol. wts. (35 K, 34 K and 14 K) and isoelectric points. Spherulation is also accompanied by the appearance of 26 prominent differentiation‐specific mRNA species translatable in the rabbit reticulocyte cell‐free system. Six of the proteins synthesized in vitro co‐migrate on two‐dimensional gels with proteins labelled in vivo, two of them being cell wall components. Blue light, which inhibits spherulation completely, inhibits also the synthesis of spherule proteins and of spherule‐specific mRNA activity. Only three protein components are induced by blue light, indicating that illumination does not induce a novel differentiated plasmodial state.