Photomorphogenesis in Physarum: induction of tubulins and sporulation-specific proteins and of their mRNAs.

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.

Blue light inhibits slime mold differentiation at the mRNA level.

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.

Blue light influences gene expression and motility in starving microplasmodia of Physarum polycephalum.

The differentiation of starving Physarum polycephalum microplasmodia into resting structures (spherules) was studied. Early events in this differentiation pathway include decreases in both plasmodial motility and protein synthesis. The starving plasmodia show a blue light avoidance response. Blue light (lambda max 450 nm, irradiance 16 W/m2) acts antagonistically to the starvation stimulus so that spherule formation is inhibited [16]. Light affects each of the above mentioned events of the differentiation pathway: the migration rate of illuminated plasmodia is stimulated, the light avoidance response is irreversibly lost. The rate of incorporation of radioactive leucine into plasmodial protein remains at a higher level in illuminated plasmodia as compared to the decreasing rate during spherule formation in the dark. Protein degradation, uptake of external leucine, and the size of the internal leucine pool are not affected by light. Analysis by SDS-polyacrylamide gel electrophoresis of pulse-labelled plasmodial proteins reveals that blue light inhibits the synthesis of distinct starvation-induced proteins and allows continued synthesis of all major plasmodial proteins. Some of the blue light responses described are mimicked by alpha-amanitin suggesting that light might influence gene expression at the level of transcription.

Blue-light receptor in a white mutant of Physarum polycephalum mediates inhibition of spherulation and regulation of glucose metabolism.

Blue light induces sporulation of Physarum 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 (lambda max approximately 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.