Volume 9 Part 1 Article 56
Title: The Action of Light and Other Factors on Sporophore Initiation in Pleurotus ostreatus
Authors: G. Eger, H.-D. Gottwald and U. von Netzer
In a few species of basidiomycetes not only the development of sporophores but also fruiting initiation is controlled by light at least under certain conditions (Alasoadura, 1963; Chapman and Fergus, 1973; Kitamoto et al., 1968; Lu, 1965, 1974; Perkins, 1969). This is true for our strains of Pleurotus ostreatus. In continuous darkness no primordia are formed (Fig. 1, left) if the inocula and the mycelia from which they have been taken have not been exposed to wavelengths shorter than 600 nm. Under illumination however (OSRAM fluorescent bulbs L30) fruiting will occur to various extents depending on the conditions used (Fig. 1, right).
Eger (1970a, b) has elaborated some prerequisites for reproduceable primordia induction in cultures on agar plates. They include i) synchronisation of growth by cutting small inocula from the growing edge of a regular, well developed mycelium, ii) cultivation of mycelia in continuous darkness, and iii) induction of primordia just at the time, when the sensitivity of the mycelia to light is at its optimum (see Madelin, 1956). As in other inducible morphogenetical processes, the phase of increased sensibility is of short duration. In our Pleurotus it is reached just at the time, when the mycelium touches the border of the petri dish, some time before or several hours later, depending on the particular strain. As in other fungi (Robbins and Hervey, 1960; Perkins, 1969) the light effect is low if luxurious aerial mycelium is present. Thus a relatively poor growth medium has to be taken. In her previous work Eger used DIFCO “malt extract broth”. However, as this product is not reliable enough for our experiments, we developed our own “malt extract broth”. Basically, we use the recipe of Reddisch (1926) – as DIFCO does. However, to eliminate excess of phosphate, which favours the unwanted aerial mycelium, we precipitate it by the aid of CaCl2 prior to first autoclaving. The required amount of the latter changes with the source of malt extract. The final pH is between 4.8 and 4.9. On this medium growth is rapid. Agar plates of 8.5 cm diameter are colonized in 7 to 8 days according to the strains. However, it allows only very few primordia to form. In order to get increased numbers, phosphate and a N-source have to be added. This is done in 1 ml buffer solution of pH 6.5 (0.05 M) which is placed as a drop in the centre of the colony. The nutrients that are applied now are not used for production of mycelium but for primordia alone (Fig. 1, right). According to Wessels (1965) depletion of glucose in the medium plays a role in fruiting initiation in Schizophyllum commune. If this would be true for Pleurotus as well, addition of glucose would inhibit fruiting initiation. This is not the case, likewise not if we apply the glucose before the mycelium has penetrated the medium or if we spray it over the whole surface of the culture prior to illumination. There are significant differences in the number of primordia when we offer various N-compounds. Eger (1970a) has shown that asparagine is superior to proline, arginine, alanine, phenylalanine, and urea. We revealed superiority even to glutamine, aspartic acid, and glutamic acid. Thus the good effect of asparagine may not be explained by the supply of two NH2-groups per molecule or by the C-skeleton. It is asparagine itself that acts.
We questioned if the phosphate buffer with asparagine supplies everything that is needed for primordia formation or if the mycelium too provides its share. We grew the mycelium on 15 ml of our malt medium and varied the quantity of asparagine. With amounts up to 125 pM per mycelium the number of primordia increased. Beyond this the process is apparently saturated. Then we kept asparagine constant at 125 pM per petri dish and varied the quantity of mycelium by the amount of medium. From 5 to 7.5 and 10 ml medium we got proportionally increasing numbers of primordia (Fig. 2). With 12.5 ml the surface was already maximally covered with fruiting body initials. It should be emphasized, that under our experimental conditions (Eger, 1976) desiccation plays no role. The linear growth rate of the mycelia is equal whether 5 or 15 ml medium is presented. This result shows, that for a given surface of mycelium not only the phosphate and asparagine but compounds from the deeper parts of the mycelium are necessary for primordia formation.Please login to download the PDF for this proceeding.