A Factor that Promotes Mycelial Development in Sporisorium reilianum In vitro

A factor that promotes mycelial growth in vitro in Sporisorium reilianum, the head smut fungus of Sorghum bicolor (L.) Moench, was found in the sorghum floral meristem, which consists of the floral primordium, its subtending node, and its surrounding leaf. The biologically active factor is a heat stable component of the primary cell wall, which becomes more available to the fungus after autoclaving. The factor was inactivated by the nonspecific protease, Pronase E. Evidence suggests that the host factor is a glycoprotein associated with the host primary cell wall. Host extract-induced mycelium formation was inhibited by the common sugars sucrose, glucose, and fructose, and associated with this inhibition there was a formation of uronic acid crystals on the surface of the culture medium. Lactose did not inhibit host extract-induced mycelium formation. Mycelia of different types were observed from S. reilianum when cultured in vitro on Murashige and Skoog medium containing various carbon sources. Host extract promoted growth of thick clusters of mycelia consisting of vigorously growing long narrow hyphae, which were richly cytoplasmic and had branches at sharp angles (type 1). Other types of mycelia were observed on some fractions of the host extract and some carbohydrates. Some had long curved hyphae with no visible cytoplasm except in their growing tips. These had their septae visible as rings at regular intervals (type 2). A third type had broader hyphae than type 1. These were also cytoplasmic, as type 1, but branched at broader angles. Two other mycelial clusters were actually sporidia joined end to end forming a branching pattern. They differed from each other in the size of individual sporidia in the cluster. Some di- and polysaccharides that have galactose or galacturonic acid as one of the monosaccharides also promoted the formation of type 2 mycelium.