Cellular Basis of Growth Rate Differences in Isolates of Rhizoctonia solani: Metabolic Processes and Growth Rates. Fu -Kuen Lin, Former Research Assistant, Department of Plant Pathology, University of Illinois at Urbana 61801, Present address of the Senior author: Tumor Biology Laboratory, 12 Plant Industry, Univ. of Nebraska, Lincoln 68503; David Gottlieb, Professor, Department of Plant Pathology, University of Illinois at Urbana 61801. Phytopathology 64:1220-1228. Accepted for publication 23 April 1974. DOI: 10.1094/Phyto-64-1220.
The pathogen, Rhizoctonia solani, includes some strains which differ in their relative growth rates, independent of their ability to utilize nutrients and plant hormones or to respond to temp changes. The vigor of these isolates is correlated with the rate of synthesis of their lipids, proteins, nucleic acids, and cell wall material. The rate of synthesis of these metabolic reactions is in turn related to the uptake of the substrates from the medium, and seems to be a function of the cell membrane. When, for example, whole cells were used, there was a positive correlation between the relative growth of slow-, medium-, and fast-growing isolates with the synthesis of protein, but no such correlation was found when the relative rates of protein synthesis in cell-free preparations were measured.
The differences in permeability do not seem to be dependent on the amount of energy that is available to the different isolates of R. solani, for previous studies have shown the lack of correlation between the ATP content of the mycelium and relative growth rate. Furthermore, the current evidence indicates that even with the nonmetabolizable sugar analogues, 2-deoxy-D-glucose and 3-O-methyl-D-glucose, the three isolates produce only insignificant amounts of carbon dioxide. Nevertheless, the uptake of these sugars by the isolates is correlated with their relative growth rates.
Other interesting positive relationships with growth are in the relative proportions of membrane-bound ribosomes, the amount of newly synthesized RNA, the rate of synthesis of the different types of RNA, and the content of CMP and UMP in the RNA. The ratio of 25S to 18S RNA is 1.4, a value much lower than has been reported for other species. The nucleotide composition of the R. solani isolates was similar to that of a number of other fungi with a G+C molar percentage of about 50. Other nucleotide data either are not correlated with growth rate of the isolates, or have a negative correlation. Uridine could be derived from exogenous uracil, and the rate of conversion to uridine is in the same order as the relative growth rates of the different isolates.