VIEW ARTICLE

Physiology and Biochemistry

Translational Activity of Polysomes of Barley Leaves During Infection by Erysiphe graminis f. sp. hordei. J. M. Manners, Queen Elizabeth II postdoctoral research fellow, Department of Biochemistry, University of Queensland, St. Lucia, Queensland 4067 Australia; K. J. Scott, professor, Department of Biochemistry, University of Queensland, St. Lucia, Queensland 4067 Australia. Phytopathology 73:1386-1392. Accepted for publication 19 April 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1386.

Polysomes were extracted from leaves of Hordeum vulgare infected by Erysiphe graminis f. sp. hordei, and their ability to direct protein synthesis in vitro was assessed. Polysome-directed protein synthesis with soluble factors (S147 fraction) from wheat germ was usually three- to fourfold greater than that with the S147 fractions of Escherichia coli. In each instance, insensitivity to aurintricarboxylic acid indicated that little or no reinitiation occurred in vitro. The inhibition of protein synthesis by chloramphenicol, lincomycin, and cycloheximide indicated that the S147 fractions of wheat germ and E. coli preferentially activated protein synthesis on cytoplasmic and chloroplast polysomes, respectively. When equal quantities of total polysomes were included in assays of protein synthesis, the amounts of protein synthesized with the S147 fraction of wheat germ by polysomes from susceptible barley cultivar Prior inoculated 1, 3, and 5 days previously with E. graminis f. sp. hordei race 3 differed only slightly from uninoculated controls. However, with the S147 fraction of E. coli, the activity of polysomes from inoculated leaves were all considerably reduced (36- 51%) below that of the controls. Similarly, when polysomes from the near-isogenic cultivars M1622 (susceptible) and M1623 (resistant) were used, a decrease (36%) in translational activity was detected in polysomes from M1622 at 1 day after inoculation with the system involving E. coli, but no differences in the activity of polysomes from control and inoculated leaves of M1622 or M1623 were detected with the S147 fraction of wheat germ. All the above results were verified by SDS-polyacrylamide gel electrophoresis and fluorography, which indicated that the decreased activity affected most of the chloroplast polypeptides that were synthesized. These results are consistent with a reduction in chloroplast polysomes per unit of total polysomes in infected leaves. Heat shock (45 C for 5- 80 min) of cultivar Prior plants had a different effect on infection, and caused a 40- 60% decrease in the activity of both cytoplasmic and chloroplast polysomes. The translational activity of thylakoid-bound polysomes was also assessed with the system involving E. coli. In Prior, no differences in activity were observed per unit of chlorophyll 1 day after inoculation, while a decrease of 31% was observed 3 days after inoculation. This indicated a decrease in the translational activity of barley chloroplast polysomes at early stages of infection by the powdery mildew fungus in susceptible, but not in resistant, hosts.