Activated synthesis of poly(A)-containing messenger RNA in soybean hypocotyls inoculated with Phytophthora megasperma var. sojae

Abstract

[ 3H]Uridine pulse-labelled RNA isolated from Harosoy 63 soybean hypocotyls infected with incompatible and compatible races of Phytophthora megasperma var. sojae was analyzed by poly(U)-Sepharose affinity chromatography and by poly(U)-filter binding assay for polyriboadenylic acid [poly(A)]-containing messenger RNA (mRNA). Poly(A)-containing mRNA constituted about 1% of the total pulse-labelled RNA in healthy hypocotyls and the remainder of the radioactivity was distributed among heavy and light ribosomal RNA (rRNA) and soluble RNA (sRNA). Plants inoculated with an incompatible race of the fungus synthesized poly(A)-containing mRNA about 6 times more rapidly than uninoculated control plants, as early as 4 h after inoculation. A similar increase occurred in plants inoculated with a compatible race of the fungus but it was of lesser magnitude. Growth of the incompatible fungus was not inhibited until about 10 h after inoculation. Following the early increase, the rate of synthesis of poly(A)-containing mRNA declined after 6 to 8 h in the incompatible interaction and reached the same rate as in the compatible interaction after 12 h. Sucrose density gradient analysis indicated that the synthesis of certain 18 S species of poly(A)-containing mRNA increased at the early stages of both the incompatible and compatible interactions, although the rate of synthesis was again higher in the former interaction. Synthesis of rRNA and sRNA also increased early after inoculation to higher degrees in the incompatible interaction than in the compatible, but the increases did not exceed about twice the rate in uninoculated control plants.