Regulation of synthesis of proteinase inhibitors I and II mRNAs in leaves of wounded tomato plants

Abstract

Levels of two wound-inducible serine proteinase inhibitors, called Inhibitor I and Inhibitor II, and their mRNAs were quantified in leaves of tomato (Lycopersicon escululentum (L.) Mill.) plants after wounding the leaves with a hemostat. A single wound on a lower leaf of 25-old tomato plants caused the accumulation of the two inhibitor proteins in wounded and non-wounded leaves beginning about 4–6 h following wounding. The rate of inhibitor accumulation was maximal in leaves for the next 4 h and then declined. By 20 h the accumulation had nearly ceased. Following a single wound, Inhibitor I mRNA [600 bases in length] and Inhibitor II mRNA (760 bases) began to accumulate in wounded leaves about 2 h before the inhibitor proteins could be detected. The levels of mRNA for both inhibitors reached a maximum at about 8 h following wounding and then decayed, both with apparent half lives of about 10 h. Four consecutive wounds, inflicted hourly, increased the levels of mRNA for both inhibitors to over twice the levels induced by a single wound. Within 4 h following multiple wounds, Inhibitor I mRNA represented about 0.5% of the total polyadenylated mRNA (poly(A⁺)mRNA) and Inhibitor II mRNA about 0.15% of the total. The rates of accumulation of the two inhibitor proteins varied depending upon the age of the plants and their environment during growth, and ranged between 3 and 10 μg Inhibitor I·h^-1·(g tissue)^-1 for Inhibitor I and about half of these rates for Inhibitor II. Nuclei were isolated from leaves of wounded and non-wounded plants, and in mRNA runoff experiments using specific inhibitor copy DNAs (cDNAs) as probes the synthesis of Inhibitor I and II mRNAs were shown to be regulated, at least in part, at the level of transcription.