Is a cysteine proteinase inhibitor involved in the regulation of petal wilting in senescing carnation (Dianthus caryophyllus L.) flowers?

Senescence of carnation petals is accompanied by autocatalytic ethylene production and wilting of the petals; the former is caused by the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase genes and the latter is related to the expression of a cysteine proteinase (CPase) gene. CPase is probably responsible for the degradation of proteins, leading to the decomposition of cell components and resultant cell death during the senescence of petals. The carnation plant also has a gene for the CPase inhibitor (DC-CPIn) that is expressed abundantly in petals at the full opening stage of flowers. In the present study, DC-CPIn cDNA was cloned and expressed in E. coli. The recombinant DC-CPIn protein completely inhibited the activities of a proteinase (CPase) extracted from carnation petals and papain. Northern blot analysis showed that the mRNA for CPase (DC-CP1) accumulated in large amounts, whereas that for DC-CPIn disappeared, corresponding to the onset of petal wilting in flowers undergoing natural senescence and exogenous ethylene-induced senescence. Based on these findings, a role of DC-CPIn in the regulation of petal wilting is suggested; DC-CPIn acts as a suppressor of petal wilting, which probably functions to fine-tune petal wilting in contrast to coarse tuning, the up-regulation of CPase activity by gene expression.