A Type I and a Type II Metacaspase Are Differentially Regulated during Corolla Development and in Response to Abiotic and Biotic Stresses in Petunia × hybrida

Metacaspases are structural homologs of the metazoan caspases that are found in plants, fungi, and protozoans. They are cysteine proteases that function during programmed cell death, stress, and cell proliferation. A putative metacaspase designated PhMC2 was cloned from Petunia × hybrida, and sequence alignment and phylogenetic analysis revealed that it encodes a type II metacaspase. PhMC2 cleaved protease substrates with an arginine residue at the P1 site and cysteine (iodoacetamide) and arginal (leupeptin) protease inhibitors nearly abolished this activity. The activity of PhMC2 was highest at pH 8, and the putative catalytic site cysteine residue was required for optimal activity. Quantitative PCR showed that PhMC2 transcripts were detectable in petunia corollas, styles, and ovaries. Expression patterns were not upregulated during petal senescence but were higher at the middle stages of development when flower corollas were fully open but not yet starting to wilt. PhMC1, a type I metacaspase previously identified in petunia, and PhMC2 were differentially regulated in vegetative tissues in response to biotic and abiotic stresses. PhMC2 expression was upregulated to a greater extent than PhMC1 following Botrytis cinerea infection, while PhMC1 was upregulated more by drought, salinity, and low nutrient stress. These results suggest that petunia metacaspases are involved in flower development, senescence, and stress responses.