Petunia Metacaspases Are Regulated during Corolla Senescence and in Response to Abiotic Stress

Metacaspases are cysteine-dependent proteases that function during programmed cell death, stress, and cell proliferation in plants.  A type I and a type II metacaspase from Petunia x hybrida (PhMC1 and PhMC2, respectively) were characterized during developmental and abiotic stress-induced senescence.   Recombinant PhMC1 and PhMC2 proteins had activity against the arginine-specific cysteine protease substrate GRR (Boc-GRR-AMC).   In vivo GRRase activity during flower development was highest in senescent petals, suggesting that metacapases are involved in flower senescence.   Immunodetection, with anti-PhMC1 and anti-PhMC2 antibodies, indicated that both proteins were up-regulated during petal senescence.  Quantitative RT-PCR experiments to characterize gene expression showed that PhMC1 transcripts were up-regulated during petal senescence, while PhMC2 expression was highest in nonsenescing corollas at anthesis.  Ethylene, drought, and nutrient stress experiments were conducted in petunias to investigate the regulation of PhMC1 and PhMC2 in response to abiotic stress.  PhMC1 but not PhMC2 transcripts were up-regulated in corollas when flowers were treated with ethylene.  Both PhMC1 and PhMC2 expression increased in response to drought stress and nutrient deficiency.  Increases in transcript abundance were detected when leaves were visibly yellowing or showing signs of senescence.  PhMC1 transcript abundance was higher than that of PhMC2 after nutrient starvation.  Nitrogen and phosphorus deficiency resulted in similar increases in PhMC2 transcript abundance in leaves, while N deficiency resulted in a greater induction of PhMC1 than P deficiency.  Metacaspases may be regulating programmed cell death during developmental and environmental stress induced senescence in both leaves and corollas.