Relationship Between Polyamines Metabolism and Cell Death in Roots of Malus hupehensis Rehd. Under Cadmium Stress

The free putrescine (Put) content, the hydrogen peroxide (H2O2) content and the polyamine oxidase (PAO) activity in roots of Malus hupehensis Rehd. var. pinyiensis Jiang (PYTC) were significantly increased, and reached its peak at 1, 2 and 6 h, respectively, under cadmium treatment. The free spermine (Spm) and spermidine (Spd) contents were dramatically decreased, and reached the minimum value at 4–6 h, then remained relatively stable. The change in total free polyamines (PAs) content was consistent with that of free Put. The number of root dead cells was gradually increased after treatment for 24 h, and the typical characteristics of programmed cell death (PCD) were displayed at 48 h. Throughout the Cd treatment process, changes in PAs metabolism appeared to be prior to cell death increase, and the H2O2 content was always maintained at a high level. These results indicated that polyamines could initiate cell death by generating H2O2 in roots of Malus hupehensis Rehd. under CdSO4 stress.