Characteristics of mitochondria and cell death in roots of Malus hupehensis var.pingyiensis under NaCl stress
NaCl胁迫下平邑甜茶根系线粒体特性和细胞死亡特征

Aims Malus hupehensis var. pingyiensis with strong ability for apomixes is unique to China and is tolerant of waterlogged conditions and salt. There is little difference among its seedlings because of apomixes. Malus hupehensis var. pingyiensis is widely used as stocks of apple cultivars and ornamental apples and has important applied value in apple production and research. Our objective was to determine the response characteristic of cells and mitochondria in M. hupehensis var. pingyiensis roots under high saline conditions. Methods Pot seedlings with at least seven mature leaves were used in this experiment. In order to avoid irreversible damage to roots caused by sudden exposure to high level saline condition, we first used a weak saline solution (0.051 mol·L–1 NaCl) to water seedlings every two days, for a total of three times. Then strong saline solution (0.085 mol·L–1 NaCl) was used to water seedlings. Tap water was used for control seedlings. H2O2 content, membrane potential of mitochondria (Δψm), ATP content, terminal deoxynucleotidyl transferase mediated dUTP nick end labelling (TUNEL) and caspase3/7 activity were determined every two days from the third day of 0.085 mol·L–1 NaCl treatment, altogether five times. Important findings Δψm of roots declined slowly in the first six days of 0.085 mol·L–1 NaCl treatment and thereafter declined rapidly. The H2O2 content in mitochondria decreased slowly in the first six days and then accumulated quickly. ATP content in roots remained constant during the treatment of 0.085 mol·L–1 NaCl, but was low compared with that in control seedlings. The results of TUNEL assay showed that root paraffin slices displayed distinct positive-reaction spots that became more numerous in the root paraffin slice of the ninth day. Caspase3/ 7 activity was examined with a detection kit; it remained at a low level for the first six days of 0.085 mol·L–1 NaCl treatment, and thereafter increased sharply. Results indicated that programmed cell death could be induced in roots of M. hupehensis var. pingyiensis by 0.085 mol·L–1 NaCl treatment, and this was closely related to the change of mitochondrial characteristics.