Caspase-like proteases regulate aluminum-induced programmed cell death in peanut

Recent evidence has proved that caspase protease activities are detected in both mammals and plants during programmed cell death (PCD). The characteristics and functions of caspase-like proteases play important roles in understanding the mechanisms of PCD in plants. In this work, we report firstly the involvement of caspase-like protease activities and effects in aluminum (Al) stress in two contrasting peanut genotypes. Caspase-like activities in the root tip cells of ‘Zhonghua 2’ (Al-sensitive) and ‘99-1507’ (Al-tolerant) were detected using synthetic caspase substrates during Al-triggered PCD. Caspase-1-, -2-, -3-, -4-, -5-, -6-, -8- and -9-like proteases were found in peanut root tip cells. VDQQDase (caspase-2-like) and WEHD (caspase-5-like) were the first detected in the plants, and almost all of the caspase-like proteases were activated during Al-induced PCD, especially caspase-3-like and caspase-1-like, which was higher in ‘Zhonghua 2’ than in ‘99-1507’. The highest activity levels of caspase-3- and caspase-1-like proteases occurred 8 and 4 h after 100 µM Al treatment, respectively. Compared with 100 µM AlCl₃ treatment alone, specific caspase-3 protease inhibitor Ac-DEVD-CHO inhibited the increase of caspase-3-like protease activity, Al content, Hsr203j expression, cell death and DNA fragmentation, and the decrease in root growth induced by 100 µM AlCl₃ treatment, but it was more obvious in ‘Zhonghua 2’ than in ‘99-1507’. In conclusion, there were different caspase-like proteases in root tips of peanut, and caspase-3-like protease was a crucial executioner in Al-induced PCD. Its effects in the ‘Zhonghua 2’ genotype were higher than in ‘99-1507’. An improved model of the mechanism of Al-induced PCD and Al tolerance differences in different genotypes is proposed.