Carbonylation-dependent protein degradation is a prerequisite for dormancy alleviation of apple (Malus domestica Borkh.) seed

To investigate the mechanism of seed dormancy alleviation in apple seeds, cold stratification for 7, 14, 21, and 40 days was applied. Despite rising reactive oxygen species levels during stratification, this study found protein carbonylation increased only transiently in embryonic axes. It was observed especially after 14 days of seed treatment, proving that this modification occurred in a time-specific and protein-specific manner. A substantial number of carbonylated protein spots in stratified seeds overlapped with dormant ones, suggesting carbonylation occurred before dormancy onset. Five proteins with carbonyl groups were present in the axes regardless of dormancy depth. Among them, globulin 11S, the storage protein, underwent degradation at each stage of seed dormancy alleviation. Interestingly, its breakdown preceded the activation of proteolytic activity. However, increased proteasomal activity, which was accompanied by a relatively low ATP pool, supported that the regulation of carbonylated protein levels involved the 20S proteasome pathway. In addition to storage proteins, carbonylation affected proteins that were no longer useful at a specific seed physiological stage or those interfering with dormancy alleviation, particularly involved in hormonal regulation, signalling, or transport. This study also indicated that carbonylation of biotin-containing proteins may trigger biotin release in plants. These findings highlight the pivotal role of ROS-mediated proteome remodelling in the embryonic axes during dormancy alleviation, a topic that has not previously been investigated in seeds with deep dormancy. The results of these studies allow the complementing of the sequence of molecular events leading from deep dormancy to a germination-ready state.