Identification of novel superoxide dismutase isoenzymes in the olive (Olea europaea L.) pollen

Abstract Background Among antioxidant enzymes, the superoxide dismutase (SOD) family is a major actor in catalysing the disproportionation of superoxide. Apart from its role as antioxidant, these enzymes have a role in cell signalling, and Cu,Zn-SOD proteins are also major pollen allergens. In order to deepen our understanding of the SOD isoenzymes present in olive pollen and to analyse the molecular variability of the pollen Cu,Zn-SOD family, we carried out biochemical, transcriptomic and localization studies of pollen grains from different olive cultivars and other allergenic species. Results Olive pollen showed a high rate of total SOD activity in all cultivars assayed, which did not correlate with pollen viability. Mass spectrometry analysis together with activity assays and Western blotting experiments enabled us to identify new forms of Cu,Zn-SOD enzyme (including chloroplastidic and peroxisomal forms) as well as differentially expressed Mn-, Fe- and Cu,Zn-SOD isoenzymes among the pollen of different olive cultivars and allergenic species. Ultrastructural localization of Cu,Zn-SOD revealed its plastidial localization in the pollen grain. We also identified the occurrence of a shorter form of one of the cytosolic Cu,Zn-SOD enzymes, likely as the result of alternative splicing. This shorter enzyme showed lower SOD activity as compared to the full length form. Conclusions The presence of multiple SOD isoenzymes in the olive pollen could be related to the need of finely tuning the ROS metabolism during the transition from its quiescent condition at maturity to a highly metabolically active state at germination.The authors would like to thank Drs. Jose Manuel Palma, Javier Corpas and Eduardo López-Huertas (EEZ-CSIC, Granada, Spain) for their support and advice with regard to the characterization of SOD activities. The English text was corrected by Michael O’Shea.This study was supported by ERDF-cofunded projects BFU2011–22779, BFU2016–77243-P and RTC-2015-4181-2, RTC-2016-4824-2 (MINECO), P2010- AGR-6274, P10-CVI-6075 and P2011-CVI-7487 (Junta de Andalucía) and 201540E065, 201840E055 (CSIC). AZ thanks the JAE-CSIC program and the Agrifood Campus of International Excellence ceiA3 for the grant funding kindly provided. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI)