Role of peroxidase activity and Ca2+ in axis growth during seed germination

Axis growth during seed germination is mediated by reactive oxygen species and apoplastic peroxidase plays a role by producing OH (center dot) from H (2) O (2) . Ca (2+) activates both apoplastic peroxidase and NADPH oxidase. Role of reactive oxygen species (ROS) in seed germination and axis growth has been demonstrated in our earlier works with Vigna radiata seeds by studying superoxide generation and its metabolism in axes (Singh et al. in Plant Signal Behav doi: 10.4161/psb.29278, 2014). In the present study, the participation of apoplastic peroxidase along with the involvement of Ca2+ in axis growth during germination and post-germination stage has been investigated. Pharmacological studies using peroxidase (POX) inhibitors (salicylhydroxamic acid, SHAM; sodium azide, NaN3) and OH center dot scavenger (sodium benzoate, NaBz) indicated that seed germination and early axis growth (phase I) depend much on POX activity. Subapical region of axes corresponding to radicle that elongated much particularly in phase II suggested high POX activity as well as high NADPH oxidase (Respiratory burst oxidase homologue, Rboh, in plants) activity as indicated from localization by staining with TMB (3,3',5,5'-tetramethyl benzidine dihydrochloride hydrate) and NBT (nitroblue tetrazolium chloride), respectively. Apoplastic class III peroxidase (Prx) and also cellular POX activity reached maximum at the time of radicle emergence as revealed by TMB staining, spectrophotometric and in-gel assay for POX activity. Treatment with Ca2+ antagonists (La3+, plasma membrane-located Ca2+ channel blocker and EGTA, Ca2+ chelator in apoplast) retarded seed germination and strongly inhibited axis growth, while Li+ (blocks endosomal Ca2+ release) was effective only in retarding phase II axis growth suggesting an involvement of Ca2+ influx during early axis growth. From the effect of Ca2+ antagonists on the localization of activities of POX and Rboh using stains, it appears that Ca2+ plays a dual role by activating Prx activity in apoplast while activating Rboh by entering into cytosol.