Oryza coarctata PROTEIN L-ISOASPARTYL METHYLTRANSFERASE (PIMT) repairs isoaspartyl modification to antioxidative enzymes and is implicated in seed traits in rice

Abstract

PROTEIN L-ISOASPARTYL METHYLTRANSFERASE (PIMT) is a protein repairing enzyme, which is highly abundant in orthodox seeds, and plays an important role in seed vigor and longevity. PIMT essentially repairs isoaspartyl modification in proteins. Despite PIMT has been characterized from several orthodox seed producing plant species, role and regulation of PIMTs in recalcitrant seed producing plants are still limited. In the present study, PIMT from Oryza coarctata, which produces recalcitrant seeds and possess both enzymatically active (OcPIMT1–1 and OcPIMT2–1) and inactive (OcPIMT1–2 and OcPIMT2–2) PIMT isoforms, are functionally characterized through biochemical and genetic approach. We show that PIMT isoforms are differentially localized in Oryza sativa and Oryza coarctata. We also report that enzymatically active OcPIMTs isoforms, but not enzymatically inactive OcPIMTs isoforms, could impart seed vigor, viability and longevity in A. thaliana. Likewise, rice transgenic lines were also generated, and ectopic overexpression of enzymatically active OcPIMT isoforms resulted in increased seed length and weight with improved seed vigor and longevity. Subsequent analysis revealed that antioxidant enzymes (OsAPX and OsCAT) are susceptible to isoAsp modification, which negatively influences their biological functions; however, OcPIMTs physically interact, repairs and protect their function from harmful isoAsp modification, and thereby modulate ROS homeostasis in seeds during aging. Collectively, our results highlight the mechanisms and importance of ectopic expression of OcPIMT isoforms in seed desiccation tolerance and subsequent vigor, viability and longevity in rice.