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Keywords:

  • alkaline α-galactosidase;
  • chloroplast;
  • degradation;
  • DGDG;
  • leaf senescence;
  • rice;
  • thylakoid membrane

Summary

  •  Here, we studied the functional role of a chloroplast alkaline α-galactosidase (OsAkαGal) in the breakdown of thylakoid membranes during rice (Oryza sativa) leaf senescence.
  •  We assayed the enzyme activity of recombinant OsAkαGal with different natural substrates and examined the effect of ectopic OsAkαGal expression in rice plants.
  •  Recombinant OsAkαGal showed at least a two-fold greater substrate-binding affinity and a 10-fold greater turnover rate to galactolipid digalactosyl diacylglycerol than the raffinose family of oligosaccharides (verbascose, stachyose, raffinose) and melibiose. The OsAkαGal null mutant (osakαgal) displayed a delayed leaf senescence phenotype. OsAkαGal complementation in osakαgal recovered OsAkαGal expression and showed a senescence phenotype similar to that of wild-type plants. Transgenic plants overexpressing OsAkαGal (UbiP-OsAkαGal) exhibited retarded plant growth and development, and showed a pale-green phenotype coupled with a reduced chlorophyll content to 42% in newly unfolded leaves. UbiP-OsAkαGal leaves also showed a 29-fold increase in alkaline α-galactosidase activity compared with wild-type leaves. An ultrastructural study of Ubi-OsAkαGal chloroplasts in newly unfolded leaves revealed abnormal grana organization.
  •  Our findings strongly suggest that OsAkαGal is a thylakoid membrane-degrading enzyme involved in the degradation of digalactosyl diacylglycerol during rice leaf senescence.