Priming and accelerated ageing affect L-isoaspartyl methyltransferase activity in tomato (Lycopersicon esculentum Mill.) seed

Damage and degradation of cellular proteins is observed during age-induced seed deterioration. L-Isoaspartyl protein methyltransferase (EC 2.1.1.77) is an enzyme hypothesized to play a role in limiting and repairing age-induced damage to proteins. Tomato (Lycopersicon esculentum Mill. 'New Yorker') seeds were assayed for changes in L-isoaspartyl methyl-transferase activity during accelerated ageing and after osmotic priming. Accelerated ageing of seeds for 1-4 d at 45°C and 100% relative humidity reduced germination from 94% to 71%, increased the mean time of germination (MTG) from 2.4 to 5.8 d, and was accompanied by a correlative decrease in L-isoaspartyl methyltransferase activity (r²=0.90). Aged and untreated seeds were primed for 7 d at 20°C in darkness using aerated solutions of 3% KNO₃ or polyethylene glycol 8000 (PEG) with equivalent osmotic potential (- 1.25 MPa). Priming with KNO₃ decreased the MTG, but did not improve germination percentage for untreated seeds. Priming did not affect L-isoaspartyl methyltransferase activity in untreated seeds, but restored activity in aged seeds primed in KNO₃ to levels near that of untreated seeds. Priming with PEG did not effectively improve the MTG or increase L-isoaspartyl methyltransferase activity. During germination, L-isoaspartyl methyltransferase activity remained constant for 48 h post-imbibition and then declined, suggesting that the enzyme was developmentally regulated and inactivated or degraded as radicle emergence occurred.