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 methyltransferase activity during accelerated ageing and after osmotic priming, Accelerated ageing of seeds for 1-4 d at 45 degrees 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 (r(2) = 0.90). Aged and untreated seeds were primed for 7 d at 20 degrees 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.