PRIMING AND ACCELERATED AGING AFFECT L-ISOASPARTYL METHYLTRANSFERASE ACTIVITY IN TOMATO (LYCOPERSICON-ESCULENTUM MILL) SEED

Damage and degradation of cellular proteins is observed during age-ind uced seed deterioration. L-isoaspartyl protein methyltransferase (EC 2 .1.1.77) is an enzyme hypothesized to play a role in limiting and repa iring age-induced damage to proteins. Tomato (Lycopersicon esculentum Mill, 'New Yorker') seeds were assayed for changes in L-isoaspartyl me thyltransferase activity during accelerated ageing and after osmotic p riming, 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 degr ees C in darkness using aerated solutions of 3% KNO3 or polyethylene g lycol 8000 (PEG) with equivalent osmotic potential (-1.25 MPa), Primin g with KNO3 decreased the MTG, but did not improve germination percent age for untreated seeds, Priming did not affect L-isoaspartyl methyltr ansferase activity in untreated seeds, but restored activity in aged s eeds primed in KNO3 to levels near that of untreated seeds, Priming wi th PEG did not effectively improve the MTG or increase L-isoaspartyl m ethyltransferase activity. During germination, L-isoaspartyl methyltra nsferase 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.