The role of endo-[beta]-mannanase activity in tomato seed germination

The role of endo-β-mannanase activity in tomato seed germination was studied using the osmotic agent PEG 6000 and the plant hormones abscisic acid (ABA) and gibberellic acid. Endo-β-mannanase is known to degrade galactomannans in cell walls, and its activity was found in the lateral endosperm upon radicle protrusion and in the endosperm cap before radicle protrusion. The former activity, involved in the mobilisation of reserves, appeared to be inhibited by ABA. No inhibition by this hormone could be detected for the latter activity, which supposedly mediates radicle protrusion. Radicle protrusion was also strongly inhibited by ABA. The isozymes that were found in the endosperm cap before radicle protrusion were different from the ones in the lateral endosperm after radicle protrusion. It was concluded that endo-β-mannanase in the endosperm cap played no role in the ABA regulated inhibition of germination. Studies with ABA analogs confirmed the observations.Puncture force measurements with endosperm caps revealed that during germination two steps can be distinguished in the endosperm cap weakening, required for radicle protrusion. The first step is not inhibited by ABA and correlates with an increase in endo-β-mannanase activity. The second step is inhibited by ABA and does not correlate with endo-β-mannanase activity. It was concluded that endo-β-mannanase activity mediates the first step of the endosperm weakening, and that a putative cell wall degrading enzyme is involved in the second step. Attempts to identify this enzyme were unsuccessful.Osmotic priming is a treatment of seeds resulting in improved germination. Its beneficial action appeared to correlate partly with endo-β-mannanase activity and step 1 of the endosperm weakening. Activity of endo-β-mannanase always correlated with a porous appearance of the cell walls in the endosperm cap, as observed with cryo-scanning electron microscopy. In high concentrations of osmotic agent no endo-β-mannanase activity and no endosperm weakening could be detected, whereas germination did improve. It was concluded that lowering of the endosperm restraint during priming positively affects the germination speed, but is not a prerequisite.The overall conclusion is that endo-β-mannanase plays a limited role in the completion of germination and acts through the first step of the endosperm weakening.