A causal role for ethylene and endo-beta-1,4-glucanase in the abscission of red-raspberry (Rubus idaeus) drupelets

During raspberry (Rubus ideaus L. cv Glen Clova) fruit ripening, endo-beta -1,4-glucanase (EGase; EC 3.2.1.4) specific activity (per g fresh weight) i ncreases approximately 15-fold. Highest activity was associated with the su rfaces of the receptacles where the,weakening abscission zones are located, Immunoblotting using antibodies raised against a bean abscission-EGase (ab .-EGase) identified a single protein of M-r 52 kDa that was present only in ripe fruit and was most prominent in the receptacle. Using reverse transcr iption-polymerase chain reaction (RT-PCR), two 497-bp partial putative EGas e clones mere obtained from ripe receptacle mRNA (termed RI-EGL1 and 2),whi ch share 53% amino acid identity. The more abundant RI-EGL1 was used to obt ain its full length clone from a ripe receptacle cDNA library, The deduced amino acid sequence of RI-EGL1 was similar to other ab,EGase sequences (ca 67%) and contained the conserved motifs present in all E2-class EGases, Nor thern analysis revealed that RI-EGL1 expression was limited to ripe-fruit r eceptacles. Application of the ethylene antagonist 1-methylcyclopropene (1- MCP) to green fruits indicated that endogenous ethylene accelerates raspber ry abscission and increases both EGase activity and RI-EGL1 expression.