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

During raspberry (Rubus ideaus L. cv Glen Clova) fruit ripening, endo-β-1,4-glucanase (EGase; EC 3.2.1.4) specific activity (per g fresh weight) increases approximately 15-fold. Highest activity was associated with the surfaces 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 transcription-polymerase chain reaction (RT-PCR), two 497-bp partial putative EGase clones were obtained from ripe receptacle mRNA (termed RI-EGL1 and 2) which share 53% amino acid identity. The more abundant RI-EGL1 was used to obtain 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. Northern analysis revealed that RI-EGL1 expression was limited to ripe-fruit receptacles. Application of the ethylene antagonist 1-methylcyclopropene (1-MCP) to green fruits indicated that endogenous ethylene accelerates raspberry abscission and increases both EGase activity and RI-EGL1 expression.