Chelator used in pectin extraction triggers ethylene production by tomato fruit

In our search for an endogenous ethylene trigger from tomato (Lycopersicon esculentum Mill. cv. Rutgers) fruit cell wall alkaline soluble pectin (ASP), we purified an active component using DEAE-Sepharose chromatography followed by elution on BioGel P-100 or Superose 12. The purified active fraction produced a single band on silver-stained SDS-PAGE of approximately M(r) 20000. Using two-dimensional proton-proton and proton-carbon correlation spectroscopy, we identified the repeating subunit as trans-1,2-diamino-cyclohexane-N,N,N',N'-tetraacetic acid (CDTA), a chelator used to extract ASP. Although the ASP undergoes extensive dialysis during its extraction which should remove CDTA, the CDTA apparently forms a large molecular weight polymer which does not diffuse out of the dialysis tubing. Infiltration of commercially prepared CDTA into mature green tomato fruit stimulated ethylene production. The ethylene stimulatory effect of CDTA was not affected by the presence of equimolar amounts of CaCl₂, or nmol g^-1 amounts of the calcium channel blockers, nifedipine or verapamil. EDTA, EGTA, and diethylenetriaminepentaacetic acid, other divalent cation chelators, also stimulated ethylene production when they were infiltrated into tomato fruit. Neither the purified material nor commercial CDTA stimulated ethylene production when they were infiltrated into leaf tissue.