Effects of NaCl, pH and Ca2+ on autolysis of isolated tomato fruit cell walls

Abstract

Cell walls isolated from ripening tomato (Lycopersicon esculentum Mill. cv. Rutgers) fruit released pectic polymers when incubated under conditions that allow activity of wall‐bound polygalacturonase (EC 3.2.1.15). Autolysis was optimally stimulated by 150–300 mM NaCl at either pH 2.5 or 4.5. This stimulation was negated by exposure to pH 6.5 or higher and by pretreatment of walls with boiling 80% ethanol. Five mM CaCl2 did not affect autolysis at pH 2.5, but significantly inhibited at pH 4.5 or higher. Inclusion of 1 M NaCl at selected steps in the extraction scheme did not inhibit subsequent autolysis of isolated walls. Exposure of isolated walls to 1 M NaCl at pH 2.5–8.5 also did not inhibit autolytic activity compared to walls that received no ionic treatment. These data support the concept that cell wall hydrolysis during tomato fruit softening is regulated by pH, Ca2+ levels and ionic strength of the apoplast.