Oxidative stress and senescence-like status of pear calli co-cultured on suspensions of incompatible quince microcalli

This work presents a simple in vitro system to study physiological, biochemical and molecular changes occurring in a pear callus (Pyrus communis L., cv. Beurre Bosc) grown in close proximity to spatially separated undifferentiated homologous (pear) or heterologous (quince; Cydonia oblonga Mill., East Malling clone C) cells in its neighboring environment. After a 7-day co-culture period, the presence of heterologous cells produced negative effects on the pear callus, whose relative weight increase and adenylate energy charge decreased by 30 and 24%, respectively. Such behavior was associated with a higher O-2 consumption rate (+125%) which did not seem to be coupled to adenosine triphosphate synthesis. Analyses of alternative oxidase and enzymatic activities involved in reactive oxygen species (ROS) detoxification strongly suggested that the higher O-2 consumption rate, measured in the pear callus grown in the heterologous combination, may probably be ascribed to extra-respiratory activities. These, in turn, might contribute to generate metabolic scenarios where ROS-induced oxidative stresses may have the upper hand. The increase in the levels of 2-thiobarbituric acid reactive metabolites, considered as diagnostic indicators of ROS-induced lipid peroxidation, seemed to confirm this hypothesis. Moreover, reverse transcription polymerase chain reaction analysis revealed that the expression levels of a few senescence-associated genes were higher in the pear callus grown in the heterologous combination than in the homologous one. Taken as a whole, physiological and molecular data strongly suggest that undifferentiated cells belonging to a pear graft-incompatible quince clone may induce an early senescence-like status in a closely co-cultured pear callus.