Effect of viroid infection on the dynamics of phenolic metabolites in the apoplast of tomato leaves

Plants are capable of producing a wide array of secondary metabolites that serve a variety of functions, due to their bioactive, redox or structural properties. Subtle changes in the external or internal environment of the plant can cause significant changes in the array of secondary metabolites present in the tissue. During the last 10 years the critical roles that these metabolites play both in plant development as well as plant defense against biological and abiotic stresses have begun to become more widely appreciated. Here we describe changes in apoplastic phenolics associated with systemic infections of Potato spindle tuber viroid (PSTVd) as it spreads through inoculated tomato plants. Apoplast fluids from leaves at different distances from the infection site were examined by HPLC–UV over a 5 week period. The phenolic composition of apoplasts in healthy plants was highly dynamic and depended on leaf position as well as plant age. Differences in the apoplast composition of PSTVd infected plants were detected only in leaves in which the viroid was present and multiplied. In those leaves, apoplast phenolics that were similar to the control generally decreased although some increased compared to control plants. Another set of phenolics that were unique to the viroid infected plants appeared simultaneously with the onset of viroid replication and then disappeared. An important basic finding of this study is that the composition of apoplastic metabolites is dynamic, changing with time and leaf development. In addition we demonstrate that the secondary metabolites of the leaf apoplast respond to the presence of the viroid in the symplast of the cell. This study provides a basis for a more detailed investigation of the dynamics and identification of the apoplastic phenolics involved in the viroid–host interaction.