Pectin methylesterase (PME) is a cell-wall enzyme that acts as a growth and morphogenesis factor in higher plants and is involved in gene silencing, plant virus reproduction, and transgenesis. A study was made of the role of PME as a stress protein in host plant-virus interactions. PME enzymatic activity was induced, not only by an additional PME gene copy, but also by an empty vector. PME suppressed tobacco mosaic virus (TMV) reproduction, including short-and long-distance virus movement in plants. Surprisingly, elevated PME activity was observed in intact stably transformed transgenic plants. For example, PME activity was increased in transgenic Nicotiana tabacum and N. benthamiana plants expressing the genes for the TMV movement protein and GFP and in tomato plants with cosuppression of the polygalacturonase gene. Activation of light-inducible psbO induced transcription of the PME gene. It was suggested that PME is involved in maintaining the stability of the plant transcriptome and restores its status quo upon viral infection, transformation with a foreign gene, or excess transcription of the cell genome.