Decreased suppression of immune stimulatory CpG motifs in plant DNA

Due to differential content of CpG motifs in genomic DNA of organisms like bacteria and mammals, CpG-containing DNA delivers a danger or immunostimulatory signal that is recognized by Toll-like receptor 9 in mammalian cells. Here we show that genomic DNA from several plants promote proliferation and CD69 expression as well as activate NFkappaB and JNK pathways in murine B lymphocytes. Plant DNA synergize with specific antigen in activating B cells in a dose-dependent manner. Using a computational method we compared the usage of CpG motif related sequences in DNA of plants, bacteria, mammals or other species. It was found that the CpG motif suppression is much less significant in plant DNA than in mammalian genomes. These computation results partially explain the immunostimulatory activity of plant DNA observed in biological experiments, and lead to the hypothesis that plants respond to plant pathogens by recognizing CpG motifs in the pathogens' genomic DNA. Collectively this work provides new evidence for further understanding the interactions between plants and the human immune system or homeostasis, and between plants and their pathogens. The hypothesis that CpG dependent immunomodulation is a feature of plant DNA that contributes to plant nutrition or food/pollen allergy is also discussed.