VIRAL AND BACTERIAL GENE DRIVERS. “Similar to the emergence and evolution of eukaryotic cells, evolution of land plants was also shaped and driven by infective and symbiotic microorganisms. These microorganisms are the driving force behind the evolution of plant synapses and other neuronal aspects of higher plants; this is especially pronounced in the root apices.”
http://journal.frontiersin.org/article/10.3389/fcimb.2013.00044/full