Research in protein tyrosine phosphorylation in higher plants has lagged behind because none of enzymes responsible (tyrosine kinases or phosphatases) have been characterized at the molecular level. There has been much controversy about the existence of tyrosine phosphorylation in plants. To date it has not been accepted that this postranslational modification does in fact, exist in plants, although a good number of papers supporting it have been published in the last three years. But is it because it does not exist, or is an artifact, or because people have not looked at the right model and/or at the right time?

The discovery of tyrosine phosphorylation in animal cells was a serendipitous event by Tony Hunter who was interested in the molecular mechanisms through which polyoma DNA virus induces malign transformation. He was studying T antigen immunoprecipitation from infected and non-infected cells. After the incubation of the immunoprecipitated with P³²P-ATP the protein became phosphorylated. One of his students was running a chromatograph with a buffer that had used many times, and he saw two spots instead of one. Using a new buffer at pH 1.9 he noticed that the phosphotryosine (PY) and phosphothreonine (PT) had coemigrated. Hunter concluded that the original buffer had made possible the separation of PY from PT because it had lower pH, around 1.7. He developed a two dimensional system to separate PY from PT. Since then, there has been a rapid rise in the number of papers describing the presence and involvement of protein tyrosine kinases (PTK), tyrosine phosphorylation (PY) and protein tyrosine phosphatases (PTPases) in important animal cell events. By contrast, the discovery of protein tyrosine kinases and protein tyrosine phosphorylation in plants remains an uncertain and perhaps an non-important event. For this reason, we want to summarize here the most important results published to date on plants, and to note that the events where PTK, PTPases and PY have been involved are as important to the plant cells as are: cell cycle, plant embryogenesis and organogenesis, cytoskeleton reorganization, sensing and response to pathogens or environmental changes, all of which are essential for plant survival.