Stressing the role of MAP kinases in mitogenic stimulation

In yeast and animal cells, distinct subfamilies of mitogen-activated protei n kinases (MAPKs) have evolved for transmitting different types of signals, such as the extracellular signal-regulated kinase (ERK) for mitogenic stim uli and differentiation, p38 and JUN kinase (JNK) for stress factors. Based on sequence analysis, the presently known plant MAPKs are most similar to ERKs, even though compelling evidence implies a role in various forms of bi otic and abiotic stress responses. However, knowledge of their involvement in controlling proliferation is just emerging. A subgroup of the plant MAPK s, containing the alfalfa MMK3 and tobacco NTF6, are only active in mitotic cells and their localisation to the cell plate suggests a role in cytokine sis. An upstream regulator of MAPKs, the tobacco NPK1, appears to be also a ctivated during mitosis. NPK1 might be associated and regulated by a microt ubule motor protein. The localisation of NPK1 to the cell plate and its mit osis-specific activation suggest that together with NTF6 it could constitut e a mitotic MAPK signalling module in tobacco. NPK1 appears to have a secon d role in repression of auxin-induced gene expression. MAPKs might also be involved in signalling within the meristems as suggested by the recruitemen t of a small G-protein to the CLAVATA 1 receptor-like protein kinase upon a ctivation. In animal and yeast cells some of the small G-proteins relay sig nals from receptors to MAPK pathways.