MAP Kinase Pathways in Phytohormone Signaling

The MAP kinase pathways are suggested to be involved in the intracellular transmission of the signals that are generated by various phytohormones, which play various roles in physiological and developmental processes in plants. Ethylene is a gas that regulates fruit ripening, leaf senescence and abscission, flowering and cell elongation (Chen et al. 2005). The CTR1 gene encodes a protein kinase that is similar in amino acid sequence to an animal MAPKKK, Raf. The ctr1 mutant shows a constitutive triple response, including short hypocotyls and roots, a thickened hypocotyl and an exaggerated apical hook; these are observed in wild-type plants only when grown in the dark in the presence of ethylene (Kieber et al. 1993). Thus, it was suggested that CTR1 is a negative regulator of the transduction pathway for ethylene signaling.

Although the direct downstream components of CTR1 have not been completely established, some MAPKKs and MAPKs have recently been reported to transduce the ethylene signal. In alfalfa, SIMK and MKK3 are MAPKs that were found to be activated upon ethylene treatment (Ouaked et al. 2003). The transgenic Arabidopsis plants expressing SIMKK, a MAPKK of SIMK and MKK3, could activate these downstream MAPKs and phenocopy ethylene treatment. Although this MAPK and MAPKK are genetically located downstream of CTR1 MAPKKK (Ouaked et al. 2003), CTR1 components have not been proven to phosphorylate SIMKK and to act as its negative regulators.

A role of MAP kinase pathways in the response to auxin has also been suggested. For example, it has been shown that tobacco NPK1 suppresses the activity of the auxin-responsive promoter in maize protoplasts (Kovtun et al. 1998). Some MAPKs from Arabidopsis are reported to be activated when auxin is applied to plants (Mizoguchi et al. 1994; Mockaitis and Howell 2000).

The AtMKK7, one of the MAPKKs of Arabidopsis, is suggested to be a negative regulator in the polar transport of auxin (Dai et al. 2005). However, these components have not been placed in the context of the respective signal cascades and the targets of the pathway have not been identified.

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