Several components described above have also been identified in other MAP kinase cascades, some of which are involved in responses against pathogen attacks and environmental stresses.
In alfalfa, SIMK appears in a variety of physiological processes. As mentioned above, SIMK is involved in the tip growth of root hairs and in response to ethylene. Since ethylene is known to stimulate the elongation of root hairs, the role of SIMK in the tip growth of root hairs might indicate its importance in the signal transduction pathway for ethylene. In addition, it is reported that SIMK is activated in response to osmotic stress (Munnik et al. 1999). It is interesting that two different signals - ethylene and hyper-osmotic stress - use the same MAP kinase cascade. It is an open question whether similar basic adaptations occur in response to different environmental changes or whether one signal represses or increases the response to another. It is also possible that the different responses depend on the types of cell expressing the genes that could be targets of a MAP kinase cascade.
NPK1 also appears in various biological processes. As described above, the NPK1 regulates phragmoplast expansion and is activated during the period from anaphase to telophase of mitosis. In addition, reduction in the expression of NPK1 by virus-induced gene silencing makes tobacco sensitive to a pathogen (Jin et al. 2002). Since the silenced plant exhibited reduced cell size and multinucleated cells, the authors report that the MAP kinase pathway regulating phragmoplast development was also impaired. However, it is uncertain whether the same MAPKKs and MAPKs are involved in both the defense response to pathogens and phragmoplast development.
A role of NPK1 in the stress response has also been suggested. Trans-genic maize expressing the catalytic domain of NPK1 has superior freezing and drought tolerance, although in these studies it was not clear which MAP kinase cascade could be activated (Shou et al. 2004a,b). Since in these experiments, the regulatory domain was removed from NPK1 and its expression was ectopic, enhanced tolerance might have been induced by the activation of MAPKs and/or MAPKKs other than the intrinsic targets of NPK1.
ANP1 of Arabidopsis, which is orthologous to NPK1 of tobacco, is suggested to be involved in the response to oxidative stress (Kovtun et al. 2000). In mesophyll protoplast cells, ANP1 without the regulatory domain activates MPK3 and MPK6, which can be activated by H2O2, suggesting that ANP1
might invoke the response to H2O2. Since H2O2 is produced on pathogenic infection of plant cells, the signal transduction pathway that includes ANP1 is expected to have a similar effect to that of NPK1 in the defense system of plants against pathogen attack.
Although multiple roles have been suggested for NPK1 and its ortholog ANP1, whether NPK1 and its orthologs have roles in responses to stresses and pathogens other than cytokinesis is not clear. It is critical for our understanding of the functions of these proteins that we should know the sites of expression of the genes for NPK1 and its orthologs in plants. It is worth noting that NPK1 is expressed only in division-competent cells in plants, such as those in which CDK genes are expressed (Nakashima et al. 1998). However, responses to environmental stresses or pathogenic attacks may occur in de-velopmentally mature tissues in plants. In addition, to understand the physiological roles of NPK1 MAPKKK it may also be crucial to identify molecules that are involved in the activation of NPK1 and those that are regulated downstream of NPK1. This may also be true for our understanding of physiological functions of the other MAP kinase cascades. The fact that there are many more MAPKKK and MAPKs than MAPKKs indicates that, rather than linear pathways, MAPK signaling presumably forms interacting networks. Although cross-talk mechanisms must therefore be operating among the several MAP kinase pathways in plants, they have yet to be demonstrated.
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