Connecting oxidative stress, auxin, and cell cycle regulation through a plant mitogen-activated protein kinase pathway.

L ike all living organisms, plants must respond to many external stimuli. Mitogen-activated protein kinases (MAPKs) mediate signal transduction of stress, cell cycle, and growth control in all eukaryotes. MAPKs perform their function as part of protein kinase modules, which in addition to other components are composed of MAPKs, MAPKKs (MAPK kinases), and MAPKKKs (MAPKK kinases) (for reviews, see refs. 1 and 2). MAPKs are serineythreonine protein kinases with a two-lobed structure. The active site is found at the domain interface and contains the MAPK-specific TXY (threonine-Xtyrosine) motif that is targeted by MAPKKs, dual-specificity protein kinases that activate MAPKs by phosphorylation of both the threonine and tyrosine residue of the TXY motif. MAPKKs are activated themselves by phosphorylation of two conserved serine or threonine residues (SyTXXXSyT) by MAPKKKs. According to their divergent structures MAPKKKs can be classified into different subfamilies. MAPKKKs contain different regulatory motifs, including pleckstrin homology domains, prolinerich sequences involved in Src homology 3 binding, zinc finger motifs, leucine zippers, and binding sites for G proteins. MAPKKKs can be activated by a wide range of stimuli and by different mechanisms, such as phosphorylation and interaction with G proteins or receptors. Whereas MAPKKKs can feed into multiple MAPK pathways and also can target other protein kinase cascades (Fig. 1), MAPKKs usually have a restricted substrate specificity, functioning mainly in a single cascade. Different kinases are assembled into distinct modules by scaffold proteins. Scaffold proteins are important for preventing cross-talk between different cascades and allow a given kinase to function in more than one module without affecting the specificity of the response. Although scaffold proteins have so far only been characterized from yeast and mammals (3), several types also exist in plants (unpublished results). Upon activation of a MAPK pathway, MAPKs often induce expression of specific sets of genes. This expression mostly occurs through translocation of the active MAPK to the nucleus where the MAPK phosphorylates and thereby regulates the activity of specific transcription factors. However, MAPKs also can be targeted to other cellular locations where they phosphorylate structural regulators such as cytoskeleton-associated proteins or regulatory enzymes, including other protein kinases, phosphatases, and phospholipases. Whereas yeast has six MAPKs, 13 are known in mammals and more than 20 in plants (2). Various MAPKKs and MAPKKKs also have been isolated in plants, but so far no MAPK module has been conclusively established in plants. A report in this issue of PNAS by Kovtun et al. (4) demonstrates for the first time the functional existence of a MAPK module in plants. The authors show that ANP1 and NPK1, orthologous MAPKKKs from Arabidopsis and tobacco, respectively, mediate H2O2 signal transduction by activating the two Arabidopsis stress MAPKs, AtMPK3, and AtMPK6. H2O2 is a wellknown product of oxidative stress playing multiple roles in plant physiology. H2O2 belongs to the class of reactive oxygen species (ROS) that are produced in photosynthetic tissues, mitochondria, and also in the cytosol under certain stress conditions, such as cold, drought, and salt stress, or pathogen attack (5, 6). ROS include singlet oxygen, superoxide, H2O2, and hy-