Two-component systems are an evolutionarily ancient means for signal transduction. These systems are comprised of a number of distinct elements, namely histidine kinases, response regulators, and in the case of multi-step phosphorelays, histidine-containing phosphotransfer proteins (HPts). Arabidopsis makes use of a two-component signaling system to mediate the response to the plant hormone cytokinin. Two-component signaling elements have also been implicated in plant responses to ethylene, abiotic stresses, and red light, and in regulating various aspects of plant growth and development. Here we present an overview of the two-component signaling elements found in Arabidopsis, including functional and phylogenetic information on both bona-fide and divergent elements.
Protein phosphorylation is a key mechanism for regulating signal transduction pathways in both prokaryotes and eukaryotes. In eukaryotes, regulatory phosphorylation predominantly occurs at serine, threonine, and tyrosine residues (Hunter, 1995; Hunter and Plowman, 1997; Plowman et al., 1999). In contrast, many signal transduction pathways in bacteria employ a so-called “two-component system” that relies upon phosphorylation of histidine and aspartic-acid residues (Mizuno, 1997). Plants also make use of two-component systems and these perform important roles in growth and development (Schaller, 2000; Mizuno, 2005; To and Kieber, 2008).
Two-component systems confer upon bacteria the ability to sense and respond to environmental stimuli and are involved in such diverse processes as chemotaxis, osmotic sensing, oxygen sensing, and host recognition (Parkinson, 1993; Stock et al., 2000; Baker et al., 2006). The simplest form of a two-component system employs a receptor with histidine-kinase activity and a response regulator (Figure 1). The receptor is located in the plasma membrane of the bacterium. In response to an environmental stimulus, the histidine kinase autophosphorylates a conserved histidine residue. This phosphate is then transferred to a conserved aspartic acid residue within the receiver domain of the response regulator. Phosphorylation of the response regulator modulates its ability to mediate downstream signaling. In bacteria, many of the response regulators are transcription factors. Thus, two proteins create a signaling circuit, capable of converting an external stimulus into a change in transcription. There are permutations on the two-component system. Of particular relevance to the plant two-component systems are multi-step phosphorelays (Figure 1) (Swanson et al., 1994; Appleby et al., 1996). These make use of a “hybrid” kinase that contains both histidine kinase and receiver domains in one protein, a histidine-containing phosphotransfer (HPt) protein, and a separate response regulator (Appleby et al., 1996). In these multi-step phosphorelays, the phosphate is transferred from amino acid to amino acid in sequence His → Asp → His → Asp.
Although originally identified in bacteria, two-component signaling elements have also been identified in fungi, slime molds, and plants (Swanson et al., 1994; Loomis et al., 1997; Schaller, 2000; Mizuno, 2005). Interestingly, the canonical histidyl-aspartyl phosphorelay is apparently not found in animals. Analysis using the SMART protein-domain search interface ( http://SMART.embl-heidelberg.de) (Schultz et al., 2000) indicates that bona-fide two-component signaling elements are lacking from the genome sequences of Homo sapiens, Drosophila melanogaster, and Caenorhabditis elegans.
In Arabidopsis, proteins with significant sequence similarities to all elements of the two-component system have been identified, including histidine kinases, response regulators, and HPt proteins (Figure 2) (Schaller, 2000; Mizuno, 2005). Phosphorylation activity has been confirmed for at least one example in each case (Gamble et al., 1998; Imamura et al., 1998; Miyata et al., 1998; Suzuki et al., 1998; Imamura et al., 1999). In recent years, multiple experimental approaches have demonstrated the action of a histidyl-aspartyl phosphorelay in mediating cytokinin signal transduction (Hwang and Sheen, 2001; Higuchi et al., 2004; Nishimura et al., 2004; Mason et al., 2005; Hutchison et al., 2006; Riefler et al., 2006; Yokoyama et al., 2007).
Arabidopsis also contains divergent two-component-like elements that are unlikely to function in histidyl-aspartyl phosphorelays (e.g. phytochromes and pseudo-response regulators) as they are missing one or more key amino-acid residues involved in a phosphorelay (Figure 2) (Makino et al., 2000; Schaller, 2000). Plants, like animals, contain pyruvate dehydrogenase kinase; this enzyme is related to histidine kinases but has an altered specificity such that it now phosphorylates serine residues (Popov et al., 1993). Thus, there is evidence from eukaryotes that two-component signaling elements have evolved to fill new functions that no longer rely upon phosphorylation of histidine and aspartic acid residues.
Analysis of the Arabidopsis genome supports the existence of eight histidine kinases that contain all the conserved residues required for enzymatic activity (Table 1, Figures 2 and 3), as well as additional diverged histidine-kinase like proteins that lack residues essential to histidine kinase activity. Some of the functional histidine-kinases have been identified as receptors for the plant hormones cytokinin (AHK2, AHK3, and AHK4) and ethylene (ETR1 and ERS1), but the ligands for the other three bona-fide histidine kinases (AHK1, CKI1, and CKI2) have yet to be determined.
Histidine Kinase-like Proteins of Arabidopsis
Cytokinin Receptor Family
The cytokinin receptor family is composed of three histidine kinases: AHK2, AHK3, and AHK4 (also called WOL1 or CRE1). Initial evidence that this family functions in cytokinin perception came from the study of AHK4 transgenically expressed in bacteria and yeast, where it was shown that AHK4 could bind cytokinins and that ligand-binding stimulated the receptor's ability to signal through a phosphorelay (Inoue et al., 2001; Suzuki et al., 2001; Ueguchi et al., 2001; Yamada et al., 2001). All three receptors contain transmembrane domains, are thought to be localized to the plasma membrane, and contain a CHASE (<u>c</u>yclases/<u>h</u>istidine kinases <u>a</u>ssociated <u>s</u>ensing <u>e</u>xtracellular) domain in their predicted extracellular portion that functions in cytokinin binding (Anantharaman and Aravind, 2001; Heyl et al., 2007). The isolation and characterization of T-DNA insertion mutations has demonstrated roles for the cytokinin receptors in diverse cytokinin-regulated processes including cell division, vascular differentiation, leaf senescence, seed size, and stress responses (Higuchi et al., 2004; Nishimura et al., 2004; Riefler et al., 2006; Tran et al., 2007).
Ethylene Receptor Family
The ethylene receptor family of Arabidopsis (ETR1, ERS1, ETR2, ERS2, and EIN4) also contains proteins with histidine kinase domains (Chang et al., 1993; Hua et al., 1995; Hua et al., 1998; Sakai et al., 1998). ETR1 and ERS1 have been demonstrated to bind ethylene when transgenically expressed in yeast (Schaller and Bleecker, 1995; Rodriguez et al., 1999; Hall et al., 2000) and both contain functional histidine kinase domains (Gamble et al., 1998; Moussatche and Klee, 2004). Although histidine-kinase activity has been implicated in subtle modulations of the ethylene response, no major role has yet been identified in ethylene signal transduction (Wang et al., 2003; Binder et al., 2004; Qu and Schaller, 2004). Mutational analysis indicates that, rather than relying on a histidyl-aspartyl phosphorelay, ethylene signal transduction incorporates the Raf-like kinase CTR1, the membrane-bound Nramp-like protein EIN2, and the EIN3 family of transcription factors (Chen et al., 2005). Histidine kinase activity of the ethylene receptors could play a direct but lesser role in ethylene signal transduction. Alternatively, histidine kinase activity could allow for cross-talk between ethylene perception and other two-component signaling pathways such as cytokinin signal transduction.
The ethylene-receptor family also includes three members (ETR2, ERS2, and EIN4) that contain divergent histidine-kinase domains (Bleecker, 1999; Schaller, 2000). Thus, one protein family of related function contains both bona-fide and divergent histidine kinases. Based on in-vitro phosphorylation assays, ETR2, ERS2, and EIN4 are now thought to function as ser/thr kinases (Moussatche and Klee, 2004). In the same study ERS1 was found to phosphorylate serine in addition to histidine, suggesting that it might be bi-functional.
CKI1 was initially identified in a mutant screen where it was found that its ectopic expression results in cytokinin-independent greening and shoot induction in callus cultures (Kakimoto, 1996). However CKI1 is not closely related to the cytokinin-receptor family and lacks the CHASE domain involved in cytokinin binding. Thus, the cytokinin-related phenotype resulting from CKI over-expression may arise due to non-physiological cross-talk with the cytokinin-signaling pathway. Analysis of loss-of-function mutations has clarified the role of CKI1 in plant growth and development, although a regulatory ligand for this putative receptor has not been identified. The homozygous cki1 mutation is lethal and examination of the mutant indicates that CKI1 is required for megagametogenesis, consistent with expression data that indicate CKI1 is expressed in developing ovules (Pischke et al., 2002; Hejatko et al., 2003).
CKI2, like CKI1, can induce cytokinin responses when ectopically expressed (Kakimoto, 1996), but is also not thought to be directly involved in cytokinin signaling. CKI2 shows strong expression in the root and weaker expression in flowers (Iwami et al., 2007). Loss-of-function mutations in CKI2 exert a subtle phenotype in which root elongation is more sensitive to growth inhibition in response to ethylene, suggesting possible cross-talk with the ethylene receptors (Iwami et al., 2007).
AHK1 was initially proposed to function as a plant osmosensor based on its ability to complement function of the yeast osmosensor SLN1, a histidine kinase that regulates the HOG1 MAP-kinase pathway for osmosensing in yeast (Urao et al., 1999). However it was subsequently found that other histidine kinases of Arabidopsis, including the cytokinin receptors, can complement yeast SLN1 mutations, indicating that this assay is not necessarily diagnostic of plant function (Reiser et al., 2003; Tran et al., 2007). Rather, genetic analysis indicates that AHK1 modulates plant growth and stress responses based on the finding that an ahk1,ahk2,ahk3 triple mutant (i.e. a combination of ahk1 with two cytokinin receptor mutations) is reduced in size compared to the ahk2,ahk3 double mutant (Tran et al., 2007). AHK1 is induced by dehydration and acts as a positive regulator of drought and salt stress responses, in contrast to the cytokinin receptors which are also induced by dehydration but act as negative regulators of these responses (Tran et al., 2007).
The phytochromes, which act as red light receptors (Sharrock and Quail, 1989; Clack et al., 1994; Rockwell et al., 2006), are divergent histidine kinases that possess ser/thr kinase activity rather than histidine kinase activity (Yeh and Lagarias, 1998).
Pyruvate dehydrogenase kinase
Pyruvate dehydrogenase kinase retains the most highly conserved residues found in histidine kinases, but has nevertheless diverged significantly in other residues, resulting in an ability to phosphorylate its substrate, pyruvate dehydrogenase, at a serine residue (Thelen et al., 2000).
HISTIDINE-CONTAINING PHOSPHOTRANSFER PROTEINS
Histidine-containing phosphotransfer (HPt) proteins function in multi-step phosphorelays, acting as signaling intermediates between hybrid histidine kinase and response regulators (Figure 1). The Arabidopsis genome encodes five HPt proteins (AHP1 through 5) that contain the conserved residues required for activity, as well as one pseudo-HPt (APHP1/AHP6) that lacks the histidine phosphorylation site (Table 2, Figures 2 and 4) (Miyata et al., 1998; Suzuki et al., 1998; Suzuki et al., 2000). The HPt proteins have been shown capable of participating in a phosphorelay with Arabidopsis response regulators (Suzuki et al., 1998). In addition, two-hybrid analysis has demonstrated their ability to interact with both hybrid histidine kinases and response regulators (Imamura et al., 1999; Urao et al., 2000; Tanaka et al., 2004; Dortay et al., 2007), consistent with an ability to function in a multi-step phosphorelay.
HPt Proteins of Arabidopsis
Analysis of loss-of-function mutations has revealed that AHP1, AHP2, AHP3, and AHP5 function as redundant positive regulators of cytokinin signaling (Hutchison et al., 2006). AHP4 only contributes slightly to the cytokinin responses, and in some cases appears to act as a negative regulator (Hutchison et al., 2006). The AHPs have been shown to accumulate in the nucleus in response to exogenous cytokinin (Hwang and Sheen, 2001; Yamada et al., 2004). APHP1, which encodes a pseudo-HPt lacking the phosphorylation site, acts as a negative regulator of cytokinin responses (Mähönen et al., 2006b). APHP1 expression is induced by cytokinin and thus functions as part of a negative feedback loop to reduce the plants sensitivity to cytokinin, with APHP1 potentially interacting with the receiver domains of the cytokinin receptors to prevent phosphotransfer to bona-fide AHPs.
There are 23 genes in the Arabidopsis genome encoding proteins predicted to be functional response regulators (Table 3, Figures 2 and 5). These authentic response regulators (ARRs) can be divided into three classes based on phylogenetic analysis and function: type-A type-B, and type-C response regulators (Imamura et al., 1999; Schaller et al., 2007). In addition, there are nine genes encoding response regulators that lack the conserved Asp for phosphorylation, and are called pseudo-response regulators (PRRs) (Makino et al., 2000).
Response Regulators of Arabidopsis
Type-A response regulators
The type-A response regulators are relatively small, containing a receiver domain along with short N- and C-terminal extensions (Brandstatter and Kieber, 1998; Imamura et al., 1998; Urao et al., 1998). Members of the type-A family are transcriptionally induced to varying extents by cytokinin (Brandstatter and Kieber, 1998; Taniguchi et al., 1998; D'Agostino et al., 2000), and cytokinin also stabilizes some type-A ARR proteins in a phosphorylation-dependent manner (To et al., 2007). Genetic analyses indicate that ARR3, 4, 5, 6, 7, 8, 9 and 15 function as negative regulators of cytokinin signaling, thus participating in a negative feedback loop to reduce the plant sensitivity to cytokinin (Kiba et al., 2003; To et al., 2004; Leibfried et al., 2005; Lee et al., 2007; To et al., 2007).
Some type-A ARRs are also implicated in other regulatory pathways. ARR4 interacts with phytochrome B to modulate its activity, allowing for cross-talk between the cytokinin and light signaling pathways (Sweere et al., 2001; Mira-Rodado et al., 2007). WUSCHEL, a regulator of stem cells in the shoot apical meristem, negatively regulates transcription of several type-A ARRs, revealing cross-talk between cytokinin signaling and a key meristem identity gene (Leibfried et al., 2005). ARR3 and ARR4 regulate the circadian period in a cytokinin-independent manner, with loss of these two genes resulting in a longer clock period (Salomé et al., 2006).
Type-B response regulators
The type-B ARRs differ from the type-A ARRs in that the type-B ARRs contain long C-terminal extensions with a Myb-like DNA binding domain referred to as the GARP domain (Imamura et al., 1999; Hosoda et al., 2002). Multiple lines of evidence support the role of the type-B ARRs as transcription factors (Sakai et al., 2000; Imamura et al., 2001; Lohrmann et al., 2001; Sakai et al., 2001; Hosoda et al., 2002; Imamura et al., 2003; Mason et al., 2004; Mason et al., 2005; Rashotte et al., 2006). Type-B ARRs are nuclear-localized in Arabidopsis and capable of transcriptional activation when expressed in yeast (Lohrmann et al., 1999; Sakai et al., 2000; Lohrmann et al., 2001). The ARR1, ARR2, and ARR10 type-B response regulators bind to a core DNA sequence 5′-(G/A)GAT(T/C)-3′ (Sakai et al., 2000; Hosoda et al., 2002).
The eleven type-B ARRs of Arabidopsis fall into three subfamilies based on phylogenetic analysis: subfamily 1 contains seven members (ARR1, ARR2, ARR10, ARR11, ARR12, ARR14, and ARR18); subfamily 2 contains two members (ARR13 and ARR21); and subfamily 3 is also comprised of two members (ARR19 and ARR20) (Mason et al., 2004). Genetic analyses indicate that at least five subfamily-1 members mediate cytokinin signaling, with ARR1, ARR10, and ARR12 appearing to play key roles (Mason et al., 2005; Yokoyama et al., 2007; Ishida et al., 2008). The cytokinin transcriptional response is substantially reduced in type-B mutant backgrounds, supporting a central role of the type-B ARRs in the cytokinin signaling pathway (Rashotte et al., 2006; Yokoyama et al., 2007). A number of primary response genes directly regulated by the type-B ARRs have been identified, including the type-A ARRs (Taniguchi et al., 2007). It has been proposed that the subfamily-1 member ARR2 may modulate ethylene signaling, but the effect is subtle and differing results have been obtained in the analysis of arr2 mutants, perhaps due to differing growth conditions (Hass et al., 2004; Mason et al., 2005).
The functions of the subfamily-2 and subfamily-3 type-B ARRs are unclear. They are not as broadly expressed as the subfamily-1 ARRs (Mason et al., 2004; Tajima et al., 2004), and the only reported mutant phenotypes arise from overexpression of activated versions of the genes (Tajima et al., 2004; Kiba et al., 2005). Overexpression of activated ARR21 (subfamily-2) results in seedlings in which cell proliferation is activated to form callus-like structures. Overexpression of activated ARR20 (subfamily-3) results in plants that develop small flowers and abnormal siliques with reduced fertility.
Type-C response regulators
Arabidopsis also contains two additional response regulators (ARR22 and 24). These lack long C-terminal extensions, like the type-A ARRs, but are not closely related to the type-A ARRs based on phylogenetic analysis (Figure 5) (Kiba et al., 2004; Schaller et al., 2007). The type-C ARRs are also not transcriptionally regulated by cytokinin. The type-C ARR sequences are more similar to the hybrid-kinase receiver domains than to other response regulators (Figure 5), raising the possibility that a histidine kinase, rather than an HPt protein, could serve as their phosphodonor. ARR22 and ARR24 are predominantly expressed in flowers and siliques (Kiba et al., 2004; Gattolin et al., 2006). Single and double loss-of-function mutants grow similarly to wild-type (Gattolin et al., 2006). Overexpression of ARR22 inhibits cytokinin signaling based on the transgenic lines having reduced shoot growth, poor root development, reduced cytokinin-responsive gene induction, and insensitivity to cytokinin under conditions for callus production (Kiba et al., 2004; Kiba et al., 2005; Gattolin et al., 2006). Whether the type-C ARRs normally antagonize the cytokinin signaling pathway is not known.
Arabidopsis contains nine pseudo-response regulators termed PRRs (Table 3, Figures 2 and 5). These contain complete receiver domains but are missing essential residues required for activity (Makino et al., 2000). In particular, the aspartate that serves as a site for phosphorylation is missing, in many cases being replaced by a glutamate residue that may mimic the phosphorylated form. These also contain C-terminal extensions, some members with a CCT-motif (PRR1/TOC1, PPR3, PRR5, PRR7, and PRR9) and others with the Myb-like motif found in the type-B response regulators (PRR2, PRR4, and PRR6).
The pseudoresponse-regulators with the CCT motif are involved in the regulation of circadian rhythms and participate in multiple regulatory feedback loops in recent models for the Arabidopsis clock (Mizuno, 2005; Gardner et al., 2006; McClung, 2006). PRR1/TOC1 was identified in a forward genetic screen due to a semi-dominant mutation (timing of cab expression 1-1) that had shortened circadian periods for leaf movement, stomatal conductance, and several molecular markers (Millar et al., 1995; Somers et al., 1998; Strayer et al., 2000). PRR1/TOC1 participates in a feedback loop involving the other well-known clock components, CCA1 and LHY (Alabadi et al., 2001). Evidence that all the CCT-motif PRRs participate in the clock came from the discovery that their expression varies in a circadian manner (Matsushika et al., 2000; Makino et al., 2001) and that loss-of-function mutants have altered circadian periods (Kaczorowski and Quail, 2003; Michael et al., 2003; Farré et al., 2005; Nakamichi et al., 2005b; Nakamichi et al., 2005a; Salomé and McClung, 2005; Ito et al., 2008). The current model places PRR5, PRR7, and PRR9 in a feedback loop that also involves CCA1 and LHY. Recent work indicates that protein stability of TOC1, PRR5, PRR7, and PRR9 is post-translationally regulated (Farré and Kay, 2007; Ito et al., 2007; Kiba et al., 2007; Para et al., 2007). Additionally, PRR7 is phosphorylated, presumably on serine/threonine residues since it lacks the canonical phospho-acceptor aspartate, which may serve as a means to regulate its stability and/or function (Farré and Kay, 2007).
Arabidopsis also contains the predicted sequence for a response regulator (ARR23) that, although containing the phosphorylated aspartate, is predicted to lack the N-terminal domain of the receiver; no EST is reported and ARR23 could be a pseudogene. The gene product of At3g04270 also shows homology to receiver domains but is missing what would be its C-terminal end.
SIGNALING THROUGH PHOSPHORELAYS
A multi-step phosphorelay, rather than a simple two-component system, appears to be the major His-Asp signaling circuit employed by Arabidopsis. This possibility was initially raised by analysis of the Arabidopsis genome, which revealed a preponderance of hybrid kinases in Arabidopsis along with the presence of HPt proteins (Schaller et al., 2002). Subsequent analyses of protein-protein interactions using the yeast two-hybrid system supports interactions among the hybrid kinases with HPt proteins, and of the HPt proteins with both type-A and type-B response regulators, consistent with what would be expected in a multi-step phosphorelay (Imamura et al., 1999; Urao et al., 2000; Dortay et al., 2007). Finally, as described below, analysis of the cytokinin signaling pathway supports use of the multi-step phosphorelay in planta.
The cytokinin signaling pathway gives us our clearest picture of how two-component signaling elements have been adapted to signaling in plants (Hwang and Sheen, 2001; Haberer and Kieber, 2002; Heyl and Schmülling, 2003; Kakimoto, 2003; To and Kieber, 2008). Genetic analyses using loss-of-function mutations indicate that the primary cytokinin signaling pathway is a positive regulatory circuit that requires hybrid histidine kinases (AHK2, AHK3, and AHK4) (Inoue et al., 2001; Suzuki et al., 2001; Ueguchi et al., 2001; Yamada et al., 2001; Kakimoto, 2003; Kim et al., 2006), HPt proteins (AHP1, AHP2, AHP3, AHP5) (Hutchison et al., 2006), and type-B response regulators (ARR1, ARR2, ARR10, ARR11, and ARR12) (Sakai et al., 2001; Mason et al., 2005; Yokoyama et al., 2007; Ishida et al., 2008) (Figure 6). The function of type-B ARRs as transcription factors indicates that signals may move from histidine kinase to the nucleus solely through elements of a two-component system, the same type of signaling circuit employed by many prokaryotes. The mobile element of this signaling circuit are the AHPs, which relocate from cytosol to nucleus in response to cytokinin (Hwang and Sheen, 2001; Yamada et al., 2004). It should be noted that additional two-component signaling elements (such as other type-B ARRs) may also contribute to this cytokinin signaling circuit but this has not yet been confirmed genetically.
Several negative regulatory circuits are also employed in cytokinin signaling. First, AHK4, like some bacterial histidine kinases, has both kinase and phosphatase activities (Mähönen et al., 2006a). In the absence of cytokinin, AHK4 acts as a phosphatase to dephosphorylate AHPs, thereby decreasing signaling through the phosphorelay. Upon cytokinin binding, AHK4 switches to act as a histidine kinase to initiate the multi-step phosphorelay, resulting in phosphorylation of AHPs and downstream response regulators. Second, one of the initial transcriptional responses mediated by the type-B ARRs in response to cytokinin is to induce the expression of the type-A ARRs (Brandstatter and Kieber, 1998; Imamura et al., 1998; D'Agostino et al., 2000; Mason et al., 2005; Rashotte et al., 2006; Taniguchi et al., 2007; Yokoyama et al., 2007). The type-A ARRs then negatively regulate cytokinin signaling, potentially by competing with the type-B ARRs for phosphorylation by the AHPs (Kiba et al., 2003; To et al., 2004; Leibfried et al., 2005; Lee et al., 2007; To et al., 2007). Furthermore, multiple type-A proteins are stabilized by cytokinin, further increasing this negative feedback loop (To et al., 2007). Third, the pseudo-HPt, APHP1, is induced by cytokinin and acts as a negative regulator of cytokinin responses (Mähönen et al., 2006b).
The presence of additional hybrid histidine-kinases in Arabidopsis, such as ETR1, AHK1, CKI1, and CKI2, suggests that multi-step phosphorelays function in relaying signals other than cytokinin. This raises the question as to which downstream components are involved in relaying the additional signals, given that a majority of the two-component signaling elements have already been implicated in cytokinin signaling. While there exists a formal possibility for unique combinations of signaling elements, it is far more likely that downstream signaling elements are shared among the receptors. In some cases, different input signals may be channeled into the same downstream signaling pathway to regulate a common response. Alternatively, a response tailored to a specific signal input could still be obtained even with shared signaling elements. One way to accomplish this would be to have varying affinities between the receptors and the AHPs, and between the AHPs and the response regulators, such that each signal would differentially activate the downstream components. Yeast two-hybrid analysis does suggest some specificity to the interactions between receptors and the AHPs (Urao et al., 2000), but the physiological relevance of these differences has not been determined. An alternative possibility would be for specificity of the downstream signaling elements to be modified by covalent modification or protein-protein interaction in ways unique to the signal input. It is also possible that protein subsets are sequestered in unique signaling complexes by scaffold proteins. Genetic, molecular, and proteomic approaches should clarify how this network of interactions functions in plants.
We thank Dennis Mathews, Mike Gribskov, and John Walker for their assistance in writing the previous version of this review (Schaller et al., 2002). We thank Takeshi Mizuno and Tatsuo Kakimoto for their assistance with naming the two-component signaling elements, which resulted in the unified gene nomenclature adopted in 2002. Research in the authors' laboratories has been supported by grants from the National Science Foundation, Department of Energy, and the U. S. Department of Agriculture.