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Select Chapter: Contents for All Chapters 1. Plant Cells 2. Energy and Enzymes 3. Water and Plant Cells 4. Water Balance of Plants 5. Mineral Nutrition 6. Solute Transport 7. Photosynthesis: The Light 8. Photosynthesis: Carbon 9. Photosynthesis: Physiological 10. Translocation in the 11. Respiration and Lipid 12. Assimilation of Mineral 13. Secondary Metabolites 14. Gene Expression and Signal 15. Cell Walls: Structure, 16. Growth and Development 17. Phytochrome and Light 18. Blue-Light Responses: 19. Auxin: The Growth Hormone 20. Gibberellins: Regulators 21. Cytokinins: Regulators 22. Ethylene: The Gaseous 23. Abscisic Acid: A Seed 24. Brassinosteroids 25. The Control of Flowering 26. Stress Physiology

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Essay 19.3

Phototropism: From Photoperception to Auxin-Dependent Changes in Gene Expression

Emmanuel Liscum, Division of Biological Sciences, University of Missouri, Columbia.

September, 2002

Phototropism—the directional curvature of organs in response to lateral differences in light intensity and/or quality—represents one of the most rapid and visually obvious responses of plants to changes in their light environment (Liscum, 2002). Although a number of plant organs appear to be responsive to directional UV-A/blue light (Iino, 1990; Koller, 1990), most studies have focused on the positive phototropic response (bending towards the actinic light) observed in seedling stems (Figure 1). In nature, stem phototropism likely provides plants with an effective means for maximizing photosynthetic light capture and thus may have appreciable adaptive significance (Iino, 1990; Liscum and Stowe-Evans, 2000). Any adaptive advantage provided by phototropic responses are likely to be particularly important during the early stages of growth and establishment of seedlings (Iino, 1990; Liscum, 2002) and during gap filling situations in dense canopy conditions (Ballare, 1999).

Figure 1   Phototropism in Arabidopsis induced by low fluence rate blue light. Photograph shows three-day-old etiolated seedlings after exposure to 8 hour unilateral blue light (λmax = 436 nm, 30 nm half-band; 0.1 µmol m-2 s-1) from the left. (Click image to enlarge.)

At the physiological level, phototropism can be distinguished from other light-modulated directional growth responses such as nastic (Satter and Galston, 1981) and circadian-regulated (McClung, 2001) leaf movements by two major criteria: (1) directionality of response and (2) cellular basis of the growth response. With respect to directionality, phototropic curvatures are oriented relative to the direction of the incident light, while nastic and circadian-regulated movements are not. Changes in cell elongation rates across the bending organ drive the development of phototropic curvatures (Baskin et al., 1985; Briggs and Baskin, 1988; Orbovic and Poff, 1993), while reversible swelling/shrinking of pulvinar cells is prerequisite for many leaf movement responses (Hart, 1988; Koller, 1990). As will be discussed later, the changes in elongation rates that mediate phototropism appear to be established through an integrated response as cells respond to a lateral gradient of auxin across the organ (lowest on the lit side, highest on the shaded side) formed as a result of the directional light stimulation (Went and Thimann, 1937; Iino, 1990; Liscum and Stowe-Evans, 2000).

This essay will address our current molecular understanding of the phototropic response, with a particular focus on components for which genetic evidence of function exists. Discussion will also be limited to signal-response components mediating the basal UV-A/blue light-induced phototropic response in seedlings. Pathways that modulate responsiveness to directional UV-A/blue light, such as the phytochrome-dependent enhancement of phototropism, will not be discussed here. For information about these modulatory pathways the reader should consult a more comprehensive review by the author recently published in The Arabidopsis Book (Liscum, 2002). Because this latter free web-based text resource is updated on a regular basis, the reviews presented therein are both the most current and thorough.

Perception of Directional Light Cues

Phototropism has been the subject of study for more than 120 years (Darwin, 1880; Sachs, 1887), however the identity of the photoreceptor(s) mediating this UV-A/blue light response remained elusive for most of this period. Much of the recent century has even seen often heated debate about the identity of the chromophore (light absorbing co-factor) utilized by the phototropic receptor(s)—flavin versus carotenoid (Liscum, 2002). Mutational studies in Arabidopsis during the last decade led to the identification of a pair of related flavin-based photoreceptor proteins, the phototropins, that mediate the perception of phototropic stimuli, finally putting to end the long search (Briggs et al., 2001a; Christie and Briggs, 2001; Briggs and Christie, 2002). The following sub-sections describe the identification and biochemical characterization of the phototropins as they relate to phototropism.

Phototropin 1 (phot1)

Historically the first significant progress towards the biochemical identification of a phototropic receptor came in 1988 when Gallagher and colleagues reported a blue-light-activated phosphorylation of a plasma membrane-localized protein in etiolated pea seedlings. A number of subsequent studies examined the photophysiological properties of this light-dependent phosphorylation reaction in a variety of species (most notably maize, oat, pea, and Arabidopsis) and associated it with the phototropic response. For example, phosphorylation reactions: occur in the most phototropically sensitive tissues; are strongest in the tissue closest to the light and decreases in strength moving away from the lit side; are fast enough to precede the development of curvature; its action spectrum matches that for phototropism; and shows similar dark-recovery kinetics as phototropism after a saturating irradiation (Short and Briggs, 1994; Briggs and Huala, 1999; Briggs and Christie, 2002).

The first genetic connection between the phosphorylation reaction and phototropism came when Reymond and colleagues (1992) showed that a phototropic mutant of Arabidopsis, strain JK224 (Khurana and Poff, 1989), exhibited little, if any, blue light-induced phosphorylation. Interestingly, strain JK224 was independently proposed, based solely upon the photophysiological properties of the mutant, to harbor a lesion in a low-fluence rate phototropic photoreceptor (Khurana and Poff, 1989; Konjevic et al., 1992). Liscum and Briggs found that null mutations in the NONPHOTOTROPIC HYPOCOTYL 1 (NPH1) locus, of which JK224 is an allele (nph1-2), lack the target protein for the blue light-dependent phosphorylation reaction, and thus proposed that the NPH1 locus encodes the apoprotein for a phototropic receptor capable of blue light-induced autophosphorylation (Liscum and Briggs, 1995).

When the NPH1 gene was isolated by positional cloning it was found to encode a protein containing the 11 signature domains of Ser/Thr protein kinases (Figure 2A) and of the proper size to be the substrate for an autophosphorylation reaction (Huala et al., 1997). Although primary sequence analyses yielded nothing obvious that indicated inherent photoreceptor activity for NPH1, a repeated amino-terminal sequence motif was identified that exhibited homology to a subfamily of PAS domains found in sensor-proteins regulated by light, oxygen, or voltage (Huala et al., 1997; Zhulin and Taylor, 1997; Taylor and Zhulin, 1999). These PAS-like domains of NPH1, designated LOV1 and LOV2 (for their relationship to light, oxygen, and voltage-regulated PAS domains; see Figure 2A), were each subsequently shown to bind one FMN molecule (Christie et al., 1998). Soluble nph1 holoprotein (NPH1 apoprotein with associated FMN cofactors) isolated from a heterologous baculovirus/insect cell expression system was shown to exhibit blue light-dependent autophosphorylation with kinetic, fluence-response, and action spectrum characteristics essentially like those obtained with native Arabidopsis nph1, and for phototropism itself (Christie et al., 1998). Various solution spectroscopic methods, as well as x-ray crystallographic studies, have been used to demonstrate that the kinase domain of nph1 is likely activated in response to a conformational change(s) associated with a light-driven, but dark-reversible, covalent bond formation between a conserved Cys within the LOV domain and the FMN chromophore (Salomon et al., 2000, 2001; Crosson and Moffat, 2001, 2002; Swartz et al., 2001, 2002). The nph1 holoprotein has been given the trivial name, phototropin 1 (phot1), in order to reflect its physiological and biochemical properties (Christie et al., 1999; see Briggs et al., 2001b for gene and protein nomenclatures).

Figure 2   Domain structures of the phototropin, NPH3 and RPT2 proteins. (A) Phototropin 1 and 2. LOV1 and LOV2 (blue regions) represent the flavin mononucleotide chromophore-binding domains (Christie et al., 1999). Both receptors contain a function serine/theornine protein kinase domain (red region) within their carboxyl-terminus (Huala et al., 1997). (B) Domain structures for members of the NPH3/RPT2 protein family. The DIa, DIb, DII, DIII, and DIV regions (black bars) represent areas of sequence conservation among members of the NPH3/RPT2 protein family. The BTB/POZ (red region) and coiled-coil (blue region) domains represent structurally conserved motifs. While not all members of the family contain all of these domains (see www.biosci.missouri.edu/liscum/nph3-rpt2figs.html), each domain is present in both NPH3 and RPT2 (Motchoulski and Liscum, 1999; Sakai et al., 2000). Inter-domain regions (grey) while not sequence conserved, exhibit moderate structural similarities from member to member (see www.biosci.missouri.edu/liscum/nph3-rpt2figs.html). (Click image to enlarge.)

Phototropin 2 (phot2): a second phototropic receptor that functions redundantly with phot1 under high fluence rate conditions

While phot1 mutants clearly lack hypocotyl and root phototropism under low fluence rate blue light (e.g., < 1 µmol m^–2 s^–1), Sakai and colleagues (2000) reported that the phot1-101 allele retains a phototropic response at higher fluence rates—one essentially like wild-type at 100 µmol m^–2 s^–1. This finding, confirmed by studies of the phot1-5 null mutant exposed to directional light under greenhouse conditions (T. Campbell and E. Liscum, unpublished), indicated the function of a second phototropic receptor under high light conditions. The most obvious candidate for a second phototropic receptor apoprotein was the sole PHOT1 paralog PHOT2 (Jarillo et al., 1998; The Arabidopsis Genome Initiative, 2000; Briggs et al., 2001b). Although the PHOT2 apoprotein is slightly smaller than PHOT1 (110 kD versus 124 kD), sequence, and structural motifs are highly conserved. In particular, PHOT2, like PHOT1, contains a Ser/Thr protein kinase domain in its carboxyl terminal region and two LOV domains in its amino terminal half (Jarillo et al., 1998; Figure 2A). Moreover, the basic photochemical properties of baculovirus/insect cell-expressed phot2 holoprotein are similar to those of phot1 (Sakai et al., 2001), suggesting that phot2 likely functions as a blue-light receptor by a mechanism analogous to that of phot1.

While no alterations in phototropic responsiveness have been observed in phot2 single mutants (Sakai et al., 2000; Jarillo et al., 2001; Sakai et al., 2001), phot1 phot2 double mutants fail to exhibit seedling phototropic responses at both low and high fluence rates (Sakai et al., 2001). The fact that phot2 single mutants retain a phototropic response indistinguishable from wild-type under all fluence rates tested (Jarillo et al., 2001; Sakai et al., 2001) while the phot1 phot2 double mutant is essentially blind (Sakai et al., 2001), demonstrates that phot1 functions to some extent under all fluence rate conditions, while phot2 has redundant function for phot1 specifically under high fluence rate conditions.

How can we explain the overlapping, yet distinct, functions of the phot1 and phot2 receptors in the perception of phototropic stimuli? While there are no definitive answers to this question there are several pieces of data that suggest biochemical and molecular bases for this observation. First, while etiolated Arabidopsis seedlings do not exhibit acute light-dependent changes in PHOT1 mRNA abundance (Harmer et al., 2000; J. M. Christie, K. Sakamoto, and W. R. Briggs, personal communication), PHOT2 mRNA levels increase upon exposure to UV-A, blue, red, or white light (Jarillo et al., 2001; Sakai et al., 2001). Most interestingly, PHOT2 message levels increase more than two-fold at fluence rates of blue light (≥ 10 µmol m^–2 s^–1) where the redundant function of phot2 is most obvious (Sakai et al., 2001). Second, there are quite dramatic differences in the photocycling properties of the Cys-FMN bond formed within the LOV domains of phot1 and phot2 (Kasahara et al., 2002). In particular, there are differences between phot1 and phot2 in both the quantum efficiencies and dark-recovery kinetics for the formation of this covalent linkage, suggesting that much higher fluences are required to drive phot2 to the same photoequilibrium established under lower fluences for phot1. Like the expression differences, these differing photochemical properties suggest that phot2 is likely to have similar physiological properties to phot1 (e.g., have redundant phototropic function) under relatively high light intensities.

What's the role of phototropin kinase activity in phototropic signal-response?

Given that both phototropins are light-activated Ser/Thr protein kinases (Christie et al., 1998; Sakai et al., 2001) it is quite reasonable to hypothesize that phototropic signal transduction might involve phot1 and phot2-activated phosphorelays (Fankhauser and Chory, 1998; Briggs and Huala, 1999; Liscum and Stowe-Evans, 2000; Briggs and Olney, 2001; Christie and Briggs, 2001; Liscum, 2002). Yet to date, no protein kinases or phosphatases have been shown to associate with the phototropins. The only known kinase substrates for the phototropins are the phototropins themselves (Christie et al., 1998; Sakai et al., 2001). While the role of phototropin autophosphorylation is not currently understood, it appears not to be required for the induction of phototropism since phototropism is 2–3 orders more sensitive to blue light than the autophosphorylation response (Palmer et al., 1993; Salomon et al., 1997; Christie et al., 1998).

If phototropin autophosphorylation is not an inductive signal, what is its function in phototropism? Given that the "dark-state," or light-sensitive state, of the phototropins is the unphosphorylated form (Christie et al., 1998; Sakai et al., 2001), autophosphorylation may be a cue for desensitization of the receptors (Liscum, 2002). Sensor adaptation, where the receptor is desensitized after an initial perception event and then is "reset" so that future signals can be utilized as a signal, is a common, if not essential, feature of photosensory biology (Galland, 1989, 1991). In Arabidopsis, phototropic desensitization occurs (Janoudi and Poff, 1991, 1993) but without reducing the sensitivity (i.e., increasing the fluence threshold) of the response (Janoudi and Poff, 1991). Thus, sensor adaptation, as classically defined (Galland, 1989, 1991), is apparently not operating in Arabidopsis phototropism. However, it is plausible that the phototropic desensitization observed in Arabidopsis reflects desensitization of a "receptor complex" rather than the receptor per se (Liscum and Stowe-Evans, 2000; Liscum, 2002). As discussed in the following section, blue light-dependent autophosphorylation of phot1 may lead to dissociation of a complex containing phot1 and NPH3, a putative scaffold/adapter protein (Motchoulski and Liscum, 1999).

Signal-response Components Acting Downstream of the Phototropins

NPH3 and RPT2: phot1- and phot2-interacting proteins that scaffold phototropin signaling complexes?

Mutations in two Arabidopsis genes have been described that apparently disrupt the function of proteins acting early in phot1 and phot2 phototropic signaling pathways. First, loss-of-function nph3 mutations appear to disrupt phot1 signaling since they decrease phototropic responses of etiolated seedlings specifically under low fluence rate conditions (Liscum and Briggs, 1996; Motchoulski and Liscum, 1999), without altering blue light-induced autophosphorylation of phot1 (Liscum and Briggs, 1995). In contrast, the loss-of-function rpt2 (mutant designation, root phototropism 2) mutant retains nearly normal phototropism in response to unilateral low fluence rate blue light, but exhibits decreasing response with increasing fluence rates (Okada and Shimura, 1992; Sakai et al., 2000). While the phototropic phenotypes of rpt2 are essentially complementary to those of the nph3 mutants, they exhibit a fluence rate dependence that mirrors the apparent range of fluence rates under which phot2 functions (Sakai et al., 2000, 2001). The congruence between RPT2 and phot2 is also observed at the level of transcript abundance. Specifically, the RPT2 and PHOT2 transcripts are barely detectable in etiolated seedlings but increase in abundance in response to light with a similar fluence and wavelength dependencies (Sakai et al., 2000, 2001). Together with the mutant phenotype, this result suggests that RPT2 functions in a phot2-specific signaling pathway (Liscum, 2002).

When the NPH3 and RPT2 genes were isolated by map-based cloning approaches they were found to encode members of the same family of novel plant-specific proteins (Motchoulski and Liscum, 1999; Sakai et al., 2000; also see www.biosci.missouri.edu/liscum/NPH3-RPT2family.html). Primary sequence conservation between members of the NPH3/RPT2 family is found in five discrete, positionally conserved, regions designated DIa, DIb, DII, DIII, and DIV (Figure 2B). DIV contains a consensus Tyr phosphorylation site ([RK]-x(2,3)-[DE]-x(2,3)-Y; Patschinsky et al., 1982) that is conserved in 29 of the 32 members of the family, including NPH3 and RPT2 (Motchoulski and Liscum, 1999; E. Liscum, unpublished). Although it is not known whether phosphorylation can occur on the conserved Tyr of NPH3 or RPT2, it is interesting to note that the nph3-2 allele, which phenotypically indistinguishable from a null mutant (nph3-6) that contains a stop codon at the Trp₂ position, carries in an in-frame deletion of this Tyr residue (Motchoulski and Liscum, 1999). NPH3 and RPT2 also contain multiple potential Ser and Thr phosphorylation sites, further raising the possibility that reversible phosphorylation may be important for their function (Motchoulski and Liscum, 1999; Sakai et al., 2000).

In addition to displaying modular sequence conservation, members of the NPH3/RPT2 family exhibit modular structural features that are positionally conserved (although sequence diverged) (Figure 2B). Two of these "modules" are represented by known protein-protein interaction motifs; a BTB (broad complex, tramtrack, bric à brac)/POZ (pox virus and zinc finger) domain (Albagli et al., 1995; Aravind and Koonin, 1999) in the amino-terminal region, and a coiled-coil domain (Cohen and Parry, 1996; Lupas, 1996) in the carboxyl-terminal region (Motchoulski and Liscum, 1999; Sakai et al., 2000; E. Liscum, unpublished). While no particular function can be inferred from the mere presence of a BTB/POZ or a coiled-coil domain, the fact that most of the NPH3/RPT2 family members contain one or both—21 of 32 members, including NPH3 and RPT2, contain both, while only two contain neither (E. Liscum, unpublished)—suggests that protein-protein interactions are an important feature of the biochemical function of this family.

NPH3, like phot1 (Briggs and Huala, 1999), has been shown to be associated with the plasmalemma (Motchoulski and Liscum, 1999). This property, along with the fact that NPH3 contains multiple potential phosphorylation sites, suggested that NPH3 might be a substrate for phot1's kinase activity. NPH3 does appear to be a phosphoprotein, however it is dephosphorylated, rather than phosphorylated, in response to blue light irradiation (Motchoulski and Liscum, 1999). Moreover, NPH3 seems to be dephosphorylated in etiolated phot1 null mutant seedlings, indicating that the protein phosphatase that dephosphorylates NPH3 in wild-type seedlings is not directly regulated by light or phot1 (Motchoulski and Liscum, 1999). Thus it appears that NPH3 is not a substrate for phot1's kinase domain.

One interpretation of the aforementioned results is that phot1 normally interacts with NPH3 in etiolated seedlings, thus "protecting" NPH3 from protein phosphatase action. Blue light-dependent changes in phot1, such as autophosphorylation (Christie et al., 1998, 1999), might disrupt this interaction, exposing sites on NPH3 for dephosphorylation. Yeast two-hybrid and in vitro co-immunoprecipitation studies have demonstrated that carboxyl-terminal coiled-coil-containing portions of NPH3 interact with amino-terminal LOV-domain-containing portions of phot1, consistent with the proposal that phot1 and NPH3 are components of a signaling complex (Motchoulski and Liscum, 1999). It is worth mentioning in the context of the model outlined above, that the phot1-NPH3 interaction in yeast is much stronger in darkness than in blue light (A. Motchoulski and E. Liscum, unpublished). These findings suggest, as introduced earlier, that phototropic "sensitivity" in low fluence rate blue light conditions may be regulated in part by the integrity of an NPH3-phot1 complex. In other words phototropic desensitization may occur through complex dissociation upon blue light-induced phot1 autophosphorylation.

NPH3 has been proposed to act as a modular scaffold bringing phot1 together with early acting proteins (e.g., protein kinases or phosphatases) whose activities are modulated by phot1 to transduce the low fluence rate phototropic signals (Motchoulski and Liscum, 1999; Liscum and Stowe-Evans, 2000). Based on its mutant phenotypes and homology to NPH3, RPT2 is proposed to scaffold phot2 and associated early signaling proteins in a situation analogous to that proposed for NPH3 and phot1 (Liscum, 2002). The assembly of multi-molecular complexes on protein scaffolds has emerged as a common mechanism of optimizing speed, specificity, and selectivity of signaling in fungi and animals (Elion, 1998; Faux and Scott, 1996; Tsunoda et al., 1998; Sim and Scott, 1999; Fisher et al., 1999). One of the "pioneer" scaffold proteins is Ste5 (Sterile5) of S. cerevisae, which is involved in mating-pheromone sensing and response. Ste5 scaffolds a functional signaling complex that includes the G-protein pheromone sensor (via the Gβ subunit, Ste4), a sensor-activated protein kinase (Ste20), and a signal amplifying MAP kinase cascade (MAPKKK, Ste11; MAPKK, Ste7; MAPK, Fus3) (Faux and Scott, 1996; Pawson and Scott, 1997). Despite being divergent in both sequence and structure, scaffold proteins are able to assemble signaling complexes via presence of multiple protein-protein interaction domains, or modules (Faux and Scott, 1996; Newton, 1996; Pawson and Scott, 1997; Tsunoda et al., 1998).

NPH3 and RPT2 are certainly modular in nature and contain two known protein-protein interaction domains: a BTB/POZ domain and a coiled-coil (Motchoulski and Liscum, 1999; Sakai et al., 2000). Scaffold proteins have also most frequently been found associated with complexes that utilize protein kinases and phosphatases as signal carriers (Faux and Scott, 1996; Pawson and Scott, 1997; Sim and Scott, 1999). The experimental demonstration that NPH3 interacts with phot1 (Motchoulski and Liscum, 1999) and likelihood that RPT2 interacts with phot2 (Liscum, 2002), are compelling since the phototropins are both sensors and protein kinases (Christie et al., 1998, 1999). Correct or incorrect, the scaffold model provides the impetus for several experimentally addressable questions.

Differential auxin localization

Most mechanistic models of tropic growth responses, including phototropism, have auxin as a linchpin that holds pieces of the model together (Estelle, 1996; Chen et al., 1999; Liscum and Stowe-Evans, 2000). Nearly all such models are based in large part on the Cholodny-Went theory which holds that tropic stimuli induce differential lateral auxin transport that leads to the unequal distribution of auxin, and hence growth, in the two sides of a curving organ (Went and Thimann, 1937). While lateral auxin transport, or accumulation, has proven remarkably difficult to demonstrate in many systems (Trewavas et al., 1992), studies of a number of Arabidopsis mutants have clearly demonstrated that the transport of, and response to, auxin is prerequisite for the development of tropic curvatures (Estelle, 1996; Leyser, 1998; Chen et al., 1999; Palme and Gälweiler, 1999; Rosen et al., 1999; Liscum and Stowe-Evans, 2000; Liscum, 2002; Friml and Palme, 2002). Most recently Friml and colleagues (2002) have shown that a member of the PIN family of putative polar auxin efflux carriers, PIN3, is localized to the lateral face of endodermal cells in hypocotyls and non-functional alleles carrying transposon insertions reduce the phototropic response of the seedling stem.

Several studies have shown that auxin efflux processes, which are necessary for the establishment of a lateral gradient of auxin (Lomax et al., 1995; Friml et al., 2002), may be regulated via reversible protein phosphorylation (Bernasconi, 1996; Garbers et al., 1996; Delbarre et al., 1998; Christensen et al., 2000; Rashotte et al., 2001; DeLong et al., 2002). These findings provide a compelling potential connection between the phototropins, which are light-activated protein kinases (Christie et al., 1998; Sakai et al., 2001), and the differential auxin gradients that have been observed in seedlings irradiated with unilateral blue light (Iino, 1990). A number of possible biochemical pathways from phototropin activation to changes in auxin transport can be postulated, including direct phosphorylation of an auxin efflux carrier by phototropins, and phototropin-dependent modulation of auxin transporter localization (Liscum, 2002). Determining exactly how phototropin activation leads to changes in auxin transportor activity/localization is likely to be one of the most active areas of phototropic research over the next few years.

Auxin-dependent changes in gene expression

What happens after a lateral auxin gradient is formed across the phototropically stimulated stem? At least one of the consequences appears to be a change in gene expression (Harper et al., 2000; Liscum and Stowe-Evans, 2000; Liscum and Reed, 2002). This conclusion is based mainly on analyses of the NPH4/MSG1/TIR5 locus (hereafter referred to as NPH4) of Arabidopsis. First, mutations in the NPH4 locus cause reduced phototropic and gravitropic responses in seedling stems (Liscum and Briggs, 1996; Watahiki and Yamamoto, 1997; Watahiki et al., 1999), as well as severely impaired auxin-induced stem bending (Watahiki and Yamamoto, 1997), stem growth inhibition (Watahiki and Yamamoto, 1997; Stowe-Evans et al., 1998), and gene expression responses (Stowe-Evans et al., 1998). Second, map-based cloning of the NPH4 gene has revealed that it encodes the auxin-responsive transcriptional activator ARF7 (Harper et al., 2000), consistent with impaired auxin-induced gene expression profiles observed in the nph4/arf7 mutants (Stowe-Evans et al., 1998). Unfortunately, to date, specific targets of NPH4/ARF7 that are necessary for the establishment of phototropic responses have not been identified. However, with functional genomics tools now at our disposal, targets for NPH4/ARF7 regulation should not remain elusive for long.

Conclusions

In recent years, tremendous advances have been made in our understanding of the molecular bases for the long-studied phototropic responses of plants. Photoreceptor molecules, the phototropins, capable of perceiving directional UV-A/blue light cues have been identified and characterized. With their identification came the end of a long debate over the molecular identity of the chromophoric unit of the phototropic receptor(s). Moreover, the finding that phototropins are light-activated protein kinases provides impetus for development of several mechanistic models of phototropic signal transduction that are eminently testable with current tools and methodologies. At least two proteins acting early in the phototropin signaling pathways have also been identified, namely NPH3 and RPT2. Both proteins are members of a novel plant-specific protein family and have been hypothesized to function as scaffold proteins to bring the phototropins together with downstream signaling elements. The physical interaction demonstrated between phot1 and NPH3 is certainly consistent with this model.

Finally, the importance of auxins in the phototropic response has been clearly demonstrated through a number of genetic studies, primarily auxin transport and auxin-dependent gene expression. If the last ten years are any indication, the next decade should be an exciting one for those studying phototropism. While the final painting of the molecular basis of phototropism is far from complete, several talented 'artists' are transforming the once blank canvas into something with recognizable form that will eventually be considered a classic piece of human endeavor.

Acknowledgments

Recent studies on phototropism in the Liscum laboratory were supported by National Science Foundation grants no. MCB-0077312 and IBN-0114992 and USDA National Research Initiative grant no. USDA-CSREES 99-35304-8060.

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