Year: 2017
Type: review
Abstract: Plasma membrane receptors play fundamental roles in shaping plant growth and development. A large proportion of these are autophosphorylating Ser/Thr kinases (De Smet et al., 2009). Some function as dual-specificity kinases, autophosphorylating additionally on Tyr residues, one of which is the extensively studied steroid receptor BRASSINOSTEROID-INSENSITIVE1 (BRI1; Oh et al., 2009). Light regulation of plant growth also is mediated by plasma membrane-bound Ser/Thr kinases known as the phototropins (phots; Fankhauser and Christie, 2015). Seed plants contain two phots (phot1 and phot2) that have important roles in regulating leaf positioning and expansion, chloroplast photorelocation movement, stomatal opening, and phototropism, all of which serve to optimize photosynthetic efficiency (Christie et al., 2015). Phots are members of the AGCVIII kinase family (Rademacher and Offringa, 2012) but are distinct from transmembrane receptor kinases such as BRI1, as they are hydrophilic and bind to the intracellular side of the plasma membrane (Kong et al., 2013) to initiate signaling (Preuten et al., 2015). Although the ability to associate with the plasma membrane is conserved in algal phots (Sullivan et al., 2016a), the mechanism underlying this attachment is still not known, but it is thought to involve some form of lipid binding/modification. Kinase-inactive versions of phot1 and phot2 are nonfunctional, highlighting the importance of receptor autophosphorylation in phot signaling (Inoue et al., 2008a, 2011). Autophosphorylation occurs predominantly on multiple Ser residues. At least 21 and 29 phosphorylation sites have been identified for Arabidopsis (Arabidopsis thaliana) phot1 and phot2, respectively (Christie et al., 2015). Most of these sites are found in the N terminus of the protein, which contains two light-sensing modules known as LIGHT, OXYGEN, OR VOLTAGE SENSING (LOV) domains known as LOV1 and LOV2 (Christie et al., 2012). Autophosphorylation of Ser-350, Ser-376, and Ser-410 within the LOV-linker region promotes the binding of 14-3-3 regulatory proteins to phot1 (Inoue et al., 2008a; Sullivan et al., 2009). However, the biological significance of this interaction is still not known. The occurrence of some these phosphorylation sites also is fluence rate dependent (Salomon et al., 2003) and is thought to play a role in receptor desensitization (Christie and Murphy, 2013). By contrast, the autophosphorylation of two conserved Ser residues within the activation loop of the C-terminal kinase domain (Ser-849 and Ser-851 in phot1 and Ser-761 or Ser-763 in phot2) is necessary for receptor signaling (Inoue et al., 2008a, 2011). Mutation of these sites to Ala impairs phot1 function in Arabidopsis, whereas phosphomimetic substitutions to Asp are without effect (Inoue et al., 2008a). Despite considerable efforts over the last two decades, the signaling events that follow receptor autophosphorylation remain poorly understood for most phot-mediated responses. Yet, it is becoming increasingly apparent that members of the NPH3/RPT2-Like family (NRL) are required to elicit several phot responses. Moreover, accumulating evidence suggests that these plant-specific proteins act in concert with different AGCVIII kinases to regulate various aspects of auxin transport and signaling. In this review, we summarize our current understanding of the biochemical properties of the NRL family in Arabidopsis and how these proteins function to coordinate different aspects of phot signaling. We also discuss to what extent these signaling processes are conserved in land plants and algae. The NRL family is named after its two founding members, NONPHOTOTROPIC HYPOCOTYL3 (NPH3) and ROOT PHOTOTROPISM2 (RPT2; Liscum et al., 2014), which were first identified from genetic screens for Arabidopsis mutants impaired in hypocotyl and root phototropism (Okada and Shimura, 1992; Liscum and Briggs, 1995; Sakai et al., 2000). The primary amino acid structure of NRL proteins can be separated into three main parts based on sequence conservation: a BTB (bric-a-brac, tramtrack, and broad complex) domain at the N terminus, followed by an NPH3 domain and a C-terminal coiled-coil domain (Liscum et al., 2014). In addition to NPH3 and RPT2, Arabidopsis contains another 31 NPH3/RPT2-Like proteins (NRL1–NRL31), all of which are defined by the presence of the NPH3 domain (Pedmale et al., 2010). Ten of these members lack the C-terminal coiled-coli domain, whereas two are devoid of the BTB domain (Pedmale et al., 2010). The reasons for these differences in domain structure are not clear at present, because many of these NRL proteins have yet to be ascribed a biological function. Functions for 10 out of the 33 Arabidopsis NRL proteins have been identified so far. Phylogenetic analysis has shown that NRL proteins can be classified into seven distinct clades in land plants (Suetsugu et al., 2016). However, in angiosperms such as Arabidopsis, only six of these clades are present (Fig. 1). NRL proteins from three of these clade classifications (Suetsugu et al., 2016) are known to interact with the phots. These include NPH3 (Motchoulski and Liscum, 1999), RPT2 (Inada et al., 2004), NRL PROTEIN FOR CHLOROPLAST MOVEMENT1 (NCH1/NRL31; Suetsugu et al., 2016), and NRL2 (Sullivan et al., 2009). NRL2 was identified as a phot1-interacting protein through yeast-two hybrid screening (Sullivan et al., 2009). However, its functional role in phot signaling remains undetermined. NPH3 and RPT2 are required for several auxin-mediated growth processes, including phototropism, leaf positioning, and leaf expansion (Christie et al., 2015). A central role for coordinating auxin distribution also has been linked to other members of the NRL family, which function independently from the phots. For instance, DEFECTIVELY ORGANIZED TRIBUTARIES3/NRL23 is associated with vascular development and leaf vein patterning (Petricka et al., 2008), whereas NRL proteins (NRL6, NRL7, NRL20, NRL21, and NRL30) known as NAKED PINS IN YUCCA (NPY; Fig. 1) function redundantly to regulate auxin movements required for organogenesis and root gravitropism in concert with AGCVIII kinases other than the phots (Cheng et al., 2008; Li et al., 2011). NRL8 (otherwise known as SETH6) is involved in pollen tube growth (Lalanne et al., 2004), which also requires AGC kinases activity (Zhang et al., 2008). Consequently, NRL proteins are proposed to function as AGC kinase modules that regulate various aspects of auxin trafficking and signaling. However, as discussed below, new evidence for the function of NRL proteins in phot signaling has begun to break down the paradigm that these proteins are solely mediators of auxin-dependent processes. Bayesian phylogenetic tree of the NRL protein family in Arabidopsis. The tree was modified from that of Suetsugu et al. (2016) with Archaeopteryx software. Six of the seven major clades of land plant NRL proteins are color coded. RPT2/NCH1, NPH3, and NPY reside in different clades. Phots regulate a range of physiological responses in Arabidopsis. These are listed in Table I and can be separated into two categories depending on the involvement of NRL proteins (NRL dependent and NRL independent). Phototropism (Motchoulski and Liscum, 1999; Sakai et al., 2000), petiole positioning and leaf expansion (Inoue et al., 2008b), and chloroplast accumulation (Suetsugu et al., 2016) are examples of NRL-dependent responses, whereas chloroplast-avoidance movement (Kong and Wada, 2014) and stomatal opening (Inoue and Kinoshita, 2017) are NRL independent. While several of these responses can be described as NRL independent, it is worth remembering that functions for 23 members of the NRL family are still lacking. Whether these processes are truly devoid of NRL involvement awaits further genetic characterization of these remaining family members. In the following sections, we summarize progress made to date in uncovering the role of NRL proteins in phot signaling. A central role for NPH3 in phototropism has been firmly established for some time now. Arabidopsis mutants lacking NPH3 fail to exhibit phototropism under a variety of different light conditions (Liscum et al., 2014; Fankhauser and Christie, 2015). A detailed discussion of phototropism is beyond the scope of this review, and readers are directed to other articles that provide a comprehensive overview of the different types of photophysiological responses involved (Sakai and Haga, 2012; Christie and Murphy, 2013). While the biochemical function of NPH3 remains poorly understood, it appears to be instrumental for establishing lateral auxin gradients required for this differential growth response (Haga et al., 2015). NPH3 is localized to the plasma membrane and has been shown to interact directly with phot1 via its C-terminal region (Motchoulski and Liscum, 1999). Like the phots, NPH3 contains no obvious transmembrane-spanning domain, so how it associates with the plasma membrane is still not known. However, truncation and transient expression analysis of NPH3-GFP in stomatal guard cells of Vicia faba suggests that the C terminus, including the coiled-coil domain, is important for localization to the plasma membrane (Inoue et al., 2008b). A recent in vivo coimmunoprecipitation analysis indicates that phot1-NPH3 interactions at the plasma membrane are transiently disrupted upon irradiation (Haga et al., 2015). Light not only impacts the ability of NPH3 to interact with phot1 but also leads to dynamic changes in its subcellular localization. The expression of YFP-NPH3 under the control of the constitutive cauliflower mosaic virus 35S promoter restores hypocotyl phototropism in the nph3 mutant (Haga et al., 2015). YFP-NPH3 is localized to the plasma membrane in darkness but is rapidly (within minutes) internalized into microdomain aggregates upon phot1 activation by blue light (Haga et al., 2015). The appearance of these microdomains is diminished in periods of prolonged irradiation or when seedlings are returned to darkness, suggesting that their formation is reversible (Haga et al., 2015). The biological significance and the role of these microdomains in phot signaling are currently unknown. However, biochemical fractionation experiments indicate that these NPH3 aggregates accumulate in the cytosol (Haga et al., 2015). These findings agree with observations that cytoskeleton inhibitors or inhibitors of vesicle trafficking fail to affect NPH3 microdomain formation (Haga et al., 2015). The above phot1-driven changes in NPH3 localization correlate well with alterations in NPH3 phosphorylation status. NPH3 is phosphorylated in darkness and becomes rapidly (within minutes) dephosphorylated following phot1 activation (Pedmale and Liscum, 2007). This dephosphorylation process is tissue, and most likely cell, autonomous (Sullivan et al., 2016b) and does not appear to be initiated by phot2 (Inada et al., 2004; Pedmale and Liscum, 2007; Tsuchida-Mayama et al., 2008; Haga et al., 2015). As found for YFP-NPH3 microdomain formation, NPH3 dephosphorylation is recoverable either in darkness (Pedmale and Liscum, 2007) or over periods of prolonged irradiation (Haga et al., 2015). The kinetics for NPH3 dephosphorylation and rephosphorylation correlate closely with the time scale described for the changes in YFP-NPH3 localization (Haga et al., 2015). This has led to the proposal that the phosphorylation of NPH3 is necessary for it to form an active signaling complex with phot1 at the plasma membrane (Fig. 2). Phot1-NPH3 interactions are disrupted following NPH3 dephosphorylation and its release from the plasma membrane (Haga et al., 2015), leading to a cessation of receptor signaling. Thus, reassembly of this complex would be necessary to reestablish and sustain phototropic signaling, especially under continuous irradiation (Fig. 2). Roles of NPH3 and RPT2 in mediating phototropic signaling in Arabidopsis. The model was modified from Haga et al. (2015), where phot1 and NPH3 form a plasma membrane (PM) complex in darkness. Phot1-NPH3 interactions are diminished upon phot1 activation and dephosphorylation of NPH3. However, efficient signaling under continuous blue light conditions requires the reconstitution of a phot1-NPH3 complex that involves RPT2 at blue light The of the for NPH3 is still not known. Moreover, have yet to NPH3 is a for phot1 kinase However, have a role for a protein in NPH3 (Pedmale and Liscum, 2007). phosphorylation sites have been identified within NPH3 through et al., 2008). The of some of these sites to phototropic signaling has been through and of the BTB domain are phosphorylated in darkness and dephosphorylated following phot1 activation et al., 2008). of these sites to Ala or of this region does not affect the ability of NPH3 to hypocotyl phototropism in Arabidopsis et al., 2008). phosphorylation sites other than and are important (Haga et al., 2015). these sites and how changes in their phosphorylation NPH3 function present a for However, recent has that the founding of the NRL family, RPT2, has a role in the phosphorylation and localization of NPH3 in Arabidopsis. RPT2 was identified from a genetic for Arabidopsis mutants impaired in root phototropism (Okada and Shimura, 1992; Sakai et al., and has been shown to interact directly with NPH3 (Inada et al., and phot1 (Inada et al., 2004; Sullivan et al., 2009). The mutant the RPT2 protein (Inada et al., and is impaired in hypocotyl phototropism at blue light of or (Haga et al., 2015). By contrast, the phototropic response of mutants is from that of seedlings at blue light of or (Haga et al., 2015). A for RPT2 in regulating phototropism to light well with its expression The expression of RPT2 is in seedlings but in response to and blue light (Inada et al., 2004; Tsuchida-Mayama et al., 2008; Haga et al., in a fluence (Sakai et al., 2000). As reconstitution of a phot1-NPH3 complex is necessary to sustain phototropic signaling under continuous irradiation (Fig. 2). The presence of RPT2 is necessary to this process at light (Haga et al., 2015). on these RPT2 is proposed to to and efficient phototropism under light conditions (Fig. 2). in either NPH3 or RPT2 exhibit leaf positioning and leaf expansion that of the phot1 phot2 mutant (Inoue et al., et al., 2013). These findings that NPH3 and RPT2 have roles in phot signaling positioning and leaf expansion differential growth and most likely from alterations in auxin The of the for these responses et al., 2013) is under light conditions when with the phot1 This of the phot1 mutant has a of the of RPT2 to phot signaling. RPT2 in with phot1 to leaf positioning and leaf In contrast, RPT2 is not required for phot2 signaling. RPT2 appears to have some role in the ability of phot1 to phot2 signaling for these responses. In this model (Fig. phot1 and phot2 signaling would be impaired in the The phot1 would be its signaling is dependent on RPT2, whereas phot2 signaling is to the of phot1 and phot2 signaling would be in a phot2 However, phot2 signaling for leaf positioning and leaf expansion still would be in this model for the phot1 mutant with the the of phot1 is (Fig. for RPT2 in phot-mediated leaf positioning in Arabidopsis. The model was from et al. the signaling in plants and mutant in of the are out for evidence suggests that this complex phot1 and phot2 signaling for other phot responses. The mutant impaired hypocotyl phototropism under and fluence of blue light (Inada et al., was at fluence in the of functional phot2 signaling. can be these and for the leaf positioning and leaf expansion of the phot1 mutant et al., 2013). However, some be when these the in as well as the apparent of the mutant (Haga et al., 2015). NPH3 and RPT2 are not the only mediators of phototropism, leaf positioning, and leaf expansion in Arabidopsis. proteins also play a role in establishing these processes. was identified as a kinase for A Fankhauser et al., 1999). Whether is protein kinase is still recent has for its role as a protein kinase in regulating et al., 2016). While proteins are required for signaling et al., et al., 2008), some of the associated with mutants of and For instance, the mutant phototropic et al., et al., 2014). and also act in concert with NPH3 to leaf positioning and leaf expansion et al., 2010). role in phot signaling appears to be to these processes, mutants are not impaired in blue stomatal opening and chloroplast photorelocation movement et al., 2010). with their involvement in phot signaling, proteins are known to to the plasma where they are known to associate with phot2, and NPH3 et al., et al., et al., 2012) have shown that is a for phot1 kinase activity et al., 2012). these that proteins are of the phot signaling associated with NPH3 and the biochemical functions of these proteins and how they with NPH3 and RPT2 be important to how phots initiate signaling from the plasma Like NPH3, proteins are to auxin transport or signaling et al., et al., 2014). Yet, their mechanism of in this remains to be chloroplast accumulation movement was shown to involve the of two NRL At blue light phot1 and phot2 in function to chloroplast to the and to light for (Sakai et al., movement is mediated solely by phot2 et al., and to of the photosynthetic in light et al., movement is by the formation of chloroplast that on the leading of the chloroplast et al., 2009). of requires the of CHLOROPLAST et al., 2003) and PROTEIN FOR CHLOROPLAST proteins (Suetsugu et al., 2010). also is required to (Suetsugu et al., 2015). Consequently, formation, and chloroplast accumulation and movement, are impaired in and light by phots is to the activity of and proteins is not known. At least for chloroplast accumulation movement, the NRL proteins and RPT2 have been shown to function as mediators of this response (Suetsugu et al., 2016). and RPT2 reside within the clade of the NRL family tree in land plants (Fig. 1). and mutants exhibit a chloroplast accumulation response with plants (Suetsugu et al., 2016). However, this process is in the that functions redundantly with RPT2 to chloroplast accumulation movement (Suetsugu et al., 2016). Moreover, chloroplast-avoidance movement is in the that and RPT2 are only necessary for the accumulation While these findings have a new function for NRL proteins in phot signaling, they also that RPT2 a central role in establishing several different phot responses. RPT2 aspects of phot signaling in with other members of the NRL genetic analysis of the NRL family in with the mutant be required to light on this The of and its role in chloroplast accumulation movement has our of how different NRL proteins to phot and AGCVIII kinase signaling. photorelocation movement is autonomous and occurs independently of expression (Kong and Wada, Wada, 2016). Thus, auxin trafficking and auxin-mediated are not required for this processes, in to responses phototropism, leaf positioning, and leaf our these findings are the first to that NRL proteins can function with AGCVIII kinases to elicit responses independently of genetic analysis in Arabidopsis has been in physiological functions for of the NRL it is still not clear how these proteins at the biochemical The of NRL proteins contain a BTB domain that can function as a to proteins for and and 2011). NPH3 has been to interact with when in cells and form of the complex that the of signaling et al., 2011). and have shown that phot1 becomes by a complex following blue light activation et al., 2014). of phot1 is associated with its in response to prolonged whereas is proposed to the trafficking of phot1 from the plasma membrane in response to light et al., 2011). Whether this in subcellular localization a role in phot1 signaling is still to 2016). at phot1 to the plasma membrane by or this not to be the as of these the of phot1 without its in Arabidopsis (Preuten et al., 2015). no other phot1 have been identified for the Yet, it is worth that phot1-driven changes in subcellular trafficking of the auxin are dependent on NPH3 et al., 2012). dynamic in distribution and have been linked to the of et al., it is to that auxin be is currently no evidence that NPH3 can interact with auxin transport NPH3 is not the only NRL protein to exhibit of a was identified as et al., 2014). and interact when transiently in is for and of the Arabidopsis which functions to plant responses to Consequently, mutants are to that as a of plant (Zhang et al., 2014). By contrast, of in to suggesting that also can act as a of plant et al., 2016). is to the role of in plant and how this function with its role in regulating chloroplast accumulation movement (Suetsugu et al., 2016). can the transport of to the via dynamic known as et al., 2015). it be of to NRL proteins such as are especially light is known to formation et al., 2012; et al., 2015). Phots are to the plant from photosynthetic to plants (Fig. but they are from et al., 2015). In to land contain one phot et al., et al., Sullivan et al., 2016), two phots have been identified in including of which function to chloroplast photorelocation movement in this (Suetsugu et al., Li et al., 2015). functions have been ascribed to phot from These include the light regulation of various aspects of the et al., and et al., 2004), the control of development and et al., and the regulation of involved in and et al., phot was shown to be for the of required for of the photosynthetic As a the mutants of under light conditions et al., 2016). Despite its phot can phototropism, chloroplast photorelocation movement, and stomatal opening when in the phot1 phot2 mutant of Arabidopsis et al., These findings that the of plant and algal phots is However, recent on phot from the are not with this While phot is of mediating chloroplast accumulation movement, stomatal opening, as well as leaf positioning and leaf expansion when in the phot1 phot2 mutant of Arabidopsis, it to phototropism and chloroplast-avoidance movement (Sullivan et al., At least for phototropism, this lack of with the of phot to complex with NPH3 (Sullivan et al., These findings in with what is known the of NPH3 and other NRL proteins involved in phot signaling. NPH3, RPT2, and are in land including and (Suetsugu et al., 2016), but they have not been identified in (Fig. The presence of these in land plants would with their to as well as chloroplast movement and stomatal responses. the of phot signaling in the plant The of the is from et al. indicate the in which of and proteins were identified from after photorelocation movement is in land plants and algae. The contains seven NRL one protein for of the seven NRL clades (Suetsugu et al., 2016). The of is for chloroplast accumulation movement in but not for the that the signaling events underlying chloroplast photorelocation movements are conserved in land plants (Suetsugu et al., 2016). In to Arabidopsis, contains one for the NPH3 clade and one for the suggesting that the of these NRL proteins in land plant (Suetsugu et al., 2016). by appear to have than NPH3 and in the land plant (Fig. and NRL proteins are present in land plants and and are for chloroplast photorelocation movement (Suetsugu and Wada, 2016). However, no of these have been found in and Thus, chloroplast movement have been of the Yet, the does contain two proteins (Suetsugu et al., 2016). is that these proteins chloroplast photorelocation in this is to how chloroplast movement is in photosynthetic algae. genetic analysis in Arabidopsis has been instrumental in light on the biological functions of NRL family members. so has their of with AGCVIII kinases and auxin signaling. have shown that NRL proteins can signaling independently of at least in the of chloroplast accumulation A major for be to how NRL proteins function at the biochemical and how changes in their localization and phosphorylation can their At least out of the 33 NRL members are known to be phosphorylated et al., 2008). NPH3 and RPT2 does not exhibit activity in et al., 2011). does its plasma membrane localization in response to blue light (Haga et al., 2015). the biochemical and localization properties for NRL family members are the biochemical functions of NPH3 and RPT2 and their role in phot signaling be we are to how auxin gradients are established to responses such as phototropism, leaf positioning, and leaf NRL2 is known to interact with phot1 (Sullivan et al., but its biological role in phot signaling awaits and genetic functional of the Arabidopsis NRL family be a the large of this protein family and the members. The and of the NRL family in a to functions to of seven NRL clade classifications found in land
Source: PLANT PHYSIOLOGY
DOI: 10.1104/pp.17.00835
Language: English
Topic: Light effects on plants
Subfield: Plant Science
Domain: Life Sciences
Open Access status: hybrid
Estimated APC paid: $4,175
OpenAlex ID: W2738907217