Arabidopsis nph1 and npl1: Blue light receptors that mediate both phototropism and chloroplast relocation

Abstract

UV-A͞blue light acts to regulate a number of physiological processes in higher plants. These include light-driven chloroplast movement and phototropism. The NPH1 gene of Arabidopsis encodes an autophosphorylating protein kinase that functions as a photoreceptor for phototropism in response to low-intensity blue light. However, nph1 mutants have been reported to exhibit normal phototropic curvature under high-intensity blue light, indicating the presence of an additional phototropic receptor. A likely candidate is the nph1 homologue, npl1, which has recently been shown to mediate the avoidance response of chloroplasts to high-intensity blue light in Arabidopsis. Here we demonstrate that npl1, like nph1, noncovalently binds the chromophore flavin mononucleotide (FMN) within two specialized PAS domains, termed LOV domains. Furthermore, when expressed in insect cells, npl1, like nph1, undergoes light-dependent autophosphorylation, indicating that npl1 also functions as a light receptor kinase. Consistent with this conclusion, we show that a nph1 npl1 double mutant exhibits an impaired phototropic response under both low-and highintensity blue light. Hence, npl1 functions as a second phototropic receptor under high fluence rate conditions and is, in part, functionally redundant to nph1. We also demonstrate that both chloroplast accumulation in response to low-intensity light and chloroplast avoidance movement in response to high-intensity light are lacking in the nph1 npl1 double mutant. Our findings therefore indicate that nph1 and npl1 show partially overlapping functions in two different responses, phototropism and chloroplast relocation, in a fluence rate-dependent manner.L ight is an important environmental factor controlling plant growth and development. In particular, wavelengths in UV-A (320-390 nm) and blue (390-500 nm) regions of the electromagnetic spectrum act to regulate a range of different plant responses. These processes include de-etiolation, photoentrainment of the circadian clock, floral initiation, phototropic curvature, chloroplast relocation, and stomatal opening (1-3). Much of our understanding of blue light perception in higher plants has come from the isolation of blue-light-response mutants of Arabidopsis thaliana. Indeed, molecular genetic studies have shown that the effects of blue light on plant development are mediated by at least four different blue-light receptors in Arabidopsis: cryptochrome 1 (cry1), cryptochrome 2 (cry2), phototropin (nph1, for non-phototropic hypocotyl 1), and the npl1 (nph1-like 1) protein.The phototropin photoreceptor, nph1, mediates both root and hypocotyl phototropism in response to low-fluence-rate unilateral blue light (Ͻ1 mol⅐m Ϫ2 ⅐s Ϫ1 ) (4, 5). Nph1 is a 120-kDa plasma-membrane-associated protein that contains a serine͞ threonine kinase domain located within its C terminus. Furthermore, the N-terminal region of nph1 contains a repeated motif of 110 aa, designated LOV1 and LOV2, that belong to the PAS domain (found in PER, ARNT, and SIM proteins) superfamil...

Light Regulation and Daytime Dependency of Inducible Plant Defenses in Arabidopsis: Phytochrome Signaling Controls Systemic Acquired Resistance Rather Than Local Defense

“…To test whether light perception by these photoreceptors is required for P. syringae-induced defense responses and disease resistance, we examined the interactions of the following Arabidopsis double mutants impaired in either cryptochrome, phototropin, or phytochrome photoperception, with an avirulent (Psm avrRpm1) or a virulent strain (Psm) of P. syringae pv. maculicola: cry1cry2 (cry1-304 cry2-1; Mockler et al, 1999), phot1phot2 (phot1-5 phot2-1; Sakai et al, 2001), and phyAphyB (phyA-211 phyB-9; Cerdán and Chory, 2003). Common genetic background for all examined mutants is accession Col (Col-0 for cry1cry2 and phyAphyB; Col-3 for phot1phot2), implicating that each line harbors the resistance gene Rpm1 whose product recognizes the bacterial avirulence protein AvrRpm1.…”

The Rice COLEOPTILE PHOTOTROPISM1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin

“…Mutants of Arabidopsis impaired in hypocotyl phototropism were isolated (Khurana and Poff, 1989;Liscum and Briggs, 1996), and molecular genetic analysis of these mutants has lead to the discovery of novel proteins that participate in the early process of phototropism. Most importantly, phototropin 1 (phot1) was uncovered as the major photoreceptor for phototropism (Huala et al, 1997) and phototropin 2 (phot2) as an additional photoreceptor that functions at high fluence rates (Sakai et al, 2001) (for the receptor nomenclature, see Briggs et al, 2001). The protein NPH3 was identified as a signaling component that is essential for hypocotyl phototropism and probably functions by directly interacting with phot1 (Motchoulski and Liscum, 1999).…”

Plant tropisms: providing the power of movement to a sessile organism

“…Its potential role as the second phototropic receptor was cemented when Sakai and colleagues (2001) determined that phot1phot2 double mutants lack phototropic response in both low and high fluence rate blue light. However, phot2 single mutants retained an essentially wildtype response under all fluence rates tested (Sakai et al, 2001). It was therefore concluded that phot1 and phot2 function redundantly as high light receptors, while phot1 acts as the low-light photoreceptor (Sakai et al, 2001).…”

“…However, phot2 single mutants retained an essentially wildtype response under all fluence rates tested (Sakai et al, 2001). It was therefore concluded that phot1 and phot2 function redundantly as high light receptors, while phot1 acts as the low-light photoreceptor (Sakai et al, 2001). The phototropins are members of a larger family of sensor proteins known as the LOV domain family (Crosson et al, 2003).…”