Most organisms use circadian oscillators to coordinate physiological and developmental processes such as growth with predictable daily environmental changes like sunrise and sunset. The importance of such coordination is highlighted by studies showing that circadian dysfunction causes reduced fitness in bacteria and plants, as well as sleep and psychological disorders in humans. Plant cell growth requires energy and water-factors that oscillate owing to diurnal environmental changes. Indeed, two important factors controlling stem growth are the internal circadian oscillator and external light levels. However, most circadian studies have been performed in constant conditions, precluding mechanistic study of interactions between the clock and diurnal variation in the environment. Studies of stem elongation in diurnal conditions have revealed complex growth patterns, but no mechanism has been described. Here we show that the growth phase of Arabidopsis seedlings in diurnal light conditions is shifted 8-12 h relative to plants in continuous light, and we describe a mechanism underlying this environmental response. We find that the clock regulates transcript levels of two basic helix-loop-helix genes, phytochrome-interacting factor 4 (PIF4) and PIF5, whereas light regulates their protein abundance. These genes function as positive growth regulators; the coincidence of high transcript levels (by the clock) and protein accumulation (in the dark) allows them to promote plant growth at the end of the night. Thus, these two genes integrate clock and light signalling, and their coordinated regulation explains the observed diurnal growth rhythms. This interaction may serve as a paradigm for understanding how endogenous and environmental signals cooperate to control other processes.

Phototropism, the bending response of plant organs to or away from a directional light source, is one of the best studied blue light responses in plants. Although phototropism has been studied for more than a century, recent advances have improved our understanding of the underlying signaling mechanisms involved. The NPH1 gene of Arabidopsis thaliana encodes a blue light-dependent autophosphorylating protein kinase with the properties of a photoreceptor for phototropism. NPH1 apoprotein noncovalently binds FMN to form the holoprotein nph1. The N-terminal region of the protein contains two LOV (light, oxygen, or voltage) domains that share homology with sensor proteins from a diverse group of organisms. These include the bacterial proteins NIFL and AER, both of which bind FAD, and the phy3 photoreceptor from Adiantium capillus-veneris. The LOV domain has therefore been proposed to ref lect a f lavin-binding site, regulating nph1 kinase activity in response to blue light-induced redox changes. Herein we demonstrate that the LOV domains of two nph1 proteins and phy3 bind stoichiometric amounts of FMN when expressed in Escherichia coli. The spectral properties of the chromopeptides are similar to the action spectrum for phototropism, implying that the LOV domain binds FMN to function as a light sensor. Thus, our findings support the earlier model that nph1 is a dual-chromophoric f lavoprotein photoreceptor regulating phototropic responses in higher plants. We therefore propose the name phototropin to designate the nph1 holoprotein.

Plants exhibit daily rhythms in their growth, providing an ideal system for the study of interactions between environmental stimuli such as light and internal regulators such as the circadian clock. We previously found that two basic loop-helix-loop transcription factors, PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5, integrate light and circadian clock signaling to generate rhythmic plant growth in Arabidopsis (Arabidopsis thaliana). Here, we use expression profiling and realtime growth assays to identify growth regulatory networks downstream of PIF4 and PIF5. Genome-wide analysis of light-, clock-, or growth-correlated genes showed significant overlap between the transcriptomes of clock-, light-, and growth-related pathways. Overrepresentation analysis of growth-correlated genes predicted that the auxin and gibberellic acid (GA) hormone pathways both contribute to diurnal growth control. Indeed, lesions of GA biosynthesis genes retarded rhythmic growth. Surprisingly, GA-responsive genes are not enriched among genes regulated by PIF4 and PIF5, whereas auxin pathway and response genes are. Consistent with this finding, the auxin response is more severely affected than the GA response in pif4 pif5 double mutants and in PIF5-overexpressing lines. We conclude that at least two downstream modules participate in diurnal rhythmic hypocotyl growth: PIF4 and/or PIF5 modulation of auxin-related pathways and PIF-independent regulation of the GA pathway.

Proteins play major roles in most biological processes; as a consequence, protein expression levels are highly regulated. While extensive post-transcriptional, translational and protein degradation control clearly influence protein concentration and functionality, it is often thought that protein abundances are primarily determined by the abundances of the corresponding mRNAs. Hence surprisingly, a recent study showed that abundances of orthologous nematode and fly proteins correlate better than their corresponding mRNA abundances. We tested if this phenomenon is general by collecting and testing matching large-scale protein and mRNA expression datasets from seven different species: two bacteria, yeast, nematode, fly, human, and plant. We find that steady-state abundances of proteins show significantly higher correlation across these diverse phylogenetic taxa than the abundances of their corresponding mRNAs (p=0.0008, paired Wilcoxon). These data support the presence of strong selective pressure to maintain protein abundances during evolution, even when mRNA abundances diverge.

In plant photomorphogenesis, it is well accepted that the perception of red͞far-red and blue light is mediated by distinct photoreceptor families, i.e., the phytochromes and blue-light photoreceptors, respectively. Here we describe the discovery of a photoreceptor gene from the fern Adiantum that encodes a protein with features of both phytochrome and NPH1, the putative blue-light receptor for secondpositive phototropism in seed plants. The fusion of a functional photosensory domain of phytochrome with a nearly full-length NPH1 homolog suggests that this polypeptide could mediate both red͞far-red and blue-light responses in Adiantum normally ascribed to distinct photoreceptors.Plants alter their growth and development in response to the light environment through a process known as photomorphogenesis. Several families of photoreceptors contribute to the whole-plant response to light from the UV-B to near-infrared region (1). Red͞far-red light is perceived by phytochrome, a biliprotein of approximately 120 kDa encoded by a multigene family both in seed plants and in cryptogams (2, 3). Blue-light perception is primarily mediated by cryptochromes (CRYs; refs. 4 and 5) and probably by the product of NPH1 locus (6). Based on the phenotypes of the nph1 (nonphototropic hypocotyl) mutant (7) and cry1cry2 double mutant (8), NPH1 has been implicated as the major photoreceptor responsible for blue light-dependent phototropic curvature in seed plants. Indeed, all of the strong nph1 mutant alleles are defective in both first-and second-positive curvature (6), whereas cry1cry2 double mutants are only impaired in first-positive phototropic curvature (8). Owing to the presence of putative flavin-binding sites on the NPH1 polypeptide (7) and epistasis analyses (9), NPH1 appears to be the primary photoreceptor mediating second-positive phototropism in plants.The influence of phytochrome and blue-light photoreceptors on each other's activity is well documented (10). Genetic analyses have clearly demonstrated an interaction between phytochrome and CRY1 signaling pathways (11). Moreover, a direct interaction between phytochrome and the CRY photoreceptors was recently documented (12). In seed plants, blue light-mediated phototropism has been shown to be affected by red-and far red-light treatments (13). Based on these and other studies, it appears likely that the signal-transduction pathways for red-and blue-light photoreceptor families share at least one common component (see ref. 14 for review).The co-action of phytochrome and blue-light photoreceptors has been examined at the cellular level in fern gametophytes, notably for the genus Adiantum (15, 16). In Adiantum, phytochrome-dependent spore germination is suppressed by blue-light irradiation (17). Phytochrome also prolongs blue light-induced cell-cycle progression in filamentous protonemal cells of Adiantum (18). By contrast, phytochrome and bluelight receptors act cooperatively to mediate phototropism of Adiantum protonemata (19) and to affect chloroplast photorelocation...