Action and function of phytochrome family members revealed through the study of mutant and transgenic plants

The adaptation of plant growth and development to changes in the light environment is dependent upon photoperception by information transducing photoreceptors. The red/far-red light-absorbing phytochromes are perhaps the best characterized of these regulatory photoreceptors. Higher plants possess multiple, discrete phytochromes, the apoprotein components of which are the products of a small, divergent gene family. Different phytochromes have different biochemical and physiological properties, and are differentially expressed in the growing plant. This has led to the proposal that different phytochromes have different physiological roles. Mutations that disrupt the normal perception of light signals have proved to be a valuable resource in assigning physiological roles to different phytochromes as well as in identifying residues/domains critical for phytochrome function and in attempting to elucidate the signal transduction pathway(s) downstream of phytochromes. This article reviews some recent progress in these areas from the study of conventional and transgenic photomorphogenic mutants.     

Right place,right time: Spatiotemporal light regulation of plant growth and development

Recent advances in our understanding of the roles of photoreceptors in light-dependent regulation of plant growth and development have been rapid and significant. Developments have been reported for numerous plant photoreceptor signaling pathways, yet researchers have made the most progress in increasing our comprehension of the roles of phytochrome family members, as well as the intracellular roles of phytochromes and phytochrome-interacting proteins in light-dependent signaling. An understudied, but vitally important, area of phytochrome biology centers on the roles phytochromes play in intercellular and interorgan signaling at the molecular level that results in the coordination of growth responses between distinct tissues and organs. This frontier of research into the spatiotemporal roles of phytochromes, and more generally plant photoreceptors, which is only beginning to be investigated and understood at the molecular genetic level, has a rich history of physiological data.Key words: cryptochrome, photomorphogenesis, photoreception, photoreceptor, phytochrome, spatiotemporal     

Nucleo-cytoplasmic partitioning of the plant photoreceptors phytochromes

Phytochromes in harmony with blue light photoreceptors play a major role in controlling plant growth and development from germination to seed maturation. Light absorption by phytochromes triggers a signaling cascade, phototransduction, which culminates in regulated gene expression. A major regulatory step at the cellular level, which affects specificities of light-induced physiological responses, seems to be the light-quality and light-quantity dependent nuclear import of the phytochromes themselves. The correlations found between the nuclear import of phytochromes (phyA and phyB) and various physiological responses regulated by these photoreceptors provides strong support for this hypothesis.     

Phytochromes confer the photoperiodic control of flowering in rice (a short-day plant)

The photoperiodic sensitivity 5 (se5) mutant of rice, a short-day plant, has a very early flowering phenotype and is completely deficient in photoperiodic response. We have cloned the SE5 gene by candidate cloning and demonstrated that it encodes a putative heme oxygenase. Lack of responses of coleoptile elongation by light pulses and photoreversible phytochromes in crude extracts of se5 indicate that SE5 may function in phytochrome chromophore biosynthesis. Ectopic expression of SE5 cDNA by the CaMV 35S promoter restored the photoperiodic response in the se5 mutant. Our results indicate that phytochromes confer the photoperiodic control of flowering in rice. Comparison of se5 with hy1, a counterpart mutant of Arabidopsis, suggests distinct roles of phytochromes in the photoperiodic control of flowering in these two species.     

Silencing threonine deaminase and JAR4 in Nicotiana attenuata impairs jasmonic acid-isoleucine-mediated defenses against Manduca sexta

Threonine deaminase (TD) catalyzes the conversion of Thr to alpha-keto butyrate in Ile biosynthesis; however, its dramatic upregulation in leaves after herbivore attack suggests a role in defense. In Nicotiana attenuata, strongly silenced TD transgenic plants were stunted, whereas mildly silenced TD transgenic plants had normal growth but were highly susceptible to Manduca sexta attack. The herbivore susceptibility was associated with the reduced levels of jasmonic acid-isoleucine (JA-Ile), trypsin proteinase inhibitors, and nicotine. Adding [(13)C(4)]Thr to wounds treated with oral secretions revealed that TD supplies Ile for JA-Ile synthesis. Applying Ile or JA-Ile to the wounds of TD-silenced plants restored herbivore resistance. Silencing JASMONATE-RESISTANT4 (JAR4), the N. attenuata homolog of the JA-Ile-conjugating enzyme JAR1, by virus-induced gene silencing confirmed that JA-Ile plays important roles in activating plant defenses. TD may also function in the insect gut as an antinutritive defense protein, decreasing the availability of Thr, because continuous supplementation of TD-silenced plants with large amounts (2 mmol) of Thr, but not Ile, increased M. sexta growth. However, the fact that the herbivore resistance of both TD- and JAR-silenced plants was completely restored by signal quantities (0.6 mumol) of JA-Ile treatment suggests that TD's defensive role can be attributed more to signaling than to antinutritive defense.     

Phytochrome phosphorylation in plant light signaling.

Reversible protein phosphorylation is a switching mechanism used in eukaryotes to regulate various cellular signalings. In plant light signaling, sophisticated photosensory receptor systems operate to modulate growth and development. The photoreceptors include phytochromes, cryptochromes and phototropins. Despite considerable progresses in defining the photosensory roles of these photoreceptors, the primary biochemical mechanisms by which the photoreceptor molecules transduce the perceived light signals into cellular responses remain to be elucidated. The signal-transducing photoreceptors in plants are all phosphoproteins and/or protein kinases, suggesting that light-dependent protein phosphorylation and dephosphorylation play important roles in the function of the photoreceptors. This review focuses on the role of phytochromes' reversible phosphorylation involved in the light signal transduction in plants.     

Detection of spatial-specific phytochrome responses using targeted expression of biliverdin reductase in Arabidopsis

To regulate levels of holophytochrome in a spatial-specific manner and investigate the major sites of action of phytochromes during seedling development, we constructed transgenic Arabidopsis (Arabidopsis thaliana) plant lines expressing plastid-targeted mammalian biliverdin IXα reductase (pBVR) under regulatory control of CAB3 and MERI5 promoters. Comparative photobiological and phenotypic analyses indicated that spatial-specific expression of pBVR led to the disruption of distinct subsets of phytochrome-regulated responses for different promoters. pBVR expression in photosynthetic tissues (CAB3pBVR lines) had intermediate effects on chlorophyll accumulation, carotenoid production, anthocyanin synthesis, and leaf development responses in white-light conditions. CAB3pBVR expression, however, resulted in distinctive phenotypes in far-red (FR) conditions. A number of FR high irradiance responses were disrupted in CABpBVR lines, including FR-dependent inhibition of hypocotyl elongation and stimulation of anthocyanin accumulation. By contrast, preferential expression of pBVR in the shoot apical meristem in MERI5pBVR lines resulted in a phytochrome-deficient, leaf development phenotype under short-day growth conditions. These results implicate leaf-localized phytochrome A as having a unique role in regulating FR-mediated hypocotyl elongation and meristem- and/or leaf primordia-localized phytochromes as having a novel role in phytochrome-dependent responses. Taken together, these studies demonstrate the efficacy of selectively inactivating distinct phytochrome-mediated responses by regulated expression of BVR in transgenic plants, a novel means to investigate the sites of phytochrome photoperception and to regulate specifically light-mediated plant growth and development.     

Ginkgo biloba retains functions of both type I and type II flowering plant phytochrome

While the photoreceptor systems of flowering plants have been well studied, the origins of these gene families and their functions are only partially understood. To begin to resolve the evolutionary origins of angiosperm photoreceptor function, we have studied the photomorphogenic responses of the early diverging gymnosperm Ginkgo biloba. Here, we describe the effects of continuous white light, red light, far-red light, and blue light on stem length, chlorophyll accumulation, Lhcb mRNA accumulation, and plastid development. Differences in the efficacy of these light regimes on de-etiolation in Ginkgo suggest separate but complementary roles for red and blue light-sensing systems. Additionally, the unique manner in which developmental regulation occurs in Ginkgo reveals a far-red high irradiance response different from both angiosperm and other gymnosperm species. We conclude from these data that Ginkgo contains a functional complement to both flowering plant type I and type II phytochromes, as well as independent blue light-sensing system(s). The implications of these findings are discussed with respect to the evolution of higher plant photoreceptors.     

Roles of different phytochromes in Arabidopsis photomorphogenesis

The red/far-red light-absorbing phytochromes play fundamental roles in photoperception of the light environment and the subsequent adaptation of plant growth and development. Higher plants possess multiple, discrete phytochromes, the apoproteins of which are the products of a family of divergent (PHY) genes. Arabidopsis thaliana has at least five PHY genes, encoding the apoproteins of phytochromes A-E. Through the analysis of mutants that are deficient in phytochrome A or B and the corresponding double mutant, it is becoming clear that these phytochromes perform both discrete and overlapping roles throughout plant development. Through analysis of the phyA phyB double mutant, it has been possible to define several responses that are mediated by other members of the phytochrome family. This article reviews some of the recent progress in the study of phytochrome-deficient mutants of the model plant Arabidopsis thaliana.