Light-dependent regulation of the synthesis of soluble and intracytoplasmic membrane proteins of Rhodopseudomonas sphaeroides.

Cells of Rhodopseudomonas sphaeroides grown under saturating light conditions (30 W/m2) and then shifted to low light intensity (3 W/m2) required 2.5 h to adapt to the new lower light conditions. After the shift, cell growth, whole cell protein accumulation, and bacteriochlorophyll accumulation ceased immediately. Approximately midway into the adaptation period, bacteriochlorophyll synthesis commenced at a new, higher rate, which continued through the beginning of the low-light growth period until new steady-state levels were reached. Immediately after the downshift, the rate of cellular protein synthesis declined to 22% of its preshift rate. Pulse-labeling of protein throughout the adaptation period and comparison with a steady-state prelabel culture revealed that synthesis of two of the three light-harvesting proteins, as well as two additional high-molecular-weight photosynthetic membrane proteins, was derepressed three- to fivefold compared with bulk cellular protein. Finally, the synthesis of at least three soluble proteins showed light-dependent regulation after the light downshift. These results are discussed in terms of the light-dependent regulation of synthesis of the photosynthetic membrane macromolecular components and the division of protein synthesis between the photosynthetic membranes and the soluble cell phase.     

Phytochrome A and Phytochrome B Have Overlapping but Distinct Functions in Arabidopsis Development

Plant responses to red and far-red light are mediated by a family of photoreceptors called phytochromes. In Arabidopsis thaliana, there are genes encoding at least five phytochromes, and it is of interest to learn if the different phytochromes have overlapping or distinct functions. To address this question for two of the phytochromes in Arabidopsis, we have compared light responses of the wild type with those of a phyA null mutant, a phyB null mutant, and a phyA phyB double mutant. We have found that both phyA and phyB mutants have a deficiency in germination, the phyA mutant in far-red light and the phyB mutant in the dark. Furthermore, the germination defect caused by the phyA mutation in far- red light could be suppressed by a phyB mutation, suggesting that phytochrome B (PHYB) can have an inhibitory as well as a stimulatory effect on germination. In red light, the phyA phyB double mutant, but neither single mutant, had poorly developed cotyledons, as well as reduced red-light induction of CAB gene expression and potentiation of chlorophyll induction. The phyA mutant was deficient in sensing a flowering response inductive photoperiod, suggesting that PHYA participates in sensing daylength. In contrast, the phyB mutant flowered earlier than the wild type (and the phyA mutant) under all photoperiods tested, but responded to an inductive photoperiod. Thus, PHYA and PHYB appear to have complementary functions in controlling germination, seedling development, and flowering. We discuss the implications of these results for possible mechanisms of PHYA and PHYB signal transduction.     

Spotlight on phytochrome nomenclature

These recommendations for genes encoding phytochromes were developed independently by Quail et al., but are broadly consistent with the Commission's guidelines. Their original article, kindly provided in advance of publication, appeared as a Letter to the Editor inPlant Cell (6:468–471, 1994) and is published with permission of the American Society of Plant Physiologists.     

Isolation and Initial Characterization of Arabidopsis Mutants That Are Deficient in Phytochrome A

Phytochrome, a red/far-red-light photoreceptor protein of plants, is encoded by a small gene family. Phytochrome A (PHYA), the product of the PHYA gene, is the predominant molecular species of phytochrome in etiolated tissue and has been best characterized biochemically. To define a role for PHYA, we isolated new mutants, designated fre1 (far-red elongated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were identified on the basis of their long hypocotyl phenotype under continuous far-red light. Although the fre1 mutants lacked the hypocotyl response to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PHYA appears to play only a minor role in the regulation of hypocotyl elongation under natural conditions. In contrast, the fre1 mutation affected greening a fre1 mutant was less able than the wild type to deetiolate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed significantly in the fre1 mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis development.     

Extragenic Suppressors of the Arabidopsis Det1 Mutant Identify Elements of Flowering-Time and Light-Response Regulatory Pathways

Light regulation of seedling morphogenesis is mediated by photoreceptors that perceive red, far-red, blue and UV light. Photomorphogenetic mutants of Arabidopsis have identified several of the primary photoreceptors, as well as a set of negative regulators of seedling photomorphogenesis, including DET1, that appear to act downstream of the photoreceptors. To study the regulatory context in which DET1 acts to repress photomorphogenesis, we used a simple morphological screen to isolate extragenic mutations in six loci, designated ted (for reversal of the det phenotype), that partially or fully suppress the seedling morphological phenotype of det1-1. Genetic analyses indicate that mutations in the ted4 and ted5 loci identify new alleles of the previously described photomorphogenetic loci hy1 and hy5, respectively. Molecular analyses indicate that the ted mutations partially suppress the dark-grown gene expression phenotype of det1-1, and that the mechanism of suppression does not involve direct remediation of the splicing defect caused by the det1-1 mutation. The ted mutations also partially suppress the light-grown morphological phenotype of mature det1-1 plants, and ted1 and ted2 suppress a daylength insensitivity phenotype of det1. TED1, TED2 and TED3 are newly described genes, whose function appears closely associated with that of DET1. In addition, alleles of ted1 are associated with a moderate late-flowering phenotype, suggesting that TED1 plays a role in the pathways that regulate both seedling morphogenesis and the initiation of flowering.     

Light receptor kinases in plants!

Plants must adapt to a capricious light environment, but the mechanism by which light signals are transmitted to cause changes in development has long eluded us. The search might be over, however, as two photoreceptors, phytochrome and NPH1, have been shown to autophosphorylate in a light-dependent fashion.     

BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), are perceived by the plasma membrane-localized leucine-rich-repeat-receptor kinase BRI1. Based on sequence similarity, we have identified three members of the BRI1 family, named BRL1, BRL2 and BRL3. BRL1 and BRL3, but not BRL2, encode functional BR receptors that bind brassinolide, the most active BR, with high affinity. In agreement, only BRL1 and BRL3 can rescue bri1 mutants when expressed under the control of the BRI1 promoter. While BRI1 is ubiquitously expressed in growing cells, the expression of BRL1 and BRL3 is restricted to non-overlapping subsets of vascular cells. Loss-of-function of brl1 causes abnormal phloem:xylem differentiation ratios and enhances the vascular defects of a weak bri1 mutant. bri1 brl1 brl3 triple mutants enhance bri1 dwarfism and also exhibit abnormal vascular differentiation. Thus, Arabidopsis contains a small number of BR receptors that have specific functions in cell growth and vascular differentiation.