Calcium/calmodulin-dependent and -independent phytochrome signal transduction pathways

Isolation of cDNAs encoding guanine nucleotide-binding protein beta-subunit homologues from maize (ZGB1) and Arabidopsis (AGB1)

We have isolated cDNAs from maize (ZGB1) and Arabidopsis (AGB1) encoding proteins homologous to beta subunits of guanine nucleotide-binding protein (G protein). The predicted ZGB1 and AGB1 gene products are 76% identical to each other and 41% or more identical to animal G protein beta subunits. Both predicted proteins contain seven repeats of the so-called "WD-40" motif, where WD is Trp-Asp. RNA blot analysis indicates that ZGB1 mRNA is present in the root, leaf, and tassel and that AGB1 mRNA is expressed in the root, leaf, and flower. DNA blot hybridizations indicate that maize and Arabidopsis genomes contain no other genes that are highly similar to ZGB1 and AGB1, respectively, suggesting that the newly isolated G protein beta-subunit homologues are likely to have unique functions. Furthermore, these G protein beta-subunit homologues are conserved among other plant species and may play important role(s) in plant signaling.

Phytochrome signal transduction pathways are regulated by reciprocal control mechanisms

Three signal transduction pathways, dependent on cGMP and/or calcium, are utilized by phytochrome to control the expression of genes required for chloroplast development and anthocyanin biosynthesis in plant cells. For example, chs is controlled by a cGMP-dependent pathway, cab is controlled by a calcium-dependent pathway, and fnr is regulated by a pathway that requires both cGMP and calcium. Using a soybean photomixotrophic cell culture and microinjection into the cells of a phytochrome-deficient tomato mutant, we have studied the regulatory mechanisms acting within and between these three signaling pathways. We provide evidence that changes in cGMP levels mediate the observed induction and desensitization of chs gene expression in response to light and demonstrate that high cGMP concentrations cause negative regulation of both the calcium- and the calcium/cGMP-dependent pathways. Conversely, high activity of the calcium-dependent pathway can negatively regulate the cGMP-dependent pathway. We have termed these opposing regulatory mechanisms reciprocal control. In all cases, the molecules that are involved appear to be downstream components of the signal transduction pathways, rather than calcium and cGMP themselves. Furthermore, we have found that the calcium/cGMP-dependent pathway has a lower requirement for cGMP than does the cGMP-dependent pathway. The role of these phenomena in the regulation of plant photoresponses is discussed.

Plant phototransduction. Phytochrome signal transduction

Signalling intermediates in plant phototransduction pathways have been identified. These new results suggest how plants maintain a hight photosyntheic efficiency despite changes in their light environment.

The FUSCA genes of Arabidopsis: negative regulators of light responses

More than 200 fusca mutants of Arabidopsis have been isolated and characterised, defining 14 complementation groups. Mutations in at least nine FUSCA genes cause light-dependent phenotypic changes in the absence of light: high levels of anthocyanin accumulation in both the embryo and the seedling, inhibition of hypocotyl elongation, apical hook opening, and unfolding of cotyledons. In double mutants, the fusca phenotype is epistatic to the hy phytochrome-deficiency phenotype, indicating that the FUSCA genes act downstream of phytochrome. By contrast, the accumulation of anthocyanin is suppressed by mutations in TT and TTG genes, which affect the biosynthesis of anthocyanin, placing the FUSCA genes upstream of those genes. Regardless of the presence or absence of anthocyanin, fusca mutations limit cell expansion and cause seedling lethality. In somatic sectors, mutant fus1 cells are viable, expressing tissue-specific phenotypes: reduced cell expansion and accumulation of anthocyanin in subepidermal tissue, formation of ectopic trichomes but no reduced cell expansion in epidermal tissue. Our results suggest a model of FUSCA gene action in light-induced signal transduction.

DET1, a negative regulator of light-mediated development and gene expression in arabidopsis, encodes a novel nuclear-localized protein

The mechanisms by which plants integrate light signals to modify endogenous developmental programs are largely unknown. One candidate for a signal transduction component that may integrate light with developmental pathways is the Arabidopsis DET1 gene product. Here we report the positional cloning of the DET1 locus and show that DET1 is a unique nuclear-localized protein. An analysis of a number of det1 mutants indicates that mutants with partial DET1 activity develop as light-grown plants in the dark. det1 null mutants share this phenotype, but also display severe defects in temporal and spatial regulation of gene expression. These results suggest that DET1 acts in the nucleus to control the cell type-specific expression of light-regulated promoters.

Approaches to understanding auxin action

The effects of auxin on plant growth and development have been studied for decades, but the molecular mechanisms of auxin action remain unknown. These mechanisms have primarily been investigated by characterization of auxin physiology mutants and analysis of auxin-binding proteins and auxin-regulated genes. These efforts are now converging, since some mutants have recently been shown to have altered expression of specific auxin-binding proteins and auxin-regulated genes. The features of these proteins and genes are providing the first tantalizing clues to the organization of auxin signal transduction pathways.

Light-regulated promoters from Synechocystis PCC6803 share a consensus motif involved in photoregulation

A library of Synechocystis PCC6803 (S.6803) DNA cloned in front of the promoterless cat reporter gene of the plasmid pFF11 was used to transform S.6803 to high light-dependent resistance to chloramphenicol. In five clones harbouring a stably replicating pFF11-derived plasmid, this phenotype occurred independently of the photosystem II electron transport and resulted from the correlated increase of CAT activity level and cat mRNA accumulation. The five promoter inserts contained no Escherichia coli sigma 70 promoter element, in agreement with their lack of activity in this organism, but shared two conserved motifs. Two secondary mutations, which restored light-regulated promoter activity to an inactive mutant of the smallest insert, mapped within one of the common motifs, emphasizing the probable involvement of this element in photoregulation.

The phytochrome apoprotein family in Arabidopsis is encoded by five genes: the sequences and expression of PHYD and PHYE

Two novel Arabidopsis phytochrome genes, PHYD and PHYE, are described and evidence is presented that, together with the previously described PHYA, PHYB and PHYC genes, the primary structures of the complete phytochrome family of this plant are now known. The PHYD- and PHYE-encoded proteins are of similar size to the other phytochrome apoproteins and show sequence similarity along their entire lengths. Hence, red/far-red light sensing in higher plants is mediated by a diverse but structurally conserved group of soluble photoreceptors. The proteins encoded by the PHYD and PHYE genes are more closely related to phytochrome B than to phytochromes A or C, indicating that the evolution of the PHY gene family in Arabidopsis includes an expansion of a PHYB-related subgroup. The PHYB and PHYD phytochromes show greater than 80% amino acid sequence identity but the phenotypes of phyB null mutants demonstrate that these receptor forms are not functionally redundant. The five PHY mRNAs are, in general, expressed constitutively under varying light conditions, in different plant organs, and over the life cycle of the plant. These observations provide the first description of the structure and expression of a complete phytochrome family in a higher plant.

Light induces rapid changes of the phosphorylation pattern in the cytosol of evacuolated parsley protoplasts

The fractionation of cells of a parsley suspension culture [Petroselinum crispum (Mill.) A. Hill] by protoplasting and subsequent removal of the vacuoles led to physiologically intact evacuolated protoplasts retaining light inducibility of chalcone synthase expression. Lysis of the evacuolated protoplasts permitted the isolation of a pure, highly concentrated cytosolic fraction containing major cytosolic membranes but only minor contamination by proplastids, mitochondria, and nuclei. Short-time irradiations of the cytosol with red or UV-containing white light resulted in very fast changes of the phosphorylation pattern of 18-, 40-, 48-, 55- to 70-, and 120-kDa proteins. Major differences were observed between the phosphorylation patterns obtained by red or UV-containing white light treatment, indicating a different primary action of the excited photoreceptors in vitro. Separation of the microsomal fraction from the cytosolic matrix established the localization of these proteins. Chase and photoreversibility experiments revealed that phytochrome in vitro regulates the phosphorylation of the 40-kDa protein by modifying a soluble cytosolic kinase/phosphatase system.

Cyclic GMP and calcium mediate phytochrome phototransduction

We have previously used single-cell assays in a phytochrome-deficient tomato mutant to demonstrate that phytochrome signaling involves heterotrimeric G proteins, calcium, and calmodulin. While G protein activation could stimulate full chloroplast development and anthocyanin pigment biosynthesis, calcium and calmodulin could not induce anthocyanins and were only able to stimulate the development of immature chloroplasts lacking cytochrome b6f and photosystem I core components. We now report that cyclic GMP is able to trigger the production of anthocyanins, and that a combination of cyclic GMP with calcium can induce the development of fully mature chloroplasts containing all the photosynthetic machinery. Furthermore, using reporter genes for these different pathways (cab-gus, chs-gus, and fnr-gus) we demonstrate that cGMP and calcium act primarily by modulating gene expression.