PHOTOMORPHOGENETIC MUTANTS OF HIGHER PLANTS

Guanosine triphosphate-binding proteins on the plasmalemma of spinach leaf cells

The molecular mechanism of light perception through phytochrome is not well understood. This red-light photosensor has been implicated in various physiological processes, including the photoinduction of flowering. A few recent studies have shown that phytochrome initiates signal transduction chains via guanosine triphosphate (GTP)-binding proteins (G-proteins). We show here by different approaches that G-proteins exist in spinach (Spinacia oleracea L. cv. Nobel). Binding of GTP on the plasmalemma has been partially characterized and its possible regulation by red light examined by in-vitro assays. These experiments indicate a clear regulation of GTP binding by red light and also by Mastoparan. At least three G-proteins or protein subunits were found to be associated with the plasmalemma of leaf cells. The use of an antibody raised against an animal Gβ subunit confirmed the presence of heterotrimeric G-proteins. Separation of a crude membrane extract by free-flow electrophoresis also showed that some G-proteins could exist on the tonoplast.

Molecular and genetic analysis of signal transduction in higher plants

Plant scientists have long recognized the complexity of responses to environmental and hormonal signals that provide the basis for plant growth and development. The systematic isolation and analysis of mutations that disrupt signal transduction and prevent the appropriate physiological response provides an important resource for studying these processes and, ultimately, for describing the molecular events that control growth and developmental responses in plants.

cop1: a regulatory locus involved in light-controlled development and gene expression in Arabidopsis

Light signals from the environment are perceived by specific regulatory photoreceptors in plants and are transduced by unknown mechanisms to genes that control growth and development. We have identified a genetic locus in Arabidopsis thaliana, which appears to play a central role in this transduction process. Mutations in this locus, designated cop1 (constitutively photomorphogenic), result in dark-grown seedlings with the morphology of wild-type seedlings grown in the light. In addition, these mutations lead to constitutive expression of an array of normally light-regulated genes in dark-grown seedlings and in light-grown adult plants placed in darkness. Promoter-reporter fusion constructs of some of these genes are constitutively expressed in dark-grown transgenic cop1 seedlings, indicating that the aberrant behavior of these genes results primarily from aberrant modulation of their promoter activities in the mutant. In contrast, light control of seed germination and diurnal control of cab gene expression is normal in the cop1 mutants. Because these mutations are recessive, we conclude that in seedlings and adult plants, the wild-type cop1 gene product normally acts in darkness to repress the expression of genes involved in the dark-adaptive developmental and that regulatory photoreceptors act to reverse this action upon exposure to light. However, photocontrol of seed germination and diurnal rhythms is apparently exerted via one or more separate pathways not involving the cop1 product. one or more separate pathways not involving the cop1 product.

Photomorphogenic responses to UV radiation: Involvement of phytochrome and UV photoreceptors in the control of hypocotyl elongation in Lycopersicon esculentum

The photo-inhibition of Lycopersicon esculentum Mill, hypocotyl growth induced by UV radiation may be mediated by both phytochrome and UV-absorbing receptors. The inhibition of growth induced by continuous irradiation with high fluence rate UV radiation is similar in the au mutant, which is severely deficient in spectrophoto metrically and immunochemically detectable phytochrome, and in the isogenic wild type. Parallel irradiation with 692 nm light, which is equivalent to UV radiation for the phytochrome system in our experimental conditions, induced at high photon fluence rates a significant increase in hypocotyl growth in the au mutant. The same light treatments inhibited the hypocotyl growth of the wild type. The responses of water-grown seedlings and chlorophyll-free seedlings (streptomycin and norflurazon treated seedlings) were compared. Water-grown and chlorophyll-free seedlings responded similarly to UV radiation. The presence of chlorophyll correlates with a significant increase in hypocotyl growth of au mutants irradiated with 692 nm light. These results support the conclusion that UV-induced inhibition of growth in the au mutant is independent of phytochrome.

Phytochrome-regulated expression of genes encoding light-harvesting chlorophyll a/b-protein in two long hypocotyl mutants and wild type plants of Arabidopsis thaliana

The cab genes which encode the light-harvesting chlorophyll a/b-protein (LHCP) are expressed normally with respect to phytochrome regulation in the hy-3 and hy-5 long hypocotyl mutants of Arabidopsis thaliana. In etiolated seedlings of these mutants as well as of the wild type, 1 min of red light elevates cab mRNA levels substantially within 2 h; this increase is reversed if far-red light is given immediately after the red light treatment. We conclude that the genetic defects in these mutants do not affect steps in the signal transduction pathway leading to the regulated expression of cab genes. Additionally, the mRNA from one of the three known A. thaliana cab genes, AB140, is similar in quantity to the mRNAs from the other two, AB165 and AB180, in dark-grown seedlings of hy-3 and hy-5 as well as the parent A. thaliana (Landsberg) after a brief red light treatment. This aspect of cab gene expression differs from the strain Columbia of A. thaliana in which AB140 mRNA is the predominant message. In mature white light-grown plants of the strain Columbia, AB140 as well as a combination of AB165 and AB180 mRNAs are expressed at high levels, suggesting that AB165 and/or AB180 may be developmentally regulated.

Phytochrome regulation of greening in wild type and long-hypocotyl mutants ofArabidopsis thaliana

A brief pulse of red light (R) given to darkgrown seedlings ofArabidopsis thaliana (L.) Heyn. potentiates rapid synthesis of chlorophyll upon transfer to continuous white light. The time course for potentiation of rapid greening shows that a R pulse in the LF (low fluence) range has maximal effect within a few hours, and that there is a small VLF (very low fluence) component as well. Partial reversal of the effect of R by far-red light (FR) indicates that the pulse acts through phytochrome. As it does in the wild-type (WT), a pulse of R accelerates greening of long-hypocotyl (hy) mutants. The extent of induction by the R pulse was about the same in the WT and in allhy mutants studied. Reversibility by FR was greatly decreased in thehy-1 andhy-2 strains. It is possible that these mutants contain a species of phytochrome with defective phototransformation kinetics. If there is such a defective phytochrome species, it nevertheless appears to be active in the potentiation of rapid greening.