The light-dependent regulation of gene expression during plastid development in higher plants

In recent years there has been a considerable increase in our understanding of the manner by which light affects gene expression during chloroplast development. In most systems that have been studied, light acts through sensitive photoreceptor molecules and quantitatively increases or represses the level of expression of specific nuclear-and plastid-encoded genes. Although the mechanisms are obscure, a picture is beginning to emerge in which the coordination of nuclear and plastid gene expression is controlled by regulatory mechanisms originating within their respective subcellular compartments. This review summarizes some of our current knowledge concerning the nature of light-regulated gene expression in higher plants and provides a prospectus for future research in this area.     

Overexpression of oat phytochrome A gene differentially affects stem growth responses to red/far-red ratio signals characteristic of sparse or dense canopies

The effects of overexpression of oat phytochrome A on neighbour detection and on stem-growth responses to changes in red light (R), far-red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild-type (WT) seedlings, stem extension growth was promoted: (1) by lowering the R:FR by means of daytime supplementary FR, end-of-day FR, neighbours reflecting FR, or selective light filters placed around the base of the shoot to reduce R without affecting FR; and (2) by lowering phytochrome-absorbable radiation (R+FR) reaching the stem. Transgenic seedlings only responded to reductions in R:FR involving no significant changes in FR irra-diance, i.e. end-of-day FR and filters placed around the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reduction of R:FR caused by FR reflected in neighbour plants. Transgenic seedlings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. Overexpression of phytochrome A impaired early neighbour detection.     

Semi-quantitative RT-PCR analysis of photoregulated gene expression in marine diatoms

The low cell densities of diatoms and other phytoplankton in culture has precluded the use of classical RNA analysis techniques for routine studies of gene expression in large numbers of samples. This has seriously hampered studies of the basic biology of such organisms. To circumvent this problem, we have developed a high-throughput semi-quantitative RT-PCR-based protocol and used it to monitor expression of a gene encoding a fucoxanthin, chlorophyll a/c-binding protein (FCP) in the centric planktonic diatom Thalassiosira weissflogii. Analysis of FCP gene expression in dark-adapted diatom cultures revealed that mRNA levels increase 5- to 6-fold in response to white light irradiation and peak around 6 to 8 h. To determine the photoreceptors involved in this response action spectra of FCP gene expression were determined using the Okazaki large spectrograph. Responses consistent with the presence of cryptochrome-, rhodopsin- and phytochrome-type receptors could be detected. The apparent presence of phytochrome-mediated responses is of particular interest given the low fluences of red and far-red light wavelengths in the marine environment.     

Mechanisms of gene expression controlled by pressure in deep-sea microorganisms

A pressure-regulated operon has been cloned and sequenced from deep-sea barophilic Shewanella strains. To understand pressure-regulated mechanisms of gene expression, a regulatory element upstream of the pressure-regulated operon from Shewanella sp. strain DSS12 was studied. Regions A and B were classified by sequence analysis. A unique octamer motif, AAGGTAAG, was found to be repeated in tandem 13 times in region B. An electrophoretic mobility shift assay demonstrated that a σ⁵⁴-like factor recognizes region A and other unknown factors recognize region B. Different shift patterns of the protein–DNA complexes were observed when extracts of cells cultured at 0.1 MPa or 50 MPa were incubated with a DNA probe specific for region B. These results indicate that the deep-sea strain DSS12 expresses different DNA-binding factors under different pressure conditions. Received: January 22, 1998 / Accepted: February 16, 1998     

Characterization of cymbidium mosaic virus coat protein gene and its expression in transgenic tobacco plants

Cymbidium mosaic virus (CyMV) is the most prevalent virus infecting orchids. Here, we report the isolation of partial cDNA clones encoding the genomic RNA of CyMV. Like most of the polyadenylated monopartite positive-strand RNA viruses, the open reading frame (ORF) coding for the viral coat protein (CP) is located at the 3 end. The ORF predicts a polypeptide chain of 220 amino acids with a molecular weight of 23 600. Sequence comparison of this ORF to the CP sequences of potato virus X(PVX) and white clover mosaic virus (WCIMV) revealed a strong amino acid homology in the mid-portion of the CP, but the overall homology was low. The CyMV CP gene was placed downstream of a cauliflower mosaic virus 35S promoter and the chimaeric gene was transferred into Nicotiana benthamiana. Transgenic plants expressing the CyMV CP were protected against CyMV infection.     

Temporal and light regulation of the expression of three phytochromes in germinating seeds and young seedlings of Avena sativa L.

An oat (Avena sativa L.) plant contains at least three phytochromes, which have monomeric masses of 125, 124, and 123 kilodaltons (kDa) (Wang et al., 1991, Planta 184, 96–104). The 124-kDa phytochrome is most abundant in dark-grown seedlings, while the other two phytochromes predominate in light-grown seedlings. Using three monoclonal antibodies, each specific to one of the three phytochromes, we have monitored by immunoblot assay the expression of these three phytochromes in the 5 d following onset of imbibition of seeds. On a per-organism basis, each of these three phytochromes increased in abundance for the first 3 d in the light, or for the first 4 d in darkness, after which they each began to decrease in quantity. When 3-d-old dark-grown seedlings were transferred to the light, the abundance of each of these three phytochromes decreased both in absolute amount and relative to the phytochrome levels in control seedlings kept in darkness. In contrast, when 3-d-old light-grown seedlings were transferred to darkness, the abundance of the 124-kDa and 125-kDa phytochromes increased while that of 123-kDa phytochrome remained unchanged. In each case, the level of phytochrome was greater than that of control seedlings maintained in the light. Thus, in addition to temporal regulation, all three phytochromes exhibit photoregulated expression at the protein level, although the magnitude of this photoregulation varies substantially. We thank Drs. Elizabeth Williams and Tammy Sage (Botany Department, University of Georgia, USA) for generously permitting us to use their image-analysis system. This research was supported by USDA NRICGP grant 91-37100-6490.