Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment

A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed.     

Stomatal sensing of the environment

The effects of environmental factors on stomatal behaviour are reviewed and the questions of whether photosynthesis and transpiration eontrol stomata or whether stomata themselves control the rates of these processes is addressed. Light affects stomata directly and indirectly. Light can act directly as an energy source resulting in ATP formation within guard cells via photophosphorylation, or as a stimulus as in the case of the blue light effects which cause guard cell H⁺ extrusion. Light also acts indirectly on stomata by affecting photosynthesis which influences the intercellular leaf CO₂ concentration ( C _i). Carbon dioxide concentrations in contact with the plasma membrane of the guard cell or within the guard cell acts directly on cell processes responsible for stomatal movements. The mechanism by which CO₂ exerts its effect is not fully understood but, at least in part, it is concerned with changing the properties of guard cell plasma membranes which influence ion transport processes. The C _i may remain fairly constant for much of the day for many species which is the result of parallel responses of stomata and photosynthesis to light. Leaf water potential also influences stomatal behaviour. Since leaf water potential is a resultant of water uptake and storage by the plant and transpirational water loss, any factor which affects these processes, such as soil water availability, temperature, atmospheric humidity and air movement, may indirectly affect stomata. Some of these factors, such as temperature and possibly humidity, may affect stomata directly. These direct and indirect effects of environmental factors interact to give a net opening response upon which is superimposed a direct effect of stomatal circadian rhythmic activity.     

Phototropic fluence-response curves for Pilobolus crystallinus sporangiophore

Fluence-response relationships were examined for positive and negative phototropism induced by blue (450 nm) and ultraviolet-B (UV-B, 280 nm) light, respectively, in the Pilobolus crystallinus sporangiophore. Fluence-response curves for both blue and UV-B light obtained by changing the fluence by varying exposure time only showed the classical first and second positive bending. However, fluence-response curves obtained by varying the fluence rate were bell-shaped irrespective of the length of the exposure time. With increasing exposure time the peak became higher along the ascendant arm and the descendant arm was shifted toward the higher fluence. The Bunsen-Roscoe reciprocity law was valid only when the fluence was less than approx. 400 pmol·m^-2 for both blue and UV-B light. Because the shapes of the fluence-response curves for blue and UV-B light were nearly the same, the photoreceptor systems for both blue and UV-B light are considered to be the same.Abbreviation UV-B ultraviolet-B     

The loci of perception for phytochrome control of internode growth in light-grown mustard: Promotion by low phytochrome photoequilibria in the internode is enhanced by blue light perceived by the leaves

Under continuous white light (WL), extension growth of the first internode in Sinapis alba L. was promoted by low red (R): far-red (FR) ratios reaching the stem and-or the leaves. Conversely, the growth promotion by end-of-day light treatments was only triggered by FR perceived by the leaves and cotyledons, while FR given to the growning internode alone was tatally ineffective. Continuous WL+FR given to the internode was also in-effective if the rest of the shoot remained in darkness. Both the background stem growth, and the growth promotion caused by either an end-of-day FR pulse or continuous WL+FR given to the internode, increased with increasing fluence rates of WL given to the rest of the shoot. The increase by WL of the growth-stimulatory effect of low phytochrome photoequilibria in the internode appears to be mediated by a specific blue-light-absorbing photoreceptor, as blue-deficient light from sodium-discharge lamps, or from filtered fluorescent tubes, promoted background stem growth similarly to WL but did not amplify the response to the R:FR ratio in the internode. Supplementing the blue-deficient light (94 mol·m^-2·s^-1) with low fluence rates of blue (<9 mol·m^-2·s^-1) restored the promotive effect of low R:FR reaching the internode.Abbreviations BL blue light - FR far-red light - PAR photosynthetically active radiation - Pfr/P ratio between the FR-absorbing form and total phytochrome - R red light - SOX low-pressure sodium lamp - WL white light Supported by the Consejo Nacional de Investigaciones Cientificas y Técnicas (República Argentina) and the ORS scheme (UK)     

Chloroplast proliferation based on their distribution and arrangement inAdiantum protonemata growing under red light

Chloroplast proliferation was investigated inAdiantum protonemata growing under continuous red light. Cell division is absent when cells are grown under red light. The chloroplast number increases as the cell length increases, therefore the chloroplasts divide in the absence of cell division. Chloroplasts in the basal part of the filamentous protonemal cell migrate gradually toward the cell apex, but there is no large net migration from the tip to the base or vice versa, indicating that chloroplast division takes place in the apical part of the protonemata. Chloroplast number in the apical 100 μm was maintained at about 200 during cell growth at least over eight days. The chloroplasts were either dumbbell- or ellipsoid-shaped. Dumbbell-shaped chloroplasts are abundant everywhere in a protonema, ranging from 30 to 50% of the total chloroplasts. The dumbbell-shaped chloroplasts attached to or very close to the plasma membrane seem to be the ones that are dividing but the dumbbell-shaped ones in the other regions do not divide. These data support the hypothesis that a signal from the plasma membrane induces the dumbbell-shaped chloroplasts to divide.     

Cell Biology and Algal Taxonomy and Evolution Absolute Orientations of Basal Bodies in Green Algae Evaluated by Light Microscopy

Using high magnification Nomarski interference microscopy a series of optical sections has been obtained through flagellated cells of several green algae in an attempt to establish the absolute orientation of their basal bodies. Using this technique we have confirmed that in Spermatozopsis similis basal bodies are displaced into the 1/7 o'clock position, whereas in gametes of Enteromorpha linza, and zoospores of Trebouxia erici basal bodies are displaced into the 11/5 o'clock position. In addition we report for the first time that in zoospores of Microthamnion kuetzingianum basal bodies are also displaced into the 11/5 o'clock position. Basal body absolute orientations can thus be determined by light microscopy and do not require serial section electron microscopy. The method should be useful for class-level recognition of unknown green algae at the light microscope level.     

Antigenic mapping of a human λ light chain: Correlation with three dimensional structure

Although the amino acid sequence and three-dimensional structure of human immunoglobulin light chains have been known for more than 15 years, the location of antigenic markers characteristic of chains has not been determined. Here, we use a set of synthetic overlapping peptides to completely model the sequence of the chain Mcg and test these for the binding or rabbit and goat antisera specific for chain determinants. We assess peptide contributions to -antigenic reactivity and also to identify a portion of C-region where conformational factors contribute to the antigenicity. Specific determinants occur both in the constant and variable (first and third framework) domains of the molecule. The fourth framework of the variable region, a segment specified by the joining gene, is also recognized and cross-reacts antigenically with the homologous region of T cell receptor chains. Major specific determinants are localized in the N- and C-terminal segments, which are linear and devoid of major conformational folding. Other segments that are strongly antigenic, such as the third framework of the V region (residue 78–93) and a segment of the constant region (residues 177–192), show strong conformational dependence in antigenicity.     

A hypothalamic activator of calmodulin-dependent enzymes is thymosin β4 (1–39)

A new class of stimulators of basal activity of a number of calmodulin-dependent enzymes have been previously isolated from bovine hypothalamus. One of these stimulators, denoted as C₃, has been purified to homogeneity by reverse phase HPLC and tentatively identified as thymosin ₄ (1–39) by mass spectrometry and Edman microsequence analysis. The stimulating effect of C₃ on rabbit skeletal muscle MLCK basal activity was compared with that of thymosin ₁ and thymosin ₄ (16–38). Evidence is presented that all the indicated compounds are Ca²⁺-independent high-affinity MLCK stimulators. The potency of the stimulators in activating the enzyme was: C₃>₄>(CaM+Ca²⁺>₁.This revised version was published online in June 2005 with corrections to the author name Gurvits.     

A nomenclature for the genes encoding the chlorophylla/b-binding proteins of higher plants

We propose a nomenclature for the genes encoding the chlorophylla/b-binding proteins of the light-harvesting complexes of photosystem I and II. The genes encoding LHC I and LHC II polypeptides are namedLhca1 throughLhca4 andLhcb1 throughLhcb6, respectively. The proposal follows the general format recommended by the Commision on Plant Gene Nomenclature. We also present a table for the conversion of old gene names to the new nomenclature.     

DNA sequence changes in mutations induced by ultraviolet light in the gpt gene on the chromosome of Escherichia coli uvr+ and urvA cells

Summary Sequence changes in mutations induced by ultraviolet light are reported for the chromosomal Escherichia coli gpt gene in almost isogenic E. coli uvr ⁺ and excision-deficient uvrA cells. Differences between the mutagenic spectra are ascribed to preferential removal of photoproducts in the transcribed strand by excision repair in uvr ⁺ cells. This conclusion is confirmed by analysis of published results for genes in both uvr ⁺ and uvr ^– cells, showing a similar selective removal of mutagenic products from the transcribed strand of the E. coli lacI gene and of the lambda phage cl repressor gene. Comparison of these data with published results for ultraviolet mutagenesis of gpt on a chromosome in Chinese hamster ovary cells showed that a mutagenic hot spot in mammalian cells is not present in E. coli; the possibility is suggested that the hot spot might arise from localized lack of excision repair. Otherwise, mutagenesis in hamster cells appeared similar to that in E. coli uvr ⁺ cells, except there appears to be a smaller fraction of single-base additions and deletions (frameshifts) in mammalian than in bacterial cells. Phenotypes of 6-thioguanine-resistant E. coli showed there is a gene (or genes) other than gpt involved in the utilization of thioguanine by bacteria.