Phytochromes in Agrobacterium fabrum

The focus of this review is on the phytochromes Agp1 and Agp2 of Agrobacterium fabrum. These are involved in regulation of conjugation, gene transfer into plants, and other effects. Since crystal structures of both phytochromes are known, the phytochrome system of A. fabrum provides a tool for following the entire signal transduction cascade starting from light induced conformational changes to protein interaction and the triggering of DNA transfer processes.

Two molecular species of phytochrome A with distinct modes of action

Adaptation of plants to environmental light conditions is achieved via operation of a highly complex photoreceptor apparatus. It includes the phytochrome system comprising phytochromes A and B (phyA and phyB) as the major components. phyA differs from phyB by several properties, including its ability to mediate all three photoresponse modes – the very low and low fluence responses (VLFR and LFR respectively) and the high irradiance responses (HIR), whereas phyB is responsible for LFR. This review discusses the uniqueness of phyA in terms of its structural and functional heterogeneity. The photoreceptor is presented in monocots and dicots by two native molecular species, phyAʹ and phyAʹʹ, differing by spectroscopic, photochemical and phenomenological properties. phyA differentiation into substates includes post-translational phosphorylation of a serine residue(s) at the N-terminal extension of the molecule with phyAʹ being the phosphorylated species and phyAʹʹ, dephosphorylated. They differ also by their mode of action, which depends on the cellular context. The current working hypothesis is that phyAʹ mediates VLFR and phyAʹʹ, HIR and LFR. The content and functional activity of the two pools are regulated by light and by phosphatase/kinase equilibrium and pH in darkness, what contributes to the fine-tuning of the phytochrome system. Detection of the native pools of the cryptogamic plant fern Adiantum capillus-veneris phy1 (phy1ʹ and phy1ʹʹ) similar to those of phyA suggests that the structural and functional heterogeneity of phyA is not a unique phenomenon and may have arisen earlier in the molecular evolution of the phytochrome system than the appearance of the angiosperm phytochromes.

Participation of labile and stabile phytochrome in the control of chlorophyll accumulation during the deetiolation of oat seedlings

The effect of the phytochrome system on the accumulation rate of chlorophyll-a and b in 96-hour-old oat seedlings during the first 3 hours of white light action was investigated. It was established that initial irradiation with red light stimulated the accumulation rate of both forms of chlorophyll. This effect depended on the level of the P_FR form of phytochrome obtained during the initial irradiation and it reached the treshold value in the case of about 20% of P_FR in relation to P_R in etiolated seedlings. Far red light stimulated only the accumulation of chlorophyll-a. The effect of red light was reversible if far red light was applied directly after red light. The reversibility diminished gradually together with the extension of the dark period between red and far red light, disappearing completely after 6 hours. The results suggest the participation of two pools of phytochrome - a labile and a stabile one - in the reaction stimulating chlorophyll accumulation. A model of labile phytochrome action through the destruction products of phytochrome is proposed.

The Heme Oxygenase(s)-Phytochrome System ofPseudomonas aeruginosa

For many pathogenic bacteria likePseudomonas aeruginosaheme is an essential source of iron. After uptake, the heme molecule is degraded by heme oxygenases to yield iron, carbon monoxide, and biliverdin. The heme oxygenase PigA is only induced under iron-limiting conditions and produces the unusual biliverdin isomers IXβ and IXδ. The gene for a second putative heme oxygenase inP. aeruginosa,bphO, occurs in an operon with the genebphPencoding a bacterial phytochrome. Here we provide biochemical evidence thatbphOencodes for a second heme oxygenase inP. aeruginosa. HPLC,1H, and13C NMR studies indicate that BphO is a “classic” heme oxygenase in that it produces biliverdin IXα. The data also suggest that the overall fold of BphO is likely to be the same as that reported for other α-hydroxylating heme oxygenases. Recombinant BphO was shown to prefer ferredoxins or ascorbate as a source of reducing equivalentsin vitroand the rate-limiting step for the oxidation of heme to biliverdin is the release of product. In eukaryotes, the release of biliverdin is driven by biliverdin reductase, the subsequent enzyme in heme catabolism. BecauseP. aeruginosalacks a biliverdin reductase homologue, data are presented indicating an involvement of the bacterial phytochrome BphP in biliverdin release from BphO and possibly from PigA.

Neighbour effects on gender variation in Ambrosia artemisiifolia

In this study we tested the prediction that male gender allocation in natural populations of common ragweed (Ambrosia artemisiifolia L.) depends more on relative plant height within the neighbouring canopy than on absolute plant height. This is consistent with the assumption that success as a pollen donor for an anemophilous plant within a crowded population will be greater when neighbours are shorter than when neighbours are taller. Data on height and proximity of neighbours, irradiance, and target plant height, biomass, and gender allocation were collected from two natural populations. In one population, these data were also recorded for a group of target plants that had local neighbours artificially removed when the target plants were seedlings. Allocation to male flowers was most strongly positively correlated with height relative to that of close neighbours and with percent irradiance in natural populations. Numerous all-female plants were recorded among the smallest individuals that were suppressed by a dense overhead canopy of neighbours. No relationships were found between plant size and gender when plants were taller than their neighbours or when neighbours were artificially removed. Hence, previously reported size-dependent gender variation in this species may depend on the presence of neighbours. It is postulated that ragweed individuals may sense the presence of neighbours through the phytochrome system, and that effects of neighbours on light quantity and quality cues a shift to increased female function. However, plants that grew from the seedling stage without neighbours were heavier and more female but were not taller than plants with neighbours left intact. The interpretation of this effect is unclear but may reflect a change in plant architecture corresponding with the removal of neighbours. Key words: gender, plant height, plasticity, pollen dispersal, phytochrome, shading.

COMMUNICATING IN PHOTOMORPHOGENESIS

Description of the light environment used in photomorphogenic research varies greatly among research teams. The environment is often described as the ratio of red (R) to far-red (FR) light, particulary when involvement of the phytochrome system is suspected. There is disagreement in the appropriate center and range of values for each ratio component. Often the center for R is reported as 660 nm. However, in chlorophyll-containing tissue 645 nm may be more appropriate because of the absorption of chlorophyll at 660. Band widths around a selected peak also vary. The widths generally are 10 or 100 nm. Comparison of experiments that describe different peaks or ranges is difficult. Much of the variation in description results from the behavior of phytochrome. Phytochrome has absorption and action spectral peaks, however wavelengths that cause absorption and/or action to a lesser extent may extend more than 50 nm from the peak. Integration formulas such as Pfr/P consider the effects of all wavelengths. However, even the integration formulas do not explain all photomorphogenic responses. A description of the entire photomorphogenic spectrum may be the most appropriate means of communication.