Thomas G. Owens

Grass roots chemistry: meta-Tyrosine, an herbicidal nonprotein amino acid

Fine fescue grasses displace neighboring plants by depositing large quantities of an aqueous phytotoxic root exudate in the soil rhizosphere. Via activity-guided fractionation, we have isolated and identified the nonprotein amino acid m-tyrosine as the major active component. m-Tyrosine is significantly more phytotoxic than its structural isomers o- and p-tyrosine. We show that m-tyrosine exposure results in growth inhibition for a wide range of plant species and propose that the release of this nonprotein amino acid interferes with root development of competing plants. Acid hydrolysis of total root protein from Arabidopsis thaliana showed incorporation of m-tyrosine, suggesting this as a possible mechanism of phytotoxicity. m-Tyrosine inhibition of A. thaliana root growth is counteracted by exogenous addition of protein amino acids, with phenylalanine having the most significant effect. The discovery of m-tyrosine, as well as a further understanding of its mode(s) of action, could lead to the development of biorational approaches to weed control.

Processing of Excitation Energy by Antenna Pigments

Absorption and transduction of light by photosynthetic organisms provides the principal energy source for all living organisms. At the same time, absorption of excess light (light in excess of the capacity of the organism to use the energy to drive photosynthesis) represents a primary site of environmental injury. Recent studies have shown that photosynthetic organisms have the ability to regulate the utilization of absorbed light energy through a group of related processes commonly called non-photochemical quenching. These process dissipate excess absorbed energy as heat. In order to remain competitive, photosynthetic organisms must seek out the delicate balance between efficient light-harvesting under limiting light conditions and regulated dissipation of energy under excess light conditions. Excess light absorption may occur as the result of increased incident intensity or a decrease in the rate of photosynthesis due to other environmental stresses. The underlying reactions of non-photochemical quenching may occur in the antennae, the reaction centers, or both, and are not well understood. Independent of the quenching site, the reactions of non-photochemical quenching must cooperate and compete with those of normal light-harvesting. Here, the proposed mechanisms of non-photochemical quenching and the common energy transfer reactions affecting both light-harvesting and non-photochemical quenching are examined in order to provide a more general framework in which the utilization of light energy can be described.

Manipulation of Root Hair Development and Sorgoleone Production in Sorghum Seedlings

Sorghum (Sorghum bicolor) roots exude a potent bioherbicide—sorgoleone. Previous work indicates that sorgoleone is produced in living root hairs. We have developed a mist system that resulted in abundant production of root hairs exuding sorgoleone and a mat system that significantly inhibited root hair development and consequently sorgoleone production. Applying Ag+ (an ethylene action inhibitor) at 1.2 mM to the seedlings grown in the mist system also inhibited root hair formation and elongation. Hypoxic conditions in the mist system did not result in the inhibition of root hair growth as compared to the standard air atmosphere (20.8% O2). Applying ethephon (an ethylene-releasing agent) at 0.031 mM to the roots of seedlings grown in the mat system with water running at 1 ml/min reversed the inhibition of root hair development by water movement. These results indicate that either water movement or ethylene can be utilized to manipulate root hair development and sorgoleone production in sorghum seedlings. It is hypothesized that water movement reduced the local ethylene concentration on the root surface and consequently inhibited root hair development of sorghum seedlings grown in the mat system.

Carotenoids in photosynthesis: structure and photochemistry

Carotenoids from phototrophic bacteria cany out light-harvesting in antenna proteins via carotenoid-to-bacteriochlorophyll singlet-singlet energy transfer and photoprotection in the reaction center via bacteriochlorophyll-to-carotenoid triplet-triplet energy transfer. Spectroscopic studies have permitted elucidation of the explicit routes of these transfers in pigment-protein complexes obtained from the bacterium Rhodobacter sphaeroides. The molecular details of these mechanisms are presented and discussed in conjunction with studies revealing the structural features of the complexes.

Excitation transport and trapping on spectrally disordered lattices

It is widely assumed that the decay of fluorescence in photosynthetic systems can be described as a sum of exponential components and that the amplitude of each component is directly related to the absorption cross-section of the antenna pigments coupled to the fluorescing species. We present exact calculations of excited state decay in two-dimensional regular lattices of different geometries containing multiple spectral forms of antenna pigments. We illustrate by these calculations that there is no simple relation between the decay amplitudes (and resulting time-resolved excitation spectra) and the steady-state absorption spectra. Only in the limit that the electronic excitations reach a rapid equilibrium among all antenna spectral forms does the excitation spectrum depend uniquely on the spectral features of the array. Using the simulations in conjunction with our recent fluorescence studies, we examine excitation transport and trapping dynamics in photosystem I and the limitations imposed by the finite time resolution in single photon counting experiments. In particular, we show that rising components, associated with excitation transfer among different spectral forms, with lifetimes <20 ps would be undetected in a typical photon counting experiment.

Expression of complementary RNA from chloroplast transgenes affects editing efficiency of transgene and endogenous chloroplast transcripts

The expression of angiosperm chloroplast genes is modified by C-to-U RNA editing. The mechanism for recognition of the ∼30 C targets of editing is not understood. There is no single consensus sequence surrounding editing sites, though sites can be grouped into small ‘clusters’ of two to five sites exhibiting some sequence similarity. While complementary RNA that guides nucleotides for alteration has been detected in other RNA modification systems, it is not known whether complementary RNA is involved in chloroplast editing site recognition. We investigated the effect of expressing RNA antisense to the sequences −20 to +6 surrounding the RpoB-2 C target of editing, which is a member of a cluster that includes the PsbL-1 and Rps14-1 sites. Previous experiments had shown that chloroplast rpoB transgene transcripts carrying only these 27 nt were edited in vivo at the proper C. Though transcripts carrying sequences −31 to +60 surrounding the RpoB-2 sites were edited in chloroplast transgenic plants, transcripts carrying the −31 to +62 region followed by the 27 nt complementary region were not edited at all. In contrast, a similar construct, in which the C target as well as the preceding and subsequent nucleotides were mismatched within the 27 nt region, was efficiently edited. The presence of any of the four transgenes carrying RpoB-2 sequences in sense and/or antisense orientation resulted in reduced editing at the PsbL-1 site. Chloroplast transgenic plants expressing the three different antisense RNA constructs exhibited abnormal growth and development, though plants expressing the 92 nt sense transcripts were phenotypically normal.

Anatomical, biochemical, and photosynthetic responses to recent allopolyploidy in Glycine dolichocarpa (Fabaceae)

PREMISE OF THE STUDY Previous studies have shown that polyploidy has pronounced effects on photosynthesis. Most of these studies have focused on synthetic or recently formed autopolyploids, and comparatively little is known about the integrated effects of natural allopolyploidy, which involves hybridity and genome doubling and often incorporates multiple genotypes through recurrent origins and lineage recombination. METHODS Glycine dolichocarpa (designated T2) is a natural allotetraploid with multiple origins. We quantified 21 anatomical, biochemical, and physiological phenotypes relating to photosynthesis in T2 and its diploid progenitors, G. tomentella (D3) and G. syndetika (D4). To assess how direction of cross affects these phenotypes, we included three T2 accessions having D3-like plastids (T2(D3)) and two accessions having D4-like plastids (T2(D4)). KEY RESULTS T2 accessions were transgressive (more extreme than any diploid accession) for 17 of 21 phenotypes, and species means differed significantly in T2 vs. both progenitors for four of 21 phenotypes (higher for guard cell length, electron transport capacity [J(max)] per palisade cell, and J(max) per mesophyll cell; lower for palisade cells per unit leaf area). Within T2, four of 21 parameters differed significantly between T2(D3) and T2(D4) (palisade cell volume; chloroplast number and volume per unit leaf area; and J(max) per unit leaf area). CONCLUSIONS T2 is characterized by transgressive photosynthesis-related phenotypes (including an ca. 2-fold increase in J(max) per cell), as well as by significant intraspecies variation correlating with plastid type. These data indicate prominent roles for both nucleotypic effects and cytoplasmic factors in photosynthetic responses to allopolyploidy.

SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF BOROHYDRIDE-TREATED D1-D2-CYTOCHROME B-559 COMPLEX OF PHOTOSYSTEM II

The D1‐D2‐cytochrome b‐559 reaction center complex of photosystem II with an altered pigment composition was prepared from the original complex by treatment with sodium borohydride (BH− 4). The absorption spectra of the modified and original complexes were compared to each other and to the spectra of purified chlorophyll a and pheophytin a (Pheo a) treated with BH− 4 in methanolic solution. The results of these comparisons are consistent with the presence in the modified complex of an irreversibly reduced Pheo a molecule, most likely 131‐deoxo‐131‐hydroxy‐Pheo a, replacing one of the two native Pheo a molecules present in the original complex. Similar to the original preparation, the modified complex was capable of a steady‐state photoaccumulation of Pheo− and P680+. It is concluded that the pheophytin a molecule which undergoes borohydride reduction is not involved in the primary charge separation and seems to represent a previously postulated photochemically inactive Pheo a molecule. The Qy and Qx transitions of this molecule were determined to be located at 5°C at 679.5–680 nm and 542 nm, respectively.

Reaction centers of photosystem II with a chemically-modified pigment composition: exchange of pheophytins with 131-deoxo-131-hydroxy-pheophytin a

Isolated reaction centers of photosystem II with an altered pigment content were obtained by chemical exchange of the native pheophytin a molecules with externally added 131‐deoxo‐131‐hydroxy‐pheophytin a. Judged from a comparison of the absorption spectra and photochemical activities of exchanged and control reaction centers, 70–80% of the pheophytin molecules active in charge separation are replaced by 131‐deoxo‐131‐hydroxy‐pheophytin a after double application of the exchange procedure. The new molecule at the active branch was not active photochemically. This appears to be the first stable preparation in which a redox active chromophore of the reaction center of photosystem II was modified by chemical substitution. The data are compatible with the presence of an active and inactive branch of cofactors, as in bacterial reaction centers. Possible applications of the 131‐deoxo‐131‐hydroxy‐pheophytin a‐exchanged preparation to the spectral and functional analysis of native reaction centers of photosystem II are discussed.

Oxygenic Photoautotrophic Growth Without Photosystem I

Contrary to the prediction of the Z-scheme model of photosynthesis, experiments demonstrated that mutants of Chlamydomonas containing photosystem II (PSII) but lacking photosystem I (PSI) can grow photoautotrophically with O2 evolution, using atmospheric CO2 as the sole carbon source. Autotrophic photosynthesis by PSI-deficient mutants was stable both under anaerobic conditions and in air (21 percent O2) at an actinic intensity of 200 microeinsteins per square meter per second. This PSII photosynthesis, which was sufficient to support cell development and mobility, may also occur in wild-type green algae and higher plants. The mutants can survive under 2000 microeinsteins per square meter per second with air, although they have less resistance to photoinhibition.