Photosynthesis Research (v.96, #2)

In plants, chlorophylls (chlorophyll a and chlorophyll b) are the most abundant tetrapyrrole molecules and are essential for photosynthesis. The first committed step of chlorophyll biosynthesis is the insertion of Mg2+ into protoporphyrin IX, and thus subsequent steps of the biosynthesis are called the Mg branch. As the Mg branch in higher plants is complex, it was not until the last decade—after many years of intensive research—that most of the genes encoding the enzymes for the pathway were identified. Biochemical and molecular genetic analyses have certainly modified the classic metabolic map of tetrapyrrole biosynthesis, and only recently have the molecular mechanisms of regulatory pathways governing chlorophyll metabolism been elucidated. As a result, novel functions of tetrapyrroles and biosynthetic enzymes have been proposed. In this review, I summarize the recent findings on enzymes involved in the Mg branch, mainly in higher plants.
Keywords: Chlorophyll; Chloroplast; Mg branch; Tetrapyrrole; Photosynthesis

Fine tuning of the spectral properties of LH2 by single amino acid residues by Martina V. Silber; Günther Gabriel; Brigitte Strohmann; Adela Garcia-Martin; Bruno Robert; Paula Braun (145-151).
The peripheral light-harvesting complex, LH2, of Rhodobacter sphaeroides consists of an assembly of membrane-spanning α and β polypeptides which assemble the photoactive bacteriochlorophyll and carotenoid molecules. In this study we systematically investigated bacteriochlorophyll-protein interactions and their effect on functional bacteriochlorophyll assembly by site-directed mutations of the LH2 α-subunit. The amino acid residues, isoleucine at position −1 and serine at position −4 were replaced by 12 and 13 other residues, respectively. All residues replacing isoleucine at position −1 supported the functional assembly of LH2. The replacement of isoleucine by glycine, glutamine or asparagine, however, produced LH2 complex with significantly altered spectral properties in comparison to LH2 WT. As indicated by resonance Raman spectroscopy extensive rearrangement of the bacteriochlorophyll-B850 macrocycle(s) took place in LH2 in which isoleucine −1 was replaced by glycine. The replacement results in disruption of the H-bond between the C3 acetyl groups and the aromatic residues +13/+14 without affecting the H-bond involving the C131 keto group. In contrast, nearly all amino acid replacements of serine at position −4 resulted in shifting of the bacteriochlorophyll-B850 red most absorption maximum. Interestingly, the extent of shifting closely correlated with the volume of the residue at position −4. These results illustrate that fine tuning of the spectral properties of the bacteriochlorophyll-B850 molecules depend on their packing with single amino acid residues at distinct positions.
Keywords: Rhodobacter sphaeroides ; Light-harvesting complex; Site-directed mutation; In situ optical absorption spectra; Spectral fine tuning

The chloroplast ATP synthase gates the flow of protons out of the thylakoid lumen. In Chlamydomonas reinhardtii deletion of any of the genes for the ATP synthase subunits, or misfolding of the peptides results in photosynthetic membranes devoid of the enzyme (Lemaire and Wollman, J Biol Chem 264:675–685, 1989). This work examines the physiologic response of an algal strain in which the epsilon subunit of the chloroplast ATP synthase has been truncated. Removal of 10 amino acids from the C-terminus of the peptide results in a sharp decrease in the content of the enzyme, but does not result in its exclusion from the thylakoid membranes. The ATP synthase of this mutant strain has a higher rate of ATP hydrolysis than the wild-type enzyme. This strain of C. reinhardtii exhibits reduced growth in the light, dependence on acetate, and a low threshold for the onset of photoinhibition. The role of the ATP synthase in regulating the proton concentration of the lumen is discussed.
Keywords: ATP synthase; CF1; Chlamydomonas ; Thylakoid

Photosynthetic limitation by CO2 diffusion in drought stressed orange leaves on three rootstocks by Norma de Magalhães Erismann; Eduardo Caruso Machado; Maria Luiza Sant’ Anna Tucci (163-172).
Photosynthetic limitations under moderate water deficit were evaluated in ‘Valência’ orange trees grafted on three different rootstocks, in pots. Net CO2 assimilation rate (A N), stomatal conductance (g s), and photosystem II (PS II) operating efficiency ( $$ {F}ifmmode{'}else$'$fi_{{ ext{q}}} /{F}ifmmode{'}else$'$fi_{{ ext{m}}} $$ ) in response to changing intercellular CO2 partial pressure (C i) were analyzed under controlled conditions. Drought decreased A N and g s, whereas $$ {F}ifmmode{'}else$'$fi_{{ ext{q}}} /{F}ifmmode{'}else$'$fi_{{ ext{m}}} $$ remained unchanged. This resulted in a higher ratio between electron transport rate (ETR) and gross CO2 assimilation rate (A G). Since the comparison of A NC i gas exchange curves can lead to incorrect conclusions, a normalization of C i values ( $$ C_{{{ ext{i}}_{{ ext{f}}} }} $$ ) of stressed leaves was applied. Then, the relationship established for irrigated trees between the ETR/A G ratio and C i was used to estimate the $$ C_{{{ ext{i}}_{{ ext{f}}} }} $$ from ETR/A G ratios measured under water stress. The response of A N to $$ C_{{{ ext{i}}_{{ ext{f}}} }} $$ suggests that the CO2 diffusional restriction is the main factor that limits photosynthesis in orange leaves under moderate water deficit.
Keywords: Chlorophyll fluorescence; Citrus; Diffusional limitation; Photosynthesis; Water stress

A new monitoring PAM fluorometer (MONI-PAM) to study the short- and long-term acclimation of photosystem II in field conditions by Albert Porcar-Castell; Erhard Pfündel; Janne F. J. Korhonen; Eija Juurola (173-179).
We present and evaluate the performance of a new field monitoring PAM fluorometer (MONI-PAM) which is intended for short- and long-term monitoring of the acclimation of photosystem II (PSII). The instrument measures chlorophyll fluorescence, photosynthetic photon flux density (PPFD), and temperature in the field, and monitors exactly the same leaf area over prolonged periods of time, facilitating the estimation of both rapidly reversible and sustained non-photochemical quenching (NPQ). The MONI-PAM performance is evaluated in the lab and under natural conditions in a Scots pine canopy during spring recovery of photosynthesis. The instrument provides a new tool to study in detail the acclimation of PSII to the environment under natural field conditions.
Keywords: Chlorophyll fluorescence; Diurnal acclimation; Scots pine; Seasonal acclimation

Honoring Andrew Benson in Paris by Hartmut K. Lichtenthaler; Bob B. Buchanan; Roland Douce (181-183).