Ikuko Iwasaki

Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO2 conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO2 conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO2 and/or HCO3− which increased the internal DIC concentration. The feature is referred to as the ‘CO2-concentrating mechanism (CCM)’. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO2 and/or HCO3−, depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO3− transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric β-type of CA composed of two internally repeating structures. An increase in the CO2 concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO2 concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO2 concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H+-transport in extremely high-CO2 conditions. This same state transition has also been observed when high-CO2 cells were transferred to low CO2 conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO2 conditions.

Role of the Aquaporin PIP1 Subfamily in the Chilling Tolerance of Rice

Although an association between chilling tolerance and aquaporins has been reported, the exact mechanisms involved in this relationship remain unclear. We compared the expression profiles of aquaporin genes between a chilling-tolerant and a low temperature-sensitive rice variety using real-time PCR and identified seven genes that closely correlated with chilling tolerance. Chemical treatment experiments, by which rice plants were induced to lose their chilling tolerance, implicated the PIP1 (plasma membrane intrinsic protein 1) subfamily member genes in chilling tolerance. Of these members, changes in expression of the OsPIP1;3 gene suggested this to be the most closely related to chilling tolerance. Although OsPIP1;3 showed a much lower water permeability than members of the OsPIP2 family, OsPIP1;3 enhanced the water permeability of OsPIP2;2 and OsPIP2;4 when co-expressed with either of these proteins in oocytes. Transgenic rice plants (OE1) overexpressing OsPIP1;3 showed an enhanced level of chilling tolerance and the ability to maintain high OsPIP1;3 expression levels under low temperature treatment, similar to that of chilling-tolerant rice plants. We assume that OsPIP1;3, constitutively overexpressed in the leaf and root of transgenic OE1 plants, interacts with members of the OsPIP2 subfamily, thereby improving the plants water balance under low temperatures and resulting in the observed chilling tolerance of the plants.

Aquaporin OsPIP1;1 promotes rice salt resistance and seed germination

OsPIP1;1 is one of the most abundant aquaporins in rice leaves and roots and is highly responsible to environmental stresses. However, its biochemical and physiological functions are still largely unknown. The oocyte assay data showed OsPIP1;1 had lower water channel activity in contrast to OsPIP2;1. EGFP and immunoelectron microscopy studies revealed OsPIP1;1 was predominantly localized in not only plasma membrane but also in some ER-like intracellular compartments in the cells. OsPIP1;1 exhibited low water channel activity in Xenopus oocytes but coexpression of OsPIP2;1 significantly enhanced its water permeability. Stop-flow assay indicated that 10His-OsPIP1;1-reconstituted proteoliposomes had significantly higher water permeability than the control liposomes. Overexpression of OsPIP1;1 greatly altered many physiological features of transgenic plants in a dosage-dependent manner. Moderate expression of OsPIP1;1 increased rice seed yield, salt resistance, root hydraulic conductivity, and seed germination rate. This work suggests OsPIP1;1 functions as an active water channel and plays important physiological roles.

Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy

The mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715-740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns(-1) or 55.0 ns(-1) energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3-12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens.

Water and CO2 permeability of SsAqpZ, the cyanobacterium Synechococcus sp. PCC7942 aquaporin

Cyanobacteria possess Aquaporin‐Z (AqpZ) membrane channels which have been suggested to mediate the water efflux underlying osmostress‐inducible gene expression and to be essential for glucose metabolism under photomixotrophic growth. However, preliminary observations suggest that the biophy‐sical properties of transport and physiological meaning of AqpZ in such photosynthetic microorganisms are not yet completely assessed.

New Compounds Induce Brassinosteroid Deficient-like Phenotypes in Rice.

Brassinosteroids (BRs) are steroidal plant hormones with potent plant growth promoting activity. Because BR-deficient mutants of rice exhibit altered plant architecture and important agronomic traits, we conducted a systemic search for specific inhibitors of BR biosynthesis to manipulate the BR levels in plant tissues. Although previous studies have been conducted with BR biosynthesis inhibitors in dicots, little is known regarding the effects of BR biosynthesis inhibition in monocot plants. In this work, we used potent inhibitors of BR biosynthesis in Arabidopsis, and we performed a hydroponic culture of rice seedlings to evaluate the effects of BR biosynthesis inhibition. Among the test compounds, we found that 1-[[2-(4-Chlorophenyl)-4-(phenoxymethyl)-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole (1) is a potent inhibitor that could induce phenotypes in rice seedlings that were similar to those observed in brassinosteroid deficient plants. The IC50 value for the retardation of plant growth in rice seedlings was approximately 1.27 ± 0.43 μM. The IC50 value for reducing the bending angle of the lamina joint was approximately 0.55 ± 0.15 μM.

Drought-Induced Ultra-Fast Fluorescence Quenching in Photosystem II in Lichens Revealed by Picosecond Time-Resolved Fluorescence Spectrophotometry

Lichens survive under the extreme drought environments. It has been suggested that dried lichens convert excess light energy into heat by unknown mechanism to prevent the accumulation of harmful photoproducts. We studied 18 lichen species by their steady-state fluorescence spectra, PAM and picosecond time-resolved fluorescence decay profiles at 4–300 K. Quantitative analyses of the decay profiles were applied. We obtained the following results: (1) All dried lichens showed a low intensity of PS II fluorescence; (2) the picosecond decays of PS II fluorescence were fast (<10 ps) in most dry lichens; (3) the excitation energy transfer from LHC II to CP43/CP47 was still active; (4) the lifetime of the PS I fluorescence was little affected; (5) the changes were fully reversed within 1 min after the re-hydration; and (6) some lichens showed no fast decay of PS II fluorescence. We noticed two different types of drought-induced energy dissipation mechanisms: Most of lichens dissipated almost all the excitation energy in a few picoseconds by an unknown quencher; some lichens decreased the antenna size of PS II by the state transition mechanism. The new type of the quencher found in this study seems to be situated in the core antenna, and is different from the well-known non-photochemical quenching mechanism.