The Arabidopsis cryptochrome 2 I404F mutant is hypersensitive and shows flavin reduction even in the absence of light

The cryptochrome photoreceptor mutant cry2I404F exhibits hyperactivity in the dark, hypersensitivity in different light conditions, and in contrast to the wild-type protein, its flavin chromophore is reducible even in the absence of light. Plant cryptochromes (cry) are blue-light photoreceptors involved in multiple signaling pathways and various photomorphogenic responses. One biologically hyperactive mutant of a plant cryptochrome that was previously characterized is Arabidopsis cry1L407F (Exner et al. in Plant Physiol 154:1633-1645, 2010). Protein sequence alignments of different cryptochromes revealed that L407 in cry1 corresponds to I404 in cry2. Point mutation of Ile to Phe in cry2 in this position created a novel mutant. The present study provided a baseline data on the elucidation of the properties of cry2I404F. This mutant was still able to bind ATP-triggering conformational changes, as confirmed by partial tryptic digestion and thermo-FAD assays. Surprisingly, the FAD cofactor of cry2I404F was reduced by the addition of reductant even in the absence of light. In vivo, cry2I404F exhibited a cop phenotype in the dark and hypersensitivity to various light conditions compared to cry2 wild type. Overall, these data suggest that the hypersensitivity to red and blue light and hyperactivity of this novel mutant in the dark can be mostly accounted to structural alterations brought forth by the Ile to Phe mutation at position 404 that allows reduction of the flavin chromophore even in the absence of light.

Seed pre-treatment in rice reduces damage, enhances carbohydrate mobilization and improves emergence and seedling establishment under flooded conditions

BACKGROUND AND AIMS

Early flooding helps control weeds but reduces seedling establishment in direct-seeded rice (Oryza sativa). When combined with appropriate management practices, the use of genotypes that better tolerate flooding during emergence can enhance crop establishment in flood-prone areas. Management options include seed pre-treatment and we tested the influence of pre-soaking for 24 h prior to sowing or of priming (soaking for 24 or 48 h followed by drying).

METHODOLOGY

The effects on seedling establishment after 21-day flooding of pre-soaking seeds for 24 h before sowing and/or of priming seeds were examined together with physiological responses connected with reactive oxygen scavenging. Seeds of four lines with contrasting abilities to tolerate flooding at the germination stage were compared. Seeds were primed using KCl solutions (48 h) or water (24 h) and pre-soaked using water. Lipid peroxidation and activities of reactive oxygen-scavenging enzymes were measured in seeds before sowing. Carbohydrate mobilization in germinating seeds and seedling growth were also monitored at intervals.

PRINCIPAL RESULTS

Seed pre-treatment by pre-soaking or by priming increased survival of flooding and accelerated and improved seedling establishment, especially in tolerant genotypes. Primed seeds had less lipid peroxidation and higher superoxide dismutase (SOD) and catalase (CAT) activities than non-primed seeds. Amylase activities and starch breakdown were also hastened in primed seeds. Survival after flooding was positively correlated with amylase activity but negatively correlated with the extent of lipid peroxidation.

CONCLUSIONS

Pre-soaking and priming improved seedling establishment in flooded soil, enhanced the capacity to scavenge reactive oxygen species in seeds by increasing SOD and CAT activities, and hastened carbohydrate mobilization. Tolerant genotypes responded better to these treatments, emphasizing the effectiveness of combining genetic tolerance with appropriate seed pre-treatment to improve seedling establishment of rice sown in flooded soils.

Photosynthesis of Prochloron as affected by environmental factors

The effects of light intensity, pH, temperature, and UV irradiation on the photosynthetic rate of Prochloron isolated from the ascidian host Lissoclinum patella, collected from Palau, were examined. Photosynthesis increased with light intensity with saturation at 500 micromol/m(2) per second. It was maximum at pH 8 to 9 but almost completely suppressed below pH 7. The optimum temperature was 35 degrees to 40 degrees C, but the photosynthesis was absent at < or =20 degrees C and at 45 degrees C. It was recovered when the symbiont was transferred from 1 hour of incubation at < or =20 degrees C to 35 degrees C but not when transferred from incubation at 45 degrees C. Ultraviolet irradiation severely inhibited the photosynthesis of Prochloron in isolation but not in vivo. This protection was brought about by the tunic covering the ascidian colony, which contains UV-absorbing mycosporine-like amino acids. These results indicate that the characteristic condition of the tropical marine environment largely determines the ecological distribution of Prochloron, and the ascidian tunic protects the organism from UV radiation.

Light-Induced Carbonic Anhydrase Expression in Chlamydomonas reinhardtii

The effect of external inorganic carbon concentration and light on carbonic anhydrase (CA) protein accumulation and steady-state mRNA levels were examined in Chlamydomonas reinhardtii. When photoautotrophically grown high-CO(2) cells were transferred to low-CO(2) conditions, they exhibited a significant accumulation of the 2.0-kilobase CA transcript after 1 hour with the maximum level reached after 2 hours. An increase in the accumulation of the 37-kilodalton CA monomer was observed after 2-hour exposure to air. Cells allowed to adapt to air levels of CO(2) in the dark showed neither an increase in CA mRNA abundance nor in the accumulation of the enzyme. Similarly, addition of 10 micromole 3-(3,4-dichlorophenyl)-1, 1-dimethylurea immediately after transferring high-CO(2) cells to low-CO(2) condition did not cause an increase in CA transcript abundance and enzyme accumulation, suggesting that photosynthesis is absolutely required in the regulation of CA transcript abundance. In addition to the photosynthesis-dependent process, a blue light stimulated mechanism is also involved in CA transcript regulation. Experiments with transcription inhibitors confirmed the notion that the gene for carbonic anhydrase enzyme is encoded by the nuclear genome and that the induction of the enzyme depends heavily on the transcription of the CA gene.

cDNA cloning, sequence, and expression of carbonic anhydrase in Chlamydomonas reinhardtii: regulation by environmental CO2 concentration

cDNA clones for the periplasmic carbonic anhydrase (CA; carbonate hydro-lyase, EC 4.2.1.1) of Chlamydomonas reinhardtii cells were isolated and characterized. The fact that the cloned cDNA fragments encoded a 377-amino acid polypeptide (41,626 Da) consisting of an NH2-terminal hydrophobic signal peptide of 20 amino acids, a large (35,603 Da) subunit polypeptide, and a small (4144 Da) subunit polypeptide indicates that the two subunits are cotranslated as a precursor polypeptide. The amino acid sequence of mature subunits deduced from the nucleotide sequence showed 20-22% homology with human CA isozymes (CAI, CAII, and CAIII). Three zinc-liganded histidine residues and those forming a hydrogen-bond network to zinc-bound solvent molecules were highly conserved in the plant and animal enzymes. Three possible asparagine-linked glycosylation sites were found in the large subunit. Northern blot analysis was carried out using the cDNA fragment as a probe. The level of 2.0-kilobase CA mRNA increased within 1 hr when CO2 concentration of the bubbling gas was changed from 5% to 0.04% (air level) in the presence of light. On the other hand, CA mRNA did not accumulate when CO2 concentration was lowered in the dark. Experiments using 3-(3,4-dichlorophenyl)-1,1-dimethylurea showed that photosynthesis is absolutely required for the accumulation of CA mRNA. These results indicate that CA biosynthesis is regulated by changes in environmental CO2 concentration as well as light at the level of mRNA abundance.