The involvement of HSP70B in the photosystem II damage and repair process in Dunaliella salina was investigated. A full-length cDNA of the D. salina hsp70B gene was cloned and sequenced. Expression patterns of the hsp70B gene were investigated upon shifting a D. salina culture from low-light to high-light growth conditions, designed to significantly accelerate the rate of PSII photodamage. Northern blot analyses and nuclear run-on transcription assays revealed a significant but transient induction of hsp70B gene transcription, followed by a subsequent increase in HSP70B protein synthesis and accumulation. Mild detergent solubilization of photoinhibited thylakoid membranes, in which photodamaged PSII centers had accumulated, followed by native gel electrophoresis revealed the formation of a 320 kDa protein complex that contained, in addition to the HSP70B, the photodamaged but as yet undegraded D1 protein as well as D2 and CP47. Evidence suggested that the 320 kDa complex is a transiently forming PSII repair intermediate. Denaturing solubilization of the 320 kDa PSII repair intermediate by SDS-urea resulted in cross-linking of its polypeptide constituents, yielding a 160 kDa protein complex. The role of the HSP70B in the repair of photodamaged PSII centers, e.g. in stabilizing the disassembled PSII-core complex and in facilitating the D1 degradation and replacement process, is discussed.

Genomic, proteomic, phylogenetic and evolutionary aspects of a novel gene encoding a putative chloroplast-targeted sulfate permease of prokaryotic origin in the green alga Chlamydomonas reinhardtii are described. This nuclear-encoded sulfate permease gene (SulP) contains four introns, whereas all other known chloroplast sulfate permease genes lack introns and are encoded by the chloroplast genome. The deduced amino acid sequence of the protein showed an extended N-terminus, which includes a putative chloroplast transit peptide. The mature protein contains seven transmembrane domains and two large hydrophilic loops. This novel prokaryotic-origin gene probably migrated from the chloroplast to the nuclear genome during evolution of C. reinhardtii. The SulP gene, or any of its homologues, has not been retained in vascular plants, e.g. Arabidopsis thaliana, although it is encountered in the chloroplast genome of a liverwort (Marchantia polymorpha). A comparative structural analysis and phylogenetic origin of chloroplast sulfate permeases in a variety of species is presented.

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The Dunaliella salina photosynthetic apparatus organization and function was investigated in wild type (WT) and a mutant (zea1) lacking all ␤,␤-epoxycarotenoids derived from zeaxanthin (Z). The zea1 mutant lacked antheraxanthin, violaxanthin, and neoxanthin from its thylakoid membranes but constitutively accumulated Z instead. It also lacked the so-called xanthophyll cycle, which, upon irradiance stress, reversibly converts violaxanthin to Z via a de-epoxidation reaction. Despite the pronounced difference observed in the composition of ␤,␤-epoxycarotenoids between WT and zea1, no discernible difference could be observed between the two strains in terms of growth, photosynthesis, organization of the photosynthetic apparatus, photo-acclimation, sensitivity to photodamage, or recovery from photo-inhibition. WT and zea1 were probed for the above parameters over a broad range of growth irradiance and upon light shift experiments (low light to high light shift and vice versa). A constitutive accumulation of Z in the zea1 strain did not affect the acclimation of the photosynthetic apparatus to irradiance, as evidenced by indistinguishable irradiance-dependent adjustments in the chlorophyll antenna size and photosystem content of WT and zea1 strain. In addition, a constitutive accumulation of Z in the zea1 strain did not affect rates of photodamage or the recovery of the photosynthetic apparatus from photo-inhibition. However, Z in the WT accumulated in parallel with the accumulation of photodamaged PSII centers in the chloroplast thylakoids and decayed in tandem with a chloroplast recovery from photo-inhibition. These results suggest a role for Z in the protection of photodamaged and disassembled PSII reaction centers, apparently needed while PSII is in the process of degradation and replacement of the D1/32-kD reaction center protein.Organisms of oxygenic photosynthesis convert the energy of sunlight into chemical energy, which supports most life on earth. In photosynthetic membranes of green algae and plants, incident irradiance is absorbed by chlorophyll (Chl)-binding lightharvesting antenna complexes (LHCs) associated with the reaction centers of PSII and PSI. However, when the photosynthetic apparatus absorbs irradiance in excess of that required for the saturation of photosynthesis, singlet oxygen is generated, and PSII is subject to an irreversible photooxidative damage (Vass et al., 1992; Telfer et al., 1994; Melis, 1999). This photodamage selectively impairs the function of the D1/32-kD reaction center protein of PSII and has the potential to lower rates of photosynthesis and diminish plant growth and productivity (Powles and Critchley, 1980; Powles, 1984). The probability of photooxidative damage in chloroplasts depends on the oxidation reduction state of the primary electron-accepting plastoquinone of PSII (Q A ), which is the parameter that controls photodamage under a variety of physiological and environmental conditions. When Q A is oxidized under continuous illumination, photochemical electron transport...

Tuberization in cassava (Manihot esculenta Crantz) occurs simultaneously with plant development, suggesting competition of photoassimilate partitioning between the shoot and the root organs. In potato, which is the most widely studied tuber crop, there is ample evidence suggesting that metabolism and regulatory processes in leaf may have an impact on tuber formation. To search for leaf proteins putatively involved in regulating tuber generation and/or development in cassava, comparative proteomic approaches have been applied to monitor differentially expressed leaf proteins during root transition from fibrous to tuberous. Stringent cross comparison and statistical analysis between two groups with different plant ages using Student's t test with 95% significance level revealed a number of protein spots whose abundance were significantly altered (P < 0.05) during week 4 to week 8 of growth. Of these, 39 spots were successfully identified by ion trap LC-MS/MS. The proteins span various functional categories from antioxidant and defense, carbohydrate metabolism, cyanogenesis, energy metabolism, miscellaneous and unknown proteins. Results suggested possible metabolic switches in the leaf that may trigger/regulate storage root initiation and growth. This study provides a basis for further functional characterization of differentially expressed leaf proteins, which can help understand how biochemical processes in cassava leaves may be involved in storage root development.