Photosynthetic organisms adapt to changes in light quality by redistributing light excitation energy between two photosystems through state transition. This reorganization of antenna systems leads to an enhanced photosynthetic yield. Using a genetic approach in Chlamydomonas reinhardtii to dissect the signal transduction pathway of state transition, we identified a chloroplast thylakoid-associated serine-threonine protein kinase, Stt7, that has homologs in land plants. Stt7 is required for the phosphorylation of the major light-harvesting protein (LHCII) and for state transition.
The CRR1 (Copper Response Regulator) locus, required for both activating and repressing target genes of a copper-and hypoxiasensing pathway in Chlamydomonas, encodes a 1,232-residue candidate transcription factor with a plant-specific DNA-binding domain named SBP, ankyrin repeats, and a C-terminal Cys-rich region, with similarity to a Drosophila metallothionein. The recombinant SBP domain of Crr1 shows zinc-dependent binding to functionally defined copper-response elements associated with the CYC6 and CPX1 promoters that contain a critical GTAC core sequence. Competition experiments indicate equivalent selectivity for copper-response elements from either promoter and 10-fold greater selectivity for the wild-type sequence vs. a sequence carrying a single mutation in the GTAC core. The SBP domain of Chlamydomonas Crr1 binds also to a related GTAC-containing sequence in the Arabidopsis AP1 promoter that is the binding site of a defining member of the SBP family of DNA-binding proteins. Chlamydomonas Crr1 is most similar to a subset of the Arabidopsis SBP domain proteins, which include SPL1, SPL7, and SPL12. The abundance of the CRR1 mRNA is only marginally copper-responsive, and although two mRNAs that differ with respect to splicing of the first intron are detected, there is no indication that the splicing event is regulated by metal nutrition or hypoxia. It is likely that the dramatic copper-responsive action of Crr1 occurs at the level of the polypeptide.acclimation ͉ copper homeostasis ͉ hypoxia ͉ metal ͉ transcription factor
In Lycopersicon esculentum, two cDNAs encoding glutathione peroxidase (GPX)-like proteins were isolated, sequenced and characterized as putative phospholipid hydroperoxide :glutathione peroxidases. After mechanical stimulation (rubbing) of the plant, each GPX tomato gene was found to behave differently in terms of inducibility and tissue distribution. The accumulation of GPX transcripts suggests that reactive oxygen species are involved in the plant response to a mechanical stress.Keywords : Lycopersicon esculentum; free-radical scavenger; gene expression ; glutathione peroxidase ; mechanical stimulation.The consumption of oxygen in normal respiratory processes ing are expressed in tomato. Two GPX-like cDNAs were cloned and the tissue distribution and accumulation of the correspondand the production of oxygen during photosynthesis results in the generation of toxic reactive oxygen species (ROS) [1,2]. ing mRNAs were investigated. Biotic and abiotic stresses can give rise to further increases in the ROS level [3,4] ; this phenomenon is described as oxidative burst. The primary constituents of protective mechanisms MATERIALS AND METHODS against ROS include enzymes, such as superoxide dismutase, Plant material, culture conditions and stimulation. Culcatalase and peroxidase, and free-radical scavengers, such as ca-ture conditions. Tomato (Lycopersicon esculentulum Mill. cv. rotenoid, tocopherol, ascorbate and glutathione [5]. VFN8) plants were raised from seeds in moist vermiculite in a Glutathione peroxidases (GPX) belong to a family of iso-controlled environment room: 16 h daylight at 60 µmol · m Ϫ2 · zymes that catalyses the reduction of H 2 O 2 , organic hydroper-s Ϫ1 , photosynthetically active radiation provided by 40-W white oxides and lipid hydroperoxides using GSH as a reducing agent daylight tubes (Mazda LDL, TF 40), 23Ϯ1°C (day) and and thus help to protect the cells against oxidative damage [6].19Ϯ1°C (night), 60Ϯ 10% relative humidity. At the cotyledon In animals, where these enzymes have been studied extensively, stage, plants were transferred to a mineral solution [13]. both selenium-dependent and selenium-independent GPX have Mechanical stimulation. Mechanical stimulus was applied to been described [7,8]. Much less is known about these enzymes 3-week-old plants, with six developed internodes. The young in plants. Recent reports have shown increases of the steady-growing internode [14] was held between the thumb and forefinstate level of mRNA or protein corresponding to GPX, and ger and rubbed back and forth. stress conditions, such as pathogenic attack, salt treatment and RNA and DNA isolation, northern blotting and Southern osmotic stress [9Ϫ11], seem to be involved. blot hybridizations. Total RNA was extracted from different The present paper reports that rubbing of tomato plants initi-organs (roots, internodes, mature and young leaves) of 3-weekates an oxidative response. Previous data in Bryonia dioica have old tomato plants by the borate method [15]. Extractions were suggested that the mechanica...
No abstract
Study of the growth responses of Lycopersicon esculentum (Mill. cv. VFN8) to mechanical stimulation applied to a single young internode showed a rapid and sharp decrease in stem elongation and an inhibition of elongation of several internodes, indicative of information transmission in the plant. A new tomato cDNA partial clone encoding calmodulin was isolated and used to study the time course of the gene induction in response to the rubbing treatment. Northern blot analysis showed a maximum accumulation of calmodulin mRNA 2 h after mechanical stimulation, not only in the rubbed internode, but also in upper and lower internodes and in young leaves. Treatment of the plant with calcium and EGTA showed the involvement of calcium and, in particular, intracellular calcium in calmodulin gene expression and cellular response.
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