Calcium signals mediate a multitude of plant responses to external stimuli and regulate a wide range of physiological processes. Calcium-binding proteins, like calcineurin B-like (CBL) proteins, represent important relays in plant calcium signaling. These proteins form a complex network with their target kinases being the CBL-interacting protein kinases (CIPKs). Here, we present a comparative genomics analysis of the full complement of CBLs and CIPKs in Arabidopsis and rice (Oryza sativa). We confirm the expression and transcript composition of the 10 CBLs and 25 CIPKs encoded in the Arabidopsis genome. Our identification of 10 CBLs and 30 CIPKs from rice indicates a similar complexity of this signaling network in both species. An analysis of the genomic evolution suggests that the extant number of gene family members largely results from segmental duplications. A phylogenetic comparison of protein sequences and intron positions indicates an early diversification of separate branches within both gene families. These branches may represent proteins with different functions. Protein interaction analyses and expression studies of closely related family members suggest that even recently duplicated representatives may fulfill different functions. This work provides a basis for a defined further functional dissection of this important plant-specific signaling system.

The ABC superfamily comprises both membrane-bound transporters and soluble proteins involved in a broad range of processes, many of which are of considerable agricultural, biotechnological and medical potential. Completion of the Arabidopsis and rice genome sequences has revealed a particularly large and diverse complement of plant ABC proteins in comparison with other organisms. Forward and reverse genetics, together with heterologous expression, have uncovered many novel roles for plant ABC proteins, but this progress has been accompanied by a confusing proliferation of names for plant ABC genes and their products. A consolidated nomenclature will provide much-needed clarity and a framework for future research.

The ABC-transporter superfamily is one of the largest protein families, and members can be found in bacteria, fungi, plants and animals. The first reports on plant ABC transporters showed that they are implicated in detoxification processes. The recent completion of the genomic sequencing of Arabidopsis thaliana (L.) Heynh. [Arabidopsis Genome Initiative (2000) Nature 408:796-815] showed that Arabidopsis contains more than 100 ABC-type proteins; 53 genes code for so-called full-size transporters, which are large proteins of about 150 kDa consisting of two hydrophobic and two hydrophilic domains. The large number of genes in the MDR/MRP and PDR5-like sub-clusters and the strong sequence homology found in many cases suggest functional redundancy. One reason for the high number of genes can be attributed to the duplication of large segments of Arabidopsis chromosomes. Recent results indicate that the function of this protein family is not restricted to detoxification processes. Plant ABC transporters have been demonstrated to participate in chlorophyll biosynthesis, formation of Fe/S clusters, stomatal movement, and probably ion fluxes; hence they may play a central role in plant growth and developmental processes.

SummaryThe Arabidopsis thaliana tpt-1 mutant which is defective in the chloroplast triose phosphate/phosphate translocator (TPT) was isolated by reverse genetics. It contains a T-DNA insertion 24 bp upstream of the start ATG of the TPT gene. The mutant lacks TPT transcripts and triose phosphate (TP)-specific transport activities are reduced to below 5% of the wild type. Analyses of diurnal variations in the contents of starch, soluble sugars and phosphorylated intermediates combined with 14 CO 2 labelling studies showed, that the lack of TP export for cytosolic sucrose biosynthesis was almost fully compensated by both continuous accelerated starch turnover and export of neutral sugars from the stroma throughout the day. The utilisation of glucose 6-phosphate (generated from exported glucose) rather than TP for sucrose biosynthesis in the light bypasses the key regulatory step catalysed by cytosolic fructose 1,6-bisphosphatase. Despite its regulatory role in the feed-forward control of sucrose biosynthesis, variations in the fructose 2,6-bisphosphate content upon illumination were similar in the mutant and the wild type. Crosses of tpt-1 with mutants unable to mobilise starch (sex1) or to synthesise starch (adg1-1) revealed that growth and photosynthesis of the double mutants was severely impaired only when starch biosynthesis, but not its mobilisation, was affected. For tpt-1/sex1 combining a lack in the TPT with a deficiency in starch mobilisation, an additional compensatory mechanism emerged, i.e. the formation and (most likely) fast turnover of high molecular weight polysaccharides. Steady-state RNA levels and transport activities of other phosphate translocators capable of transporting TP remained unaffected in the mutants.

SummaryCalcium ions represent both an integrative signal and an important convergence point of many disparate signaling pathways. Calcium-binding proteins, like calcineurin B-like (CBL) proteins, have been implicated as important relays in calcium signaling. Here, we report the in vivo study of CBL1 function in Arabidopsis. Analyses of loss-of-function as well as CBL1-overexpressing lines indicate a crucial function of this calcium sensor protein in abiotic stress responses. Mutation of CBL1 impairs plant responses to drought and salt stresses and affects gene expression of cold-regulated genes, but does not affect abscisic acid (ABA) responsiveness. Conversely, overexpression of CBL1 reduces transpirational water loss and induces the expression of early stress-responsive transcription factors and stress adaptation genes in non-stressed plants. Together, our data indicate that the calcium sensor protein CBL1 may constitute an integrative node in plant responses to abiotic stimuli and contributes to the regulation of early stress-related transcription factors of the C-Repeat-Binding Factor/dehydration-responsive element (CBF/DREB) type.

Background: WRKY proteins belong to the WRKY-GCM1 superfamily of zinc finger transcription factors that have been subject to a large plant-specific diversification. For the cereal crop barley (Hordeum vulgare), three different WRKY proteins have been characterized so far as regulators in sucrose signaling, pathogen defense, and in response to cold and drought. However, their phylogenetic relationship remained unresolved.

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The mitochondrial multienzyme glycine decarboxylase (GDC) catalyzes the tetrahydrofolate-dependent catabolism of glycine to 5,10-methylene-tetrahydrofolate and the side products NADH, CO 2 , and NH 3 . This reaction forms part of the photorespiratory cycle and contributes to one-carbon metabolism. While the important role of GDC for these two metabolic pathways is well established, the existence of bypassing reactions has also been suggested. Therefore, it is not clear to what extent GDC is obligatory for these processes. Here, we report on features of individual and combined T-DNA insertion mutants for one of the GDC subunits, P protein, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana). The individual knockout of either of these two genes does not significantly alter metabolism and photosynthetic performance indicating functional redundancy. In contrast, the double mutant does not develop beyond the cotyledon stage in air enriched with 0.9% CO 2 . Rosette leaves do not appear and the seedlings do not survive for longer than about 3 to 4 weeks under these nonphotorespiratory conditions. This feature distinguishes the GDC-lacking double mutant from all other known photorespiratory mutants and provides evidence for the nonreplaceable function of GDC in vital metabolic processes other than photorespiration.The mitochondrial multienzyme complex Gly decarboxylase (GDC) contributes to the two strategically important metabolic pathways of (1) photorespiration in all photosynthesizing organs and (2) one-carbon metabolism in all biosynthetically active tissues. In each of these two metabolic contexts, GDC closely cooperates with a second mitochondrial enzyme, Ser hydroxymethyltransferase (SHM), in the conversion of Gly to Ser. In the course of the tetrahydrofolate (THF)-dependent GDC reaction cycle comprising three individual reactions, CO 2 and NH 3 are released, and NAD 1 becomes reduced to NADH. The remaining methylene moiety becomes attached to THF to produce the one-carbon donor compound 5,10-methylene-THF (CH 2 -THF). SHM subsequently synthesizes Ser from CH 2 -THF and a second molecule of Gly in a fully reversible reaction (Douce et al., 2001;Hanson and Roje, 2001).The combined GDC/SHM reaction represents the mitochondrial part of the photorespiratory C 2 cycle, which occurs in all photosynthesizing tissues of C 3 plants, extends over three cellular compartments, and converts Rubisco-generated 2-phosphoglycolate into the Calvin cycle metabolite 3-phosphoglycerate (Tolbert, 1997;Douce and Neuburger, 1999). The importance of GDC and SHM for photorespiration becomes apparent from the fact that all as yet-reported mutants and antisense plants show strong metabolic disturbations in normal air, but grow well in the nonphotorespiratory conditions of approximately 1% CO 2 (Somerville and