The light stress-induced protein ELIP2 is a regulator of chlorophyll synthesis in Arabidopsis thaliana

in Plant Journal (2007), 50(5), 795-809

The early light-induced proteins (ELIPs) belong to the multigenic family of pigment-binding light-harvesting complexes. ELIPs accumulate transiently and are believed to play a protective role in plants ... [more ▼]

The early light-induced proteins (ELIPs) belong to the multigenic family of pigment-binding light-harvesting complexes. ELIPs accumulate transiently and are believed to play a protective role in plants exposed to high levels of light. Constitutive expression of the ELIP2 gene in Arabidopsis resulted in a marked reduction of the pigment content of the chloroplasts, both in mature leaves and during greening of etiolated seedlings. The chlorophyll loss was associated with a decrease in the number of photosystems in the thylakoid membranes, but the photosystems present were fully assembled and functional. A detailed analysis of the chlorophyll-synthesizing pathway indicated that ELIP2 accumulation downregulated the level and activity of two important regulatory steps: 5-aminolevulinate synthesis and Mg-protoporphyrin IX (Mg-Proto IX) chelatase activity. The contents of glutamyl tRNA reductase and Mg chelatase subunits CHLH and CHLI were lowered in response to ELIP2 accumulation. In contrast, ferrochelatase activity was not affected and the inhibition of Heme synthesis was null or very moderate. As a result of reduced metabolic flow from 5-aminolevulinic acid, the steady state levels of various chlorophyll precursors (from protoporphyrin IX to protochlorophyllide) were strongly reduced in the ELIP2 overexpressors. Taken together, our results indicate that the physiological function of ELIPs could be related to the regulation of chlorophyll concentration in thylakoids. This seems to occur through an inhibition of the entire chlorophyll biosynthesis pathway from the initial precursor of tetrapyrroles, 5-aminolevulinic acid. We suggest that ELIPs work as chlorophyll sensors that modulate chlorophyll synthesis to prevent accumulation of free chlorophyll, and hence prevent photooxidative stress. [less ▲]

Functional distribution and dynamics of Arabidopsis SR splicing factors in living plant cells

in Plant Journal (2005), 41(4), 567-582

Serine/arginine-rich (SR) proteins constitute an important class of splicing regulators in higher eukaryotes that share a modular structure consisting of one or two N-terminal RNA recognition motif (RRM ... [more ▼]

Serine/arginine-rich (SR) proteins constitute an important class of splicing regulators in higher eukaryotes that share a modular structure consisting of one or two N-terminal RNA recognition motif (RRM) domains and a C-terminal RS-rich domain. Herein, we have investigated the in vivo functional distribution of Arabidopsis SR factors. Agrobacterium-mediated transient transformation revealed nuclear speckled distribution and the overall colocalization of fluorescent protein (FP)-tagged SR factors in both tobacco and Arabidopsis cells. Their overall colocalization in larger nucleoplasmic domains was further observed after transcriptional and phosphorylation/dephosphorylation inhibition, indicating a close functional association between SR factors, independent of their phosphorylation state. Furthermore, we demonstrated in vivo the conserved role of the RS and RRM domains in the efficient targeting of Arabidopsis SR proteins to nuclear speckles by using a series of structural domain-deleted mutants of atRSp31 and atRSZp22. We suggest additional roles of RS domain such as the shuttling of atRSZp22 between nucleoplasm and nucleolus through its phosphorylation level. The coexpression of deletion mutants with wild-type SR proteins revealed potential complex associations between them. Fluorescence recovery after photobleaching demonstrated similar dynamic properties of SR factors in both tobacco transiently expressing cells and Arabidopsis transgenics. Cell cycle phase-dependent organization of FP-tagged SR proteins was observed in living tobacco BY-2 cells. We showed that atRSp31 is degraded at metaphase by fluorescence quantification. SR proteins also localized within small foci at anaphase. These results demonstrate interesting related features as well as potentially important differences between plant and animal SR proteins. [less ▲]