Photomorphogenesis, B-Box transcription factors, and the legacy of Magnus Holm

The Arabidopsis B-box protein BZS1/BBX20 interacts with HY5 and mediates strigolactone regulation of photomorphogenesis

Plant growth is controlled by integration of hormonal and light-signaling pathways. BZS1 is a B-box zinc finger protein previously characterized as a negative regulator in the brassinosteroid (BR)-signaling pathway and a positive regulator in the light-signaling pathway. However, the mechanisms by which BZS1/BBX20 integrates light and hormonal pathways are not fully understood. Here, using a quantitative proteomic workflow, we identified several BZS1-associated proteins, including light-signaling components COP1 and HY5. Direct interactions of BZS1 with COP1 and HY5 were verified by yeast two-hybrid and co-immunoprecipitation assays. Overexpression of BZS1 causes a dwarf phenotype that is suppressed by the hy5 mutation, while overexpression of BZS1 fused with the SRDX transcription repressor domain (BZS1-SRDX) causes a long-hypocotyl phenotype similar to hy5, indicating that BZS1's function requires HY5. BZS1 positively regulates HY5 expression, whereas HY5 negatively regulates BZS1 protein level, forming a feedback loop that potentially contributes to signaling dynamics. In contrast to BR, strigolactone (SL) increases BZS1 level, whereas the SL responses of hypocotyl elongation, chlorophyll and HY5 accumulation are diminished in the BZS1-SRDX seedlings, indicating that BZS1 is involved in these SL responses. These results demonstrate that BZS1 interacts with HY5 and plays a central role in integrating light and multiple hormone signals for photomorphogenesis in Arabidopsis.

Transcription factors in microalgae: genome-wide prediction and comparative analysis

Background

Studying transcription factors, which are some of the key players in gene expression, is of outstanding interest for the investigation of the evolutionary history of organisms through lineage-specific features. In this study we performed the first genome-wide TF identification and comparison between haptophytes and other algal lineages.

Results

For TF identification and classification, we created a comprehensive pipeline using a combination of BLAST, HMMER and InterProScan software. The accuracy evaluation of the pipeline shows its applicability for every alga, plant and cyanobacterium, with very good PPV and sensitivity. This pipeline allowed us to identify and classified the transcription factor complement of the three haptophytes Tisochrysis lutea, Emiliania huxleyi and Pavlova sp.; the two stramenopiles Phaeodactylum tricornutum and Nannochloropsis gaditana; the chlorophyte Chlamydomonas reinhardtii and the rhodophyte Porphyridium purpureum. By using T. lutea and Porphyridium purpureum, this work extends the variety of species included in such comparative studies, allowing the detection and detailed study of lineage-specific features, such as the presence of TF families specific to the green lineage in Porphyridium purpureum, haptophytes and stramenopiles. Our comprehensive pipeline also allowed us to identify fungal and cyanobacterial TF families in the algal nuclear genomes.

Conclusions

This study provides examples illustrating the complex evolutionary history of algae, some of which support the involvement of a green alga in haptophyte and stramenopile evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2610-9) contains supplementary material, which is available to authorized users.

Identification and functional characterization of the BBX24 promoter and gene from chrysanthemum in Arabidopsis

The B-box (BBX) family is a subgroup of zinc finger transcription factors that regulate flowering time, light-regulated morphogenesis, and abiotic stress in Arabidopsis. Overexpression of CmBBX24, a zinc finger transcription factor gene in chrysanthemum, results in abiotic stress tolerance. We have investigated and characterized the promoter of CmBBX24, isolating a 2.7-kb CmBBX24 promoter sequence and annotating a number of abiotic stress-related cis-regulatory elements, such as DRE, MYB, MYC, as well as cis-elements which respond to plant hormones, such as GARE, ABRE, and CARE. We also observed a number of cis-elements related to light, such as TBOX and GBOX, and some tissue-specific cis-elements, such as those for guard cells (TAAAG). Expression of the CmBBX24 promoter produced a clear response in leaves and a lower response in roots, based on β-glucuronidase histochemical staining and fluorometric analysis. The CmBBX24 promoter was induced by abiotic stresses (mannitol, cold temperature), hormones (gibberellic acid, abscisic acid), and different light treatments (white, blue, red); activation was measured by fluorometric analysis in the leaves and roots. The deletion of fragments from the 5'-end of the promoter led to different responses under various stress conditions. Some CmBBX24 promoter segments were found to be more important than others for regulating all stresses, while other segments were relatively more specific to stress type. D0-, D1-, D2-, D3-, and D4-proCmBBX24::CmBBX24 transgenic Arabidopsis lines developed for further study were found to be more tolerant to the low temperature and drought stresses than the controls. We therefore speculate that CmBBX24 is of prime importance in the regulation of abiotic stress in Arabidopsis and that the CmBBX24 promoter is inductive in abiotic stress conditions. Consequently, we suggest that CmBBX24 is a potential candidate for the use in breeding programs of important ornamental plants.

Effect of low-intensity white light mediated de-etiolation on the biosynthesis of polyphenols in tea seedlings

Light is an important source of energy as well as environmental signal for the regulation of biosynthesis and accumulation of multiple secondary metabolites in plants. Polyphenols are the major class of secondary metabolites in tea, which possess potential antioxidant properties. In order to investigate the effect of light signal on the regulation of biosynthesis and accumulation of polyphenols in tea seedlings, a low-intensity white light was used and the change in trends of polyphenol contents, patterns of gene expression, and corresponding enzymatic activities were studied. LC-TOF/MS analysis revealed that light signal promoted the accumulation of hydroxycinnamic acid derivatives and nongalloylated catechin (EGC), while it restrained the accumulation of β-glucogallin and galloylated catechins. The quantitative reverse transcription-PCR analysis showed that the expression levels of the regulator genes and some structural genes involved in photomorphogenesis and biosynthetic pathway of nongalloylated catechins, respectively, were up-regulated. In contrast, the expression of DHD/SDH and UGT genes, which may be involved in biosynthetic pathway of βG, was down-regulated. The corresponding in vitro enzyme assays revealed decrease in the activity of ECGT (galloylates nongalloylated catechins) and an increase in activity of GCH (hydrolyzes galloylated catechins) during de-etiolation. The present study yielded inconsistent accumulation patterns of phenolic acids, flavan-3-ols, and flavonols in tea seedlings during de-etiolation. In addition, the accumulation of catechins was possibly jointly influenced by the biosynthesis, hydrolysis, glycosylation, and galloylation of polyphenols in tea plants.

Gene expression regulation in photomorphogenesis from the perspective of the central dogma

Depending on the environment a young seedling encounters, the developmental program following seed germination could be skotomorphogenesis in the dark or photomorphogenesis in the light. Light signals are interpreted by a repertoire of photoreceptors followed by sophisticated gene expression networks, eventually resulting in developmental changes. The expression and functions of photoreceptors and key signaling molecules are highly coordinated and regulated at multiple levels of the central dogma in molecular biology. Light activates gene expression through the actions of positive transcriptional regulators and the relaxation of chromatin by histone acetylation. Small regulatory RNAs help attenuate the expression of light-responsive genes. Alternative splicing, protein phosphorylation/dephosphorylation, the formation of diverse transcriptional complexes, and selective protein degradation all contribute to proteome diversity and change the functions of individual proteins.