RED AND FAR-RED INSENSITIVE 2, a RING-domain zinc finger protein, mediates phytochrome-controlled seedling deetiolation responses

The Pleiades are a cluster of fungal effectors that inhibit host defenses

Biotrophic plant pathogens secrete effector proteins to manipulate the host physiology. Effectors suppress defenses and induce an environment favorable to disease development. Sequence-based prediction of effector function is impeded by their rapid evolution rate. In the maize pathogen Ustilago maydis, effector-coding genes frequently organize in clusters. Here we describe the functional characterization of the pleiades, a cluster of ten effector genes, by analyzing the micro- and macroscopic phenotype of the cluster deletion and expressing these proteins in planta. Deletion of the pleiades leads to strongly impaired virulence and accumulation of reactive oxygen species (ROS) in infected tissue. Eight of the Pleiades suppress the production of ROS upon perception of pathogen associated molecular patterns (PAMPs). Although functionally redundant, the Pleiades target different host components. The paralogs Taygeta1 and Merope1 suppress ROS production in either the cytoplasm or nucleus, respectively. Merope1 targets and promotes the auto-ubiquitination activity of RFI2, a conserved family of E3 ligases that regulates the production of PAMP-triggered ROS burst in plants.

Nuclear-encoded sigma factor 6 (SIG6) is involved in the block of greening response in Arabidopsis thaliana

PREMISE

Light is critical in the ability of plants to accumulate chlorophyll. When exposed to far-red (FR) light and then grown in white light in the absence of sucrose, wild-type seedlings fail to green in a response known as the FR block of greening (BOG). This response is controlled by phytochrome A through repression of protochlorophyllide reductase-encoding (POR) genes by FR light coupled with irreversible plastid damage. Sigma (SIG) factors are nuclear-encoded proteins that contribute to plant greening and plastid development through regulating gene transcription in chloroplasts and impacting retrograde signaling from the plastid to nucleus. SIGs are regulated by phytochromes, and the expression of some SIG factors is reduced in phytochrome mutant lines, including phyA. Given the association of phyA with the FR BOG and its regulation of SIG factors, we investigated the potential regulatory role of SIG factors in the FR BOG response.

METHODS

We examined FR BOG responses in sig mutants, phytochrome-deficient lines, and mutant lines for several phy-associated factors. We quantified chlorophyll levels and examined expression of key BOG-associated genes.

RESULTS

Among six sig mutants, only the sig6 mutant significantly accumulated chlorophyll after FR BOG treatment, similar to the phyA mutant. SIG6 appears to control protochlorophyllide accumulation by contributing to the regulation of tetrapyrrole biosynthesis associated with glutamyl-tRNA reductase (HEMA1) function, select phytochrome-interacting factor genes (PIF4 and PIF6), and PENTA1, which regulates PORA mRNA translation after FR exposure.

CONCLUSIONS

Regulation of SIG6 plays a significant role in plant responses to FR exposure during the BOG response.

Research Progress on Plant RING-Finger Proteins

E3 ubiquitin ligases are the most expanded components of the ubiquitin proteasome system (UPS). They mediate the recognition of substrates and later transfer the ubiquitin (Ub) of the system. Really Interesting New Gene (RING) finger proteins characterized by the RING domain, which contains 40–60 residues, are thought to be E3 ubiquitin ligase. RING-finger proteins play significant roles in plant growth, stress resistance, and signal transduction. In this study, we mainly describe the structural characteristics, classifications, and subcellular localizations of RING-finger proteins, as well the physiological processes of RING-finger proteins in plant growth and development. We also summarize the functions of plant RING-finger proteins in plant stress resistance. Finally, further research on plant RING-finger proteins is suggested, thereby establishing a strong foundation for the future study of plant RING-finger proteins.

Zinc finger protein transcription factors: Integrated line of action for plant antimicrobial activity

The plants resist/tolerate unfavorable conditions in their natural habitats by using different but aligned and integrated defense mechanisms. Such defense responses include not only morphological and physiological adaptations but also the genomic and transcriptomic reconfiguration. Microbial attack on plants activates multiple pro-survival pathways such as transcriptional reprogramming, hypersensitive response (HR), antioxidant defense system and metabolic remodeling. Up-regulation of these processes during biotic stress conditions directly relates with plant survival. Over the years, hundreds of plant transcription factors (TFs) belonging to diverse families have been identified. Zinc finger protein (ZFP) TFs have crucial role in phytohormone response, plant growth and development, stress tolerance, transcriptional regulation, RNA binding and protein-protein interactions. Recent research progress has revealed regulatory and biological functions of ZFPs in incrementing plant resistance to pathogens. Integration of transcriptional activity with metabolic modulations has miniaturized plant innate immunity. However, the precise roles of different zinc finger TFs in plant immunity to pathogens have not been thoroughly analyzed. This review consolidates the pivotal functioning of zinc finger TFs and proposes the integrative understanding as foundation for the plant growth and development including the stress responses.

SbRFP1 regulates cold-induced sweetening of potato tubers by inactivation of StBAM1

Potato cold-induced sweetening (CIS) is a major drawback restricting potato process industry. Starch degradation and sucrose decomposition are considered to be the key pathways in potato CIS. Our previous study showed that the RING finger gene SbRFP1 could slow down starch degradation and the accumulation of reducing sugars (RS) through inhibiting amylase and invertase activity in cold-stored tubers. However, the regulation mechanism of SbRFP1 is not clear. In this paper, we first proved that SbRFP1 could promote starch synthesis and modify the shape of starch granules. By further yeast two hybrid, GST-pull down and inhibition of enzyme activity assays, we confirmed that SbRFP1 could slow down the transformation of starch to RS in tubers mainly through the inhibition of β-amylase StBAM1 activity. SbRFP1 was also proved to possess E3 ubiquitin ligase activity by ubiquitination assay. Thus, SbRFP1 may regulate the accumulation of RS in cold-stored tubers by ubiquitination and degradation of StBAM1. Therefore, our study reveals the regulatory mechanism of SbRFP1 in the process of CIS and provides more powerful evidence for the effect of starch degradation on potato CIS.

Genomic and Phenomic Screens for Flower Related RING Type Ubiquitin E3 Ligases in Arabidopsis

Flowering time control integrates endogenous as well as environmental signals to promote flower development. The pathways and molecular networks involved are complex and integrate many modes of signal transduction. In plants ubiquitin mediated protein degradation pathway has been proposed to be as important mode of signaling as phosphorylation and transcription. To systematically study the role of ubiquitin signaling in the molecular regulation of flowering we have taken a genomic approach to identify flower related Ubiquitin Proteasome System components. As a large and versatile gene family the RING type ubiquitin E3 ligases were chosen as targets of the genomic screen. The complete list of Arabidopsis RING E3 ligases were retrieved and verified in the Arabidopsis genome v11 and their differential expression was used for their categorization into flower organs or developmental stages. Known regulators of flowering time or floral organ development were identified in these categories through literature search and representative mutants for each category were purchased for functional characterization by growth and morphological phenotyping. To this end, a workflow was developed for high throughput phenotypic screening of growth, morphology and flowering of nearly a thousand Arabidopsis plants in one experimental round.

Gene copy silencing and DNA methylation in natural and artificially produced allopolyploid fish

Allelic silencing is an important mechanism for coping with gene dosage changes in polyploid organisms that is well known in allopolyploid plants. Only recently, it was shown in the allotriploid fish Squalius alburnoides that this process also occurs in vertebrates. However, it is still unknown whether this silencing mechanism is common to other allopolyploid fish, and which mechanisms might be responsible for allelic silencing. We addressed these questions in a comparative study between Squalius alburnoides and another allopolyploid complex, the Amazon molly (Poecilia formosa). We examined the allelic expression patterns for three target genes in four somatic tissues of natural allo-anorthoploids and laboratory-produced tri-genomic hybrids of S. alburnoides and P. formosa. Also, for both complexes, we evaluated the correlation between total DNA methylation level and the ploidy status and genomic composition of the individuals. We found that allelic silencing also occurs in other allopolyploid organisms besides the single one that was previously known. We found and discuss disparities within and between the two considered complexes concerning the pattern of allele-specific expression and DNA methylation levels. Disparities might be due to intrinsic characteristics of each genome involved in the hybridization process. Our findings also support the idea that long-term evolutionary processes have an effect on the allele expression patterns and possibly also on DNA methylation levels.

Holophytochrome-Interacting Proteins in Physcomitrella: Putative Actors in Phytochrome Cytoplasmic Signaling

Phytochromes are the principle photoreceptors in light-regulated plant development, primarily acting via translocation of the light-activated photoreceptor into the nucleus and subsequent gene regulation. However, several independent lines of evidence indicate unambiguously that an additional cytoplasmic signaling mechanism must exist. Directional responses in filament tip cells of the moss Physcomitrella patens are steered by phy4 which has been shown to interact physically with the blue light receptor phototropin at the plasma membrane. This complex might perceive and transduce vectorial information leading to cytoskeleton reorganization and finally a directional growth response. We developed yeast two-hybrid procedures using photochemically functional, full-length phy4 as bait in Physcomitrella cDNA library screens and growth assays under different light conditions, revealing Pfr-dependent interactions possibly associated with phytochrome cytoplasmic signaling. Candidate proteins were then expressed in planta with fluorescent protein tags to determine their intracellular localization in darkness and red light. Of 14 candidates, 12 were confirmed to interact with phy4 in planta using bimolecular fluorescence complementation. We also used database information to study their expression patterns relative to those of phy4. We discuss the likely functional characteristics of these holophytochrome-interacting proteins (HIP's) and their possible roles in signaling.

Genome-wide analysis of RING finger proteins in the smallest free-living photosynthetic eukaryote Ostreococus tauri

RING finger proteins and ubiquitination marks are widely involved in diverse aspects of growth and development, biological processes, and stress or environmental responses. As the smallest free-living photosynthetic eukaryote known so far, the green alga Ostreococus tauri has become an excellent model for investigating the origin of different gene families in the green lineage. Here, 65 RING domains in 65 predicted proteins were identified from O. tauri and on the basis of one or more substitutions at the metal ligand positions and spacing between them they were divided into eight canonical or modified types (RING-CH, -H2, -v, -C2, -C3HCHC2, -C2HC5, -C3GC3S, and -C2SHC4), in which the latter four were newly identified and might represent the intermediate states between RING domain and other similar domains, respectively. RING finger proteins were classified into eight classes based on the presence of additional domains, including RING-Only, -Plus, -C3H1, -PHD, -WD40, -PEX, -TM, and -DEXDc classes. These RING family genes usually lack introns and are distributed over 17 chromosomes. In addition, 29 RING-finger proteins in O. tauri share different degrees of homology with those in the model flowering plant Arabidopsis, indicating they might be necessary for the basic survival of free-living eukaryotes. Therefore, our results provide new insight into the general classification and evolutionary conservation of RING domain-containing proteins in O. tauri.

A reverse genetics approach identifies novel mutants in light responses and anthocyanin metabolism in petunia

Flower color and plant architecture are important commercially valuable features for ornamental petunias (Petunia x hybrida Vilm.). Photoperception and light signaling are the major environmental factors controlling anthocyanin and chlorophyll biosynthesis and shade-avoidance responses in higher plants. The genetic regulators of these processes were investigated in petunia by in silico analyses and the sequence information was used to devise a reverse genetics approach to probe mutant populations. Petunia orthologs of photoreceptor, light-signaling components and anthocyanin metabolism genes were identified and investigated for functional conservation by phylogenetic and protein motif analyses. The expression profiles of photoreceptor gene families and of transcription factors regulating anthocyanin biosynthesis were obtained by bioinformatic tools. Two mutant populations, generated by an alkalyting agent and by gamma irradiation, were screened using a phenotype-independent, sequence-based method by high-throughput PCR-based assay. The strategy allowed the identification of novel mutant alleles for anthocyanin biosynthesis (CHALCONE SYNTHASE) and regulation (PH4), and for light signaling (CONSTANS) genes.

Divergence of flowering genes in soybean

Soybean genome sequences were blasted with Arabidopsis thaliana regulatory genes involved in photoperioddependent flowering. This approach enabled the identification of 118 genes involved in the flowering pathway. Two genome sequences of cultivated (Williams 82) and wild (IT182932) soybeans were employed to survey functional DNA variations in the flowering-related homologs. Forty genes exhibiting nonsynonymous substitutions between G. max and G. soja were catalogued. In addition, 22 genes were found to co-localize with QTLs for six traits including flowering time, first flower, pod maturity, beginning of pod, reproductive period, and seed filling period. Among the genes overlapping the QTL regions, two LHY/CCA1 genes, GI and SFR6 contained amino acid changes. The recently duplicated sequence regions of the soybean genome were used as additional criteria for the speculation of the putative function of the homologs. Two duplicated regions showed redundancy of both flowering-related genes and QTLs. ID 12398025, which contains the homeologous regions between chr 7 and chr 16, was redundant for the LHY/CCA1 and SPA1 homologs and the QTLs. Retaining of the CRY1 gene and the pod maturity QTLs were observed in the duplicated region of ID 23546507 on chr 4 and chr 6. Functional DNA variation of the LHY/CCA1 gene (Glyma07g05410) was present in a counterpart of the duplicated region on chr 7, while the gene (Glyma16g01980) present in the other portion of the duplicated region on chr 16 did not show a functional sequence change. The gene list catalogued in this study provides primary insight for understanding the regulation of flowering time and maturity in soybean.

Early gene expression events in the laminar abscission zone of abscission-promoted citrus leaves after a cycle of water stress/rehydration: involvement of CitbHLH1

Leaf abscission is a common response of plants to drought stress. Some species, such as citrus, have evolved a specific behaviour in this respect, keeping their leaves attached to the plant body during water stress until this is released by irrigation or rain. This study successfully reproduced this phenomenon under controlled conditions (24h of water stress followed by 24h of rehydration) and used it to construct a suppression subtractive hybridization cDNA library enriched in genes involved in the early stages of rehydration-promoted leaf abscission after water stress. Sequencing of the library yielded 314 unigenes, which were spotted onto nylon membranes. Membrane hybridization with petiole (Pet)- and laminar abscission zone (LAZ)-enriched RNA samples corresponding to early steps in leaf abscission revealed an almost exclusive preferential gene expression programme in the LAZ. The data identified major processes such as protein metabolism, cell-wall modification, signalling, control of transcription and vesicle production, and transport as the main biological processes activated in LAZs during the early steps of rehydration-promoted leaf abscission after water stress. Based on these findings, a model for the early steps of citrus leaf abscission is proposed. In addition, it is suggested that CitbHLH1, the putative citrus orthologue of Arabidopsis BIGPETAL, may play major roles in the control of abscission-related events in citrus abscission zones.

The E3 ubiquitin ligase WVIP2 highlights the versatility of protein ubiquitination

Plant cells regulate many cellular processes controlling the half-life of critical proteins through ubiquitination. Previously, we characterized two interacting RING-type E3 ubiquitin ligases of Triticum durum, TdRF1 and WVIP2. We revealed their role in tolerance to dehydration, and existing knowledge about their partners also indicated their involvement in the regulation of some aspects of plant development. Here we located WVIP2 in the regulation of the ABA signaling, based on sequence similarities. Further we acquired general evidence about the versatility of ubiquitination in plant cells. A protein can be target of different E3 ligases for a perfect tuning of its abundance as well as the same E3 ligase can ubiquitinate different and unrelated proteins, thus representing a cross-connections between different signaling pathways for a global coordination of cellular processes.

DICER-like proteins and their role in plant-herbivore interactions in Nicotiana attenuata

DICER-like (DCL) proteins produce small RNAs that silence genes involved in development and defenses against viruses and pathogens. Which DCLs participate in plant-herbivore interactions remains unstudied. We identified and stably silenced four distinct DCL genes by RNAi in Nicotiana attenuata (Torrey ex. Watson), a model for the study of plant-herbivore interactions. Silencing DCL1 expression was lethal. Manduca sexta larvae performed significantly better on ir-dcl3 and ir-dcl4 plants, but not on ir-dcl2 plants compared to wild type plants. Phytohormones, defense metabolites and microarray analyses revealed that when DCL3 and DCL4 were silenced separately, herbivore resistance traits were regulated in distinctly different ways. Crossing of the lines revealed complex interactions in the patterns of regulation. Single ir-dcl4 and double ir-dcl2 ir-dcl3 plants were impaired in JA accumulation, while JA-Ile was increased in ir-dcl3 plants. Ir-dcl3 and ir-dcl4 plants were impaired in nicotine accumulation; silencing DCL2 in combination with either DCL3 or DCL4 restored nicotine levels to those of WT. Trypsin proteinase inhibitor activity and transcripts were only silenced in ir-dcl3 plants. We conclude that DCL2/3/4 interact in a complex manner to regulate anti-herbivore defenses and that these interactions significantly complicate the already challenging task of understanding smRNA function in the regulation of biotic interactions.

Recruitment of a ribosomal release factor for light- and stress-dependent regulation of petB transcript stability in Arabidopsis chloroplasts

Land plant genomes encode four functional ribosomal peptide chain release factors (Prf) of eubacterial origin, two (PrfA and PrfB homologs) for each endosymbiotic organelle. Formerly, we have shown that the Arabidopsis thaliana chloroplast-localized PrfB homolog, PrfB1, is required not only for termination of translation but also for stabilization of UGA stop codon-containing chloroplast transcripts. A previously undiscovered PrfB-like protein, PrfB3, is localized to the chloroplast stroma in a petB RNA-containing complex and found only in vascular plants. Highly conserved positions of introns unequivocally indicate that PrfB3 arose from a duplication of PrfB1. Notably, PrfB3 is lacking the two most important tripeptide motifs characteristic for all eubacterial and organellar PrfB homologs described so far: the stop codon recognition motif SPF and the catalytic center GGQ for peptidyl-tRNA hydrolysis. Complementation studies, as well as functional and molecular analyses of two allelic mutations in Arabidopsis, both of which lead to a specific deficiency of the cytochrome b₆f complex, revealed that PrfB3 is essentially required for photoautotrophic growth. Plastid transcript, polysome, and translation analyses indicate that PrfB3 has been recruited in vascular plants for light- and stress-dependent regulation of stability of 3' processed petB transcripts to adjust cytochrome b₆ levels.

Gene expression profiling identifies two regulatory genes controlling dormancy and ABA sensitivity in Arabidopsis seeds

Seed dormancy is a very important trait that maximizes the survival of seed in nature, the control of which can have important repercussions on the yield of many crop species. We have used gene expression profiling to identify genes that are involved in dormancy regulation in Arabidopsis thaliana. RNA was isolated from imbibed dormant (D) and after-ripened (AR) ecotype C24 seeds, and then screened by quantitative RT-PCR (qRT-PCR) for differentially expressed transcription factors (TFs) and other regulatory genes. Out of 2207 genes screened, we have identified 39 that were differentially expressed during the first few hours of imbibition. After analyzing T-DNA insertion mutants for 22 of these genes, two displayed altered dormancy compared with the wild type. These mutants are affected in genes that encode a RING finger and an HDZip protein. The first, named DESPIERTO, is involved in ABA sensitivity during seed development, regulates the expression of ABI3, and produces a complete loss of dormancy when mutated. The second, the HDZip (ATHB20), is expressed during seed germination in the micropylar endosperm and in the root cap, and increases ABA sensitivity and seed dormancy when mutated.

RFI2, a RING-domain zinc finger protein, negatively regulates CONSTANS expression and photoperiodic flowering

The red and far-red light-absorbing phytochromes interact with the circadian clock, a central oscillator that sustains a 24-h period, to measure accurately seasonal changes in day-length and regulate the expression of several key flowering genes. The interactions and subsequent signalling steps upstream of the flowering genes such as CONSTANS (CO) and FLOWERING LOCUS T (FT) remain largely unknown. We report here that a photomorphogenic mutant, red and far-red insensitive 2-1 ( rfi2-1), flowered early particularly under long days. The rfi2-1 mutation also enhanced the expression of CO and FT under day/night cycles or constant light. Both co-2 and gigantea-2 (gi-2) were epistatic to rfi2-1 in their flowering responses. The gi-2 mutation was also epistatic to the rfi2-1 mutation in the expression of CO and hypocotyl elongation. However, the rfi2-1 mutation did not affect the expression of GI, a gene that mediates between the circadian clock and the expression of CO. Like many other flowering genes, the expression of RFI2 oscillated under day/night cycles and was rhythmic under constant light. The amplitude of the rhythmic expression of RFI2 was significantly reduced in phyB-9 or lhy-20 plants, and was also affected by the gi-2 mutation. As previously reported, the gi-2 mutation affects the period length and amplitude of CCA1 and LHY expression, and GI may act through a feedback loop to maintain a proper circadian function. We propose a regulatory step in which RFI2 represses the expression of CO, whereas GI may maintain the proper expression of RFI2 through its positive action on the circadian clock. The regulatory step serves to tune the circadian outputs that control the expression of CO and photoperiodic flowering.