The RING-Finger Ubiquitin Ligase HAF1 Mediates Heading date 1 Degradation during Photoperiodic Flowering in Rice

Hairy Leaf 6, an AP2/ERF Transcription Factor, Interacts with OsWOX3B and Regulates Trichome Formation in Rice

Trichome formation has been extensively studied as a mechanistic model for epidermal cell differentiation and cell morphogenesis in plants. However, the genetic and molecular mechanisms underlying trichome formation (i.e., initiation and elongation) in rice remain largely unclear. Here, we report an AP2/ERF transcription factor, Hairy Leaf 6 (HL6), which controls trichome formation in rice. Functional analyses revealed that HL6 transcriptionally regulates trichome elongation in rice, which is dependent on functional OsWOX3B, a homeodomain-containing protein that acts as a key regulator in trichome initiation. Biochemical and molecular genetic analyses demonstrated that HL6 physically interacts with OsWOX3B, and both of them regulate the expression of some auxin-related genes during trichome formation, in which OsWOX3B likely enhances the binding ability of HL6 with one of its direct target gene, OsYUCCA5. Population genetic analysis indicated that HL6 was under negative selection during rice domestication. Taken together, our findings provide new insights into the molecular regulatory network of trichome formation in rice.

E3 Ubiquitin Ligases: Ubiquitous Actors in Plant Development and Abiotic Stress Responses

Understanding the precise regulatory mechanisms of plant development and stress responses at the post-translational level is currently a topic of intensive research. Protein ubiquitination, including the sequential performances of ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes, is a refined post-translational modification ubiquitous in all eukaryotes. Plants are an integral part of our ecosystem and, as sessile organisms, the ability to perceive internal and external signals and to adapt well to various environmental challenges is crucial for their survival. Over recent decades, extensive studies have demonstrated that protein ubiquitination plays key roles in multiple plant developmental stages (e.g. seed dormancy and germination, root growth, flowering time control, self-incompatibility and chloroplast development) and several abiotic stress responses (e.g. drought and high salinity), by regulating the abundance, activities or subcellular localizations of a variety of regulatory polypeptides and enzymes. Importantly, diverse E3 ligases are involved in these regulatory pathways by mediating phytohormone and light signaling or other pathways. In this updated review, we mainly summarize recent advances in our understanding of the regulatory roles of protein ubiquitination in plant development and plant-environment interactions, and primarily focus on different types of E3 ligases because they play critical roles in determining substrate specificity.

Alternative functions of Hd1 in repressing or promoting heading are determined by Ghd7 status under long-day conditions

Previous studies suggested that Hd1 promoted heading under short-day conditions (SD) and delayed heading under long-day conditions (LD). However in this study, Hd1 was demonstrated to consistently promote heading date in Zhenshan 97 (ZS97) background by upregulating Ehd1, Hd3a and RFT1 expression under both SD and LD. While the high photoperiod sensitivity of Hd1 was observed in Minghui 63 (MH63) background, with heading being suppressed in LD but promoted in SD. Comparative analysis of two sets of near isogenic lines of Hd1 in MH63 and ZS97 backgrounds indicated that the alternative functions of Hd1 in promoting or suppressing heading under LD are dependent on the previously cloned flowering repressor gene Ghd7. The interaction between proteins Ghd7 and Hd1 occurred through binding of the CCT domain of Ghd7 to the transcription-activating domain of Hd1, resulting in suppression of Ehd1 and florigen gene expression. The involvement of the transcription-activating domain of Hd1 in this protein-protein interaction probably blocked or weakened its transcriptional activity. These findings suggest that Hd1 alone essentially acts as a promoter of heading date, and the protein interaction between Ghd7 and Hd1 determines photoperiod sensitivity and integrated Hd1-mediated and Ehd1-mediated flowering pathways in rice.

The DTH8-Hd1 Module Mediates Day-Length-Dependent Regulation of Rice Flowering

Photoperiodic flowering is one of the most important pathways to govern flowering in rice (Oryza sativa), in which Heading date 1 (Hd1), an ortholog of the Arabidopsis CONSTANS gene, encodes a pivotal regulator. Hd1 promotes flowering under short-day conditions (SD) but represses flowering under long-day conditions (LD) by regulating the expression of Heading date 3a (Hd3a), the FLOWERING LOCUS T (FT) ortholog in rice. However, the molecular mechanism of how Hd1 changes its regulatory activity in response to day length remains largely unknown. In this study, we demonstrated that the repression of flowering in LD by Hd1 is dependent on the transcription factor DAYS TO HEADING 8 (DTH8). Loss of DTH8 function results in the activation of Hd3a by Hd1, leading to early flowering. We found that Hd1 directly interacts with DTH8 and that the formation of the DTH8-Hd1 complex is necessary for the transcriptional repression of Hd3a by Hd1 in LD, implicating that the switch of Hd1 function is mediated by DTH8 in LD rather than in SD. Furthermore, we revealed that DTH8 associates with the Hd3a promoter to modulate the level of H3K27 trimethylation (H3K27me3) at the Hd3a locus. In the presence of the DTH8-Hd1 complex, the H3K27me3 level was increased at Hd3a, whereas loss of DTH8 function resulted in decreased H3K27me3 level at Hd3a. Taken together, our findings indicate that, in response to day length, DTH8 plays a critical role in mediating the transcriptional regulation of Hd3a by Hd1 through the DTH8-Hd1 module to shape epigenetic modifications in photoperiodic flowering.

ThDof1.4 and ThZFP1 constitute a transcriptional regulatory cascade involved in salt or osmotic stress in Tamarix hispida

Identification of the upstream regulators of a gene is important to characterize the transcriptional pathway and the function of the gene. Previously, we found that a zinc finger protein (ThZFP1) is involved in abiotic stress tolerance of Tamarix hispida. In the present study, we further investigated the transcriptional pathway of ThZFP1. Dof motifs are abundant in the ThZFP1 promoter; therefore, we used them to screen for transcriptional regulators of ThZFP1. A Dof protein, ThDof1.4, binds to the Dof motif specifically, and was hypothesized as the upstream regulator of ThZFP1. Further study showed that overexpression of ThDof1.4 in T. hispida activated the expression of GUS controlled by the ThZFP1 promoter. In T. hispida, transient overexpression of ThDof1.4 increased the transcripts of ThZFP1; conversely, transient RNAi-silencing of ThDof1.4 reduced the expression of ThZFP1. Chromatin immunoprecipitation indicated that ThDof1.4 binds to the ThZFP1 promoter. Additionally, ThDof1.4 and ThZFP1 share similar expression patterns in response to salt or drought stress. Furthermore, like ThZFP1, ThDof1.4 could increase the proline level and enhance ROS scavenging capability to improve salt and osmotic stress tolerance. Together, these results suggested that ThDof1.4 and ThZFP1 form a transcriptional regulatory cascade involved in abiotic stress resistance in T. hispida.

The Pepper RING Finger E3 Ligase, CaDIR1, Regulates the Drought Stress Response via ABA-Mediated Signaling

Drought stress from soil or air limits plant growth and development, leading to a reduction in crop productivity. Several E3 ligases positively or negatively regulate the drought stress response. In the present study, we show that the pepper (Capsicum annuum) Drought Induced RING type E3 ligase 1, CaDIR1, regulates the drought stress response via abscisic acid (ABA)-mediated signaling. CaDIR1 contains a C3HC4-type RING finger domain in the N-terminal region; this domain functions during protein degradation via attachment of ubiquitins to the substrate target proteins. The expression levels of the CaDIR1 gene were suppressed and induced by ABA and drought treatments, respectively. We conducted loss-of-function and gain-of function genetic studies to examine the in vivo function of CaDIR1 in response to ABA and drought stress. CaDIR1-silenced pepper plants displayed a drought-tolerant phenotype characterized by a low level of transpirational water loss via increased stomatal closure and elevated leaf temperatures. CaDIR1-overexpressing (OX) Arabidopsis plants exhibited an ABA-hypersensitive phenotype during the germination stage, but an ABA-hyposensitive phenotype-characterized by decreased stomatal closure and reduced leaf temperatures-at the adult stage. Moreover, adult CaDIR1-OX plants exhibited a drought-sensitive phenotype characterized by high levels of transpirational water loss. Our results indicate that CaDIR1 functions as a negative regulator of the drought stress response via ABA-mediated signaling. Our findings provide a valuable insight into the plant defense mechanism that operates during drought stress.

Y flowering? Regulation and activity of CONSTANS and CCT-domain proteins in Arabidopsis and crop species

Changes in day length regulate the proper timing of flowering in several plant species. The genetic architecture of this process is based on CCT-domain proteins, many of which interact with NF-Y subunits to regulate transcription of target genes. In the model plant Arabidopsis thaliana, the CONSTANS CCT-domain protein is a central photoperiodic sensor. We will discuss how the diurnal rhythms of its transcription and protein accumulation are generated, and how the protein engages into multiple complexes to control production of a systemic flowering signal. Regulatory parallels will be drawn between Arabidopsis and major crops that indicate conservation of some CCT/NF-Y modules during plant evolution. This article is part of a Special Issue entitled: Nuclear Factor Y in Development and Disease, edited by Prof. Roberto Mantovani.