Publikationen - Molekulare Signalverarbeitung

The preprophase band (PPB) is a transient cytokinetic structure that marks the future division plane at the onset of mitosis. The PPB forms a dense cortical ring of mainly microtubules, actin filaments, endoplasmic reticulum, and associated proteins that encircles the nucleus of mitotic cells. After PPB disassembly, the positional information is preserved by the cortical division zone (CDZ). The formation of the PPB and its contribution to timely CDZ set-up involves activities of functionally distinct microtubule-associated proteins (MAPs) that interact physically and genetically to support robust division plane orientation in plants. Recent studies identified two types of plant-specific MAPs as key regulators of PPB formation, the TON1 RECRUITMENT MOTIF (TRM) and IQ67 DOMAIN (IQD) families. Both families share hallmarks of disordered scaffold proteins. Interactions of IQDs and TRMs with multiple binding partners, including the microtubule severing KATANIN1, may provide a molecular framework to coordinate PPB formation, maturation, and disassembly.

Chemistry assigns phosphate (Pi) dominant roles in metabolism; however, it also renders the macronutrient a genuinely limiting factor of plant productivity. Pi bioavailability is restricted by low Pi mobility in soil and antagonized by metallic toxicities, which force roots to actively seek and selectively acquire the vital element. During the past few years, a first conceptual outline has emerged of the sensory mechanisms at root tips, which monitor external Pi and transmit the edaphic cue to inform root development. This review highlights new aspects of the Pi acquisition strategy of Arabidopsis roots, as well as a framework of local Pi sensing in the context of antagonistic interactions between Pi and its major associated metallic cations, Fe3+ and Al3+.

BackgroundThe formation of flowers is one of the main model systems to elucidate the molecular mechanisms that control developmental processes in plants. Although several studies have explored gene expression during flower development in the model plant Arabidopsis thaliana on a genome-wide scale, a continuous series of expression data from the earliest floral stages until maturation has been lacking. Here, we used a floral induction system to close this information gap and to generate a reference dataset for stage-specific gene expression during flower formation.ResultsUsing a floral induction system, we collected floral buds at 14 different stages from the time of initiation until maturation. Using whole-genome microarray analysis, we identified 7,405 genes that exhibit rapid expression changes during flower development. These genes comprise many known floral regulators and we found that the expression profiles for these regulators match their known expression patterns, thus validating the dataset. We analyzed groups of co-expressed genes for over-represented cellular and developmental functions through Gene Ontology analysis and found that they could be assigned specific patterns of activities, which are in agreement with the progression of flower development. Furthermore, by mapping binding sites of floral organ identity factors onto our dataset, we were able to identify gene groups that are likely predominantly under control of these transcriptional regulators. We further found that the distribution of paralogs among groups of co-expressed genes varies considerably, with genes expressed predominantly at early and intermediate stages of flower development showing the highest proportion of such genes.ConclusionsOur results highlight and describe the dynamic expression changes undergone by a large number of genes during flower development. They further provide a comprehensive reference dataset for temporal gene expression during flower formation and we demonstrate that it can be used to integrate data from other genomics approaches such as genome-wide localization studies of transcription factor binding sites.

The phytohormone jasmonate (JA) plays essential roles in plant growth, development and defense. In response to the JA signal, the CORONATINE INSENSITIVE 1 (COI1)-based SCF complexes recruit JASMONATE ZIM-domain (JAZ) repressors for ubiquitination and degradation, and subsequently regulate their downstream signaling components essential for various JA responses. Tremendous progress has been made in understanding the JA signaling pathway and its crosstalk with other phytohormone pathways during the past two decades. Recent studies have revealed that a variety of positive and negative regulators act as targets of JAZs to control distinctive JA responses, and that JAZs and these regulators function as crucial interfaces to mediate synergy and antagonism between JA and other phytohormones. Owing to different regulatory players in JA perception and JA signaling, a fine-tuning of JA-dependent processes in plant growth, development and defense is achieved. In this review, we will summarize the latest progresses in JA signaling and its crosstalk with gibberellin and ethylene.

Animal and plant development starts with a constituting phase called embryogenesis, which evolved independently in both lineages1. Comparative anatomy of vertebrate development—based on the Meckel-Serrès law2 and von Baer’s laws of embryology3 from the early nineteenth century—shows that embryos from various taxa appear different in early stages, converge to a similar form during mid-embryogenesis, and again diverge in later stages. This morphogenetic series is known as the embryonic ‘hourglass’4,5, and its bottleneck of high conservation in mid-embryogenesis is referred to as the phylotypic stage6. Recent analyses in zebrafish and Drosophila embryos provided convincing molecular support for the hourglass model, because during the phylotypic stage the transcriptome was dominated by ancient genes7 and global gene expression profiles were reported to be most conserved8. Although extensively explored in animals, an embryonic hourglass has not been reported in plants, which represent the second major kingdom in the tree of life that evolved embryogenesis. Here we provide phylotranscriptomic evidence for a molecular embryonic hourglass in Arabidopsis thaliana, using two complementary approaches. This is particularly significant because the possible absence of an hourglass based on morphological features in plants suggests that morphological and molecular patterns might be uncoupled. Together with the reported developmental hourglass patterns in animals, these findings indicate convergent evolution of the molecular hourglass and a conserved logic of embryogenesis across kingdoms.

Phosphate (Pi) and its anhydrides constitute major nodes in metabolism. Thus, plant performance depends directly on Pi nutrition. Inadequate Pi availability in the rhizosphere is a common challenge to plants, which activate metabolic and developmental responses to maximize Pi usage and acquisition. The sensory mechanisms that monitor environmental Pi and transmit the nutritional signal to adjust root development have increasingly come into focus. Recent transcriptomic analyses and genetic approaches have highlighted complex antagonistic interactions between external Pi and Fe bioavailability and have implicated the stem cell niche as a target of Pi sensing to regulate root meristem activity.

Auxin is a pivotal plant hormone that controls many aspects of plant growth and development. Perceived by a small family of F-box proteins including transport inhibitor response 1 (TIR1), auxin regulates gene expression by promoting SCF ubiquitin-ligase-catalysed degradation of the Aux/IAA transcription repressors, but how the TIR1 F-box protein senses and becomes activated by auxin remains unclear. Here we present the crystal structures of the Arabidopsis TIR1–ASK1 complex, free and in complexes with three different auxin compounds and an Aux/IAA substrate peptide. These structures show that the leucine-rich repeat domain of TIR1 contains an unexpected inositol hexakisphosphate co-factor and recognizes auxin and the Aux/IAA polypeptide substrate through a single surface pocket. Anchored to the base of the TIR1 pocket, auxin binds to a partially promiscuous site, which can also accommodate various auxin analogues. Docked on top of auxin, the Aux/IAA substrate peptide occupies the rest of the TIR1 pocket and completely encloses the hormone-binding site. By filling in a hydrophobic cavity at the protein interface, auxin enhances the TIR1–substrate interactions by acting as a ‘molecular glue’. Our results establish the first structural model of a plant hormone receptor.

Auxin regulates a host of plant developmental and physiological processes, including embryogenesis, vascular differentiation, organogenesis, tropic growth, and root and shoot architecture. Genetic and biochemical studies carried out over the past decade have revealed that much of this regulation involves the SCFTIR1/AFB-mediated proteolysis of the Aux/IAA family of transcriptional regulators. With the recent finding that the TRANSPORT INHIBITOR RESPONSE1 (TIR1)/AUXIN SIGNALING F-BOX (AFB) proteins also function as auxin receptors, a potentially complete, and surprisingly simple, signaling pathway from perception to transcriptional response is now before us. However, understanding how this seemingly simple pathway controls the myriad of specific auxin responses remains a daunting challenge, and compelling evidence exists for SCFTIR1/AFB-independent auxin signaling pathways.

In eukaryotes, the ubiquitin–proteasome system participates in the control of signal transduction events by selectively eliminating regulatory proteins. E3 ubiquitin ligases specifically bind degradation substrates and mediate their poly-ubiquitylation, a prerequisite for their degradation by the 26S proteasome. On the basis of the analysis of the Arabidopsis genome sequence, it is predicted that there are more than 1000 E3 ubiquitin ligases in plants. Several types of E3 ubiquitin ligases have already been characterized in eukaryotes. Recently, some of these E3 enzymes have been implicated in specific plant signaling pathways.