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+.

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.

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.

Several mammalian peptide hormones and proteins from plant and animal origin contain an N-terminal pyroglutamic acid (pGlu) residue. Frequently, the moiety is important in exerting biological function in either mediating interaction with receptors or stabilizing against N-terminal degradation. Glutaminyl cyclases (QCs) were isolated from different plants and animals catalyzing pGlu formation. The recent resolution of the 3D structures of Carica papaya and human QCs clearly supports different evolutionary origins of the proteins, which is also reflected by different enzymatic mechanisms. The broad substrate specificity is revealed by the heterogeneity of physiological substrates of plant and animal QCs, including cytokines, matrix proteins and pathogenesis-related proteins. Moreover, recent evidence also suggests human QC as a catalyst of pGlu formation at the N-terminus of amyloid peptides, which contribute to Alzheimer's disease. Obviously, owing to its biophysical properties, the function of pGlu in plant and animal proteins is very similar in terms of stabilizing or mediating protein and peptide structure. It is possible that the requirement for catalysis of pGlu formation under physiological conditions may have triggered separate evolution of QCs in plants and animals.

Glutaminyl cyclases (QCs) catalyze the formation of pyroglutamic acid at the N-terminus of several peptides and proteins. On the basis of the amino acid sequence of Carica papaya QC, we identified cDNAs of the putative counterparts from Solanum tuberosum and Arabidopsis thaliana. Upon expression of the corresponding cDNAs from both plants via the secretory pathway of Pichia pastoris, two active QC proteins were isolated. The specificity of the purified proteins was assessed using various substrates with different amino acid composition and length. Highest specificities were observed with substrates possessing large hydrophobic residues adjacent to the N-terminal glutamine and for fluorogenic dipeptide surrogates. However, compared to Carica papaya QC, the specificity constants were approximately one order of magnitude lower for most of the QC substrates analyzed. The QCs also catalyzed the conversion of N-terminal glutamic acid to pyroglutamic acid, but with approximately 105- to 106-fold lower specificity. The ubiquitous distribution of plant QCs prompted a search for potential substrates in plants. Based on database entries, numerous proteins, e.g., pathogenesis-related proteins, were found that carry a pyroglutamate residue at the N-terminus, suggesting QC involvement. The putative relevance of QCs and pyroglutamic acid for plant defense reactions is discussed.

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.

Glutaminyl cyclases (QC) catalyze the intramolecular cyclization of N-terminal glutamine residues of peptides and proteins. For a comparison of the substrate specificity of human and papaya QC enzymes, a novel continuous assay was established by adapting an existing discontinuous method. Specificity constants (kcat/Km) of dipeptides and dipeptide surrogates were higher for plant QC, whereas the selectivity for oligopeptides was similar for both enzymes. However, only the specificity constants of mammalian QC were dependent on size and composition of the substrates. Specificity constants of both enzymes were equally pH-dependent in the acidic pH-region, revealing a pKa value identical to the pKa of the substrate, suggesting similarities in the substrate conversion mode. Accordingly, both QCs converted the L-?homoglutaminyl residue in the peptide H-?homoGln-Phe-Lys-Arg-Leu-Ala-NH2 and the glutaminyl residues of the branched peptide H-Gln-Lys(Gln)-Arg-Leu-Ala-NH2 as well as the partially cyclized peptide H-Gln-cyclo( N?-Lys-Arg-Pro-Ala-Gly-Phe). In contrast, only QC from C. papaya was able to cyclize a methylated glutamine residue, while this compound did not even inhibit human QC-catalysis, suggesting distinct substrate recognition pattern. The conversion of the potential physiological substrates gastrin, neurotensin and [GlN1]-fertilization promoting peptide indicates that human QC may play a key role in posttranslational modification of most if not all pGlu-containing hormones.

In addition to a previously characterized 13-lipoxygenase of 100 kDa encoded by LOX2:Hv:1 [Vörös et al., Eur. J. Biochem. 251 (1998), 36 44], two fulllength cDNAs (LOX2:Hv:2, LOX2:Hv:3) were isolated from barley leaves (Hordeum vulgare cv. Salome) and characterized. Both of them encode 13-lipoxygenases with putative target sequences for chloroplast import. Immunogold labeling revealed preferential, if not exclusive, localization of lipoxygenase proteins in the stroma. The ultrastructure of the chloroplast was dramatically altered following methyl jasmonate treatment, indicated by a loss of thylakoid membranes, decreased number of stacks and appearance of numerous osmiophilic globuli. The three 13-lipoxygenases are differentially expressed during treatment with jasmonate, salicylate, glucose or sorbitol. Metabolite profiling of free linolenic acid and free linoleic acid, the substrates of lipoxygenases, in water floated or jasmonatetreated leaves revealed preferential accumulation of linolenic acid. Remarkable amounts of free 9- as well as 13-hydroperoxy linolenic acid were found. In addition, metabolites of these hydroperoxides, such as the hydroxy derivatives and the respective aldehydes, appeared following methyl jasmonate treatment. These findings were substantiated by metabolite profiling of isolated chloroplasts, and subfractions including the envelope, the stroma and the thylakoids, indicating a preferential occurrence of lipoxygenasederived products in the stroma and in the envelope. These data revealed jasmonateinduced activation of the hydroperoxide lyase and reductase branch within the lipoxygenase pathway and suggest differential activity of the three 13-lipoxygenases under different stress conditions.