Bedeutung und Regulation des Methioninzyklus in Pflanzen

Im Methioninzyklus wird Methylthioadenosin (MTA), das Nebenprodukt der Ethylen-, Polyamin- und Nicotianaminbiosynthese, über eine Reihe von Zwischenschritten zu Methionin (Met) recycelt. Die Regulation und Bedeutung dieses Zyklus sind bislang nur wenig verstanden. In dieser Arbeit wurde die Bedeutung des Methioninzyklus für die Ethylensynthese, sowie für die Metabolisierung von MTA über die Analyse von Mutanten in Arabidopsis untersucht.As a by-product of ethylene, polyamine and nicotianamine synthesis, methylthioadenosine (MTA) is released and subsequently recycled to methionine (Met) through the Met cycle. The role of the Met cycle in plant sulfur metabolism as well as its regulation are only poorly understood. In this study Arabidopsis mutants were employed in order to analyze the role of the Met cycle for ethlylene synthesis and for the metabolization of MTA

A novel plant actin-microtubule bridging complex regulates cytoskeletal and ER structure at ER-PM contact sites

In plants, the cortical endoplasmic reticulum (ER) network is connected to the plasma membrane (PM) through the ER-PM contact sites (EPCSs), whose structures are maintained by EPCS resident proteins and the cytoskeleton.1, 2, 3, 4, 5, 6, 7 Strong co-alignment between EPCSs and the cytoskeleton is observed in plants,1,8 but little is known of how the cytoskeleton is maintained and regulated at the EPCS. Here, we have used a yeast-two-hybrid screen and subsequent in vivo interaction studies in plants by fluorescence resonance energy transfer (FRET)-fluorescence lifetime imaging microscopy (FLIM) analysis to identify two microtubule binding proteins, KLCR1 (kinesin-light-chain-related protein 1) and IQD2 (IQ67-domain 2), that interact with the actin binding protein NET3C and form a component of plant EPCS that mediates the link between the actin and microtubule networks. The NET3C-KLCR1-IQD2 module, acting as an actin-microtubule bridging complex, has a direct influence on ER morphology and EPCS structure. Their loss-of-function mutants, net3a/NET3C RNAi, klcr1, or iqd2, exhibit defects in pavement cell morphology, which we suggest is linked to the disorganization of both actin filaments and microtubules. In conclusion, our results reveal a novel cytoskeletal-associated complex, which is essential for the maintenance and organization of cytoskeletal structure and ER morphology at the EPCS and for normal plant cell morphogenesis

SAM levels, gene expression of SAM synthetase, methionine synthase and ACC oxidase, and ethylene emission from N. suaveolens flowers

S′adenosyl-l-methionine (SAM) is a ubiquitous methyl donor and a precursor in the biosynthesis of ethylene, polyamines, biotin, and nicotianamine in plants. Only limited information is available regarding its synthesis (SAM cycle) and its concentrations in plant tissues. The SAM concentrations in flowers of Nicotiana suaveolens were determined during day/night cycles and found to fluctuate rhythmically between 10 and 50 nmol g−1 fresh weight. Troughs of SAM levels were measured in the evening and night, which corresponds to the time when the major floral scent compound, methyl benzoate, is synthesized by a SAM dependent methyltransferase (NsBSMT) and when this enzyme possesses its highest activity. The SAM synthetase (NsSAMS1) and methionine synthase (NsMS1) are enzymes, among others, which are involved in the synthesis and regeneration of SAM. Respective genes were isolated from a N. suaveolens petal cDNA library. Transcript accumulation patterns of both SAM regenerating enzymes matched perfectly those of the bifunctional NsBSMT; maximum mRNA accumulations of NsMS1 and NsSAMS1 were attained in the evening. Ethylene, which is synthesized from SAM, reached only low levels of 1–2 ppbv in N. suaveolens flowers. It is emitted in a burst at the end of the life span of the flowers, which correlates with the increased expression of the 1-aminocyclopropane-1-carboxylate oxidase (NsACO)

OsMTN Encodes a 5′-methylthioadenosine Nucleosidase that is Up-Regulated During Submergence-Induced Ethylene Synthesis in Rice (\u3cem\u3eOryza sativa\u3c/em\u3e L.)

Methylthioadenosine (MTA) is released as a by-product of S-adenosylmethionine (AdoMet)-dependent reactions central to ethylene, polyamine, or phytosiderophore biosynthesis. MTA is hydrolysed by methylthioadenosine nucleosidase (MTN; EC 3.2.2.16) into adenine and methylthioribose which is processed through the methionine (Met) cycle to produce a new molecule of AdoMet. In deepwater rice, submergence enhances ethylene biosynthesis, and ethylene in turn influences the methionine cycle through positive feedback regulation of the acireductone dioxygenase gene OsARD1. In rice, MTN is encoded by a single gene designated OsMTN. Recombinant OsMTN enzyme had a KM for MTA of 2.1 mM and accepted a wide array of 5′ substitutions of the substrate. OsMTN also metabolized S-adenosylhomocysteine (AdoHcy) with 15.9% the rate of MTA. OsMTN transcripts and OsMTN-specific activity increased slowly and in parallel upon submergence, indicating that regulation occurred mainly at the transcriptional level. Neither ethylene, MTA, nor Met regulated OsMTN expression. Analysis of steady-state metabolite levels showed that MTN activity was sufficiently high to prevent Met and AdoMet depletion during long-term ethylene biosynthesis

The secreted PAMP-induced peptide StPIP1_1 activates immune responses in potato

Abstract Treatment of potato plants with the pathogen-associated molecular pattern Pep-13 leads to the activation of more than 1200 genes. One of these, StPIP1_1, encodes a protein of 76 amino acids with sequence homology to PAMP-induced secreted peptides (PIPs) from Arabidopsis thaliana. Expression of StPIP1_1 is also induced in response to infection with Phytophthora infestans, the causal agent of late blight disease. Apoplastic localization of StPIP1_1-mCherry fusion proteins is dependent on the presence of the predicted signal peptide. A synthetic peptide corresponding to the last 13 amino acids of StPIP1_1 elicits the expression of the StPIP1_1 gene itself, as well as that of pathogenesis related genes. The oxidative burst induced by exogenously applied StPIP1_1 peptide in potato leaf disks is dependent on functional StSERK3A/B, suggesting that StPIP1_1 perception occurs via a receptor complex involving the co-receptor StSERK3A/B. Moreover, StPIP1_1 induces expression of FRK1 in Arabidopsis in an RLK7-dependent manner. Expression of an RLK from potato with high sequence homology to AtRLK7 is induced by StPIP1_1, by Pep-13 and in response to infection with P. infestans. These observations are consistent with the hypothesis that, upon secretion, StPIP1_1 acts as an endogenous peptide required for amplification of the defense response

Auxin-dependent control of cytoskeleton and cell shape regulates division orientation in the Arabidopsis embryo

Premitotic control of cell division orientation is critical for plant development, as cell walls prevent extensive cell remodeling or migration. While many divisions are proliferative and add cells to existing tissues, some divisions are formative and generate new tissue layers or growth axes. Such formative divisions are often asymmetric in nature, producing daughters with different fates. We have previously shown that, in the Arabidopsis thaliana embryo, developmental asymmetry is correlated with geometric asymmetry, creating daughter cells of unequal volume. Such divisions are generated by division planes that deviate from a default “minimal surface area” rule. Inhibition of auxin response leads to reversal to this default, yet the mechanisms underlying division plane choice in the embryo have been unclear. Here, we show that auxin-dependent division plane control involves alterations in cell geometry, but not in cell polarity axis or nuclear position. Through transcriptome profiling, we find that auxin regulates genes controlling cell wall and cytoskeleton properties. We confirm the involvement of microtubule (MT)-binding proteins in embryo division control. Organization of both MT and actin cytoskeleton depends on auxin response, and genetically controlled MT or actin depolymerization in embryos leads to disruption of asymmetric divisions, including reversion to the default. Our work shows how auxin-dependent control of MT and actin cytoskeleton properties interacts with cell geometry to generate asymmetric divisions during the earliest steps in plant development