Publikationen - Molekulare Signalverarbeitung

Jasmonates (JAs) are plant hormones that integrate external stress stimuli with physiological responses. (+)-7-iso-JA-L-Ile is the natural JA ligand of COI1, a component of a known JA receptor. The upstream JA biosynthetic precursor cis-(+)-12-oxo-phytodienoic acid (cis-(+)-OPDA) has been reported to act independently of COI1 as an essential signal in several stress-induced and developmental processes. Wound-induced increases in the endogenous levels of JA/JA-Ile are accompanied by two to tenfold increases in the concentration of OPDA, but its means of perception and metabolism are unknown. To screen for putative OPDA metabolites, vegetative tissues of flowering Arabidopsis thaliana were extracted with 25% aqueous methanol (v/v), purified by single-step reversed-phase polymer-based solid-phase extraction, and analyzed by high throughput mass spectrometry. This enabled the detection and quantitation of a low abundant OPDA analog of the biologically active (+)-7-iso-JA-L-Ile in plant tissue samples. Levels of the newly identified compound and the related phytohormones JA, JA-Ile and cis-(+)-OPDA were monitored in wounded leaves of flowering Arabidopsis lines (Col-0 and Ws) and compared to the levels observed in Arabidopsis mutants deficient in the biosynthesis of JA (dde2-2, opr3) and JA-Ile (jar1). The observed cis-(+)-OPDA-Ile levels varied widely, raising questions concerning its role in Arabidopsis stress responses.

The historic developmental hourglass concept depicts the convergence of animal embryos to a common form during the phylotypic period. Recently, it has been shown that a transcriptomic hourglass is associated with this morphological pattern, consistent with the idea of underlying selective constraints due to intense molecular interactions during body plan establishment. Although plants do not exhibit a morphological hourglass during embryogenesis, a transcriptomic hourglass has nevertheless been identified in the model plant Arabidopsis thaliana. Here, we investigated whether plant hourglass patterns are also found postembryonically. We found that the two main phase changes during the life cycle of Arabidopsis, from embryonic to vegetative and from vegetative to reproductive development, are associated with transcriptomic hourglass patterns. In contrast, flower development, a process dominated by organ formation, is not. This suggests that plant hourglass patterns are decoupled from organogenesis and body plan establishment. Instead, they may reflect general transitions through organizational checkpoints.

Auxin coordinates plant development largely via hierarchical control of gene expression. During the past decades, the study of early auxin genes paired with the power of Arabidopsis genetics have unraveled key nuclear components and molecular interactions that perceive the hormone and activate primary response genes. Recent research in the realm of structural biology allowed unprecedented insight into: (i) the recognition of auxin-responsive DNA elements by auxin transcription factors; (ii) the inactivation of those auxin response factors by early auxin-inducible repressors; and (iii) the activation of target genes by auxin-triggered repressor degradation. The biophysical studies reviewed here provide an impetus for elucidating the molecular determinants of the intricate interactions between core components of the nuclear auxin response module.

Oxylipins of the jasmonate family are active as signals in plant responses to biotic and abiotic stresses as well as in development. Jasmonic acid (JA), its precursor cis-12-oxo-phytodienoic acid (OPDA) and the isoleucine conjugate of JA (JA-Ile) are the most prominent members. OPDA and JA-Ile have individual signalling properties in several processes and differ in their pattern of gene expression. JA-Ile, but not OPDA, is perceived by the SCFCOI1-JAZ co-receptor complex. There are, however, numerous processes and genes specifically induced by OPDA. The recently identified OPDA-Ile suggests that OPDA specific responses might be mediated upon formation of OPDA-Ile. Here, we tested OPDA-Ile-induced gene expression in wild type and JA-deficient, JA-insensitive and JA-Ile-deficient mutant background. Tests on putative conversion of OPDA-Ile during treatments revealed only negligible conversion. Expression of two OPDA-inducible genes, GRX480 and ZAT10, by OPDA-Ile could be detected in a JA-independent manner in Arabidopsis seedlings but less in flowering plants. The data suggest a bioactivity in planta of OPDA-Ile.

Jasmonates (JAs) are lipid-derived signals in plant stress responses and development. A crucial step in JA biosynthesis is catalyzed by allene oxide cyclase (AOC). Four genes encoding functional AOCs (AOC1, AOC2, AOC3 and AOC4) have been characterized for Arabidopsis thaliana in terms of organ- and tissue-specific expression, mutant phenotypes, promoter activities and initial in vivo protein interaction studies suggesting functional redundancy and diversification, including first hints at enzyme activity control by protein-protein interaction. Here, these analyses were extended by detailed analysis of recombinant proteins produced in Escherichia coli. Treatment of purified AOC2 with SDS at different temperatures, chemical cross-linking experiments and protein structure analysis by molecular modelling approaches were performed. Several salt bridges between monomers and a hydrophobic core within the AOC2 trimer were identified and functionally proven by site-directed mutagenesis. The data obtained showed that AOC2 acts as a trimer. Finally, AOC activity was determined in heteromers formed by pairwise combinations of the four AOC isoforms. The highest activities were found for heteromers containing AOC4 + AOC1 and AOC4 + AOC2, respectively. All data are in line with an enzyme activity control of all four AOCs by heteromerization, thereby supporting a putative fine-tuning in JA formation by various regulatory principles.

Eggplant latent viroid (ELVd), like other members of family Avsunviroidae, replicates in plastids through a symmetric rolling-circle mechanism in which elongation of RNA strands is most likely catalyzed by a nuclear-encoded polymerase (NEP) translocated to plastids. Here we have addressed where NEP initiates transcription of viroid strands. Because this step is presumably directed by sequence/structural motifs, we have previously determined the conformation of the monomeric linear (+) and (−) RNAs of ELVd resulting from hammerhead-mediated self-cleavage. In silico predictions with 3 softwares led to similar bifurcated conformations for both ELVd strands. In vitro examination by non-denaturing PAGE showed that they migrate as prominent single bands, with the ELVd (+) RNA displaying a more compact conformation as revealed by its faster electrophoretic mobility. In vitro SHAPE analysis corroborated the ELVd conformations derived from thermodynamics-based predictions in silico. Moreover, sequence analysis of 94 full-length natural ELVd variants disclosed co-variations, and mutations converting canonical into wobble pairs or vice versa, which confirmed in vivo most of the stems predicted in silico and in vitro, and additionally helped to introduce minor structural refinements. Therefore, results from the 3 experimental approaches were essentially consistent among themselves. Application to RNA preparations from ELVd-infected tissue of RNA ligase-mediated rapid amplification of cDNA ends, combined with pretreatments to modify the 5′ ends of viroid strands, mapped the transcription initiation sites of ELVd (+) and (−) strands in vivo at different sequence/structural motifs, in contrast with the situation previously observed in 2 other members of the family Avsunviroidae.

Reduced height 8 (Rht8) is the main alternative to the GA-insensitive Rht alleles in hot and dry environments where it reduces plant height without yield penalty. The potential of Rht8 in northern-European wheat breeding remains unclear, since the close linkage with the photoperiod-insensitive allele Ppd-D1a is unfavourable in the relatively cool summers. In the present study, two near-isogenic lines (NILs) contrasting for the Rht8/tall allele from Mara in a UK-adapted and photoperiod-sensitive wheat variety were evaluated in trials with varying nitrogen fertiliser (N) treatments and water regimes across sites in the UK and Spain.The Rht8 introgression was associated with a robust height reduction of 11% regardless of N treatment and water regime and the Rht8 NIL was more resistant to root-lodging at agronomically-relevant N levels than the tall NIL. In the UK with reduced solar radiation over the growing season than the site in Spain, the Rht8 NIL showed a 10% yield penalty at standard agronomic N levels due to concomitant reduction in grain number and spike number whereas grain weight and harvest index were not significantly different to the tall NIL. The yield penalty associated with the Rht8 introgression was overcome at low N and in irrigated conditions in the UK, and in the high-temperature site in Spain. Decreased spike length and constant spikelet number in the Rht8 NIL resulted in spike compaction of 15%, independent of N and water regime. The genetic interval of Rht8 overlaps with the compactum gene on 2DS, raising the possibility of the same causative gene. Further genetic dissection of these loci is required.

BackgroundPlant adaptation to limited phosphate availability comprises a wide range of responses to conserve and remobilize internal phosphate sources and to enhance phosphate acquisition. Vigorous restructuring of root system architecture provides a developmental strategy for topsoil exploration and phosphate scavenging. Changes in external phosphate availability are locally sensed at root tips and adjust root growth by modulating cell expansion and cell division. The functionally interacting Arabidopsis genes, LOW PHOSPHATE RESPONSE 1 and 2 (LPR1/LPR2) and PHOSPHATE DEFICIENCY RESPONSE 2 (PDR2), are key components of root phosphate sensing. We recently demonstrated that the LOW PHOSPHATE RESPONSE 1 - PHOSPHATE DEFICIENCY RESPONSE 2 (LPR1-PDR2) module mediates apoplastic deposition of ferric iron (Fe3+) in the growing root tip during phosphate limitation. Iron deposition coincides with sites of reactive oxygen species generation and triggers cell wall thickening and callose accumulation, which interfere with cell-to-cell communication and inhibit root growth.ResultsWe took advantage of the opposite phosphate-conditional root phenotype of the phosphate deficiency response 2 mutant (hypersensitive) and low phosphate response 1 and 2 double mutant (insensitive) to investigate the phosphate dependent regulation of gene and protein expression in roots using genome-wide transcriptome and proteome analysis. We observed an overrepresentation of genes and proteins that are involved in the regulation of iron homeostasis, cell wall remodeling and reactive oxygen species formation, and we highlight a number of candidate genes with a potential function in root adaptation to limited phosphate availability. Our experiments reveal that FERRIC REDUCTASE DEFECTIVE 3 mediated, apoplastic iron redistribution, but not intracellular iron uptake and iron storage, triggers phosphate-dependent root growth modulation. We further highlight expressional changes of several cell wall-modifying enzymes and provide evidence for adjustment of the pectin network at sites of iron accumulation in the root.ConclusionOur study reveals new aspects of the elaborate interplay between phosphate starvation responses and changes in iron homeostasis. The results emphasize the importance of apoplastic iron redistribution to mediate phosphate-dependent root growth adjustment and suggest an important role for citrate in phosphate-dependent apoplastic iron transport. We further demonstrate that root growth modulation correlates with an altered expression of cell wall modifying enzymes and changes in the pectin network of the phosphate-deprived root tip, supporting the hypothesis that pectins are involved in iron binding and/or phosphate mobilization.

Jasmonic acid and other fatty‐acid‐derived compounds called oxylipins are signals in stress responses and development of plants. The receptor complex, signal transduction components as well as repressors and activators in jasmonate‐induced gene expression have been elucidated. Different regulatory levels and cross‐talk with other hormones are responsible for the multiplicity of plant responses to environmental and developmental cues.

Mutualistic symbioses between plant roots and microorganisms can reduce the demand for chemical fertilizers in agriculture. Most crops are able to establish arbuscular mycorrhiza (AM) symbiosis with fungi to take up phosphate more efficiently. A second symbiosis, nitrogen-fixing root nodule symbiosis, supersedes energy-intensive nitrogen fertilization: Legumes such as peas, clover and soybeans take up rhizobia – special bacteria that are capable of converting atmospheric nitrogen into ammonium – into their root cells. Plant root cells perceive rhizobia and AM fungi via very similar signaling molecules (N-acetylglucosamine tetra- or pentamers), even though the resultant developmental processes differ strongly. Interestingly, N-acetylglucosamine containing signals including fungal chitin- and bacterial peptidoglycan-fragments from their cell walls, also play a role in the recognition of pathogenic microorganisms.Despite the intrinsic sustainability potential of the nitrogen-fixing root nodule symbiosis, too much of a good thing, however, has led to global problems: The massive increase in global meat production is largely based on soybean. Large scale soybean monoculture destroyed ecosystems in South America. Large scale animal production results in excessive methane and nitrogen release into the environment, which causes climate change and death zones in marine ecosystems, respectively. This calls for a considerable reduction in meat consumption.