Markus C. Baier

Antisense Repression of the Medicago truncatula Nodule-Enhanced Sucrose Synthase Leads to a Handicapped Nitrogen Fixation Mirrored by Specific Alterations in the Symbiotic Transcriptome and Metabolome

We analyzed the role of the sucrose (Suc) synthase MtSucS1 during nodulation of the model legume Medicago truncatula, integrating data for the developmental, transcriptional, and metabolic processes affected downstream of an impaired Suc cleavage in root nodules. To reduce carbohydrate supply to nodule tissues, transgenic plants expressing a p35S-driven MtSucS1-antisense fusion were constructed. These plants displayed an up to 90% reduction of MtSucS1 proteins in roots and nodules. Phenotypic studies of two independent MtSucS1-reduced lines demonstrated that only under conditions depending on nodulation, these plants appeared to be impaired in above-ground growth. Specifically plant height, shoot weight, leaf development, flowering, as well as seed maturation were reduced, and the efficiency of photosynthesis was affected. Concomitantly, a significantly enhanced root to shoot ratio with a marked increase in root tip numbers was observed. Root nodule formation was found retarded and the impaired nodulation was accompanied by a less efficient nitrogen (N) acquisition. The decreased total N content of MtSucS1-antisense lines and an enhanced carbon to N ratio in roots, nodules, and shoots correlated with the extent of MtSucS1 knockdown. On the level of transcription, effects of an MtSucS1 reduction were evident for genes representing important nodes of the nodule carbon and N metabolism, while metabolite profiling revealed significantly lower levels of amino acids and their derivatives particularly in strongly MtSucS1-reduced nodules. Our results support the model that nodule-enhanced Suc synthase 1 of the model legume M. truncatula is required for the establishment and maintenance of an efficient N-fixing symbiosis.

Knockdown of the Symbiotic Sucrose Synthase MtSucS1 Affects Arbuscule Maturation and Maintenance in Mycorrhizal Roots of Medicago truncatula

The relevance of the symbiosis-induced Medicago truncatula sucrose synthase gene MtSucS1 for an efficient arbuscular mycorrhiza (AM) was studied using two independent antisense lines that displayed up to 10-fold reduced SucS1 levels in roots. Mycorrhizal MtSucS1-reduced lines exhibited an overall stunted aboveground growth under inorganic phosphorus limitation. Apart from a reduced plant height, shoot weight, and leaf development, a delayed flowering, resulting in a lower seed yield, was observed. In addition, the root-to-shoot and root weight ratios increased significantly. Gene expression studies demonstrated a major reversion of AM-associated transcription, exhibiting a significant repression of well-known plant AM marker and mycosymbiont genes, together indicating a diminished AM fungus colonization of MtSucS1-antisense lines. Concomitantly, gas chromatography-mass spectrometry-based metabolite profiling revealed that mycorrhizal MtSucS1-reduced lines were affected in important nodes of the carbon, nitrogen, and phosphorus metabolism, accentuating a physiological significance of MtSucS1 for AM. In fact, antisensing MtSucS1 provoked an impaired fungal colonization within the less abundant infected regions, evident from strongly reduced frequencies of internal hyphae, vesicles, and arbuscules. Moreover, arbuscules were early senescing, accompanied with a reduced development of mature arbuscules. This defective mycorrhiza status correlated with reduced phosphorus and nitrogen levels and was proportional to the extent of MtSucS1 knockdown. Together, our results point to an important role for MtSucS1 in the establishment and maintenance of arbuscules in the AM symbiosis.

Chemical fingerprints encode mother–offspring similarity, colony membership, relatedness, and genetic quality in fur seals

Significance Understanding olfactory communication in natural vertebrate populations requires knowledge of how genes and the environment influence highly complex individual chemical fingerprints. To understand how relevant information is chemically encoded and may feed into mother–offspring recognition, we therefore generated chemical and genetic data for Antarctic fur seal mother–pup pairs. We show that pups are chemically highly similar to their mothers, reflecting a combination of genetic and environmental influences. We also reveal associations between chemical fingerprints and both genetic quality and relatedness, the former correlating positively with substance diversity and the latter encoded mainly by a small subset of substances. Dissecting apart chemical fingerprints to reveal subsets of potential biological relevance has broad implications for understanding vertebrate chemical communication. Chemical communication underpins virtually all aspects of vertebrate social life, yet remains poorly understood because of its highly complex mechanistic basis. We therefore used chemical fingerprinting of skin swabs and genetic analysis to explore the chemical cues that may underlie mother–offspring recognition in colonially breeding Antarctic fur seals. By sampling mother–offspring pairs from two different colonies, using a variety of statistical approaches and genotyping a large panel of microsatellite loci, we show that colony membership, mother–offspring similarity, heterozygosity, and genetic relatedness are all chemically encoded. Moreover, chemical similarity between mothers and offspring reflects a combination of genetic and environmental influences, the former partly encoded by substances resembling known pheromones. Our findings reveal the diversity of information contained within chemical fingerprints and have implications for understanding mother–offspring communication, kin recognition, and mate choice.

The signal peptide of the Medicago truncatula modular nodulin MtNOD25 operates as an address label for the specific targeting of proteins to nitrogen-fixing symbiosomes.

The nodule-specific MtNOD25 gene of the model legume Medicago truncatula encodes a modular nodulin composed of different repetitive modules flanked by distinct N- and C-termini. Although similarities are low with respect to all repetitive modules, both the N-terminal signal peptide (SP) and the C-terminus are highly conserved in modular nodulins from different legumes. On the cellular level, MtNOD25 is only transcribed in the infected cells of root nodules, and this activation is mediated by a 299-bp minimal promoter containing an organ-specific element. By expressing mGFP6 translational fusions in transgenic nodules, we show that MtNOD25 proteins are exclusively translocated to the symbiosomes of infected cells. This specific targeting only requires an N-terminal MtNOD25 SP that is highly conserved across a family of legume-specific symbiosome proteins. Our finding sheds light on one possible mechanism for the delivery of host proteins to the symbiosomes of infected root nodule cells and, in addition, defines a short molecular address label of only 24 amino acids whose N-terminal presence is sufficient to translocate proteins across the peribacteroid membrane.

Differences in olfactory species recognition in the females of two Australian songbird species

Although birds have recently been shown to possess olfactory abilities and to use chemical cues in communication, limited effort has been made to demonstrate the use of odorants in social contexts. Even less is known regarding the use of odorants in species recognition. The ability to recognize conspecifics should be more pronounced in social species. This study investigated the importance of olfactory cues in species recognition in females of two estrildid finch species with different levels of sociality. Combining odor preference tests with chemical analyses, we surveyed whether female zebra finches and diamond firetails are able to distinguish between the species based on volatile traits and whether individuals exhibit species-specific differences in body odorants. Zebra finches are more social than diamond firetails; nevertheless, both species have an overlapping distribution area. Applying an experimental Y-maze paradigm, we showed that zebra finches can use differences in their species odor fingerprints and displayed a significant preference for the odor of conspecifics over that of heterospecifics, whereas diamond firetails did not reveal a preference. Using gas chromatography and mass spectrometry, we demonstrated that body odorants of the two species were significantly different in relative composition. This finding demonstrates the potential importance of olfactory cues in species recognition, at least in social bird species. Even these two closely related species displayed remarkable differences in their responsiveness to similar chemical cues, which might be caused by species-specific differences in ecology, physiology, or evolution.

Arbuscular mycorrhiza-induced shifts in foliar metabolism and photosynthesis mirror the developmental stage of the symbiosis and are only partly driven by improved phosphate uptake.

In arbuscular mycorrhizal (AM) plants, the plant delivers photoassimilates to the arbuscular mycorrhizal fungus (AMF), whereas the mycosymbiont contributes, in addition to other beneficial effects, to phosphate (PO4(3-)) uptake from the soil. Thereby, the additional fungal carbon (C) sink strength in roots and improved plant PO4(3-) nutrition may influence aboveground traits. We investigated how the foliar metabolome of Plantago major is affected along with the development of root symbiosis, whether the photosynthetic performance is affected by AM, and whether these effects are mediated by improved PO4(3-) nutrition. Therefore, we studied PO4(3-)-limited and PO4(3-)-supplemented controls in comparison with mycorrhizal plants at 20, 30, and 62 days postinoculation with the AMF Rhizophagus irregularis. Foliar metabolome modifications were determined by the developmental stage of symbiosis, with changes becoming more pronounced over time. In a well-established stage of mature mutualism, about 60% of the metabolic changes and an increase in foliar CO2 assimilation were unrelated to the significantly increased foliar phosphorus (P) content. We propose a framework relating the time-dependent metabolic changes to the shifts in C costs and P benefits for the plant. Besides P-mediated effects, the strong fungal C sink activity may drive the changes in the leaf traits.

Larval food composition affects courtship song and sperm expenditure in a lekking moth

1. Variation in larval food composition can have far‐reaching effects on the adult phenotype of insects. To maximise reproductive output, it is therefore beneficial if insects are able to plastically adapt to nutritional cues in their larval diet.

Chemical Analyses Reveal Family-Specific Nest Odor Profiles in Zebra Finches ( Taeniopygia guttata ): A Pilot Study

Several bird species, including zebra finches, have been shown to be able to recognize their own nest, based on olfactory cues. This suggests that differences in nest odor profiles exist. One might assume that these differences are due to differences in food or nesting material. However, laboratory studies led to the assumption that individual body odors of parents and offspring are causing the nest odor differences. In a pilot study, we collected volatile nest odors of zebra finch nests using closed-loop stripping and analyzed them by gas chromatography coupled with mass spectrometry (GC-MS). Our analysis revealed the existence of family-specific nest odor profiles that vary in concentrations of different compounds, providing the basis for odor-based nest recognition.