Arbuscular mycorrhiza (AM) is a widespread symbiosis between roots of the majority of land plants and Glomeromycotina fungi. AM is important for ecosystem health and functioning as the fungi critically support plant performance by providing essential mineral nutrients, particularly the poorly accessible phosphate, in exchange for organic carbon. AM fungi colonize the inside of roots and this is promoted at low but inhibited at high plant phosphate status, while the mechanistic basis for this phosphate-dependence remained obscure. Here we demonstrate that a major transcriptional regulator of phosphate starvation responses in rice PHOSPHATE STARVATION RESPONSE 2 (PHR2) regulates AM. Root colonization of phr2 mutants is drastically reduced, and PHR2 is required for root colonization, mycorrhizal phosphate uptake, and yield increase in field soil. PHR2 promotes AM by targeting genes required for pre-contact signaling, root colonization, and AM function. Thus, this important symbiosis is directly wired to the PHR2-controlled plant phosphate starvation response.

No abstract

Freshly brewed coffee is appreciated by consumers all over the world because of its stimulating effect, its characteristic taste centring on sourness and pleasant bitterness and its alluring aroma with characteristic "roasty/sulphurous" odour notes, culminating in the unique flavour sensation of coffee. The molecules responsible for the olfactory sensation of roasted coffee beans and percolated coffee beverages, analysed by means of the molecular sensory science approach are well understood. With previous studies providing qualitative and quantitative data, using aroma extract dilution analysis (AEDA), gas chromatography-olfactometry (GC-O) and headspace GC-MS techniques resulting in comprehensive aroma recombinates consisting of not more than 30 odorants [1][2][3]. Although the aroma of coffee can be reconstituted rather well, the impact of the melanoidin containing high molecular weight fractions (HMW) on the sensory quality of coffee beverages, is still mostly unclear on a molecular basis. Whereas former studies clearly indicated that especially odour active thiols exhibit high binding affinity to high molecular weight melanoidin fractions of coffee, only covalent interactions have been considered so far. The impact of non-covalent π-π interactions on coffee flavour is still completely unclear [4,5]. To get detailed insight into the molecular phenomenon of odorant polymer interactions and the sensory impact on coffee flavour perception, a quantitative 1 H-NMR based screening approach was developed, which allowed the direct and non-invasive analysis of molecular interactions between key coffee odorants, like 2-furfurylthiol and high molecular weight melanoidin polymers (>10 kDa). A clear distinction between covalent and non-covalent interactions was achieved by monitoring time dependency of odorant polymer interactions, with 2-furfurylthiol exhibiting π-π interactions as well as covalent bindings. In contrast, pyrazines and hydroxyphenols showed only non-covalent π-π stacking, whereas aldehydes incubated with HMW material showed only covalent interactions at prolonged incubation times. Furanones, as well as diketones showed no interactions with the HMW. Human sensory experiments with isolated HMW material >10 kDa and a full aroma recombinate of coffee were well in the line with the findings from the NMR based approach. A drastic reduction of "roasty/sulphurous" aroma notes in combination with a decrease in overall coffee-like odour quality, as well as an increased "sweetish/caramel-like" flavour was perceivable upon incubation of coffee melanoidins with the aroma recombinate. The lack of binding affinity of the sweetish/caramel smelling 4-hydroxy-2,5-dimethyl-3(2H)-furanone in combination with the high binding affinity of coffee thiols provides explanation of the sensory evaluation and might be the reason for the fast disappearance of the "roasty/sulphurous" aroma impressions of a freshly prepared coffee brew.

To gain comprehensive insight into the interactions of key coffee odorants, like the Strecker aldehydes, acetaldehyde, propanal, methylpropanal, 2- and 3-methylbutanal, and methional, and the nonvolatile fraction of coffee, an untargeted metabolomics approach was applied. Ultra performance liquid chromatography (UPLC)–time of flight (TOF)-mass spectrometry (ESI–) profiling followed by statistical data analysis revealed a marker substance for a coffee beverage spiked with acetaldehyde with an accurate mass of 217.0703 [M – H]−. This compound could be identified as a reaction product of quinic acid (QA) and acetaldehyde linked by acetalization at the cis-diol function of QA. Consequently, the acetalization of aldehydes, QA, 5-O-caffeoyl quinic acid (CQA), and quinic acid γ-lactone (QAL) was investigated by means of model reactions, followed by synthesis, isolation, and structure elucidation via UPLC–TOF-MS and 1D and 2D NMR techniques. UHPLC–MS/MSMRM screening and the quantification of aldehyde adducts in coffee beverages revealed the presence of QA/acetaldehyde, -/propanal, -/methylpropanal, and -/methional reaction products and CQA/acetaldehyde, -/propanal, -/methylpropanal, -/2- and 3-methylbutanal, and -/methional and QAL/acetaldehyde adducts for the first time, in concentrations of 12–270 μg/L for QA/aldehydes, 5–225 μg/L for CQA/aldehydes, and 62–173 μg/L for QAL/acetaldehyde. The sensory characterization of the identified compounds showed bitter taste recognition thresholds of 48–297 μmol/L for CQA adducts and 658 μmol/L for QAL/acetaldehyde, while the QA adducts showed no bitter taste (<2000 μmol/L).