Almond (
) is the principal
species in which the consumed and thus commercially important part of the fruit is the kernel. As a result of continued selection, the vast majority of almonds have a ...nonbitter kernel. However, in the field, there are trees carrying bitter kernels, which are toxic to humans and, consequently, need to be removed. The toxicity of bitter almonds is caused by the accumulation of the cyanogenic diglucoside amygdalin, which releases toxic hydrogen cyanide upon hydrolysis. In this study, we identified and characterized the enzymes involved in the amygdalin biosynthetic pathway: PdCYP79D16 and PdCYP71AN24 as the cytochrome P450 (CYP) enzymes catalyzing phenylalanine-to-mandelonitrile conversion, PdUGT94AF3 as an additional monoglucosyl transferase (UGT) catalyzing prunasin formation, and PdUGT94AF1 and PdUGT94AF2 as the two enzymes catalyzing amygdalin formation from prunasin. This was accomplished by constructing a sequence database containing UGTs known, or predicted, to catalyze a β(1→6)-
-glycosylation reaction and a Basic Local Alignment Search Tool search of the draft version of the almond genome versus these sequences. Functional characterization of candidate genes was achieved by transient expression in
Reverse transcription quantitative polymerase chain reaction demonstrated that the expression of
and
was not detectable or only reached minute levels in the sweet almond genotype during fruit development, while it was high and consistent in the bitter genotype. Therefore, the basis for the sweet kernel phenotype is a lack of expression of the genes encoding the two CYPs catalyzing the first steps in amygdalin biosynthesis.
Cyanogenic glycosides are phytoanticipins involved in plant defence against herbivores by virtue of their ability to release toxic hydrogen cyanide (HCN) upon tissue disruption. In addition, ...endogenous turnover of cyanogenic glycosides without the liberation of HCN may offer plants an important source of reduced nitrogen at specific developmental stages. To investigate the presence of putative turnover products of cyanogenic glycosides, comparative metabolic profiling using LC-MS/MS and high resolution MS (HR-MS) complemented by ion-mobility MS was carried out in three cyanogenic plant species: cassava, almond and sorghum. In total, the endogenous formation of 36 different chemical structures related to the cyanogenic glucosides linamarin, lotaustralin, prunasin, amygdalin and dhurrin was discovered, including di- and tri-glycosides derived from these compounds. The relative abundance of the compounds was assessed in different tissues and developmental stages. Based on results common to the three phylogenetically unrelated species, a potential recycling endogenous turnover pathway for cyanogenic glycosides is described in which reduced nitrogen and carbon are recovered for primary metabolism without the liberation of free HCN. Glycosides of amides, carboxylic acids and 'anitriles' derived from cyanogenic glycosides appear as common intermediates in this pathway and may also have individual functions in the plant. The recycling of cyanogenic glycosides and the biological significance of the presence of the turnover products in cyanogenic plants open entirely new insights into the multiplicity of biological roles cyanogenic glycosides may play in plants.
The use of high-throughput phenotyping systems and non-destructive imaging is widely regarded as a key technology allowing scientists and breeders to develop crops with the ability to perform well ...under diverse environmental conditions. However, many of these phenotyping studies have been optimized using the model plant Arabidopsis thaliana. In this study, The Plant Accelerator® at The University of Adelaide, Australia, was used to investigate the growth and phenotypic response of the important cereal crop, Sorghum bicolor L. Moench and related hybrids to water-limited conditions and different levels of fertilizer. Imaging in different spectral ranges was used to monitor plant composition, chlorophyll, and moisture content. Phenotypic image analysis accurately measured plant biomass. The data set obtained enabled the responses of the different sorghum varieties to the experimental treatments to be differentiated and modelled. Plant architectural instead of architecture elements were determined using imaging and found to correlate with an improved tolerance to stress, for example diurnal leaf curling and leaf area index. Analysis of colour images revealed that leaf ‘greenness’ correlated with foliar nitrogen and chlorophyll, while near infrared reflectance (NIR) analysis was a good predictor of water content and leaf thickness, and correlated with plant moisture content. It is shown that imaging sorghum using a high-throughput system can accurately identify and differentiate between growth and specific phenotypic traits. R scripts for robust, parsimonious models are provided to allow other users of phenomic imaging systems to extract useful data readily, and thus relieve a bottleneck in phenotypic screening of multiple genotypes of key crop plants.
Summary
Cyanogenic glucosides are nitrogen‐containing specialized metabolites that provide chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide. It has been ...suggested that cyanogenic glucosides are also a store of nitrogen that can be remobilized for general metabolism via a previously unknown pathway. Here we reveal a recycling pathway for the cyanogenic glucoside dhurrin in sorghum (Sorghum bicolor) that avoids hydrogen cyanide formation. As demonstrated in vitro, the pathway proceeds via spontaneous formation of a dhurrin‐derived glutathione conjugate, which undergoes reductive cleavage by glutathione transferases of the plant‐specific lambda class (GSTLs) to produce p‐hydroxyphenyl acetonitrile. This is further metabolized to p‐hydroxyphenylacetic acid and free ammonia by nitrilases, and then glucosylated to form p‐glucosyloxyphenylacetic acid. Two of the four GSTLs in sorghum exhibited high stereospecific catalytic activity towards the glutathione conjugate, and form a subclade in a phylogenetic tree of GSTLs in higher plants. The expression of the corresponding two GSTLs co‐localized with expression of the genes encoding the p‐hydroxyphenyl acetonitrile‐metabolizing nitrilases at the cellular level. The elucidation of this pathway places GSTs as key players in a remarkable scheme for metabolic plasticity allowing plants to reverse the resource flow between general and specialized metabolism in actively growing tissue.
Significance Statement
Glutathione transferase enzymes (GSTs) are highly abundant in plants, but only very few have been assigned a physiological function. Here we demonstrate that GSTs are key players in a pathway for endogenous recycling of a cyanogenic glucoside defense compound in sorghum, revealing that plants continuously optimize, and can even reverse, their allocation of resources between general and specialized metabolism.
Flavin-dependent monooxygenases (FMOs) are ancient enzymes present in all kingdoms of life. FMOs typically catalyze the incorporation of an oxygen atom from molecular oxygen into small molecules. To ...date, the majority of functional characterization studies have been performed on mammalian, fungal and bacterial FMOs, showing that they play fundamental roles in drug and xenobiotic metabolism. By contrast, our understanding of FMOs across the plant kingdom is very limited, despite plants possessing far greater FMO diversity compared to both bacteria and other multicellular organisms. Here, we review the progress of plant FMO research, with a focus on FMO diversity and functionality. Significantly, of the FMOs characterized to date, they all perform oxygenation reactions that are crucial steps within hormone metabolism, pathogen resistance, signaling and chemical defense. This demonstrates the fundamental role FMOs have within plant metabolism, and presents significant opportunities for future research pursuits and downstream applications.
Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the ...Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013, ApJ, 762, L8).
Model stellar atmospheres are fundamental tools for understanding stellar observations from Interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also ...Include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.
Context. A longstanding challenge for understanding classical Cepheids is the Cepheid mass discrepancy, where theoretical mass estimates using stellar evolution and stellar pulsation calculations ...have been found to differ by approximately 10−20%. Aims. We study the role of pulsation-driven mass loss during the Cepheid stage of evolution as a possible solution to this mass discrepancy. Methods. We computed stellar evolution models with a Cepheid mass-loss prescription and various amounts of convective core overshooting. The contribution of mass loss towards the mass discrepancy is determined using these models, Results. Pulsation-driven mass loss is found to trap Cepheid evolution on the instability strip, allowing them to lose about 5 − 10% of their total mass when moderate convective core overshooting, an amount consistent with observations of other stars, is included in the stellar models. Conclusions. We find that the combination of moderate convective core overshooting and pulsation-driven mass loss can solve the Cepheid mass discrepancy.
Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling ...traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients.
ABSTRACT
Classical Cepheids are essential objects in the study of stellar evolution and cosmology; however, we know little about their magnetic properties. We report the detection of Stokes V ...features interpreted as Zeeman signatures in four classical Cepheids using high-resolution spectropolarimetric observations obtained with ESPaDOnS at the Canada–France–Hawaii Telescope. Eight observations of η Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, ζ Gem, δ Cep, and RT Aur were obtained in 2020 as part of our ongoing systematic survey. We use mean circular polarization Stokes V profiles generated using the least-squares deconvolution procedure to diagnose Zeeman signatures and measure mean longitudinal field strengths 〈Bz〉. We detect magnetic signatures across all pulsation phases of η Aql (−0.89 ± 0.47 G$\, \lt \langle B_{z}\rangle \lt 1.27\pm 0.40$ G), as well as in the single observations of Polaris (0.59 ± 0.16 G), ζ Gem (0.41 ± 0.16 G), and δ Cep (0.43 ± 0.19 G). The Stokes V profile of Polaris is detected at extremely high signal-to-noise ratio and implies a complex magnetic field topology. It stands in stark contrast to all other detected Stokes V profiles, which show unusual approximately unipolar positive circular polarization lobes analogous to those observed in some Am stars.
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