Amaranthus caudatus lectin with polyproline II fold: conformational and functional transitions and molecular dynamics

Polyproline II (PPII) fold, a peculiar structural element was detected in the Amaranthus caudatus seed lectin (ACL) based on far UV circular dichroism spectrum, conformational transitions of the lectin, and a distinct isodichroic point in thermal denaturation. It was confirmed using PolyprOnline database to estimate the percentage of amino acids contributing to PPII fold and showed the values as 13.5 and 13.9% for PROSS and XTLSSTR, respectively. Investigations of the functional and conformational transitions of ACL during thermal-, pH-, and guanidine hydrochloride (GdnHCl)-induced denaturation were carried out using biochemical and biophysical techniques and molecular dynamics (MD) simulations approach. The lectin got aggregated at 60°C with instantaneous structural alterations. The aggregation-prone regions in ACL were predicted using online servers viz. AGGRESCAN, AmylPred, FoldAmyloid, and Waltz that were represented by Visual Molecular Dynamics tools. Nine conserved regions were identified by these softwares as being ‘hot-spots’ for aggregation. MD simulation studies of the lectin at 60°C revealed increase in radius of gyration. The loss of PPII fold in 2.0 M GdnHCl was reversible. The partially unfolded intermediate of ACL with diminished PPII fold formed at pH 1.0 was stable up to 90°C. The polyproline II fold has been rarely detected in lectins, ACL being the second after the potato lectin.     

The pectic disaccharides lepidimoic acid and β-d-xylopyranosyl-(1→3)-d-galacturonic acid occur in cress-seed exudate but lack allelochemical activity

Background and aims Cress-seed (Lepidium sativum) exudate exerts an allelochemical effect, promoting excessive hypocotyl elongation and inhibiting root growth in neighbouring Amaranthus caudatus seedlings. We investigated acidic disaccharides present in cress-seed exudate, testing the proposal that the allelochemical is an oligosaccharin—lepidimoic acid (LMA; 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→2)-l-rhamnose).Methods Cress-seed exudate was variously treated [heating, ethanolic precipitation, solvent partitioning, high-voltage paper electrophoresis and gel-permeation chromatography (GPC)], and the products were bioassayed for effects on dark-grown Amaranthus seedlings. Two acidic disaccharides, including LMA, were isolated and characterized by electrophoresis, thin-layer chromatography (TLC) and nuclear magnetic resonance (NMR) spectroscopy, and then bioassayed.Key Results Cress-seed exudate contained low-M_r, hydrophilic, heat-stable material that strongly promoted Amaranthus hypocotyl elongation and inhibited root growth, but that separated from LMA on electrophoresis and GPC. Cress-seed exudate contained ∼250 µm LMA, whose TLC and electrophoretic mobilities, susceptibility to mild acid hydrolysis and NMR spectra are reported. A second acidic disaccharide, present at ∼120 µm, was similarly characterized, and shown to be β-d-xylopyranosyl-(1→3)-d-galacturonic acid (Xyl→GalA), a repeat unit of xylogalacturonan. Purified LMA and Xyl→GalA when applied at 360 and 740 µm, respectively, only slightly promoted Amaranthus hypocotyl growth, but equally promoted root growth and thus had no effect on the hypocotyl:root ratio, unlike total cress-seed exudate.Conclusions LMA is present in cress seeds, probably formed by rhamnogalacturonan lyase action on rhamnogalacturonan-I during seed development. Our results contradict the hypothesis that LMA is a cress allelochemical that appreciably perturbs the growth of potentially competing seedlings. Since LMA and Xyl→GalA slightly promoted both hypocotyl and root elongation, their effect could be nutritional. We conclude that rhamnogalacturonan-I and xylogalacturonan (pectin domains) are not sources of oligosaccharins with allelochemical activity, and the biological roles (if any) of the disaccharides derived from them are unknown. The main allelochemical principle in cress-seed exudate remains to be identified.     

In vitro alpha-amylase inhibition and in vivo antioxidant potential of Amaranthus spinosus in alloxan-induced oxidative stress in diabetic rats

Amaranthus spinosus Linn. (Amaranthaceae), commonly known as “Mulluharivesoppu” in Kannada, is used in the Indian traditional system of medicine for the treatment of diabetes. The present study deals with the scientific evaluation of alpha amylase and the antioxidant potential of methanol extract of A. spinosus (MEAS). The aim of this study was to investigate in vitro alpha-amylase enzyme inhibition by CNPG3 (2-chloro-4-nitrophenol α-d-maltotrioside) and in vivo antioxidant potential of malondialdehyde (MDA), glutathione (GSH), catalase (CAT) and total thiols (TT) in alloxan-induced diabetic rats of a methanolic extract of A. spinosus. Blood sugar was also determined in MEAS-treated alloxan-induced diabetic rats. MEAS showed significant inhibition of alpha-amylase activity and IC₅₀ 46.02 μg/ml. Oral administration of MEAS (200 and 400 mg/kg) for 15 days showed significant reduction in the elevated blood glucose, MDA and restores GSH, CAT and TT levels as compared with a diabetic control. The present study provides evidence that the methanolic extract of A. spinosus has potent alpha amylase, anti-diabetic and antioxidant activities.     

Conidial germination and germ tube elongation of Phomopsis amaranthicola and Microsphaeropsis amaranthi on leaf surfaces of seven Amaranthus species: Implications for biological control

Microsphaeropsis amaranthi and Phomopsis amaranthicola are potential biological control agents for several Amaranthus species. In an effort to understand the initial infection processes with these pathogens, a study was conducted of the conidial germination and germ tube length (μm) on the weed leaf surfaces at 21 °C and 28 °C. Weeds included Amaranthus rudis, A. palmeri, A. powellii, A. retroflexus, A. spinosus, A. hybridus, and A. albus. For P. amaranthicola, conidial germination and germ tube length varied among the seven weed species at both temperatures, while for M. amaranthi the differences in germ tube lengths were significant among weed species only at 21 °C. While the conidia of M. amaranthi and P. amaranthicola germinated on the leaf surfaces of all seven weed species, temperature appeared to impact the number and length of germ tubes on the leaf surfaces. The percentage of germinated conidia and the length of germ tubes at both temperatures were often greater for M. amaranthi than for P. amaranthicola. In order for the fungal pathogen to successfully infect and kill a weedy host, conidia must germinate and form a germ tube, two processes that vary with host species and temperature for M. amaranthi and P. amaranthicola. The extent to which successive infection processes, e.g., penetration, invasion and colonization, contribute to host specificity warrants study.     

Fitness effects of a photosynthetic mutation across contrasting environments

To test the hypothesis that variation in photosynthesis can cause differences in fitness, we compared wild‐type (WT) Amaranthus hybridus genotypes to those having a single‐gene mutation (R) that affects photosynthetic rate. By using light and water treatments, we generated a range of differences between WT and R genotypes in photosynthetic rate, growth and reproduction at three developmental stages. In two cases photosynthetic differences were in the expected direction (WT > R), they did not differ in others, and in one case the R genotype had a higher rate than the WT. Across light and water treatments, higher rates of photosynthesis were related to increases in specific leaf area, leaf nitrogen content and stomatal conductance relative to the other genotype. Differences between genotypes in growth and allocation paralleled those in photosynthesis; in treatments where photosynthetic rate differed between the genotypes (high light), growth and reproduction did as well. In high light, the effects of genotype on fitness were indirect with high‐water availability, but were direct with low‐water availability. When photosynthetic rate did not differ between genotypes (low light), neither did growth and reproduction. These results demonstrate that variation in photosynthesis can cause differences in growth and reproduction. Furthermore, resource availability can moderate the ways in which selection operates on photosynthetic traits.     

The identity of Amaranthus taishanensis F. Z. Li et C. K. Ni

Examination of the type specimens of Amaranthus taishanensis F. Z. Li et C. K. Ni shows that it is actually conspecific with A. polygonoides L., a North American species.     

Rapid recovery of photosynthetic cell suspension cultures following heterotrophic bleaching

Summary Seven suspension-cultured lines of five different species (Amaranthus powellii Datura innoxia, Glycine max, Gossypium hirsutum, andNicotiana tabacum × Nicotiana glutinosa fusion hybrid), which had been grown under photomixotrophic conditions, were placed under heterotrophic conditions (darkness and media with 3% sucrose or starch) where the chlorophyll levels declined to near zero. After three transfers over a 70-d period, the cells were placed back into photomixotrophic or photoautotrophic conditions where regreening occurred rapidly and continued growth was observed. This rapid adaptation to photosynthetic conditions contrasts with the original initiation process for these cultures, which required many months and an apparent selection since many of the original cells died. Thus, these seven photosynthetic cell suspension cultures appear to be different from the original cultures due possible to genetic or adaptive changes.     

Sex expression, sex-specific traits, and the effects of salinity on growth and reproduction of Amaranthus cannabinus (Amaranthaceae), a dioecious annual

Amaranthus cannabinus was studied to investigate some of the ecological factors thought to be involved in the evolution of dioecy and to investigate the effects of salinity on sex expression and sex-specific selection. In the field portion of this study, sex ratios, stability of sex expression, spatial distribution, allocation strategies, and phenologies of the sexes were investigated in New Jersey freshwater and salt marsh populations of water hemp. To examine the effects of salinity on vegetative and reproductive development of males and females, plants were grown in the greenhouse at three salinity levels. Adult sex ratios were found to be 1:1. Temporal deviations from a 1:1 sex ratio varied by population and were due to differences in flowering phenology and mortality between the sexes. No plants were observed to change sex expression, and there was no evidence of spatial segregation of the sexes in the field. In both the field and the greenhouse, females allocated more resources to vegetative tissues and had a longer growing period than males. The results of this study suggest that increased reproductive efficiency through sex-specific growth patterns may have been an important selective factor involved in the evolution of dioecy in A. cannabinus.     

Structural characterization of a heteropolysaccharide isolated from hot water extract of the stems of Amaranthus tricolor Linn. (Amaranthus gangeticus L.)

A water-soluble polysaccharide (PS-I), isolated from the aqueous extract of the stems of Amaranthus tricolor Linn. (Amaranthus gangeticus L.), was found to consist of l-arabinose, methyl-d-galacturonate, d-galactose, and 3-O-Ac-l-rhamnose in a molar ratio of nearly 1:1:1:1. On the basis of total acid hydrolysis, methylation analysis, periodate oxidation, and NMR studies (¹H, ¹³C, TOCSY, DQF-COSY, NOESY, ROESY, HMQC, and HMBC), the structure of the repeating unit of the PS-I is determined as:     

1,3,7-Trimethylxanthine,an allelochemical from seeds ofCoffea arabica: Some aspects of its mode of action as a natural herbicide

Summary Certain aspects of selective toxicity of 1,3,7-trimethylxanthine (1,3,7-T), a natural herbicide, were studied inPhaseolus mungo andAmaranthus spinosus. Treatment of seeds ofA. spinosus with 1,3,7-T (sub-lethal concentration) caused a marked decrease (29.5%) in amylase activity. Kinetic studies with respect to substrate saturation and Km values as well asin vitro treatments of the cell-free enzyme indicated no effect on its catalytic property. The inhibitory effect of 1,3,7-T on amylase activity could not be counteracted with treatments of GA₃. A general biochemical analysis revealed that the starch/sugar ratio increased in the seedlings grown from treated seeds. Analysis also showed a higher ratio for insoluble/soluble nitrogen. It is suggested that inhibition of germination is caused by reduced synthesis of amylase which is not mediated through gibberelic acid. Treatment of seeds ofPhaseolus mungo with, 1,3,7-T. caused no change in amylase activity,in vivo nitrate reductase activity, ethanol soluble and insoluble nitrogen and protein content.