The ABC-like vacuolar transporter for rye mesophyll flavone glucuronides is not species-specific

Antibacterial and Antibiofilm Effects of Allelopathic Compounds Identified in Medicago sativa L. Seedling Exudate against Escherichia coli

In this study, the allelopathic properties of Medicago sativa L. (alfalfa) seedling exudates on the germination of seeds of various species were investigated. The compounds responsible for the allelopathic effects of alfalfa were identified and characterized by employing liquid chromatography ion mobility high-resolution mass spectrometry. Crude exudates inhibited the germination of seeds of all various plant species tested. Overall, nine compounds in alfalfa were identified and quantified. The most predominant compounds were a hyperoside representing a flavonoid glucoside, the non-proteinogenic amino acid canavanine, and two dipeptides, identified as H-Glu-Tyr-OH and H-Phe-Glu-OH. The latter corresponds to the first finding that dipeptides are exuded from alfalfa seedlings. In addition, the antibacterial and antibiofilm activities of alfalfa exudate and its identified compounds were elucidated. Both hyperoside and canavanine revealed the best antibacterial activity with minimum inhibitory concentration (MIC) values that ranged from 8 to 32 and 32 to 256 µg/mL, respectively. Regarding the antibiofilm action, hyperoside and canavanine caused a decline in the percentage of E. coli isolates that possessed a strong and moderate biofilm-forming potential from 68.42% to 21.05% and 31.58%, respectively. Studies on their inhibiting effects exhibit that these major substances are predominantly responsible for the allelopathic and antimicrobial effects of the crude exudates.

Biochemistry and Molecular Basis of Intracellular Flavonoid Transport in Plants

Flavonoids are a biochemically diverse group of specialized metabolites in plants that are derived from phenylalanine. While the biosynthesis of the flavonoid aglycone is highly conserved across species and well characterized, numerous species-specific decoration steps and their relevance remained largely unexplored. The flavonoid biosynthesis takes place at the cytosolic side of the endoplasmatic reticulum (ER), but accumulation of various flavonoids was observed in the central vacuole. A universal explanation for the subcellular transport of flavonoids has eluded researchers for decades. Current knowledge suggests that a glutathione S-transferase-like protein (ligandin) protects anthocyanins and potentially proanthocyanidin precursors during the transport to the central vacuole. ABCC transporters and to a lower extend MATE transporters sequester anthocyanins into the vacuole. Glycosides of specific proanthocyanidin precursors are sequestered through MATE transporters. A P-ATPase in the tonoplast and potentially other proteins generate the proton gradient that is required for the MATE-mediated antiport. Vesicle-mediated transport of flavonoids from the ER to the vacuole is considered as an alternative or additional route.

Accumulation of furanocoumarins by Bituminaria bituminosa in relation to plant development and environmental stress

Bituminaria bituminosa (L.) C.H. Stirton (Fabaceae) accumulates high concentrations of the furanocoumarins (FCs) angelicin and psoralen, which protect against infection and herbivory. The effects on FC accumulation of the exposure of two populations of B. bituminosa to abiotic stress (cold, heat and drought) under field conditions were determined, as well as the effect of temperature under controlled conditions, in hydroponic culture. In field conditions, psoralen and angelicin levels in the leaf dry matter were 400-6000 and 1500-11,000 μg g(-), respectively. There were significant effects of population on the psoralen concentration and psoralen:angelicin ratio. In hydroponic culture, exposure to a diurnal temperature of 33 °C increased FC levels in population Calnegre but not in Llano del Beal, compared with plants grown at 22 °C; however, high summer temperatures in the field (>30 °C) did not coincide with the highest leaf FC levels, since the plants accumulated FCs preferentially in the fruits. Hence, leaf FC levels were higher in winter. Irrigation, to alleviate water stress in the semi-arid conditions, increased the fruit psoralen concentration but produced only minor decreases in leaf FC levels. There was a significant, positive correlation (P < 0.001) between the FC and nitrogen concentrations in the plant organs analysed (both increased in the order: fruits > growing leaves > mature leaves), reflecting their respective contributions to plant fitness. The genetically- and developmentally-regulated accumulation of FCs by B. bituminosa is altered by short-term variations in environmental conditions, particularly temperature.

Characterization of anthocyanic vacuolar inclusions in Vitis vinifera L. cell suspension cultures

Anthocyanic vacuolar inclusions (AVIs) are intra-vacuolar structures capable of concentrating anthocyanins and are present in over 50 of the highest anthocyanin-accumulating plant species. Presence of AVIs alters pigment intensity, total anthocyanin levels, pigment hue and causes bathochromic shifts in a spatio-temporal manner within various flowers, vegetables and fruits. A year-long study on Vitis vinifera cell suspension cultures found a strong correlation between AVI prevalence and anthocyanin content, but not the number of pigmented cells, growth rate or stilbene content. Furthermore, enhancement of the prevalence of AVIs and anthocyanins was achieved by treatment of V. vinifera cell suspension cultures with sucrose, jasmonic acid and white light. A unique autofluorescence of anthocyanins was used to demonstrate microscopically that AVIs proceed from the cytosol across the tonoplast and were able to coalesce intravacuolarly, with fewer, larger AVIs predominating as cells mature. Purification and characterisation of these bodies were performed, showing that they were dense, highly organic structures, with a lipid component indicative of membrane-encasement. These purified AVIs were also shown to comprise long-chain tannins and possessed an increased affinity for binding acylated anthocyanins, though no unique protein component was detected.

Involvement of a soybean ATP-binding cassette-type transporter in the secretion of genistein, a signal flavonoid in legume-Rhizobium symbiosis

Legume plants have an ability to fix atmospheric nitrogen into nutrients via symbiosis with soil microbes. As the initial event of the symbiosis, legume plants secrete flavonoids into the rhizosphere to attract rhizobia. Secretion of flavonoids is indispensable for the establishment of symbiotic nitrogen fixation, but almost nothing is known about the membrane transport mechanism of flavonoid secretion from legume root cells. In this study, we performed biochemical analyses to characterize the transport mechanism of flavonoid secretion using soybean (Glycine max) in which genistein is a signal flavonoid. Plasma membrane vesicles prepared from soybean roots showed clear transport activity of genistein in an ATP-dependent manner. This transport activity was inhibited by sodium orthovanadate, a typical inhibitor of ATP-binding cassette (ABC) transporters, but was hardly affected by various ionophores, such as gramicidin D, nigericin, or valinomycin, suggesting involvement of an ABC transporter in the secretion of flavonoids from soybean roots. The K(m) and V(max) values of this transport were calculated to be 158 mum and 322 pmol mg protein(-1) min(-1), respectively. Competition experiments using various flavonoids of both aglycone and glucoside varieties suggested that this ABC-type transporter recognizes genistein and daidzein, another signaling compound in soybean root exudates, as well as other isoflavonoid aglycones as its substrates. Transport activity was constitutive regardless of the availability of nitrogen nutrition. This is, to our knowledge, the first biochemical characterization of the membrane transport of flavonoid secretion from roots.

Uptake of salicylic acid 2-O-beta-D-glucose into soybean tonoplast vesicles by an ATP-binding cassette transporter-type mechanism

In soybean (Glycine max L.), salicylic acid (SA) is converted primarily to SA 2-O-beta-d-glucose (SAG) in the cytoplasm and then accumulates exclusively in the vacuole. However, the mechanism involved in the vacuolar transport of SAG has not been investigated. The vacuolar transport of SAG was characterized by measuring the uptake of [(14)C]SAG into tonoplast vesicles isolated from etiolated soybean hypocotyls. The uptake of SAG was stimulated about six-fold when MgATP was included in the assay media. In contrast, the uptake of SA was only stimulated 1.25-fold by the addition of MgATP and was 2.2-fold less than the uptake of SAG providing an indication that the vacuolar uptake of SA is promoted by glucosylation. The ATP-dependent uptake of SAG was inhibited by increasing concentrations of vanadate (64% inhibition in the presence of 500 microM) but was not very sensitive to inhibition by bafilomycin A(1) (a specific inhibitor of vacuolar H(+)-ATPase; EC 3.6.1.3), and dissipation of the transtonoplast H(+)-electrochemical gradient. The SAG uptake exhibited Michaelis-Menten-type saturation kinetics with a K(m) value of 90 microM for SAG. SAG uptake was inhibited 60% by beta-estradiol 17-(beta-d-glucuronide), but glutathione conjugates and uncharged glucose conjugates were only slightly inhibitory. Based on the characteristics of SAG uptake into soybean tonoplast vesicles it is likely that this uptake occurs through an ATP-binding cassette transporter-type mechanism. However, this vacuolar uptake mechanism is not universal since the uptake of SAG by red beet (Beta vulgaris L) tonoplast vesicles appears to involve an H(+)-antiport mechanism.

The ATP-binding cassette (ABC) transporter Bpt1p mediates vacuolar sequestration of glutathione conjugates in yeast

Vacuolar sequestration or cellular extrusion of glutathione-conjugated xenobiotics and catabolites by ATP-binding cassette (ABC) transporters is an important detoxification mechanism operating in many species. In this study, we show that the yeast ABC transporter Bpt1p, a paralogue of Ycf1p, acts as an ATP-dependent vacuolar pump for glutathione conjugates. Bpt1p, which is inhibited by vanadate and glibenclamide, accounts for one third of the total vacuolar transport of glutathione conjugates. Furthermore, immunoblot analyses show that Bpt1p levels are strongly elevated in early stationary phase, consistent with a function of Bpt1p in vacuolar detoxification.

Flavone glucoside uptake into barley mesophyll and Arabidopsis cell culture vacuoles. Energization occurs by H(+)-antiport and ATP-binding cassette-type mechanisms

In many cases, secondary plant products accumulate in the large central vacuole of plant cells. However, the mechanisms involved in the transport of secondary compounds are only poorly understood. Here, we demonstrate that the transport mechanisms for the major barley (Hordeum vulgare) flavonoid saponarin (apigenin 6-C-glucosyl-7-O-glucoside) are different in various plant species: Uptake into barley vacuoles occurs via a proton antiport and is competitively inhibited by isovitexin (apigenin 6-C-glucoside), suggesting that both flavone glucosides are recognized by the same transporter. In contrast, the transport into vacuoles from Arabidopsis, which does not synthesize flavone glucosides, displays typical characteristics of ATP-binding cassette transporters. Transport of saponarin into vacuoles of both the species is saturable with a K(m) of 50 to 100 microM. Furthermore, the uptake of saponarin into vacuoles from a barley mutant exhibiting a strongly reduced flavone glucoside biosynthesis is drastically decreased when compared with the parent variety. Thus, the barley vacuolar flavone glucoside/H(+) antiporter could be modulated by the availability of the substrate. We propose that different vacuolar transporters may be responsible for the sequestration of species-specific/endogenous and nonspecific/xenobiotic secondary compounds in planta.