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Foodborne illnesses have been a heavy burden in the United States and globally. Many medicinal herbs have been cultivated in the US and many of which contain antimicrobial compounds with the potential to be used for food preservation. Methanol/water extracts of pomegranate peel (“PP”, Punica Granatum L.), Chinese gallnut (“CG”, Galla chinensis), Forsythia fruit (“FF”, Forsythia suspensa) and Baikal skullcap root (“BS”, Scutellaria baicalensis) were tested for antimicrobial activity using the agar diffusion assay on tryptic soy agar (TSA) and microdilution assay in tryptic soy broth (TSB). CG and PP extracts showed good to excellent inhibitory effect against Vibrio parahaemolyticus and Listeria monocytogenes in both assays, with a minimum inhibitory concentration (MIC) range from 0.04 to 5 mg/mL. BS had moderate inhibitory effects against V. parahaemolyticus with an MIC of 5 mg/mL in TSB, and against L. monocytogenes with an MIC of 20 mg/mL on TSA. CG was analyzed using LC-MS and fractionated using HPLC. The major components were identified as gallic acid, digallic acid, methyl gallate, and gallotannins (oligo-galloyl-D-glucose, nGG, n = 1~10). Six fractions (I - VI) were collected and their antibacterial activities were tested against L. monocytogenes, and V. parahaemolyticus both on TSA and in TSB. On TSA, fraction III, IV and V inhibited V. parahaemolyticus but no fraction inhibited L. monocytogenes. In TSB, all fractions inhibited V. parahaemolyticus and fractions II - V inhibited L. monocytogenes. Future studies are needed to investigate the effects of medicinal plants on food products.

Desiccating and germinating seeds are particularly prone to oxidative stress. Glutathione (GSH) is one of the most abundant and ubiquitous antioxidants in plants. To better understand GSH's role in developing and germinating seeds, wildtype (WT) and a line of seed antisense for glutathione reductase 2 (anGR2), one of two glutathione reductase (GR) genes characterized in the genetic model plant Arabidopsis thaliana, were compared. GSH levels in maturing and germinating seeds were measured by high performance liquid chromatography (HPLC) and GR activity by native Polyacrylamide gel electrophoresis (PAGE). Natural and accelerated aging and germination at high temperature and under water stress were compared for WT and anGR2 seeds. Total GSH pools were two-fold less in anGR2 than in WT after three hours of imbibition. Under high temperature and osmotic stress, germination percentages were lower in anGR2, suggesting a specific relationship between GSH metabolism and stress defense mechanisms early in germination. Germination of anGR2 seeds declined while that of WT seeds increased slightly, during nine months of conventional storage. The relationship between GR2 transcript and activity levels during early imbibition and total GSH pools suggest a key role for GR2 in protection against environmental stress. This study shows that GSH metabolism is one factor that determines the stress tolerance of germinating Arabidopsis seeds. [ABSTRACT FROM AUTHOR]

Acidovorax citrulli causes bacterial fruit blotch of cucurbits, a serious economic threat to watermelon (Citrullus lanatus) and melon (Cucumis melo) production worldwide. Based on genetic and biochemical traits, A. citrulli strains have been divided into two distinct groups: group I strains have been mainly isolated from various non-watermelon hosts, while group II strains have been generally isolated from and are highly virulent on watermelon. The pathogen depends on a functional type III secretion system for pathogenicity. Annotation of the genome of the group II strain AAC00-1 revealed 11 genes encoding putative type III secreted (T3S) effectors. Due to the crucial role of type III secretion for A. citrulli pathogenicity, we hypothesized that group I and II strains differ in their T3S effector repertoire. Comparative analysis of the 11 effector genes from a collection of 22 A. citrulli strains confirmed this hypothesis. Moreover, this analysis led to the identification of a third A. citrulli group, which was supported by DNA:DNA hybridization, DNA fingerprinting, multilocus sequence analysis of conserved genes, and virulence assays. The effector genes assessed in this study are homologous to effectors from other plant-pathogenic bacteria, mainly belonging to Xanthomonas spp. and Ralstonia solanacearum. Analyses of the effective number of codons and gas chromatography content of effector genes relative to a representative set of housekeeping genes support the idea that these effector genes were acquired by lateral gene transfer. Further investigation is required to identify new T3S effectors of A. citrulli and to determine their contribution to virulence and host preferential association.

Listeria monocytogenes is a foodborne pathogen that represents a high risk for consumers because it can grow under refrigeration conditions and can also develop acid tolerance. Fresh blueberries are hand-picked, packed, and transported under refrigeration without receiving a microbial inactivation treatment. The aim of this work was to study the survival of L. monocytogenes in fresh highbush blueberries stored at 4 or 12 °C under different controlled atmosphere conditions, including air (control); 5% O2, 15% CO2, 80% N2 (controlled atmosphere storage [CAS]); or ozone gas (O3), 4 ppm at 4 °C or 2.5 ppm at 12 °C, at high relative humidity (90 to 95%) for a total of 10 days. Fresh blueberries inside a plastic clamshell were spot inoculated with the bacteria and were stored at 4 or 12 °C in isolated cabinets under air, CAS, and O3 atmospheric conditions. Samples were evaluated on days 0, 1, 4, 7, and 10 for microbial growth using modified Oxford agar. CAS did not delay or inhibit L. monocytogenes growth in fresh blueberries after 10 days. O3 achieved 3- and 2-log reductions when compared with air treatment at 4 and 12 °C, respectively. Low concentrations of O3 together with proper refrigeration temperature can ensure product safety throughout transportation. O3 is a strong antimicrobial that safely decomposes to oxygen and water without leaving residues and can be used as an alternative method to prevent bacterial growth during a long transport period.

Previous studies have suggested that the apoplast solution of sugarcane stalk tissue contains high concentrations of sucrose, but the accuracy of these reports has been questioned because sucrose leakage from damaged cells may have influenced the results. In this study, the solute potential of the apoplast and symplast of the second (immature), tenth, twentieth, thirtieth, and fortieth internodes of field-grown sugarcane (Saccharum spp. hybrid) stalk tissue was determined by two independent methods. Solute potential of the apoplast was measured either directly by osmometry from solution collected by centrifugation, or inferred from the initial water potential of fully hydrated tissue determined by thermocouple psychrometry before the tissue was progressively dehydrated for generation of water potential isotherms. Both methods produced nearly identical values ranging from -0.6 to -1.8 megapascals for immature and mature tissue, respectively. The solute potential of the symplast determined by either method ranged from -1.0 to approximately -2.2 megapascals for immature and mature internodes, respectively. Solute quantitation by HPLC agreed with concentrations inferred from osmometry. Washing thirtieth internode tissue in deionized water increased pressure potential from 0.29 to 1.96 megapascals. The apoplast of mature sugarcane stalk tissue is a significant storage compartment for sucrose containing as much as 25% of the total tissue water volume and as much as 21% of the stored sucrose.

The initiation of radicle growth during seed germination may be driven by solute accumulation and increased turgor pressure, by cell wall relaxation, or by weakening of tissues surrounding the embryo. To investigate these possibilities, imbibition kinetics, water contents, and water (Psi) and solute (psi(s)) potentials of intact muskmelon (Cucumis melo L.) seeds, decoated seeds (testa removed, but a thin perisperm/endosperm envelope remains around the embryo), and isolated cotyledons and embryonic axes were measured. Cotyledons and embryonic axes excised and imbibed as isolated tissues attained water contents 25 and 50% greater, respectively, than the same tissues hydrated within intact seeds. The effect of the testa and perisperm on embryo water content was due to mechanical restriction of embryo swelling and not to impermeability to water. The Psi and psi(s) of embryo tissues were measured by psychrometry after excision from imbibed intact seeds. For intact or decoated seeds and excised cotyledons, Psi values were >-0.2 MPa just prior to radicle emergence. The Psi of excised embryonic axes, however, averaged only -0.6 MPa over the same period. The embryonic axis apparently is mechanically constrained within the testa/perisperm, increasing its total pressure potential until axis Psi is in equilibrium with cotyledon Psi, but reducing its water content and resulting in a low Psi when the constraint is removed. There was no evidence of decreasing psi(s) or increasing turgor pressure (Psi-psi(s)) prior to radicle growth for either intact seeds or excised tissues. Given the low relative water content of the axes within intact seeds, cell wall relaxation would be ineffective in creating a Psi gradient for water uptake. Rather, axis growth may be initiated by weakening of the perisperm, thus releasing the external pressure and creating a Psi gradient for water uptake into the axis. The perisperm envelope contains a cap of small, thin-walled endosperm cells adjacent to the radicle tip. We hypothesize that weakening or separation of cells in this region could initiate radicle expansion.

Muskmelon (Cucumis melo L.) seeds are germinable 15 to 20 days before fruit maturity and are held at relatively high water content within the fruit, yet little precocious germination is observed. To investigate two possible factors preventing precocious germination, the inhibitory effects of abscisic acid and osmoticum on muskmelon seed germination were determined throughout development. Seeds were harvested at 5-day intervals from 30 to 65 days after anthesis (DAA) and incubated either fresh or after drying on factorial combinations of 0, 1, 3.3, 10, or 33 micromolar abscisic acid (ABA) and 0, -0.2, -0.4, -0.6, or -0.8 megapascals polyethylene glycol 8000 solutions at 30 degrees C. Radicle emergence was scored at 12-hour intervals for 10 days. In the absence of ABA, the water potential (Psi) required to inhibit fresh seed germination by 50% decreased from -0.3 to -0.8 megapascals between 30 and 60 DAA. The Psi inside developing fruits was from 0.4 to 1.4 megapascals lower than that required for germination at all stages of development, indicating that the fruit Psi is sufficiently low to prevent precocious germination. At 0 megapascal, the ABA concentration required to inhibit germination by 50% was approximately 10 micromolar up to 50 DAA and increased to >33 micromolar thereafter. Dehydration improved subsequent germination of immature seeds in ABA or low Psi. There was a linear additive interaction between ABA and Psi such that 10 micromolar ABA or -0.5 megapascal osmotic potential resulted in equivalent, and additive, reductions in germination rate and percentage of mature seeds. Abscisic acid had no effect on embryo solute potential or water content, but increased the apparent minimum turgor required for germination. ABA and osmoticum appear to influence germination rates and percentages by reducing the embryo growth potential (turgor in excess of a minimum threshold turgor) but via different mechanisms. Abscisic acid apparently increases the minimum turgor threshold, while low Psi reduces turgor by reducing seed water content.

We previously reported that an apparent water potential disequilibrium is maintained late in muskmelon (Cucumis melo L.) seed development between the embryo and the surrounding fruit tissue (mesocarp). To further investigate the basis of this phenomenon, the permeability characteristics of the tissues surrounding muskmelon embryos (the mucilaginous endocarp, the testa, a 2- to 4-cell-layered perisperm and a single cell layer of endosperm) were examined from 20 to 65 days after anthesis (DAA). Water passes readily through the perisperm envelope (endosperm + perisperm), testa, and endocarp at all stages of development. Electrolyte leakage (conductivity of imbibition solutions) of individual intact seeds, decoated seeds (testa removed), and embryos (testa and perisperm envelope removed) was measured during imbibition of freshly harvested seeds. The testa accounted for up to 80% of the total electrolyte leakage. Leakage from decoated seeds fell by 8- to 10-fold between 25 and 45 DAA. Presence of the perisperm envelope prior to 40 DAA had little effect on leakage, while in more mature seeds, it reduced leakage by 2- to 3-fold. In mature seeds, freezing, soaking in methanol, autoclaving, accelerated aging, and other treatments which killed the embryos had little effect on leakage of intact or decoated seeds, but caused osmotic swelling of the perisperm envelope due to the leakage of solutes from the embryo into the space between the embryo and perisperm. The semipermeability of the perisperm envelope of mature seeds did not depend upon cellular viability or lipid membrane integrity. After maximum seed dry weight is attained (35-40 DAA), the perisperm envelope prevents the diffusion of solutes, but not of water, between the embryo and the surrounding testa, endocarp, and mesocarp tissue.

Total water potential (psi), solute potential, and turgor potential of field-grown muskmelon (Cucumis melo L.) fruit tissue (pericarp) and seeds were determined by thermocouple psychrometry at 5-day intervals from 10 to 65 days after anthesis (DAA). Fruit maturity occurred between 44 and 49 DAA, and seed germination ability developed between 35 and 45 DAA. Pericarp psi was essentially constant at approximately -0.75 megapascal (MPa) from 10 to 25 DAA, then decreased to a minimum value of -1.89 MPa at 50 DAA before increasing to -1.58 MPa at 65 DAA. Seed psi remained relatively constant at approximately -0.5 MPa from 10 to 30 DAA then decreased to -2.26 MPa at 50 to 60 DAA before increasing to -2.01 MPa at 65 DAA. After a rapid increase to 20 DAA, seed fresh weight declined until 30 DAA due to net water loss, despite continuing dry weight gain. As fruit and seed growth rates decreased, turgor potential initially increased, then declined to small values when growth ceased. A disequilibrium in psi was measured between seeds and pericarp both early and late in development. From 20 to 40 DAA, the psi gradient was from the seed to the tissue, coinciding with water loss from the seeds. From 50 to 65 DAA, seed psi decreased, causing a reversal of the psi gradient and a slight increase in seed water content. The partitioning of solutes between symplast and apoplast may create and maintain psi gradients between the pericarp and seed. The low solute potential within the pericarp due to solute accumulation and loss of cellular compartmentation during ripening and sensecence may be involved in prevention of precocious germination of mature seeds.