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PubMed1998

The major resistance gene cluster in lettuce is highly duplicated and spans several megabases.

At least 10 Dm genes conferring resistance to the oomycete downy mildew fungus Bremia lactucae map to the major resistance cluster in lettuce. We investigated the structure of this cluster in the lettuce cultivar Diana, which contains Dm3. A deletion breakpoint map of the chromosomal region flanking Dm3 was saturated with a variety of molecular markers. Several of these markers are components of a family of resistance gene candidates (RGC2) that encode a nucleotide binding site and a leucine-rich repeat region. These motifs are characteristic of plant disease resistance genes. Bacterial artificial chromosome clones were identified by using duplicated restriction fragment length polymorphism markers from the region, including the nucleotide binding site-encoding region of RGC2. Twenty-two distinct members of the RGC2 family were characterized from the bacterial artificial chromosomes; at least two additional family members exist. The RGC2 family is highly divergent; the nucleotide identity was as low as 53% between the most distantly related copies. These RGC2 genes span at least 3.5 Mb. Eighteen members were mapped on the deletion breakpoint map. A comparison between the phylogenetic and physical relationships of these sequences demonstrated that closely related copies are physically separated from one another and indicated that complex rearrangements have shaped this region. Analysis of low-copy genomic sequences detected no genes, including RGC2, in the Dm3 region, other than sequences related to retrotransposons and transposable elements. The related but divergent family of RGC2 genes may act as a resource for the generation of new resistance phenotypes through infrequent recombination or unequal crossing over.

Base Sequence↗
PubMed2005

Three-dimensional organization of higher-plant chloroplast thylakoid membranes revealed by electron tomography.

The light-harvesting and energy-transducing functions of the chloroplast are performed within an intricate lamellar system of membranes, called thylakoid membranes, which are differentiated into granum and stroma lamellar domains. Using dual-axis electron microscope tomography, we determined the three-dimensional organization of the chloroplast thylakoid membranes within cryo-immobilized, freeze-substituted lettuce (Lactuca sativa) leaves. We found that the grana are built of repeating units that consist of paired layers formed by bifurcations of stroma lamellar sheets, which fuse within the granum body. These units are rotated relative to each other around the axis of the granum cylinder. One of the layers that makes up the pair bends upwards at its edge and fuses with the layer above it, whereas the other layer bends in the opposite direction and merges with the layer below. As a result, each unit in the granum is directly connected to its neighbors as well as to the surrounding stroma lamellae. This highly connected morphology has important consequences for the formation and function of the thylakoid membranes as well as for their stacking/unstacking response to variations in light conditions.

Chloroplasts↗
PubMed2004

Isolation and characterisation of a class of carbohydrate oxidases from higher plants, with a role in active defence.

In a search for novel plant-derived antimicrobial proteins, we screened extracts from salicylic acid (SA)-treated lettuce and sunflower leaves. These extracts displayed very potent antimicrobial activity against a set of phytopathogens. Characterisation of these extracts revealed that in both extracts, proteins of approximately 60 kDa were responsible for the antimicrobial activity. Further characterisation of these proteins and cloning of the respective cDNAs revealed close homology to a range of (plant) oxidases. Dissection of the enzymatic activity of both proteins revealed them to be carbohydrate oxidases (Helianthus annuus carbohydrate oxidase (Ha-CHOX) and Lactuca sativa carbohydrate oxidase (Ls-CHOX)) with broad substrate specificity and with hydrogen peroxide (H(2)O(2)) as one of the reaction products. The sunflower transcript, in addition to being SA inducible, was also inducible by fungal pathogens but not by ethylene and jasmonate. To determine whether Ha-CHOX plays a role in pathogen defence, it was transformed into tobacco and the effect of resistance to Pectobacterium carotovorum ssp. carotovorum was examined. Transgenic plants overexpressing Ha-CHOX displayed enhanced resistance to infection by this pathogen, and the resistance level was proportional to enzyme expression.

Amino Acid Sequence↗
PubMed2005

Isotopic discrimination of zinc in higher plants.

* The extent of isotopic discrimination of transition metals in biological processes is poorly understood but potentially has important applications in plant and biogeochemical studies. * Using multicollector inductively coupled plasma (ICP) mass spectrometry, we measured isotopic fractionation of zinc (Zn) during uptake from nutrient solutions by rice (Oryza sativa), lettuce (Lactuca sativa) and tomato (Lycopersicon esculentum) plants. * For all three species, the roots showed a similar extent of heavy Zn enrichment relative to the nutrient solution, probably reflecting preferential adsorption on external root surfaces. By contrast, a plant-species specific enrichment of the light Zn isotope occurred in the shoots, indicative of a biological, membrane-transport controlled uptake into plant cells. The extent of the fractionation in the shoots further depended on the Zn speciation in the nutrient solution. * The observed isotopic depletion in heavy Zn from root to shoot (-0.13 to -0.26 per atomic mass unit) is equivalent to roughly a quarter of the total reported terrestrial variability of Zn isotopic compositions (c. 0.84 per atomic mass unit). Plant uptake therefore represents an important source of isotopic variation in biogeochemical cycling of Zn.

Biological Transport↗
PubMed2006

Evidence of an antilisterial factor induced by wounding of iceberg lettuce tissues.

AIMS: To examine the influence of wound-associated reactions in cut iceberg lettuce (Lactuca sativa L.) tissues on the fate of Listeria monocytogenes. METHODS AND RESULTS: Aqueous extracts prepared from shredded iceberg lettuce before and after storage in high oxygen permeability film were inoculated with L. monocytogenes. Listeria monocytogenes grew in extracts prepared from fresh lettuce. In contrast, inhibition ranging from arrested growth to a decline in cell viability was observed in extracts prepared from samples stored for 1-3 days. Similar behaviour was evident in lettuce shreds inoculated with 10(5) CFU g(-1)L. monocytogenes immediately after processing or after 3 days in storage. Heat treatment of the cut tissues at 47 degrees C for 3 min before storage diminished the inhibitory effect. CONCLUSIONS: The results provided evidence that an antilisterial factor or factors are released by wounded iceberg lettuce tissues. Antilisterial activity was mitigated by heat treatment of the lettuce. SIGNIFICANCE AND IMPACT OF STUDY: This study indicates that intrinsic factors associated with plant metabolism could play a significant role in the ecology of human pathogens in packaged horticultural products.

Aerobiosis↗
PubMed1997

Environmental modification of yield and nutrient composition of 'Waldmann's Green' leaf lettuce.

Leaf number, dry weight, and nutrient composition of Lactuca sativa L. cv. Waldmann's Green leaves were compared following 9 days of treatment in a controlled environment room under various combinations of photosynthetic photon flux (PPF:350 vs 800 micromoles m-2 s-1), atmospheric CO2 level (ambient vs 1500 micromoles mol-1), and single-strength (1X:15 mM) vs double-strength (2X:30 mM) nitrogen (N) as NO3- alone or as NH4(+) + NO3- (1:5 molar ratio). CO2 enrichment greatly enhanced leaf number under all PPF and N conditions, but increased leaf dry weight only at high PPF. Conditions favoring high photosynthesis enhanced leaf starch content 3-fold, and protein content increased as much as 64% with 2X NH4(+)+NO3-. Free sugar content was 6 to 9% of leaf dry weight for all treatment combinations, while fat was 1.5 to 3.5%. Ash content varied from 15 to 20% of leaf dry weight. Modified controlled environments can be used to enhance the nutritional content as well as the yield of crops to be used for life support in space-deployed, self-sustaining human habitats. Leaf lettuce is a useful model crop for demonstrating the potential of nutritional value added by environmental manipulation.

Biomass↗
PubMed1996

Rhizobium leguminosarum as a plant growth-promoting rhizobacterium: direct growth promotion of canola and lettuce.

Early seedling root growth of the nonlegumes canola (Brassica campestris cv. Tobin, Brassica napus cv. Westar) and lettuce (Lactuca sativa cv. Grand Rapids) was significantly promoted by inoculation of seeds with certain strains of Rhizobium leguminosarum, including nitrogen- and nonnitrogen-fixing derivatives under gnotobiotic conditions. The growth-promotive effect appears to be direct, with possible involvement of the plant growth regulators indole-3-acetic acid and cytokinin. Auxotrophic Rhizobium mutants requiring tryptophan or adenosine (precursors for indole-3-acetic acid and demonstrate a new facet of the Rhizobium-plant relationship and that Rhizobium leguminosarum can be considered a plant growth-promoting rhizobacterium (PGPR).

Indoleacetic Acids↗
PubMed2001

A comparison of sole carbon source utilization patterns and phospholipid fatty acid profiles to detect changes in the root microflora of hydroponically grown crops.

Sole carbon source utilization (SCSU) patterns and phospholipid fatty acid (PLFA) profiles were compared with respect to their potential to characterize root-inhabiting microbial communities of hydroponically grown crops. Sweet pepper (Capsicum annum cv. Evident), lettuce (Lactuca sativa cv. Grand Rapids), and four different cultivars of tomato (Lycopersicon esculentum cvs. Gitana, Armada, Aromata, and Elin) were grown in 1-L black plastic beakers placed in a cultivation chamber with artificial light. In addition to the harvest of the plants after 6 weeks, plants of one tomato cultivar, cv. Gitana, were also harvested after 4 and 8 weeks. The cultivation in this study was performed twice. Principal component analysis was used to analyze the data. Both characterization methods had the ability to discriminate between the root microflora of different plant species, cultivars, and one tomato cultivar at different ages. Differences in both SCSU patterns and PLFA profiles were larger between plant species than between cultivars, but for both methods the largest differences were between the two cultivations. Still, the differences between treatments were always due to differences in the same PLFAs in both cultivations. This was not the case for the SCSU patterns when different plant ages were studied. Furthermore, PLFA profiles showed less variation between replicates than did SCSU patterns. This larger variation observed among the SCSU data indicates that PLFA may be more useful to detect changes in the root microflora of hydroponically grown crops than the SCSU technique.

Capsicum↗
PubMed1996

Activation of aniline by extracts from plants and induction of chromosomal damages in Chinese hamster ovary cells.

Activation of aniline by plant extracts was studied by a chromosomal damage induction assay in Chinese hamster ovary (CHO) cells in vitro. Extracts from roots of Vicia faba activated aniline and the activation caused increases in chromosomal aberrations (CAs) and endoreduplicated cells (ERCs), but did not cause sister-chromatid exchanges (SCEs). Extracts from Pisum sativum and Lactuca sativa, however, did not activate aniline. All C-hydroxylated metabolites of aniline, o-aminophenol, m-aminophenol and p-aminophenol, induced not only CAs but also SCEs in CHO cells. These results show that the pathway for aniline activation by Vicia extracts is by means other than the C-hydroxylation.

Aminophenols↗
PubMed2003

Deactivation of gibberellin by 2-oxidation during germination of photoblastic lettuce seeds.

Gibberellin (GA) plays an important role in the induction of germination of photoblastic lettuce (Lactuca sativa L. cv. Grand Rapids) seeds. We have previously shown that gene expression of a GA 3-oxidase (Ls3h1) increased after a red light treatment, resulting in an increase in the endogenous content of GA1, bioactive GA. Since the metabolism of GAs is also important for determining the endogenous levels of bioactive GAs, cDNAs encoding GA 2-oxidases (LsGA2ox1 and LsGA2ox2, for L. sativa GA 2-oxidase), which catalyze the deactivation of GAs, were isolated from lettuce seeds to investigate the regulation of these genes by light. An expression analysis shows that the mRNA levels of both enzymes was not markedly altered under different light conditions during germination. However, the amount of LsGA2ox2 transcripts was decreased to approximately half the level by red light. This reduction might play a role in the increase in GA1 level by red light in the lettuce seeds.

Amino Acid Sequence↗
PubMed2004

Distribution and population development of Nasonovia ribisnigri (Homoptera: Aphididae) in iceberg lettuce.

A field study was conducted to determine the distribution and development of aphid Nasonovia ribisnigri (Mosley) (Homoptera: Aphididae) populations in iceberg lettuce, Lactuca sativa L. 'Salinas'. Lettuce plants were transplanted and caged individually in the field and inoculated with apterous N. ribisnigri at 0, 1, 2, 3, and 4 wk after transplanting in spring and fall 2002. Plants were harvested 15-50 d after inoculations; numbers of alates and apterous N. ribisnigri were counted or estimated on each leaf for each plant. Inoculations during all 5 wk of plant development resulted in successful colonization of lettuce heads. Results indicated that head formation did not reduce the risk of colonization by N. ribisnigri to iceberg lettuce; plants were susceptible to colonization by N. ribisnigri throughout their development. For later inoculations, N. ribisnigri populations were relatively smaller, and aphids were found mostly within the heads. For earlier inoculations, N. ribisnigri populations were larger, and within-plant distributions shifted toward frame leaves. The shift of population distributions toward frame leaves correlated significantly with increases in N. ribisnigri population density. For most inoculations, more aphids were present on wrapper leaves than on other leaves. The proportion of alates did not vary significantly with population density. Population development of N ribisnigri also correlated significantly with heat unit accumulation. Yellow sticky cards were used to monitor alates in each cage. Catches of N. ribisnigri alates on yellow sticky cards were significantly correlated with total numbers of alates as well as with total population sizes on individual lettuce plants.

Animals↗
PubMed2000

Impact of insecticides and surfactant on lettuce physiology and yield.

Insecticides are used extensively on lettuce, Lactuca sativa L., grown in southwestern Arizona because of heavy insect pressure that can potentially reduce lettuce productivity. Multiple sprays are made per season to manage these insects in lettuce. One of the major concerns related to extensive insecticide applications in lettuce is the potential subtle impact of insecticides that may reduce lettuce photosynthesis and yield. We conducted field and greenhouse experiments to examine the impact of multiple insecticides and surfactant spray applications on lettuce photosynthesis and yield. Lettuce was planted in the field in 1998, insecticides and surfactant were applied, and lettuce gas-exchange and dry weights were determined. Treatments were arranged in a split-plot consisting of insecticides as main plot and surfactant as subplot treatments in a randomized complete block design with four replications. Photosynthetic rates of lettuce were significantly reduced by endosulfan, methomyl, acephate, and surfactant at seedling stage 4 h and 2 d after the spray application was made. However, the reduction in lettuce photosynthesis by these insecticides and surfactant was only transient, and lettuce photosynthesis recovered 5 d after the spray application was made. Photosynthetic rates were not altered by zeta-cypermethrin, emamectin benzoate, and spinosad at the seedling stage. Insecticides or surfactant (Kinetic, a nonionic surfactant) did not significantly affect lettuce photosynthesis after rosette formation. In addition, lettuce dry weight was not significantly altered. These studies suggest that lettuce photosynthesis may be susceptible to some insecticides at the seedling stage. Consequently, we found that biorational insecticides, introduced to manage insect pests in lettuce, have no influence on lettuce physiology at the seedling stage, unlike the chlorinated hydrocarbons, organophosphates, or carbamates tested in this study. In a greenhouse study, we found that lettuce photosynthesis and yield were not altered by Bacillus thuringiensis application. Our results indicate that B. thuringiensis and the newer insecticides, particularly biorationals, can be used to manage lettuce insect pests without significantly altering lettuce gas-exchange and yield.

Animals↗
PubMed2002

Resistance to adult banded cucumber beetle (Coleoptera: Chrysomelidae) in romaine lettuce.

Four cultivars of lettuce, Lactuca sativa L., were evaluated for their resistance to the adult banded cucumber beetle, Diabrotica balteata LeConte, under laboratory conditions. When paired with each of the other three cultivars, leaf consumption in all possible combinations of short-term (48 h) two-choice tests among the cultivars was significantly reduced only for 'Valmaine'. However, in a 48-h no-choice situation, beetles fed Valmaine or 'Short Guzmaine' had similarly low leaf consumption, followed by 'Parris White', with the greatest consumption occurring on 'Tall Guzmaine'. In longer term experiments, female beetles fed Valmaine for 10, 13, or 16 d generally had lower survival and the lowest body weights compared with beetles fed any of the other three cultivars. No mature eggs were found in the ovaries of females fed Valmaine, whereas from 14% (Short Guzmaine, day 10) to 100% (Tall Guzmaine, day 13) of females fed the other cultivars produced mature eggs. In a starvation test, most D. balteata of either sex did not survive after 7 d with access only to water. Moreover, starved females did not produce mature eggs. Thus, food consumption by adult D. balteata is very important to their survival and reproductive performance, and it is likely that females fed Valmaine failed to produce mature eggs because they did not consume a sufficient amount of this cultivar. However, because Valmaine-fed beetles maintained their body weight and lived significantly longer than starved beetles, it appears that they can obtain some nourishment from their limited feeding on this cultivar. Overall, these results suggest that Valmaine, and to a lesser extent short Guzmaine (a cultivar produced by crossing Valmaine with two other cultivars), exhibit antixenosis-based resistance against D. balteata.

Animals↗
PubMed1996

Effect of dissolved oxygen concentration on lettuce growth in floating hydroponics.

Lettuce (Lactuca sativa L., cv. Ostinata) growth experiments were carried out to study the effect of dissolved oxygen (DO) concentration on plant growth in a floating hydroponic system. Pure O2 and N2 gas were supplied to the hydroponic system for precise DO control. This system made it easy to increase the DO concentration beyond the maximum (or saturation) concentration possible when bubbling air into water. Eleven day old lettuce seedlings were grown for 24 days under various DO concentrations: sub-saturated, saturated, and super-saturated. There was no significant difference in fresh weight, shoot and root dry weights among the DO concentrations: 2.1 (25% of saturated at 24 degrees C), 4.2 (50%), 8.4 (saturated), and 16.8 (200%) mg/L. The critical DO concentration for vigorous lettuce growth was considered to be lower than 2.1 mg/L. Neither root damage nor delay of shoot growth was observed at any of the studied DO concentrations.

Biomass↗
PubMed1996

An automated plant monitoring system using machine vision.

A plant growth chamber equipped with a machine vision (MV) system was developed for the continuous, non-contact sampling and near-real-time evaluation of the top projected leaf area (TPLA) of lettuce (Lactuca sativa, cv. Ostinata) seedlings. A rotary table enabled automatic, individual presentation of the lettuce plants to the imaging system. Hourly measurements were continuously made for 16 plants from the first true leaf stage through 30 days from seeding. A near-infrared radiation source illuminated the plants during the dark period, permitting measurements without interrupting the 12 hour photoperiod. Daily minimum hourly change of TPLA for the plants occurred from 3 to 4 hours after the start of the light period. Most rapid increase in TPLA occurred from 4 to 5 hours after the onset of the dark period. The machine vision system was capable of determining a plant physiological response to the nutrient stress within 24 hours of the change of the nutrient regime.

Ecological Systems, Closed↗
PubMed2003

Determining toxicity of lead and zinc runoff in soils: salinity effects on metal partitioning and on phytotoxicity.

When assessing cationic metal toxicity in soils, metals are often added to soil as the chloride, nitrate, or sulfate salts. In many studies, the effects of these anions are ignored or discounted; rarely are appropriate controls included. This study used five soils varying in pH, clay content, and organic matter to determine whether salinity from counter-ions contributed to or confounded metal phytotoxicity. Varying rates of Pb and Zn were applied to soils with or without a leaching treatment to remove the metal counter-ion (NO3-). Lactuca sativa (lettuce) plants were grown in metal-treated soils, and plant dry weights were used to determine median effective concentrations where there was a 50% reduction in yield (EC50s) on the basis of total metals measured in the soil after harvest. In two of the five soils, leaching increased the EC50s significantly for Zn by 1.4- to 3.7-fold. In three of the five soils, leaching increased the EC50s significantly for Pb by 1.6- to 3.0-fold. The shift in EC50s was not a direct result of toxicity of the nitrate ion but was an indirect effect of the salinity increasing metal concentrations in soil solution and increasing its bioavailability for a given total metal concentration. In addition, calculation of potential salinity changes in toxicological studies from the addition of metals exhibiting strong sorption to soil suggested that if the anion associated with the metal is not leached from the soil, direct salinity responses could also lead to significant overestimation of the EC50 for those metals. These findings question the relevance of the application of single-metal salts to soils as a method of assessing metal phytotoxicity when, in many cases in our environment, Zn and Pb accumulate in soil over a period of time and the associated counter-ions are commonly removed from the soil during the accumulation process (e.g.. roof and galvanized tower runoff).

Aluminum Silicates↗
PubMed2006

Transfer of Cd, Cu, Ni, Pb, and Zn in a soil-plant-invertebrate food chain: a microcosm study.

The transfer of Cd, Cu, Ni, Pb, and Zn was evaluated in a soil-plant (lettuce, Lactuca sativa)-invertebrate (snail, Helix aspersa) food chain during a microcosm experiment. Two agricultural soils, polluted and unpolluted, were studied. Lettuce was cultivated for eight weeks before introduction of snails into the microcosms (M-snails). In a parallel experiment, snails were exposed to lettuce only (i.e., without soil) in simpler exposure devices called containers (C-snails). Snail exposure duration was eight weeks for both M- and C-snails. No effects on snail survival were found. Both M- and C-snails exposed to polluted soil showed a growth reduction, but only after two weeks of exposure. Time-dependent accumulation in M-snails exposed to the polluted environment showed a regular increase of Cd and Zn concentrations over time and a rapid increase of Pb concentrations within the first two weeks, which then remained stable. Copper and Ni concentrations did not increase during any of the experiments. Concentrations in M- and C-snails were compared to estimate the relative contribution of soil and plant to the total bioaccumulation. The results suggest that the soil contribution may be higher than 80% for Pb, from 30 to 60% for Zn, and from 2 to 40% for Cd.

Animals↗
PubMed2006

Evaluating the contribution of soil properties to modifying lead phytoavailability and phytotoxicity.

Soil properties affect Pb bioavailability to human and ecological receptors and should be considered during ecological risk assessment of contaminated soil. We used path analysis (PA) to determine the relative contribution of soil properties (pH, organic C [OC], amorphous Fe and Al oxides [FEAL], and cation-exchange capacity [CEC]) in modifying Pb bioavailability. The response of biological endpoints (bioaccumulation and dry matter growth [DMG]) of lettuce (Lactuca sativa) grown on 21 Pb-spiked (2,000 mg/kg) soils were determined. Lettuce tissue Pb ranged from 3.22 to 233 mg/kg, and relative DMG ranged from 2.5 to 88.5% of their respective controls. Simple correlation showed strong relationships between CEC and OC (p < 0.01) and weaker relationships between pH and FEAL (p < 0.05) and Pb bioaccumulation. Results of PA suggest that soil pH increased the negative surface charge of organic matter and clay, thereby increasing CEC and decreasing Pb bioaccumulation. Also, the direct effect of OC on tissue Pb can be attributed to formation of surface Pb complexes by organic matter functional group ligands. Increased OC and/or CEC reduced Pb solubility and bioavailability in the 21 soils in the present study. The relative importance of soil properties likely will vary between studies employing different soils. Soil properties should be considered during the ecological risk assessment of metal in contaminated soils. Path analysis is useful for ecological studies involving soils with a wide range of physicochemical properties and can assist in site risk assessment of metals and remediation decisions on contaminated sites.

Aluminum↗