Induction of resistance in tomato against cucumber mosaic cucumovirus by plant growth-promoting rhizobacteria

Studies were done to evaluate specific strains of plant growth promoting rhizobacteria (PGPR) for induced resistance against cucumber mosaic cucumovirus(CMV) in tomato. In greenhouse experiments where plants were challenged by mechanical inoculation of CMV, the percentage of symptomatic plants in the most effective PGPR treatments ranged from 32 to 58%,compared with 88 to 98% in the nonbacterized, challenged disease control treatment. Field experiments were conducted in 1996 and 1997 to evaluate 4 PGPR strain treatments based on superior performance in the greenhouse studies. In the 1996field experiment, tomato plants treated with 3 PGPR strains exhibited a significantly lower incidence of CMV infection and significantly higher yields, compared with nonbacterized, CMV-challenged controls. In 1997, the overall percentages of plants infected with CMV in the control and PGPR treatments was higher than in 1996. CMV symptom development was significantly reduced on PGPR-treated plants in 1997compared with the control, but the percentage of infected plants and tomato yields were not significantly different among treatments. These results suggest that PGPR-mediated induced resistance against CMV infection following mechanical inoculation onto tomato can be maintained under field conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

ARTIFICIAL HYBRIDIZATION OF RUBBER-BEARING GUAYULE WITH COLD-TOLERANT PARTHENIUM LIGULATUM

ABSTRACT Caespitose and cold-tolerant plants of Parthenium ligulatum (Jones) Barneby (Asteraceae) from a native population in the Uinta Basin, Utah, were uprooted, potted, and transferred to a greenhouse in California. Approximately two years after transfer, the plants flowered and subsequently were crossed to diploid guayule (Parthenium argentatum Gray), the rubber-bearing species, native to the state of Durango, Mexico. Only female guayule × male P. ligulatum crosses produced F₁ hybrids. Only crosses involving guayule as female parent and F₁ plants as male parents produced backcross (BC,) plants. Hybrid plants were variable with respect to their growth habit, inflorescence, and leaf shape. Both parents and F₁ hybrids had 2n = 36 chromosomes. Unlike the parents, however, meiosis was irregular in the hybrids which showed a range of 0–5 and an average of 2.1 univalents at metaphase I. Hybrids averaged 0.87 laggards at anaphase I and 0.83 micronuclei at the tetrad stage. The crossability of guayule and P. ligulatum, the high degree of chromosome pairing of the F₁ hybrids, and the production of BC₁ plants indicate that the two species are related in spite of their distinct morphological and ecological differences. This study suggests that the cold-tolerance trait of P. ligulatum may be transferred to guayule through interspecific hybridization followed by backcrossing. The development of cold-tolerant guayule cultivars is expected to expand the areas of guayule production beyond that of the Chihuahuan desert and similar climates.     

Biological control of bacterial wilt caused by Pseudomonas solanacearum in India with antagonistic bacteria

From among 125 strains of fluorescent and 52 strains of nonfluorescent bacteria initially screened in the laboratory for their antibiosis towards the bacterial wilt pathogen, Pseudomonas solanacearum, strain Pfcp of Pseudomonas fluorescens and strains B33 and B36 of Bacillus spp., were chosen and evaluated further in greenhouse and field tests. Pfcp treated banana (Musa balbisiana), eggplant and tomato plants were protected from wilt upto 50, 61 and 95% in greenhouse and upto 50, 49 and 36% respectively in field. Protection afforded by the Bacillus strains was lower. In bacteria-treated plants which were subsequently inoculated with P. solanacearum plant height and biomass values increased and were close to those of nontreated and noninoculated control plants.     

Evaluation of selected geocarposphere bacteria for biological control of Aspergillus flavus in peanut

Selected bacterial strains isolated from the region of peanut pod development (geocarposphere) and two additional bacterial strains were screened as potential biological control agents against Aspergillus flavus invasion and subsequent aflatoxin contamination of peanut in laboratory, greenhouse, and field trials. All 17 geocarposphere strains tested delayed invasion of young roots and reduced colonization by the fungus in a root-radicle assay used as a rapid laboratory prescreen. In a greenhouse study, seven bacterial strains significantly reduced pod colonization by A. flavus compared to the control. In a field trial, conducted similarly to the greenhouse assay, pods sampled at mid-peg from plants seed-treated with suspensions of either 91A-539 or 91A-550 were not colonized by A. flavus, and the incidence of pods invaded from plants treated with either 91A-539 or 91A-599 was consistently lower than nonbacterized plants at each of five sampling dates. At harvest, 8 geocarposphere bacterial strains significantly lowered the percentage of pods colonized (> 51%) compared to the control. Levels of seed colonization ranged from 1.3% to 45% and did not appear related to aflatoxin concentrations in the kernels.     

Screening rhizobacteria for biological control of Ralstonia solanacearum in Ethiopia

Bacterial wilt caused by Ralstonia solanacearum (Smith) has become a severe problem mainly on potato and tomato in Ethiopia and no effective control measure is available yet. To explore possibilities for the development of biological control for the disease, 118 rhizobacteria, most of them collected from Ethiopia, were screened against an Ethiopian R. solanacearum strain. On the basis of in vitro screening, six strains (RP87, B2G, APF1, APF2, APF3, and APF4) with good inhibitory effect were selected for in planta testing in a greenhouse. In the greenhouse, soil and tomato seedlings were treated with the antagonists and their effects studied. The study showed that APF1 and B2G strains significantly reduced disease incidence and increased weight of tomato plants. Area under disease progress curves (AUDPC) was reduced by 60% and 56% in plants inoculated with APF1 and B2G strains, respectively. Plant dry weight increase in plants inoculated with APF1 and B2G strains was 96% and 75%, respectively. APF1 was found to be the most beneficial strain in disease suppression and also growth promotion resulting in 63% dry weight increase compared to untreated control. The study revealed that APF1 and B2G strains are promising strains whose effectiveness under field conditions and their mode of action should be investigated.     

Effects of some pure and mixedFrankia strains on seedling growth in differentAlnus species

In greenhouse experiments plants of eightAlnus species, from various parts of the world, and from different taxonomic sections, were inoculated with threeFrankia strains in order to show any possible interaction. Mixtures in equal parts of theseFrankia strains were also tried. The growth of inoculated plants was significantly higher than of the controls, with one of the three strains being superior. Mixtures of strains generally provided higher growth than the best individual strain. No interaction betweenFrankia strains andAlnus species was detected in the young plants 60 days after inoculation. Three clones ofAlnus glutinosa were inoculated with the same pure cultures ofFrankia, without producing any interaction. Inoculation time was studied in one clone and one progeny ofAlnus glutinosa. The best results were obtained with the earlier inoculation (at sowing for the progeny and at transfer to soil for thein vitro-propagated clone). The results are discussed in terms of nursery practice and field experiments for selection in breeding programmes.     

Bacterization of peanut withPseudomonas fluorescens for biological control ofRhizoctonia solani and for enhanced yield

Severity of stem-rot disease of peanut caused byRhizoctonia solani was reduced by 54.9 and 68% in plants of two cultivars treated in the greenhouse with antagonistic strains ofPseudomonas fluorescens. These strains were selected based on theirin vitro toxicity to mycelial growth and sclerotial germination ofR. solani. In field experiments, bacterization of peanuts withP. fluorescens resulted in taller plants (by 25.7%) and increased yields (by 59.0%).     

Effects of tall fescue turf on growth and nitrogen fixation potential of the woody legume Lupinus albifrons

The effect of tall fescue turf on growth, flowering, nodulation, and nitrogen fixing potential of Lupinus albifrons Benth. was examined for greenhouse and field grown plants. No allelopathic effect was observed for lupine plants treated with tall fescue leachates. The nitrogen-fixing potential measured by nodule dry weight and acetylene reduction rates was not significantly affected by tall fescue turf.Both the greenhouse and field studies showed that the growth, sexual reproductive allocation and number of inflorescences were significantly reduced when lupine plants were grown with tall fescue. The root-length densities of tall fescue turf and lupine monoculture were measured. The tall fescue turf had 20 times higher root-length density (20 cm cm^-3 soil) than the lupine plant monoculture. This suggests that intense competition at the root zone may be a dominant factor which limits the growth of the lupine plants.The flowering characters of the lupine plants were improved by phosphorus fertilization. Transplanting of older lupine plants into the turf substantially alleviated the tall fescue turf competitive effect.     

Agrobacterium-mediated transformation and regeneration of guayule

In vitro grown shoot tissue of facultative apomictic lines of guayule (Parthenium argentatum Gray), a rubber producing desert shrub, were transformed by Agrobacterium-mediated DNA transfer and regenerated into complete plants. Guayule shoots of lines 11591, UC101 and UC104 were inoculated with A. tumefaciens strains LBA4404 or PC2760 harboring the binary vector pCGN1557. Axillary shoots were regenerated from transformed cells and rooted in vitro in the presence of kanamycin. Genetic transformation in all cases was verified by Southern blot analysis. Transgenic plants were grown to maturity in the greenhouse and, as predicted for apomictic species, all seed produced possessed kanamycin resistance. Because apomicts have limitations for gene transfer by normal sexual crosses, this method offers a new means of transferring genes into this species.Abbreviations BA benzyladenine - EDTA ethylene diamine tetraacetate - kan_R kanamycin resistance - MS salts salts of Murashige and Skoog medium (1962) - NAA naphthalene acetic acid - NPT-II neomycin phosphotransferase - SDS sodium dodecyl sulfate     

Maize root response to N in the greenhouse and field

Summary Root growth and morphology were compared between seven week old maize plants grown in the greenhouse and in the field. The plants were similar in shoot dry weight and the partioning of N and dry matter to roots were similar except for the field grown plants in 1983. Field grown plants had greater root mass per length and greater calculated diameter than greenhouse plants. Nitrogen fertilization decreased N and dry matter partitioning to the root system in all three environments.     

Latex content and biomass increase in mycorrhizal guayule (Partheniurn argentatum) under field conditions*

Guayule seedlings were inoculated with two Glomus species in pasteurised soil and grown in the glasshouse without added fertiliser for 8 wk prior to transplanting to the field. The survival rate of transplanted guayule seedlings was increased by inoculation with vesicular arbuscular mycorrhizal fungi compared with uninoculated controls. Inoculated guayule had greater concentrations of Ca, Fe, Mg and Mn at six months of age, and greater concentrations of Ca, Mg, and Zn at 12 months of age than did uninoculated plants. The latex content of both roots and shoots of guayule was greater in inoculated than in uninoculated guayule plants at 12 and 18 months of age. The resin content remained unchanged between treatments irrespective of sampling date.     

The uptake,transport and metabolism of the bioregulator 2-(3,4-dichlorophenoxy) ethyldimethylamine in guayule

The bioregulator 2-(3,4-dichlorophenoxy) ethyldimethylamine was applied to five-month-old summer and winter guayule plants. Uptake of this molecule depended on the presence of viable trichomes and a well-developed cuticle, in the leaves. Winter plants absorbed the bioregulator more successfully than summer plants. The stem proved to be an active absorption site in young plants. Six days after bioregulator application, transport of the molecule was restricted to the lower stem in summer plants, and stem and leaves in winter plants. Transport was governed by the availability and development of conduits. The intact molecule was recovered two days after application but was not detectable after 4 and 6 days indicating that it is metabolized fairly rapidly. The significance of these findings is discussed in terms of the use of bioregulators to stimulate rubber production in guayule plants.     

Low light and low ammonium are key factors for guayule leaf tissue shoot organogenesis and transformation

A new method has been developed for guayule tissue culture and transformation. Guayule leaf explants have a poor survival rate when placed on normal MS medium and under normal culture room light conditions. Low light and low ammonium treatment greatly improved shoot organogenesis and transformation from leaf tissues. Using this method, a 35S promoter driven BAR gene and an ubiquitin-3 promoter driven GUS gene (with intron) have been successfully introduced into guayule. These transgenic guayule plants were resistant to the herbicide ammonium-glufosinate and were positive to GUS staining. Molecular analysis showed the expected band and signal in all GUS positive transformants. The transformation efficiency with glufosinate selection ranged from 3 to 6%. Transformation with a pBIN19-based plasmid containing a NPTII gene and then selection with kanamycin also works well using this method. The ratio of kanamycin-resistant calli to total starting explants reached 50% in some experiments.     

Photosynthesis and assimilate partitioning between carbohydrates and isoprenoid products in vegetatively active and dormant guayule: physiological and environmental constraints on rubber accumulation in a semiarid shrub

The stems and roots of the semiarid shrub guayule, Parthenium argentatum, contain a significant amount of natural rubber. Rubber accumulates in guayule when plants are vegetatively and reproductively dormant, complicating the relationship between growth/reproduction and product synthesis. To evaluate the factors regulating the partitioning of carbon to rubber, carbon assimilation and partitioning were measured in guayule plants that were grown under simulated summer‐ and winter‐like conditions and under winter‐like conditions with CO₂ enrichment. These conditions were used to induce vegetatively active and dormant states and to increase the source strength of vegetatively dormant plants, respectively. Rates of CO₂ assimilation, measured under growth temperatures and CO₂, were similar for plants grown under summer‐ and winter‐like conditions, but were higher with elevated CO₂. After 5 months, plants grown under summer‐like conditions had the greatest aboveground biomass, but the lowest levels of non‐structural carbohydrates and rubber. In contrast, the amount of resin in the stems was similar under all growth conditions. Emission of biogenic volatile compounds was more than three‐fold higher in plants grown under summer‐ compared with winter‐like conditions. Taken together, the results show that guayule plants maintain a high rate of photosynthesis and accumulate non‐structural carbohydrates and rubber in the vegetatively dormant state, but emit volatile compounds at a lower rate when compared with more vegetatively active plants. Enrichment with CO₂ in the vegetatively dormant state increased carbohydrate content but not the amount of rubber, suggesting that partitioning of assimilate to rubber is limited by sink strength in guayule.     

Early season root production and zoospore infection of cultivars of flue-cured tobacco that differ in level of partial resistance to Phytophthora parasitica var. nicotianae

Root production of four cultivars of flue-cured tobacco was quantified in the field, greenhouse and phytotron. The cultivars ranged in level of partial resistance to the black shank pathogen, Phytophthora parasitica var. nicotianae, from susceptible to highly resistant. In the field, root-observation plates were installed approximately 10 cm from plants, and in greenhouse and phytotron studies, plants were grown in 4-liter containers with one sloping transparent side for root observation. Root growth was determined weekly for four weeks after transplanting in the field and daily up to 14 days after transplanting in the greenhouse and phytotron. Root tracings were made on acetate sheets placed against the sloping transparent side of the containers or against the transparent observation plates in the field following removal of soil from the outside of the observation plate. Root growth was quantified by retracing the root pattern on the acetate sheets over a digitizing tablet attached to a personal computer. Numbers of roots, root length, and mean and maximum rate of root growth were determined. Cultivars Hicks (susceptible) and K-326 (low level of resistance) had significantly larger root systems than moderately resistant G-28 or highly resistant NC 82. Differences in total root length were due to increased branching that resulted in development of significantly greater numbers of roots in Hicks and K-326. For example, between day 21 and 28, Hicks produced more than three times the number of new roots as NC 82 in the field. The mean rate of root extension observed (2.17 mm hr^–1) was similar in all four cultivars. Infection efficiency on the different cultivars was determined in the field by inoculating roots with zoospores of P. p. nicotianae. Lesions were visible as water soaked areas within 24 hr of inoculation. At 48 hr after inoculation, percentages of inoculations that resulted in lesion formation were 57, 46, 23, and 16% for Hicks, K-326, G-28 and NC 82, respectively. The possible role of rooting intensity as a mechanism of avoidance to P. p. nicotianae in tobacco cultivars is discussed.     

EFFECTS OF CULTIVAR AND PLANT AGE ON VEGETATIVE GROWTH OF THE STRAWBERRY,FRAGARIA ANANASSA

Growth of strawberry plants (Dunlap, Sparkle, and Catskill) in the greenhouse was affected by the time of rooting of the runners in the field. This was shown by more rapid leaf and leaf-area production, larger leaf size, more runners and root growth on the older compared to the younger runner plants. The oldest plants also produced the greatest number of flower buds, but the time of bloom was not appreciably affected. The growth responses of plants of different sizes were similar to those of plants of different ages. These growth responses were most apparent during the first stages of growth, but the resulting differences frequently persisted for several months. Significant differences between cultivars were also found. Initially the greatest leaf size and leaf area were produced by Catskill plants. Because of a higher leaf production rate, Sparkle plants eventually had the largest leaf area. Sparkle plants were the last to flower and also to initiate runner production. Dunlap plants were the first to flower and produced the largest number of flowers. In the field, leaf production records indicated responses from cultivars and plant age similar to those found in the greenhouse.     

Selective plant growth suppression by shoot application of soil bacteria

Selected rhizosphere bacterial isolates, previously determined as plant growth deleterious, were tested for their ability to suppress plant growth after foliar spray applications, for selectivity with regard to plant species, and in pilot field experiments for their potential as weed biocontrol agents. Inundative foliar applications of aqueous bacterial suspension were performed on a range of weed and crop species. Plant symptoms after spraying ranged from rapid necrosis and wilting to an overall growth suppression or stunting. Significant and selective reductions in biomass of up to 90% fresh weight, as well as large reductions in plant survival and plant height were recorded in greenhouse pot experiments. However, monocotyledonous plants were affected weakly or not at all by two isolates extensively tested. Effects of these were dose- and plant age-dependent, and were for some plants enhanced by high relative humidity. For one isolate, A153, effects were also expressed in cell-free culture filtrates pointing to involvement of specific metabolites. In pilot field experiments, strong growth suppression was observed on broad-leaved plants, while barley crop plants were unaffected.     

TRICHOME MORPHOLOGY AS AN INDICATOR OF HIGH RUBBER BEARING GUAYULE (PARTHENIUM ARGENTATUM GRAY) PLANTS IN NATIVE POPULATIONS

Parthenium argentatum Gray (guayule) and P. incanum (mariola) grow in close associations in their native habitat of Mexico. Variation in rubber concentration and morphology of guayule has been known for a long time. Studies of over 75 native guayule plants indicated the presence of at least three prominent forms of guayule plants. Group I guayule plants have oblanceolate leaves and leaf margins are entire to two-toothed. T-shaped leaf trichomes are with centrally attached stalk and cap cell with two blunt ends. Plants in this group are high in rubber, containing on the average 17% rubber. Group II guayule plants have narrow elliptic leaves and leaf margins are entire to four-toothed. T-shaped trichomes have an acentrally attached stalk and a cap cell with short end blunt, long end pointed and straight. Rubber content in these plants averages 10%. Group III guayule plants have ovate leaves and leaf margins are four- to eight-toothed. T-trichomes have an acentrally attached stalk and a cap cell with short end blunt, long end pointed and wavy or curved. These plants on the average contain 6% rubber. Morphological as well as biochemical data indicate the presence of mariola genes in the last two groups of plants and this has resulted in an increase in trichome length and a decrease in the rubber content. Group II guayule plants are of more common occurrence than the others. Based on the data presented here, high rubber bearing guayule plants in native stands can be easily selected by analyzing trichome morphology.     

Selection and evaluation of the potential of choline-metabolizing microbial strains to reduce Fusarium head blight

Choline and betaine are found in wheat flower tissues and have been implicated in stimulating hyphal growth of the primary causal agent of Fusarium head blight (FHB), Gibberella zeae. Choline metabolizing strains (CMS) from wheat anthers may therefore be a useful source of antagonists of G. zeae. One-hundred twenty-three of 738 microbial strains that were recovered from wheat anthers collected from plants grown in Illinois and Ohio were CMS as determined by growth in a liquid medium containing choline as a sole carbon and nitrogen source and a colorimetric, choline oxidase-based assay of culture filtrate. Thirty-one out of 123 CMS reduced FHB disease severity by at least 25% in greenhouse tests on wheat and 17 reduced FHB severity by at least 50%. All five CMS selected for field testing in 2003 reduced disease severity compared to the untreated check at both field locations on moderately resistant cultivar Freedom. Freedom wheat treated with Pseudomonas sp. AS 64.4 had 63% and 46% less FHB severity than untreated wheat at the two sites. Three of five CMS reduced severity at both locations on susceptible cultivar Pioneer Brand 2545. Disease control was comparable to that obtained using the fungicide Folicur 3.6F. Selection of wheat anther colonists for ability to utilize choline as a sole carbon and nitrogen source has utility as a screening tool in the search for efficacious antagonists of G. zeae although choline utilization does not insure that an isolate will be an effective biocontrol agent against Fusarium head blight.     

Biological control of bacterial spot disease and plant growth-promoting effects of lactic acid bacteria on pepper

In our previous studies, we observed the biological control effect of lactic acid bacteria strains (LABs) KLF01, KLC02 and KPD03 against different plant pathogenic bacteria in vitro against Ralstonia solanacearum, and strains KLF01 and KLC02 against Pectobacterium carotovorum under greenhouse and field experiments, respectively. In this study, we observed the efficacy of these bacteria against bacterial spot pathogen (Xanthomonas campestris pv. vesicatoria) and their plant growth-promoting activities in pepper (Capsicum annuum L. var. annuum), under greenhouse and field conditions. LABs significantly (P < 0.05) reduced bacterial spot on pepper plants in comparison to untreated plants in both the greenhouse and the field experiments. The plant growth-promoting effect of LABs on pepper varied; some strains had a significant effect on growth promotion (P < 0.05) compared with untreated plants, while some showed no significant effect in the greenhouse and field experiments. Additionally, LABs were able to colonise roots, produce indole-3-acetic acid (IAA), siderophores and solubilise phosphate. These findings indicate that application of LABs could provide a promising alternative for the management of bacterial spot disease in pepper plants and could therefore be used as a healthy plant growth-promoting agent.