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Penaloza-Vazquez A.,Oklahoma State University | Sreedharan A.,Oklahoma State University | Sreedharan A.,Citrus Research and Education Center | Bender C.L.,Oklahoma State University
Environmental Microbiology | Year: 2010

Pseudomonas syringae pv. syringae strain FF5 is a phytopathogen that causes a rapid dieback on ornamental pear trees. In the present study, the transcriptional expression of hrpM/opgH, algD, hrpR and rpoD was evaluated in P. syringae FF5 and FF5.M2 (hrpM/ opgH mutant). The temporal expression of these genes was evaluated during biofilm formation, the hypersensitive reaction (HR) on tobacco plants, and when the bacteria were subjected to different environmental stresses. The results indicate that mutations in hrpM negatively impair several traits including biofilm formation, the ability to cause disease in host plants and the HR in non-host plants, and the expression of hrpR, a regulatory gene modulating the latter two traits. Furthermore, FF5.M2 was decreased in swarming motility and unable to respond to different environmental challenges. Interestingly, FF5.M2 showed an exponential increase in the expression of algD, which is the first gene to be transcribed during the biosynthesis of the alginate, a virulence factor in P. syringae. The expression of both hrpM and algD were required for biofilm formation, and hrpM was expressed earlier than algD during biofilm develop ment. These findings indicate that hrpM expression is required for several traits in P. syringae and plays an important role in how this bacterium responds to environmental challenges. © 2010 Society for Applied Microbiology and Blackwell Publishing Ltd.

Zambrosi F.C.B.,Instituto Agronomico | Mattos Jr. D.,Instituto Agronomico | Boaretto R.M.,Instituto Agronomico | Quaggio J.A.,Instituto Agronomico | And 2 more authors.
Plant and Soil | Year: 2012

Background and aims: Phosphorus (P) is a mobile nutrient in the plant so growth depends on its internal remobilization and a plant's ability to respond to its availability in the growing media. This study was conducted to evaluate the influence of P status and rootstocks on the patterns of P uptake and remobilization in orange trees. Methods: Sweet orange trees on Cleopatra mandarin (CM) or Rangpur lime (RL) rootstocks were grown for nine months in nutrient solution (NS) that was either P-deficient (DNS) or was P-sufficient (SNS). After this period, half of the trees were reciprocally transferred between DNS and SNS (from D to S and S to D), while the others remained in their initial P availability. Results: Trees on RL had more shoot and root growth, accumulated more P and had greater efficiency of P absorption and transport to the shoot (PAE) than those on CM. The major source of P for growth was previously stored P even with an adequate current P supply to the roots. This suggested the dominance of P remobilization over P uptake and the requirement that trees had sufficient stored P to meet P demand of new growth. Trees on CM had greater concentrations of remobilized P in new shoots than trees on RL. Conclusion: Trees grafted on rootstocks less able to take up P (CM) were more dependent on the internal reserves of P for new growth than rootstocks with higher PAE (RL). © 2012 Springer Science+Business Media B.V.

Cesar Bachiega Zambrosi F.,Instituto Agronomico | Mattos D.,Instituto Agronomico | Syvertsen J.P.,Citrus Research and Education Center
Journal of Plant Nutrition and Soil Science | Year: 2011

Some formulations of phosphite (Phi) have been recommended as a source of P nutrition for several crops including citrus even though there are known negative effects of Phi on plant growth. Changes in plant growth and metabolism after Phi application should be reflected in altered nutrient-use efficiency and leaf photosynthesis. We carried out a greenhouse study using seedlings of two contrasting citrus (Citrus spp.) rootstocks, Carrizo citrange (CC) and Smooth Flat Seville (SFS), growing in either aerated hydroponic culture or sterilized native sandy soil. Plants were subjected to four P treatments: No P (control, P0); 0.5 mM Pi (PO4-P); 0.25 mM Pi + 0.25 mM Phi (Pi + Phi), or 0.5 mM Phi (Phi). Photosynthetic characteristics, concentrations of total P (Pt) and soluble PO4-P or PO3-P in leaves and roots, and plant growth were evaluated after 80-83 d P treatments. Overall, the Pi plants had the highest Pt (total P) and total plant dry weight while the P0 plants had the lowest Pt but highest total root length and root-to-shoot ratio. Leaf chlorophyll (SPAD readings) and net assimilation of CO2 (ACO2) of the P0 and Phi plants were similarly lower than those of Pi and Pi + Phi plants. Growth responses of the Pi + Phi treatment were intermediate between the Pi and Phi treatments. Although Phi increased Pt and soluble-PO4-P concentration in leaves and roots above the P0 treatment, this did not translate into increased plant growth. In fact, the Phi treatment had some phytotoxic symptoms, impaired P- and N-utilization efficiency for biomass production as well as lower nutrient-use efficiency in the photosynthetic process. Thus, these two rootstocks could not use Phi as a nutritional source of P. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Jude Grosser, a professor of plant cell genetics at UF's Institute of Food and Agricultural Sciences Citrus Research and Education Center, and Manjul Dutt, a research assistant scientist at the CREC, used a gene isolated from the Arabidopsis plant, a member of the mustard family, to create the new trees. Their experiment resulted in trees that exhibited enhanced resistance to greening, reduced disease severity and even several trees that remained disease-free after 36 months of planting in a field with a high number of diseased trees. The journal PLOS ONE recently published a paper on their study. "Citrus crop improvement using conventional breeding methods is difficult and time consuming due to the long juvenile phase in citrus, which can vary from four to twelve years, "Grosser said. "Improvement of citrus through genetic engineering remains the fastest method for improvement of existing citrus cultivars and has been a key component in the University of Florida's genetic improvement strategy." Citrus greening threatens to destroy Florida's $10.7 billion citrus industry. The diseased bacterium first enters the tree via the tiny Asian citrus psyllid, which sucks on leaf sap and leaves behind the greening bacteria. The bacteria then move through the tree via the phloem - the veins of the tree. The disease starves the tree of nutrients, damages its roots and the tree produces fruits that are green and misshapen, unsuitable for sale as fresh fruit or, for the most part, juice. Most infected trees eventually die and the disease has already affected millions of citrus trees in North America. Citrus greening was first detected in Florida in 2005. Florida has lost approximately 100,000 citrus acres and $3.6 billion in revenues since 2007, according to researchers with UF/IFAS. Grosser and Dutt's research team used sweet orange cultivars Hamlin and Valencia and created plants that defend themselves against pathogens utilizing a process called systemic acquired resistance, or SAR. SAR provides protection against a broad spectrum of microorganisms and is associated with the production of anti-pathogen proteins. Utilizing SAR has already resulted in the production of transgenic canker-resistant trees. Transgenic trees are those into which DNA from an unrelated organism has been artificially introduced. Disease resistance to greening, also known as huanglongbing or HLB, in this study was evaluated in two ways. First, in a greenhouse study conducted with Southern Gardens Citrus in Clewiston, several hundred trees (clones from several independent transgenic plant lines) were exposed continuously for two years to free-flying, greening-positive psyllids. Trees were routinely pruned and fertilized to stimulate new leaf production. These trees were evaluated every six months for two years for the presence of greening. The insects were also randomly evaluated during this study for the presence of the greening bacterium. Approximately 45 percent of the trees expressing the Arabidopsis gene tested negative for greening. In three of the transgenic lines, the greening bacterium was not detected at all. Control trees tested positive for the presence of greening within six months and remained positive for the entire duration of the study. In the second concurrent study, selected transgenic trees and controls were cloned, grown and planted in fields with a 90-percent HLB infection rate. These trees were similarly evaluated every six months for three years for the presence of the greening bacterium. In this study, one transgenic line remained greening-free for the duration of the study, except for the 24-month sampling period when it tested positive. A second line tested positive at the 30-month sampling period while a third line tested positive at 30 months, but was greening-free at 36 months. Neither of these lines declined in health, and both showed continued growth with periodic flushes. "In addition to inducing resistance to greening, this transgenic line could potentially protect our trees from other important citrus fungal and bacterial diseases such as citrus canker and black spot," Dutt said. The next steps include transferring this gene into additional commercial varieties and rootstocks that are commonly grown in Florida. In addition, researchers must 'stack' this gene with another transgene that provides resistance to the greening bacterium by a completely different mechanism. That will prevent the pathogen from overcoming the resistance in the field. It will still be several years before such trees will be available for commercial use. Explore further: Citrus greening bacterium may 'ring the dinner bell' to attract insect

Mishra A.R.,Citrus Research and Education Center | Karimi D.,Citrus Research and Education Center | Ehsani R.,Citrus Research and Education Center | Lee W.S.,University of Florida
Transactions of the ASABE | Year: 2012

Citrus greening, also known as Huanglongbing or HLB, is a major threat to the U.S. citrus industry. Currently, scouting and visual inspection are used for screening infected trees. However, this is a time-consuming and expensive method for HLB disease detection. Moreover, as it is subjective, the current method exhibits high detection error rates. The objective of this study was to investigate the potential of visible and near-infrared (VIS-NIR) spectroscopy for identifying HLB-infected citrus trees. The spectral data from infected and healthy orange trees of the Valencia variety were collected from four different orchards in Florida. Two different spectroradiometers with a spectral range of 350 to 2500 nm were used to collect the canopy reflectance spectral data. Three classification techniques were used to classify the data: k-nearest neighbors (KNN), logistic regression (LR), and support vector machines (SVM). Analysis showed that using only one canopy reflectance observation per tree was inadequate. None of the classification methods was successful in discriminating healthy trees from HLB-infected trees because of the large variability in the canopy reflectance spectral data. When five spectra from the same tree were used for classification, the SVM and weighted KNN methods classified spectra with 3.0% and 6.5% error rates, respectively. The results from this study indicate that canopy VIS-NIR spectral reflectance data can be used to detect HLB-infected citrus trees; however, high classification accuracy (>90%) requires multiple measurements from a single tree. © 2012 American Society of Agricultural and Biological Engineers.

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