Stimulation of the oxidative decarboxylation of indole-3-acetic acid in citrus tissues by ethylene

Ethylene has been shown to stimulate the degradation of indole-3-acetic acid (IAA) in citrus leaf tissues via the oxidative decarboxylation pathway, resulting in the accumulation of indole-3-carboxylic acid (ICA). Preliminary data indicated that ethylene stimulates only the first step of this pathway, i.e. the decarboxylation of IAA which leads to the formation of indole-3-methanol. The effect of ethylene seems to be a specific one since 2,5-norbornadiene, an ethylene action inhibitor, significantly inhibited the stimulation of IAA decarboxylation by ethylene. It has long been suggested that peroxidase or a specific form of the peroxidase complex (`IAA oxidase') catalyse this step. However, we did not observe a clear effect of ethylene on the peroxidase system. An alternative possibility, that the stimulatory effect of ethylene on IAA catabolism results from increased formation of hydrogen peroxide (H₂O₂), a co-factor for peroxidase activity, was verified by direct measurements of H₂O₂ in the tissues or by assaying the activity of gluthathione reductase, which has been shown to be induced by oxygen species. This possibility is further supported by the observations showing that IAA decarboxylation in control tissues was enhanced to the level detected in ethylene-treated tissues by application of H₂O₂.     

RAPD-based genetic analysis of offsprings from the sexual cross using allotetraploid citrus somatic hybrid as pollen parent

Thirty-one polymorphic decamer primers were selected to genotype 92 progenies from the cross between Yiben No.4, a monoembryonic diploid F₁ hybrid of Citrus reticulata Blanco cv Huanongbendizao tangerine and C. ichangensis Swingle, and [Hamlin sweet orange + Rough lemon], an allotetraploid somatic hybrid of Citrus sinensis Osbeck cv. Hamlin and C. jambhiri Lush cv. Rough Lemon. χ² (Chi-square) analysis of RAPD markers in the progenies indicated they were randomly transmitted from the four donor parents, without significant difference between the diploids and triploids. However, these progenies were clustered into three major groups using dendrogram constructed by UPGMA, skewed to three parents in certain degrees, 15 (13 triploids and 2 diploids) to Hamlin, 16 (9 and 7) to Yiben No. 4, and 61 (57 and 4) to [Hamlin sweet orange + Rough Lemon] from which genomic contribution was predominant in progenies, respectively.     

Antagonistic effects of the endophytic fungus Meira geulakonigii on the citrus rust mite Phyllocoptruta oleivora

AIMS: The fungus Meira geulakonigii has been shown to reduce populations of citrus rust mite (CRM; Phyllocoptruta oleivora) on citrus leaves and fruits, in both the field and laboratory. However, attempts to isolate the fungus from leaves and fruits have been unsuccessful. The aims of this study were therefore to determine whether M. geulakonigii is a citrus endophyte, and to assess possible mechanisms involved in its mite-antagonist activity. METHODS AND RESULTS: A quantitative real-time PCR and regular PCR approaches were developed to detect M. geulakonigii in both the field and laboratory. The fungus was detected throughout. Different methods revealed that M. geulakonigii is an endophyte, which colonizes both the peel of grapefruits. Applications of conidia protected the grapefruits against CRM, and fungal secretions extracted from growth media caused 100% CRM mortality. CONCLUSIONS: Meira geulakonigii is a beneficial endophyte of grapefruits that colonizes the fruit's peel, and protects it from CRM. SIGNIFICANCE AND IMPACT OF THE STUDY: Findings from this study demonstrate the endophytic nature of M. geulakonigii in its interaction with grapefruits. In addition, a molecular approach was developed to specifically detect the fungus inside the grapefruit peel. This approach can be used to assess the natural occurrence of M. geulakonigii in grapefruit.     

Sampling and distribution pattern of Trioza erytreae Del Guercio, 1918 (Hemiptera: Triozidae) in citrus orchard

Developing efficient sampling protocols is essential to monitor crop pests. One vector of the citrus disease HLB, the African citrus psyllid Trioza erytreae Del Guercio, 1918 (Hemiptera: Triozidae), currently threatens the lemon industry throughout the Mediterranean region. In this work, a pool of sampling methods devoted to monitoring the population of T. erytreae was compared, its spatial distribution in the orchard was assessed, and the minimum sampling effort for the best sampling method was estimated. Three lemon orchards in North-western Portugal were sampled for one year using two types of yellow sticky traps (standard yellow and fluorescent Saturn yellow), B-vac sampling and sweep net sampling. The method that best performed, in terms of cost-efficiency, was the yellow sticky traps. The two colours of the sticky traps tested did not yield a significantly different number of catches. The spatial distribution throughout the orchards was found to be aggregated towards the borders. A minimum of three sticky traps per hectare was found to be enough to estimate the population at 90% accuracy for the mean during the outbreak. These results should help to monitor and anticipate outbreaks that may even colonize neighbour orchards. Studies on the local dispersion patterns of T. erytreae throughout the orchard are mandatory to further refine and optimize efficient monitoring protocols.     

The WD-40 repeat protein PkwA of Thermomonospora curvata is associated with rapid growth and is localized in the tips of growing hyphae

The PkwA protein of the thermophilic actinomycete Thermomonospora curvata has already been reported as the first instance of a WD-40 module-containing protein of prokaryotic origin. This protein is composed of an N-terminal eukaryotic-type protein kinase domain and of seven C-terminal WD-40 repeats. PkwA is a peripheral membrane protein that is linked to the early exponential growth phase of the bacterium. Its intracellular concentrations are extremely low. We have shown that the protein forms high molecular weight complexes and is localized mainly in the tips of the young Thermomonospora vegetative hyphae.     

Activity and mode of action against fungal phytopathogens of bovine lactoferricin-derived peptides

AIM: To evaluate the activity against fungal phytopathogens of two synthetic peptides derived from the protein bovine lactoferricin: the antibacterial active core of six amino acid residues (LfcinB(20-25)) and an extension of 15 amino acids (LfcinB(17-31)). METHODS AND RESULTS: In vitro activity against fungal pathogens was determined and compared with that against model micro-organisms. Activity was demonstrated against fungi of agronomic relevance. Distinct antimicrobial properties in vitro were found for the two peptides. LfcinB(17-31) had growth inhibitory activity higher than LfcinB(20-25). However, LfcinB(17-31) was not fungicidal to quiescent conidia of Penicillium digitatum at the concentrations assayed, while LfcinB(20-25) killed conidia more efficiently. Microscopical observations showed that the mycelium of P. digitatum treated with LfcinB(17-31) developed alterations of growth, sporulation and chitin deposition, and permeation of hyphal cells. In experimental inoculations of mandarins, both peptides showed limited protective effect against the disease caused by P. digitatum. CONCLUSIONS: LfcinB(20-25) and LfcinB(17-31) peptides were shown to have antimicrobial activity against plant pathogenic filamentous fungi, with distinct properties and mode of action. SIGNIFICANCE AND IMPACT OF THE STUDY: LfcinB(20-25) and LfcinB(17-31) peptides offer novel alternatives to develop resistant plants by molecular breeding.     

Molecular Typing and Genetic Characterization of Pathogenic Variants of Xanthomonas axonopodis pv. citri in Taiwan

Molecular typing was applied and optimized for genetic characterization for three pathogenic variants of Xanthomonas axonopodis pv. citri (Xac) from Taiwan. These three novel variants of atypical symptom–producing X. axonopodis pv. citri were designated as Xac‐A^f, Xac‐A^p and Xac‐A^r. Based on polymerase chain reaction (PCR) with primers specific to X. axonopodis pv. citri, leucine‐responsive regulatory protein (lrp) gene assay and DNA fingerprintings generated by repetitive‐sequence PCR (rep‐PCR) and amplified fragment length polymorphism (AFLP) were used to compare strains including the three types of atypical symptom–producing strains Xac‐A^f, Xac‐A^p and Xac‐A^r, and additional reference strains from pathotypes Xac‐A, Xac‐A*, Xac‐A^w, X. axonopodis pv. auruantifolii and X. axonopodis pv. citrumelo. These three types of X. axonopodis pv. citri variants can be detected with six sets of primer specific for X. axonopodis pv. citri. Cluster analyses by lrp sequence assay, AFLP and combing the band patterns of rep‐PCR clearly grouped the atypical symptom–producing variants in types Xac‐ A^f, Xac‐A^r and Xac‐A^p into the same cluster with typical symptom‐producing strains in pathotype Xac‐A. These three types of X. axonopodis pv. citri variants could be excluded from strains of Xac‐A* and Xac‐A^w in these genotypic analyses. Strains of Xac‐A* and Xac‐A^w were closely related to Xac‐A strains in our results. No Taiwan isolate was related to X. axonopodis pv. auruantifolii or X. axonopodis pv. citrumelo. The results further confirmed the atypical symptom–producing variants of X. axonopodis pv. citri in Taiwan belong to pathotype Xac‐A.     

Field efficacy of Ceratitis capitata (Diptera: Tephritidae) mass trapping technique on clementine groves in Spain

Mass trapping is being used in Mediterranean regions to control Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) in citrus. The technique is based on placing a high density of traps with an attractant (Ferag CC D TM^®, a three‐membrane dispensers of trimethylamine, ammonium acetate and diaminoalkane), and a toxicant, aiming to capture the highest numbers of adults in the grove. From 2006 to 2008, field trials were conducted in commercial Clementine (Citrus reticulata Blanco) groves to evaluate the efficacy against medfly of using different trap densities, 25, 50, 75 and 100 traps per ha. Based on the number of adults captured, fruit maturity parameters and medfly fruit damage, a 25 trap per ha density appears to be a valid stand‐alone method to protect mid‐season varieties (Clemenules) from the attack of C. capitata, because <0.5% of fruits on average were damaged at harvest. For early‐season varieties (Loretina and Marisol), mass trapping technique alone did not offer a satisfactory medfly control, because medfly populations were higher in the warmer months of the early‐season variety production, which led to a higher percentage of attacked fruits, even when increasing the trap density from 50 to 100 per ha. However, using 50 traps per ha density combined with chemical treatments only to the perimeter row of the grove gave good results, because <2% of fruits in average were damaged at harvest.     

植物保护剂防治害虫效果的评价方法

在以作用因子组建的生命表和干扰作用控制指数(IIPC)的基础上建立植物保护剂的研究方法和综合评价指标,以表示驱避成虫选择寄主,干扰成虫的产卵行为,对卵孵化的影响,以及干扰幼虫的行为,包括逃避、拒食、幼虫特别是初孵幼虫中毒死亡的作用.采用所建立的研究方法和评价指标的试验结果表明,菜田附近大多数非寄主植物的乙醇抽提物(干重1g·100ml^-1)如大叶桉(Eucalytus rubusta)、蟛蜞菊(Wedelia chinensis)等对保护十字花科蔬菜免受小菜蛾为害起着明显的作用.印楝油对小菜蛾幼虫的防治效果相当显著.机油乳剂(CAL TEX产品)对桔全爪螨具有明显的控制效果.