Hydrogen peroxide localization and antioxidant status in the recovery of apricot plants from European Stone Fruit Yellows

Hydrogen peroxide (H₂O₂) localization and roles of peroxidases, malondialdehyde and reduced glutathione were compared in leaves of apricot (Prunus armeniaca) plants asymptomatic, European Stone Fruits Yellows (ESFY)-symptomatic and recovered. Nested PCR analysis revealed that Candidatus Phytoplasma prunorum, is present in asymptomatic, symptomatic and recovered apricot trees, confirming previous observations on this species, in which recovery does not seem to be related to the disappearance of phytoplasma from the plant.H₂O₂was detected cytochemically by its reaction with cerium chloride, which produces electron-dense deposits of cerium perhydroxides. H₂O₂was present in the plasmalemma of the phloem cells of recovered apricot plant leaves, but not in the asymptomatic or symptomatic material. Furthermore, by labelling apricot leaf tissues with diaminobenzidine DAB, no differences were found in the localization of peroxidases.Protein content in asymptomatic, symptomatic and recovered leaves was not significantly different from one another. In contrast, guaiacol peroxidase activity had the following trend: symptomatic > recovered > asymptomatic, whereas reduced glutathione content followed the opposite trend: asymptomatic > recovered > symptomatic. Moreover, no differences were observed in malondialdehyde concentrations between asymptomatic, symptomatic and recovered leaves. The overall results suggest that H₂O₂ and related metabolites and enzymes appear to be involved in lessening both pathogen virulence and disease symptom expression in ESFY-infected apricot plants.     

Detection and characterization of phytoplasmas in diseased stone fruits and pear by PCR-RFLP analysis in Turkey

During the late summer-early autumn of 2002, surveys were carried out in Turkey to determine the presence of phytoplasma diseases in fruit trees. Phytoplasmas were detected and characterized by PCR-RFLP analysis and TEM technique in stone fruit and pear trees in the eastern Mediterranean region of the country. Six out of 24 samples, including almond, apricot, peach, pear and plum, gave positive results in PCR assays. RFLP analysis usingSspI andBsaAI enzymes of PCR products obtained with primer pair f01/r01 enabled identification of the phytoplasmas involved in the diseases. Stone fruit trees, including a local apricot variety (‘Sakıt’) and a pear sample, were found to be infected with European stone fruit yellows (ESFY, 16SrX-B) and pear decline (PD, 16SrX-C) phytoplasmas, respectively. This is the first report in Turkey of PD phytoplasma infecting pear and of ESFY phytoplasma infecting almond, apricot, myrobalan plum and peach; ESFY phytoplasma infecting Japanese plum was previously reported. http://www.phytoparasitica.org posting July 21, 2005.     

Association of vegetation and soil mite assemblages with isolated Scots pine trees on a Scottish wet heath

Isolated trees may significantly enhance biodiversity at the landscape level. However, our understanding of their impacts is still poor, particularly in environments with high soil moisture where research on this topic has been comparatively limited. We examined understorey vegetation and soil oribatid mite assemblages under live and dead Scots pine trees and in open treeless areas, all within the same Scottish upland wet heath system, to determine whether isolated live trees affected the understorey and mite components of the ecosystem, and whether these effects occurred in parallel. We also explored whether these responses might result from tree-driven reductions in soil moisture content. Live trees reduced soil moisture (relative to wet heath and beneath dead trees) and appeared to change vegetation from wet heath to dry heath type communities. These effects were strongly related to tree trunk diameter (tree size). No major effects of dead trees on understorey vegetation or soil moisture were apparent. Higher mite species abundance and richness were found under live trees than in treeless open heath. Although mite abundances were lower under dead trees than live trees, richness remained similar, thus different factors seem to be regulating mite abundance and community composition. These findings indicate that landscape-level biodiversity responses to environmental change such as habitat fragmentation cannot be predicted from vegetation patterns alone, and that even in heavily fragmented landscapes comparatively small patches such as isolated individual trees can enhance biodiversity.     

High tolerance of European plum varieties to plum leptonecrosis

A 13 year comparative study was carried out on the behaviour of four European and two Japanese plum varieties grown in adjacent rows in an area of northern Italy where plum leptonecrosis is epidemic. Within seven years, 100% of the Japanese plum trees became symptomatically infected. Nine years after planting, five trees of each of the European cvs, which were asymptomatic, were top-grafted with healthy buds of the cv Ozark Premier, which is an indicator for plum leptonecrosis. Based on the results of PCR analysis, DAPI staining and on the reaction of the top-grafted Ozark Premier indicators, 50% of the European plum trees, despite their healthy appearance, were shown to be infected with plum leptonecrosis. The detectable presence and graft transmissibility of the plum leptonecrosis phytoplasma in the asymptomatic European plum trees means that the European plum trees are not resistant to the infection but that they are tolerant. The active presence of a still unknown vector/s in the investigated area is stressed as well as the important role of Prunus domestica L. played in the conservation and spread of plum leptonecrosis.     

Macropsis mendax as a vector of elm yellows phytoplasma of Ulmus species

A 3-year study was carried out in north-east Italy, the site of recent elm yellows epidemics, to identify vectors for the elm yellows phytoplasma. Using PCR analysis, Ulmus minor and Ulmus pumila , each with and without symptoms, were positive for the elm yellows phytoplasma. Macropsis mendax , a univoltine and monophagous leafhopper, was shown to be the vector of the elm yellows-associated disease agent. PCR analyses demonstrated that the insect was infected both in natural conditions and in the screenhouse after acquisition-feeding on infected elm plants. Groups of M. mendax , collected from naturally infected elm trees, transmitted elm yellows phytoplasma to elm test plants. In nature, Alnus glutinosa trees affected by alder yellows were found in the surroundings of yellows-affected elm trees; the associated disease agent of alder yellows was transmitted under controlled conditions from alder to elm test plants by grafting.     

Predictors of fine-scale spatial variation in soil mite and microbe community composition differ between biotic groups and habitats

Soils support highly diverse communities, but due to the great spatial complexity at very fine scales and small size of soil organisms, our knowledge of what structures belowground communities remains limited. We used a spatially nested sampling regime (distances between samples of 0.06–122 m) to develop an understanding of the relationship between soil mite and microbial community composition and properties of vegetation and underlying soil within two habitats: birch woodland and heather moorland. We found that variation in community composition of all four biotic groups investigated (Oribatida, Mesostigmata, fungi and bacteria – thus representing multiple trophic levels) was related to variation in soil properties and vegetation, but also space. These three factors (i.e. vegetation, soil properties and space) accounted for 17–36% of the variation in community composition of the biotic groups. The remaining, rather large, proportion of unexplained variation in community composition (64–87%) is likely to represent either random variation or variation related to unmeasured variables that are not spatially auto-correlated. Soil properties generally explained more variance in belowground communities than could vegetation composition. This should not be interpreted as evidence that soil properties exert a greater influence on belowground communities than vegetation composition per se, as the vegetation composition within a habitat is likely to vary at a larger spatial scale than the soil properties we measured. Vegetation composition had stronger explanatory power in the birch woodland and was more strongly related to fungal community composition than that of any other biotic group, particularly in heather moorland. This relationship may reflect the strong association between plants and their fungal symbionts and pathogens. We conclude that both vegetation composition and variation in soil properties influence belowground communities. However, the relative importance of these two factors depends on both habitat and the type of organism being studied.     

Differentially-regulated defence genes in Malus domestica during phytoplasma infection and recovery

To improve knowledge about plant/phytoplasma interactions and, in particular, about the ‘recovery’ phenomenon in previously-infected plants, we investigated and compared expression levels of several defence-related genes (four pathogenesis-related proteins and three jasmonate-pathway marker enzymes) in apple plants showing different states of health: vigorous (healthy), phytoplasma-infected, and recovered. Real Time-PCR analyses demonstrated that genes are differentially expressed in apple leaf tissue according to the plants’ state of health. Malus domestica Pathogenesis-Related protein (MdPR) 1, MdPR 2 and MdPR 5 were significantly induced in leaves of diseased and symptomatic plants compared to leaves of those plants that were healthy or recovered. On the other hand, levels of all the jasmonate (JA)-pathway marker genes that we selected for this study, were up-regulated in the leaves of recovered plants compared to the diseased ones. In conclusion, our study demonstrated that two different sets of defence genes are involved in the interactions between apple plants and ‘Candidatus Phytoplasma mali’ (‘Ca. P. mali’) and that these genes are differentially expressed during phytoplasma infection or recovery.     

Correlation between mite community structure and gross N fluxes

Net nitrogen (N) mineralisation rate is the result of two independent processes: gross N mineralisation and N immobilisation. Techniques for determining these rates have only been developed recently and no studies have examined the relationship between soil fauna and these two components of net N mineralisation. We present data which demonstrates that mite community structure, described using the Shannon index, is correlated with gross N immobilisation rates in the Western Australian wheat belt. These results suggest that examining the impacts of mites on gross N fluxes is warranted.