Vascular Colonization Research Articles

Influence of seed- borne and soil- carried inocula of Dickeya spp. on potato plant transpiration and symptom expression

Pectinolytic bacteria such as Dickeya spp. and Pectobacterium spp. represent a significant threat for seed potato production in Europe. Dickeya spp. induces various symptoms such as plant wilting, stem rot (usually referred to as blackleg) and tuber soft rot. Expression of wilting and blackleg symptoms depends on a range of factors: the susceptibility of the cultivar, the aggressiveness of the Dickeya strain and environmental factors such as soil moisture, temperature and plant evapotranspiration. Using the flow of sap driven by plant transpiration, these bacteria can colonize the entire plant vascular system. However, it is not known whether this vascular colonization differs according to the nature of the inoculum (soil-carried or tuber-borne) and whether this has an impact on the expression of symptoms. To better characterize the consequences of colonization of the xylem vessels by Dickeya, greenhouse and field trials were carried out with plants infected via mother tuber inoculation or root inoculation. The development of symptoms was tracked, as were changes in plants’ transpiration rate, and plant transpiration was used as an index of the early stages of symptom expression. Our trials revealed that seed- and soil- carried inocula can both induce wilting and blackleg symptoms in potato plants. It was also found that blackleg symptoms appearing during the first three weeks after plant emergence are caused mainly by a tuber-borne inoculum. Finally, it was established that the presence of a latent inoculum in the xylem vessels does not alter the plant transpiration process, as long as there are no wilting symptoms on rotting lesions.

Resistance to Verticillium wilt in olive progenies from open-pollination

Verticillium wilt, a vascular disease caused by the soilborne fungus Verticillium dahliae Kleb., constitutes currently the major cultivation constraint in many olive growing areas. An integrated disease management strategy is recommended for Verticillium wilt control including the use of resistant cultivars as one of the most efficient control measures. However, only a few traditional cultivars have showed high levels of disease resistance. For this reason, an olive breeding program was initiated aiming at obtaining new cultivars displaying both high levels of disease resistance and good agronomic characteristics. In this work, the resistance to the defoliating pathotype of V. dahliae was evaluated in 64 genotypes that included 12 cultivars from the World Olive Germplasm Bank of Córdoba (Spain) and 52 genotypes coming from open-pollination of these cultivar plus three other cultivars. These genotypes were previously selected from a wider initial population on the basis of their favourable agronomic performance, mainly early crop (short juvenile period) and high oil content. Inoculation experiments were carried out under controlled conditions in a growth chamber by dipping the root system of plant cuttings in a conidial suspension of the pathogen. A Relative Susceptibility Index (RSI), which summarized several disease parameters, and plant colonization parameters were used to assess the resistance response. Eight of the evaluated genotypes showed similar resistance level to external development of symptoms than ‘Frantoio’ (the control resistant cultivar). Vascular colonization estimated by isolation of the fungus suggests that a tolerant more than resistant plant defence mechanism may be operating in some genotypes. The correlation between values for female parents and their progenies suggests a high heritability in some disease parameters.

Evaluation of Malaysian oil palm progenies for susceptibility, resistance or tolerance to Fusarium oxysporum f. sp. elaeidis and defence‐related gene expression in roots

Vascular wilt of oil palm caused by Fusarium oxysporum f. sp. elaeidis (Foe) is a devastating disease in West and Central Africa. As the oil palm industry in southeast Asia is still expanding, so is the oil palm germplasm collection through the importation of seed and pollen from Africa, the centre of diversity for Elaeis guineensis. There is a risk of inadvertent spread of the disease on contaminated seed or pollen. Regular re‐evaluation of the reaction of currently grown palm genotypes towards Foe is clearly required for biosecurity. This study has demonstrated that four Malaysian oil palm progenies, three in current or recent commercial use, are highly susceptible to infection by at least one of two African isolates of Foe, representing different countries, aggressiveness and vegetative compatibility groups. Symptoms and reduction of palm growth generally reflected the extent and intensity of systemic colonization by Foe. Progeny PK 5463 expressed partial resistance to Foe isolate F3, but not to isolate 16F, displaying significantly milder symptoms and supporting less widespread vascular colonization. This relatively incompatible interaction was used to study expression of potential defence‐related genes during root infection when compared to a susceptible palm–isolate combination. The only significant response was an early up‐regulation of chitinase in resistant palms. The research revealed at least one progeny–isolate differential interaction, and the associated resistance expression suggests a component of tolerance, because colonization by Foe was systemic in both compatible and incompatible combinations.

A Newly Developed Real-Time PCR Assay for Detection and Quantification of Fusarium oxysporum and Its Use in Compatible and Incompatible Interactions with Grafted Melon Genotypes

A reliable and species-specific real-time quantitative polymerase chain reaction (qPCR) assay was developed for detection of the complex soilborne anamorphic fungus Fusarium oxysporum. The new primer pair, designed on the translation elongation factor 1-α gene with an amplicon of 142 bp, was highly specific to F. oxysporum without cross reactions with other Fusarium spp. The protocol was applied to grafted melon plants for the detection and quantification of F. oxysporum f. sp. melonis, a devastating pathogen of this cucurbit. Grafting technologies are widely used in melon to confer resistance against new virulent races of F. oxysporum f. sp. melonis, while maintaining the properties of valuable commercial varieties. However, the effects on the vascular pathogen colonization have not been fully investigated. Analyses were performed on 'Charentais-T' (susceptible) and 'Nad-1' (resistant) melon cultivars, both used either as rootstock and scion, and inoculated with F. oxysporum f. sp. melonis race 1 and race 1,2. Pathogen development was compared using qPCR and isolations from stem tissues. Early asymptomatic melon infections were detected with a quantification limit of 1 pg of fungal DNA. The qPCR protocol clearly showed that fungal development was highly affected by host-pathogen interaction (compatible or incompatible) and time (days postinoculation). The principal significant effect (P ≤ 0.01) on fungal development was due to the melon genotype used as rootstock, and this effect had a significant interaction with time and F. oxysporum f. sp. melonis race. In particular, the amount of race 1,2 DNA was significantly higher compared with that estimated for race 1 in the incompatible interaction at 18 days postinoculation. The two fungal races were always present in both the rootstock and scion of grafted plants in either the compatible or incompatible interaction.

Morphological evidences of Bio‐Oss® colonization by CD44‐positive cells

Our objectives were to examine vascular and cellular colonization in anorganic bovine bone (ABB) after 6months of healing in human maxillary sinus augmentation grafts. Fifty unilateral maxillary sinus augmentation procedures were performed on 50 consecutive patients. Bone cores were obtained through the implant receptor sites 6months later and evaluated with morphological image analysis and immunohistochemical techniques. Image analysis revealed a mean of 35.44±16% vital bone, 31.66±15% non-mineralized tissue, and 32.72±25% remnant ABB particles. In our patients, neovascularization in 46.3% of cases was demonstrated within ABB particles 6months after sinus floor augmentation surgery. Neovascularization of ABB particles was inversely related to age, directly to osteoclast number per mm², and not influenced by habits or disease. CD44-positive cells colonization was found in 74% of cases. An important correlation was found regarding CD44 expression and number of vessel in ABB particles (r=0.624 P<0.001, Pearson). Osteopontin expression was detected on the interstitial boundary of bone with ABB particles and within the osteocyte lacunae and bone canaliculi and was relationship with presence of CD44-positive cells inside ABB particles (r=0.388, P=0.046, Pearson). In conclusion, images compatible with osteone colonized by osteocytes CD44 positive and neovascularization in ABB particles were observed after 6months of graft maturation. These biologic events have not previously been morphologically and immunohistochemically documented.

In vitro activity and durability of a combination of an antibiofilm and an antibiotic against vascular catheter colonization.

Catheter-associated infections can cause severe complications and even death. Effective antimicrobial modification of catheters that can prevent device colonization has the potential of preventing clinical infection. We studied in vitro the antimicrobial activities of central venous catheters impregnated with N-acetylcysteine (NAC), an antibiofilm agent, and a broad-spectrum antibiotic against a range of important clinical pathogens. NAC-levofloxacin-impregnated (NACLEV) catheters were also evaluated for their antiadherence activity. NACLEV catheters produced the most active and durable antimicrobial effect against both Gram-positive and Gram-negative isolates and significantly reduced colonization (P < 0.0001) by all tested pathogens compared to control catheters. These in vitro results suggest that this antimicrobial combination can potentially be used to combat catheter colonization and catheter-associated infection.

Impact of Seed Lot Infection on the Development of Black Dot and Verticillium Wilt of Potato in Washington.

The fungi Colletotrichum coccodes and Verticillium dahliae are two pathogens of potato which are prevalent in potato field soils and seed tubers. The objective of this study was to investigate the influence of seed lot infection by C. coccodes and V. dahliae on disease severity and yield using infested seed lots and fields with and without potato in their recent cropping histories. Greater incidence of C. coccodes in seed led to more root colonization in 2008 (both field types) but not in 2007 and did not influence tuber quantity or yield. Incidence of infection in seed lots by V. dahliae was not related to canopy necrosis, infection of progeny tubers, or yield. However, seed lots exhibited more V. dahliae aboveground vascular colonization, canopy necrosis, infested plant debris, and infected progeny tubers in 5-year potato rotation fields compared with long potato rotation fields. In general, foliar disease symptoms and yield reductions were not related to levels of either pathogen in seed lots. Symptoms of both diseases were more severe and yields were lower in fields with potato in their recent cropping histories, emphasizing the importance of soilborne inoculum in the development of black dot and Verticillium wilt of potato.

Loss of cAMP-Dependent Protein Kinase A Affects Multiple Traits Important for Root Pathogenesis by Fusarium oxysporum

The soilborne fungal pathogen Fusarium oxysporum causes vascular wilt and root rot diseases in many plant species. We investigated the role of cyclic AMP-dependent protein kinase A of F. oxysporum (FoCPKA) in growth, morphology, and root attachment, penetration, and pathogenesis in Arabidopsis thaliana. Affinity of spore attachment to root surfaces of A. thaliana, observed microscopically and measured by atomic force microscopy, was reduced by a loss-of-function mutation in the gene encoding the catalytic subunit of FoCPKA. The resulting mutants also failed to penetrate into the vascular system of A. thaliana roots and lost virulence. Even when the mutants managed to enter the vascular system via physically wounded roots, the degree of vascular colonization was significantly lower than that of the corresponding wild-type strain O-685 and no noticeable disease symptoms were observed. The mutants also had reduced vegetative growth and spore production, and their hyphal growth patterns were distinct from those of O-685. Coinoculation of O-685 with an focpkA mutant or a strain nonpathogenic to A. thaliana significantly reduced disease severity and the degree of root colonization by O-685. Several experimental tools useful for studying mechanisms of fungal root pathogenesis are also introduced.

Carbon dioxide and methane fluxes from arctic mudboils

Climate change is expected to alter the Arctic’s carbon (C) balance and changes in these C-rich ecosystems may contribute to a positive feedback on global climate change. Low-center mudboils, a form of patterned ground in the Arctic, are distinct landforms in which the exchange of greenhouse gases between the atmosphere and soil has not been fully characterized, but which may have an important influence on the overall C balance of tundra ecosystems. Chamber systems were used to sample net ecosystem exchange of CO2 (NEE) and CO2 and CH4 effluxes along a 35-m transect intersecting two mudboils in a wet sedge fen in Canada’s Southern Arctic (lat. 64°52′N, long. 111°34′W) during the summer months in 2008. Mudboil features gave rise to dramatic variations in vegetation, soil temperature and thaw depth, and soil organic matter content along this transect. Variations in NEE were driven by variations in the amount of vascular vegetation, while CO2 and CH4 effluxes were remarkably similar among the two mudboil (CO2 effluxes: 1.1 ± 0.9 and 1.4 ± 0.7 µmol m-2 s-1; CH4 effluxes: 83.1 ± 189.4 and 23.1 ± 9.4 nmol m-2 s-1, ± 1 standard deviation) and the sedge fen (CO2 effluxes: 1.6 ± 0.7 mol m-2 s-1 ; CH4 effluxes: 28.0 ± 62.0 nmol m-2 s-1) sampling areas. Vegetation appeared to play an important role in limiting temporal variations in CH4 effluxes through plant mediated transport in both mudboil and sedge fen sampling areas. One of the mudboils had negligible vascular plant colonization presumably due to more active frost heave processes. The relatively high CO2 and CH4 efflux in this mudboil area was speculated to be a result of growth and decomposition of cryptogamic organisms, inflow of dissolved organic C, and warmer soil temperatures. Key words: Patterned ground, nonsorted circle, tundra, net ecosystem exchange, methane, carbon dioxide

Differential colonization and defence responses of resistant and susceptible melon lines infected by Fusarium oxysporum race 1·2

A green fluorescent protein (GFP)‐expressing strain of Fusarium oxysporum f.sp. melonis race 1·2 (FOM 1·2‐GFP) was used to visualize infection of a susceptible melon cultivar, Ein Dor (ED). At 1–2 days post‐inoculation (d.p.i.), the fungus grew on the root epidermis and adhered to epidermal cell borders. By day 4, the mycelium crossed the cortex and endodermis through narrow pores in cell walls and reached xylem vessels, where it sporulated and produced secondary hyphae that grew upwards. Colonization dynamics of ED seedlings were compared with those of a genetically resistant line, BIZ. FOM 1·2‐GFP colonized the resistant plant’s vascular system, but the incidence of seedling infection was lower than in ED, suggesting stronger defence responses in BIZ expressed at the pre‐xylem stage of infection. Infection of the vascular system of BIZ was slower: at 11 d.p.i., FOM 1·2‐GFP only colonized the lower hypocotyl sections of BIZ, whilst the upper hypocotyls of ED were already infected, indicating that BIZ also restricted FOM 1·2 movement in the xylem. The expression patterns of three defence genes were compared between the resistant and susceptible genotypes using real‐time PCR. Transcript levels of phenylalanine ammonia lyase (PAL), chitinase (CHI) and hydroperoxide lyase (HPL) were induced to a greater extent in BIZ than in ED. A constitutive two‐ to fourfold difference between BIZ and ED in the basal levels of all three transcripts was also apparent. Both the constitutive and inducible defence responses could contribute to reduced vascular colonization of the resistant genotype.

Comment on: Use of chlorhexidine-impregnated dressing to prevent vascular and epidural catheter colonization and infection: a meta-analysis

Objectives: Vascular and epidural catheter-related infections cause significant morbidities and mortality in hospitalized patients. This meta-analysis assessed the effect of chlorhexidine-impregnated dressing on the risk of vascular and epidural catheter bacterial colonization and infection. Methods: Literature search was based on MEDLINE (1966 to 1 November 2005), EMBASE and Cochrane Controlled Trials Register (2005 issue 3) databases. Only randomized controlled clinical trials comparing chlorhexidine-impregnated dressing with placebo or povidine-iodine dressing were included in this meta-analysis. Two reviewers reviewed and extracted the data independently. Results: Eight studies assessing a single type of chlorhexidine-impregnated dressing were identified and subjected to meta-analysis. The chlorhexidine-impregnated dressing reduced the risk of epidural [3.6% versus 35%, odds ratio (OR) 0.07, 95% CI: 0.02–0.31, P = 0.0005] and intravascular catheter or exit-site bacterial colonization (14.8% versus 26.9%, OR 0.47, 95% CI: 0.34–0.65, P < 0.00001) (overall 14.3% versus 27.2%, OR 0.40, 95% CI: 0.26–0.61; P < 0.0001). The use of chlorhexidine-impregnated dressing was associated with a trend towards reduction in catheter-related bloodstream or CNS infections (2.2% versus 3.8%, OR 0.58, 95% CI: 0.29–1.14, P = 0.11). Local cutaneous reactions to chlorhexidine-impregnated dressing were reported in 5.6% of the patients in three studies (OR 8.17, 95% CI: 1.19–56.14, P = 0.04), and 96% of these reactions occurred in neonatal patients. The number needed to prevent one episode of intravascular catheter-related bloodstream infection was 142 for an average period of catheter in situ of 10 days and a change of dressing every 5 days. The cost of preventing one vascular catheter-related bloodstream infection was estimated to be £298 (US$532.5). Conclusions: Chlorhexidine-impregnated dressing is effective in reducing vascular and epidural catheter bacterial colonization and is also associated with a trend towards reduction in catheter-related bloodstream or CNS infections. A large randomized controlled trial is needed to confirm whether chlorhexidine-impregnated dressing is cost-effective in preventing bacterial infection related to vascular and epidural catheters.

Use of chlorhexidine-impregnated dressing to prevent vascular and epidural catheter colonization and infection: a meta-analysis

Use of chlorhexidine-impregnated dressing to prevent vascular and epidural catheter colonization and infection: a meta-analysis

Mode of action of a non-pathogenic Fusarium oxysporum strain against Verticillium dahliae using Real Time QPCR analysis and biomarker transformation

Verticillium wilt is a devastating disease of a wide range of herbaceous and woody plant hosts. It is incited by the soilborne fungus Verticillium dahliae. Management strategies are mainly focused on preventive measures. In a previous study, the efficacy of a non-pathogenic Fusarium oxysporum strain, designated as F2, isolated from a suppressive compost amendment, has been reported to reduce Verticillium wilt symptom development in eggplants under greenhouse and field conditions; in addition, antibiosis or parasitism were ruled out by using a dual culture test. In the present study, we investigated the mode of action of F2 against V. dahliae. For this purpose, the F2 and V. dahliae strains were transformed with the EGFP and DsRed2 reporter genes respectively, so as to visualize their presence on the root surface of eggplants. In addition, the ramification of both fungi into the plant vascular system was monitored by Real Time QPCR analysis. It was shown that F2 colonizes the root surface along the intercellular junctions excluding V. dahliae from the same ecological niche. In parallel, QPCR analysis showed that application of F2 reduces the levels of V. dahliae vascular colonization along with the disease severity. In a split root experiment it was demonstrated that F2 does not trigger the defense mechanisms of eggplants against V. dahliae. Therefore, it seems that competition for space or nutrients on the root surface are the main mechanism of action of F2 against V. dahliae.

β‐tubulin affects cellulose microfibril orientation in plant secondary fibre cell walls

Cellulose microfibrils are the major structural component of plant secondary cell walls. Their arrangement in plant primary cell walls, and its consequent influence on cell expansion and cellular morphology, is directed by cortical microtubules; cylindrical protein filaments composed of heterodimers of alpha- and beta-tubulin. In secondary cell walls of woody plant stems the orientation of cellulose microfibrils influences the strength and flexibility of wood, providing the physical support that has been instrumental in vascular plant colonization of the troposphere. Here we show that a Eucalyptus grandisbeta-tubulin gene (EgrTUB1) is involved in determining the orientation of cellulose microfibrils in plant secondary fibre cell walls. This finding is based on RNA expression studies in mature trees, where we identified and isolated EgrTUB1 as a candidate for association with wood-fibre formation, and on the analysis of somatically derived transgenic wood sectors in Eucalyptus. We show that cellulose microfibril angle (MFA) is correlated with EgrTUB1 expression, and that MFA was significantly altered as a consequence of stable transformation with EgrTUB1. Our findings present an important step towards the production of fibres with altered tensile strength, stiffness and elastic properties, and shed light on one of the molecular mechanisms that has enabled trees to dominate terrestrial ecosystems.

Use of chlorhexidine-impregnated dressing to prevent vascular and epidural catheter colonization and infection: a meta-analysis

Vascular and epidural catheter-related infections cause significant morbidities and mortality in hospitalized patients. This meta-analysis assessed the effect of chlorhexidine-impregnated dressing on the risk of vascular and epidural catheter bacterial colonization and infection. Literature search was based on MEDLINE (1966 to 1 November 2005), EMBASE and Cochrane Controlled Trials Register (2005 issue 3) databases. Only randomized controlled clinical trials comparing chlorhexidine-impregnated dressing with placebo or povidine-iodine dressing were included in this meta-analysis. Two reviewers reviewed and extracted the data independently. Eight studies assessing a single type of chlorhexidine-impregnated dressing were identified and subjected to meta-analysis. The chlorhexidine-impregnated dressing reduced the risk of epidural [3.6% versus 35%, odds ratio (OR) 0.07, 95% CI: 0.02-0.31, P=0.0005] and intravascular catheter or exit-site bacterial colonization (14.8% versus 26.9%, OR 0.47, 95% CI: 0.34-0.65, P<0.00001) (overall 14.3% versus 27.2%, OR 0.40, 95% CI: 0.26-0.61; P<0.0001). The use of chlorhexidine-impregnated dressing was associated with a trend towards reduction in catheter-related bloodstream or CNS infections (2.2% versus 3.8%, OR 0.58, 95% CI: 0.29-1.14, P=0.11). Local cutaneous reactions to chlorhexidine-impregnated dressing were reported in 5.6% of the patients in three studies (OR 8.17, 95% CI: 1.19-56.14, P=0.04), and 96% of these reactions occurred in neonatal patients. The number needed to prevent one episode of intravascular catheter-related bloodstream infection was 142 for an average period of catheter in situ of 10 days and a change of dressing every 5 days. The cost of preventing one vascular catheter-related bloodstream infection was estimated to be pound298 (US$532.5). Chlorhexidine-impregnated dressing is effective in reducing vascular and epidural catheter bacterial colonization and is also associated with a trend towards reduction in catheter-related bloodstream or CNS infections. A large randomized controlled trial is needed to confirm whether chlorhexidine-impregnated dressing is cost-effective in preventing bacterial infection related to vascular and epidural catheters.

Mouse Polycomb M33 is required for splenic vascular and adrenal gland formation through regulating Ad4BP/SF1 expression

AbstractMice with disrupted mammalian PcG (Polycomb group) genes commonly show skeletal transformation of anterior-posterior identities. Disruption of the murine M33 gene, a PcG member, displayed posterior transformation of the vertebral columns and sternal ribs. In addition, failure of T-cell expansion and hypoplasia and sex-reversal of the gonads, have been observed. In the present study, we identified defects in the splenic and adrenal formation of M33–knock-out (KO) mice on a C57BL/6 genetic background. The spleen in these animals was smaller than in the wild-type mice and was spotted red because of nonuniform distribution of blood cells. Histologic examination revealed disorganization of the vascular endothelium and its surrounding structures, and immunohistochemistry demonstrated disturbances in vascular formation and colonization of immature hematopoietic cells. These splenic phenotypes observed in the M33-KO mice were quite similar to those seen in Ad4BP/SF1 (Nr5a1) knock-outs. Moreover, the adrenal glands of M33-KO and Ad4BP/SF1 heterozygous KO mice were smaller than those of the wild-type mice. Western blot, immunohistochemistry, and reverse transcriptase–polymerase chain reaction (RT-PCR) analyses of the M33 knock-outs all indicated significantly low expression of adrenal 4 binding protein/steroidogenic factor-1 (Ad4BP/SF-1), indicating that M33 is an essential upstream regulator of Ad4BP/SF1. In agreement with these observations, chromatin immunoprecipitation assays with adrenocortical Y-1 cells revealed direct binding of the M33-containing PcG to the Ad4BP/SF1 gene locus.

DIFFERENTIAL TEMPORAL AND WALL SHEAR STRESS MODULATION OF ARTERIAL CYTOKINE EXPRESSION—IMPACT ON REMODELING

The behaviour of two biomaterials polytetrafluoroethylene (PTFE) and polypropylene (PL) has been studied, focussing especially on the macrophage response to the implant, as well as on certain aspects of the process of scar formation. A total of 24 animals (white New Zealand rabbits) received prostheses implanted into the anterior abdominal wall in such a way as to involve every layer over an area of 7cm × 5cm. The interfaces formed with the visceral peritoneum, subcutaneous tissue layer and the recipient muscle-aponeurotic tissue were assessed. The techniques employed were light microscopy, scanning electron microscopy and immunohistochemical methods, the latter using a monoclonal antibody specific for rabbit macrophages (RAM-11). From the results obtained, it can be seen that: (a) the structure of PL allows its total integration with the reparative tissue, while PTFE becomes encapsulated, on both the subcutaneous and the peritoneal aspects, by orderly connective tissue; (b) the macrophage response, determined on the basis of the presence of labelled macrophages, shows a similar pattern in both biomaterials; and (c) angiogenesis is very intense in the PL mesh, whereas the PTFE prosthesis undergoes almost no vascular colonization.

Use of Scots pine seedling roots as an experimental model to investigate gene expression during interaction with the conifer pathogen Heterobasidion annosum (P-type).

The root-rot fungus Heterobasidion annosum is a major pathogen of woody trees in temperate regions of the world. In this study, seedling root of Scots pine was used as an experimental model to investigate gene expression in conifer trees during challenge with H. annosum. Initial cellular and histochemical studies have established the systems and indicated the key sequence of events during the infection process. Also, to correlate histochemical observations with the time-dependent pattern of events in host gene expression, a transcriptome profiling of a selected set of host genes from a pine-root subtraction cDNA library was conducted. Differential screening of the subset of genes arrayed on nylon membrane with cDNA probes made from seedling roots infected for 1, 3, 7 and 15 days revealed a number of up-regulated genes [disease-resistance gene analog, antimicrobial peptide (AMP) gene homolog etc.] following inoculation. The results also showed strong expression of genes involved in cell defense and protein synthesis at the early stages of the infection (3-7 days) with a decline at late stages of infection (15 days). The decline in expression of key defense genes at late stages of infection correlated well with the period of vascular colonization and subsequent loss of root turgor. Northern analyses with two of the major induced genes (AMP homolog and disease-resistance gene analog) indicated a several-fold increase in host gene expression following infection. In addition, a particular single gene (thaumatin-like protein) was consistently expressed throughout the four sampling periods of the experiment. BlastX analyses revealed that the Scots-pine thaumatin-like gene shared 51-77% sequence homology with other thaumatin-like proteins in GenBank. The importance of these results in tree defense and use of conifer seedling root in host-parasite interaction in forest trees is discussed.

Vascular colonization patterns in susceptible and resistant tomato cultivars inoculated with Fusarium oxysporum f.sp. lycopersici races 0 and 1

The vascular colonization pattern of Fusarium oxysporum f.sp. lycopersici races 0 and 1 in tomato was studied in five susceptible and five resistant cultivar–fungus combinations during a 26‐day period after inoculation by root immersion. Propagules spread discontinuously along the stems in all five cultivars 1 day after inoculation, irrespective of cultivar resistance. Five days later the fungus was limited to the stem bases in all cultivars. Between the fifth and 12th days, stem colonization by the fungus stopped in all cultivar–race combinations. Thereafter, the situation remained stable in resistant combinations, with inoculum distributed discontinuously, and no disease symptoms were apparent. By contrast, in the susceptible combinations a gradual upward colonization of the stems was seen such that fungal distribution was no longer discontinuous and disease symptoms appeared. These results suggest that a fungal ‘incubation’ period in the base of the vascular system is required before a secondary invasion of tissues occurs in susceptible genotypes. The slope of the regression line fitted between the height reached by the fungus up the stem (y) and the time after inoculation (x) provides a measure of the horizontal (polygenic) resistance in tomato cultivars

Mulberry Shoot Blight: Axenic Culture, Pathogenicity and Plant-Pathogen Interaction

Occurrence of a new disease, shoot blight was observed afflicting mulberry (Morus spp.) in the southern peninsula of India. The disease initiated as marginal burning or blackening of leaf lamina which later spread across the whole lamina and then to the stems of affected shoots, resulting in the drooping of the entire shoot and complete death of the affected plants. On isolation of the causal organism, a bacterial isolate was found to be associated with the disease symptoms, which was identified as Xanthomonas campestris pv. mori. Scanning electron microscopic (SEM) examination of cross sections of stems revealed the presence of rod-shaped bacteria inside xylem vessels of affected plants. This indicates the vascular translocation and colonization of X. campestris pv. mori which resulted in shoot blight in mulberry.