Cell Wall Carbohydrate Dynamics during the Differentiation of Infection Structures by the Apple Scab Fungus, Venturia inaequalis

Scab, caused by the biotrophic fungal pathogen Venturia inaequalis, is the most economically important disease of apples. During infection, V. inaequalis colonizes the subcuticular host environment, where it develops specialized infection structures called runner hyphae and stromata. These structures are thought to be involved in nutrient acquisition and effector (virulence factor) delivery, but also give rise to conidia that further the infection cycle. Despite their importance, very little is known about how these structures are differentiated. Likewise, nothing is known about how these structures are protected from host defenses or recognition by the host immune system. To better understand these processes, we first performed a glycosidic linkage analysis of sporulating tubular hyphae from V. inaequalis developed in culture. This analysis revealed that the V. inaequalis cell wall is mostly composed of glucans (44%) and mannans (37%), whereas chitin represents a much smaller proportion (4%). Next, we used transcriptomics and confocal laser scanning microscopy to provide insights into the cell wall carbohydrate composition of runner hyphae and stromata. These analyses revealed that, during subcuticular host colonization, genes of V. inaequalis putatively associated with the biosynthesis of immunogenic carbohydrates, such as chitin and β-1,6-glucan, are downregulated relative to growth in culture, while on the surface of runner hyphae and stromata, chitin is deacetylated to the less-immunogenic carbohydrate chitosan. These changes are anticipated to enable the subcuticular differentiation of runner hyphae and stromata by V. inaequalis, as well as to protect these structures from host defenses and recognition by the host immune system. IMPORTANCE Plant-pathogenic fungi are a major threat to food security. Among these are subcuticular pathogens, which often cause latent asymptomatic infections, making them difficult to control. A key feature of these pathogens is their ability to differentiate specialized subcuticular infection structures that, to date, remain largely understudied. This is typified by Venturia inaequalis, which causes scab, the most economically important disease of apples. In this study, we show that, during subcuticular host colonization, V. inaequalis downregulates genes associated with the biosynthesis of two immunogenic cell wall carbohydrates, chitin and β-1,6-glucan, and coats its subcuticular infection structures with a less-immunogenic carbohydrate, chitosan. These changes are anticipated to enable host colonization by V. inaequalis and provide a foundation for understanding subcuticular host colonization by other plant-pathogenic fungi. Such an understanding is important, as it may inform the development of novel control strategies against subcuticular plant-pathogenic fungi.

Effector loss drives adaptation of Pseudomonas syringae pv. actinidiae biovar 3 to Actinidia arguta

A pandemic isolate of Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) has devastated kiwifruit orchards growing cultivars of Actinidia chinensis. In contrast, A. arguta (kiwiberry) is not a host of Psa3. Resistance is mediated via effector-triggered immunity, as demonstrated by induction of the hypersensitive response in infected A. arguta leaves, observed by microscopy and quantified by ion-leakage assays. Isolates of Psa3 that cause disease in A. arguta have been isolated and analyzed, revealing a 51 kb deletion in the exchangeable effector locus (EEL). This natural EEL-mutant isolate and strains with synthetic knockouts of the EEL were more virulent in A. arguta plantlets than wild-type Psa3. Screening of a complete library of Psa3 effector knockout strains identified increased growth in planta for knockouts of four effectors–AvrRpm1a, HopF1c, HopZ5a, and the EEL effector HopAW1a –suggesting a resistance response in A. arguta. Hypersensitive response (HR) assays indicate that three of these effectors trigger a host species-specific HR. A Psa3 strain with all four effectors knocked out escaped host recognition, but a cumulative increase in bacterial pathogenicity and virulence was not observed. These avirulence effectors can be used in turn to identify the first cognate resistance genes in Actinidia for breeding durable resistance into future kiwifruit cultivars.

Clonally propagated monoculture crop plants facilitate the emergence and spread of new diseases. Plant pathogens cause disease by the secretion of effectors that function by repressing the host defense response. While the last few decades have seen a huge increase in our understanding of the role effectors play in mediating plant-pathogen interactions, the combinations of effectors required for the establishment of plant disease and that account for host specificity are less well understood. Breeding genetic resistance is often used to protect plants from disease but it is frequently evaded by rapidly evolving pathogens. Pseudomonas syringae pv. actinidiae (Psa) which causes bacterial canker disease of kiwifruit has spread rapidly throughout the world’s kiwifruit orchards, particularly those growing cultivars of Actinidia chinensis. Other Actinidia species including A. arguta display strong resistance conferred by recognition of effectors delivered by Psa. We explore the depth and dynamics of Psa effector recognition by A. arguta and show that there is a trade-off between losses of effector recognition by A. arguta versus the retention of pathogenicity. Our findings should aid in the understanding of how to breed durable resistance into perennial plants challenged by swiftly evolving pathogens.

Reference genes for gene expression analysis in the fungal pathogen Neonectria ditissima and their use demonstrating expression up-regulation of candidate virulence genes

European canker, caused by the necrotrophic fungal phytopathogen Neonectria ditissima, is one of the most damaging apple diseases worldwide. An understanding of the molecular basis of N. ditissima virulence is currently lacking. Identification of genes with an up-regulation of expression during infection, which are therefore probably involved in virulence, is a first step towards this understanding. Reverse transcription quantitative real-time PCR (RT-qPCR) can be used to identify these candidate virulence genes, but relies on the use of reference genes for relative gene expression data normalisation. However, no report that addresses selecting appropriate fungal reference genes for use in the N. ditissima-apple pathosystem has been published to date. In this study, eight N. ditissima genes were selected as candidate RT-qPCR reference genes for gene expression analysis. A subset of the primers (six) designed to amplify regions from these genes were specific for N. ditissima, failing to amplify PCR products with template from other fungal pathogens present in the apple orchard. The efficiency of amplification of these six primer sets was satisfactory, ranging from 81.8 to 107.53%. Analysis of expression stability when a highly pathogenic N. ditissima isolate was cultured under 10 regimes, using the statistical algorithms geNorm, NormFinder and BestKeeper, indicated that actin and myo-inositol-1-phosphate synthase (mips), or their combination, could be utilised as the most suitable reference genes for normalisation of N. ditissima gene expression. As a test case, these reference genes were used to study expression of three candidate virulence genes during a time course of infection. All three, which shared traits with fungal effector genes, had up-regulated expression in planta compared to in vitro with expression peaking between five and six weeks post inoculation (wpi). Thus, these three genes may well be involved in N. ditissima pathogenicity and are priority candidates for further functional characterization.

Cell type-specific gene expression underpins remodelling of cell wall pectin in exocarp and cortex during apple fruit development

In apple (Malus×domestica) fruit, the different layers of the exocarp (cuticle, epidermis, and hypodermis) protect and maintain fruit integrity, and resist the turgor-driven expansion of the underlying thin-walled cortical cells during growth. Using in situ immunolocalization and size exclusion epitope detection chromatography, distinct cell type differences in cell wall composition in the exocarp were revealed during apple fruit development. Epidermal cell walls lacked pectic (1→4)-β-d-galactan (associated with rigidity), whereas linear (1→5)-α-l-arabinan (associated with flexibility) was exclusively present in the epidermal cell walls in expanding fruit and then appeared in all cell types during ripening. Branched (1→5)-α-l-arabinan was uniformly distributed between cell types. Laser capture microdissection and RNA sequencing (RNA-seq) were used to explore transcriptomic differences controlling cell type-specific wall modification. The RNA-seq data indicate that the control of cell wall composition is achieved through cell-specific gene expression of hydrolases. In epidermal cells, this results in the degradation of galactan side chains by possibly five β-galactosidases (BGAL2, BGAL7, BGAL10, BGAL11, and BGAL103) and debranching of arabinans by α-arabinofuranosidases AF1 and AF2. Together, these results demonstrate that flexibility and rigidity of the different cell layers in apple fruit during development and ripening are determined, at least in part, by the control of cell wall pectin remodelling.

Simultaneous knock-down of six β-galactosidase genes in petunia petals prevents loss of pectic galactan but decreases petal strength

Galactose (Gal) is incorporated into cell wall polysaccharides as flowers open, but then is lost because of β-galactosidase activity as flowers mature and wilt. The significance of this for flower physiology resides in the role of galactan-containing polysaccharides in the cell wall, which is still largely unresolved. To investigate this, transcript accumulation of six cell wall-associated β-galactosidases was simultaneously knocked down in 'Mitchell' petunia (Petunia axillaris x (P. axillaris x P. hybrida)) flower petals. The multi-PhBGAL RNAi construct targeted three bud- and three senescence-associated β-galactosidase genes. The petals of the most down-regulated line (GA19) were significantly disrupted in galactose turnover during flower opening, and at the onset of senescence had retained 86% of their galactose compared with 20% in the controls. The Gal content of Na₂CO₃-soluble cell wall extracts and the highly insoluble polysaccharides associated with cellulose were particularly affected. Immunodetection with the antibody LM5 showed that much of the cell wall Gal in GA19 was retained as galactan, presumably the side-chains of rhamnogalacturonan-I. The flowers of GA19, despite having retained substantially more galactan, were no different from controls in their internal cell arrangement, dimensions, weight or timing of opening and senescence. However, the GA19 petals had less petal integrity (as judged by force required to cause petal fracture) after opening and showed a greater decline in this integrity with time than controls, raising the possibility that galactan loss is a mechanism for helping to maintain petal tissue cohesion after flower opening.

Lower cell wall pectin solubilisation and galactose loss during early fruit development in apple (Malus x domestica) cultivar 'Scifresh' are associated with slower softening rate

Substantial differences in softening behaviour can exist between fruit even within the same species. Apple cultivars 'Royal Gala' and 'Scifresh' soften at different rates despite having a similar genetic background and producing similar amounts of ethylene during ripening. An examination of cell wall metabolism from the fruitlet to the ripe stages showed that in both cultivars pectin solubilisation increased during cell expansion, declined at the mature stage and then increased again during ripening. This process was much less pronounced in the slower softening 'Scifresh' than in 'Royal Gala' at every developmental stage examined, consistent with less cell separation and softening in this cultivar. Both cultivars also exhibited a progressive loss of pectic galactan and arabinan side chains during development. The cell wall content of arabinose residues was similar in both cultivars, but the galactose residue content in 'Scifresh' remained higher than that of 'Royal Gala' at every developmental stage. The higher content of cell wall galactose residue in 'Scifresh' cell walls correlated with a lower β-galactosidase activity and more intense immunolabelling of RG-I galactan side chains in both microscopy sections and glycan microarrays. A high cell wall galactan content has been associated with reduced cell wall porosity, which may restrict access of cell wall-modifying enzymes and thus maintain better structural integrity later in development. The data suggest that the composition and structure of the cell wall at very early development stages may influence subsequent cell wall loosening, and may even predispose the wall's ensuing properties.

Cell wall structures leading to cultivar differences in softening rates develop early during apple (Malus x domestica) fruit growth

BACKGROUND

There is a paucity of information regarding development of fruit tissue microstructure and changes in the cell walls during fruit growth, and how these developmental processes differ between cultivars with contrasting softening behaviour. In this study we compare two apple cultivars that show different softening rates during fruit development and ripening. We investigate whether these different softening behaviours manifest themselves late during ethylene-induced softening in the ripening phase, or early during fruit expansion and maturation.

RESULTS

'Scifresh' (slow softening) and 'Royal Gala' (rapid softening) apples show differences in cortical microstructure and cell adhesion as early as the cell expansion phase. 'Scifresh' apples showed reduced loss of firmness and greater dry matter accumulation compared with 'Royal Gala' during early fruit development, suggesting differences in resource allocation that influence tissue structural properties. Tricellular junctions in 'Scifresh' were rich in highly-esterified pectin, contributing to stronger cell adhesion and an increased resistance to the development of large airspaces during cell expansion. Consequently, mature fruit of 'Scifresh' showed larger, more angular shaped cells than 'Royal Gala', with less airspaces and denser tissue. Stronger cell adhesion in ripe 'Scifresh' resulted in tissue fracture by cell rupture rather than by cell-to-cell-separation as seen in 'Royal Gala'. CDTA-soluble pectin differed in both cultivars during development, implicating its involvement in cell adhesion. Low pectin methylesterase activity during early stages of fruit development coupled with the lack of immuno-detectable PG was associated with increased cell adhesion in 'Scifresh'.

CONCLUSIONS

Our results indicate that cell wall structures leading to differences in softening rates of apple fruit develop early during fruit growth and well before the induction of the ripening process.

Apple SEPALLATA1/2-like genes control fruit flesh development and ripening

Flowering plants utilize different floral structures to develop flesh tissue in fruits. Here we show that suppression of the homeologous SEPALLATA1/2-like genes MADS8 and MADS9 in the fleshy fruit apple (Malus x domestica) leads to sepaloid petals and greatly reduced fruit flesh. Immunolabelling of cell-wall epitopes and differential staining showed that the developing hypanthium (from which the apple flesh develops) of MADS8/9-suppressed apple flowers lacks a tissue layer, and the remaining flesh tissue of fully developed apples has considerably smaller cells. From these observations, it is proposed that MADS8 and MADS9 control the development of discrete zones within the hypanthium tissue, and therefore fruit flesh, and also act as foundations for development of different floral organs. At fruit maturity, the MADS8/9-suppressed apples do not ripen in terms of both developmentally controlled ripening characters, such as starch degradation, and ethylene-modulated ripening traits. Transient assays suggest that, like the RIN gene in tomato, the MADS9 gene acts as a transcriptional activator of the ethylene biosynthesis enzyme, 1-aminocyclopropane-1-carboxylate (ACC) synthase 1. The existence of a single class of genes that regulate both flesh formation and ripening provides an evolutionary tool for controlling two critical aspects of fleshy fruit development.

Down-regulation of POLYGALACTURONASE1 alters firmness, tensile strength and water loss in apple (Malus x domestica) fruit

BACKGROUND

While there is now a significant body of research correlating apple (Malus x domestica) fruit softening with the cell wall hydrolase ENDO-POLYGALACTURONASE1 (PG1), there is currently little knowledge of its physiological effects in planta. This study examined the effect of down regulation of PG1 expression in 'Royal Gala' apples, a cultivar that typically has high levels of PG1, and softens during fruit ripening.

RESULTS

PG1-suppressed 'Royal Gala' apples harvested from multiple seasons were firmer than controls after ripening, and intercellular adhesion was higher. Cell wall analyses indicated changes in yield and composition of pectin, and a higher molecular weight distribution of CDTA-soluble pectin. Structural analyses revealed more ruptured cells and free juice in pulled apart sections, suggesting improved integrity of intercellular connections and consequent cell rupture due to failure of the primary cell walls under stress. PG1-suppressed lines also had reduced expansion of cells in the hypodermis of ripe apples, resulting in more densely packed cells in this layer. This change in morphology appears to be linked with reduced transpirational water loss in the fruit.

CONCLUSIONS

These findings confirm PG1's role in apple fruit softening and suggests that this is achieved in part by reducing cellular adhesion. This is consistent with previous studies carried out in strawberry but not with those performed in tomato. In apple PG1 also appears to influence other fruit texture characters such as juiciness and water loss.

Cell wall polysaccharide distribution in Sandersonia aurantiaca flowers using immuno-detection

The localization of cell wall polysaccharides of the fused petals of monocotyledonous Sandersonia aurantiaca flowers has been identified using antibodies directed to pectin and xyloglucan epitopes and detection by fluorescence microscopy. Cross sections of the petal tissue were taken from cut flowers in bud and at various stages of maturity and senescence. Patterns of esterification in pectin backbones were identified by JIM5 and 2F4 labelling. Pectic galactan and arabinan side branches were detected by LM5 and LM6, respectively, while fucosylated xyloglucan was identified by CCRC-M1. The labelling patterns highlighted compositional differences between walls of the outer/inner epidermis compared to the spongy parenchyma cells of the interior mesophyll for fucosylated xyloglucan and arabinan. Partially esterified homogalacturonan was present in the junction zones of the outer epidermis and points of contact between cells of the mesophyll, and persisted throughout senescence. Pectic galactans were ubiquitous in the outer and inner epidermal cell walls and walls of the interior mesophyll at flower opening, whereas pectic arabinan was found predominantly in the epidermal cells. Galactan was lost from walls of all cells as flowers began to senesce, while fucosylated xyloglucan appeared to increase over this time. Such differences in the location of polysaccharides and the timing of changes suggest distinct combinations of certain polysaccharides offer mechanical and rheological advantages that may assist with flower opening and senescence.

A New 'Candidatus Liberibacter' Species Associated with Diseases of Solanaceous Crops

A new disease of glasshouse-grown tomato and pepper in New Zealand has resulted in plant decline and yield loss. Affected plants are characterized by spiky, chlorotic apical growth, curling or cupping of the leaves, and overall stunting. Transmission electron microscopy revealed the presence of phloem-limited bacterium-like organisms in symptomatic plants. The strategy used to identify the bacterium involved using specific prokaryote polymerase chain reaction (PCR) primers in combination with universal 16S rRNA primers. Sequence analysis of the 16S rRNA gene, the 16S/23S rRNA spacer region, and the rplKAJL-rpoBC operon revealed that the bacterium shared high identity with 'Candidatus Liberibacter' species. Phylogenetic analysis showed that the bacterium is distinct from the three citrus liberibacter species previously described and has been named 'Candidatus Liberibacter solanacearum'. This is the first report of a liberibacter naturally infecting a host outside the Rutaceae family. A specific PCR primer pair was developed for its detection.

Identification and characterisation of acidic and novel basic forms of actinidin, the highly abundant cysteine protease from kiwifruit

Actinidin is a cysteine protease found in Actinidia Lindl. (kiwifruit) species that affects the nutraceutical properties, processing characteristics and allergenicity of the fruit. Given the increased consumption of kiwifruit worldwide and the release of new varieties from different Actinidia species, the expression of actinidin mRNA and protein in a range of kiwifruit tissues was examined. Ten different actinidin mRNAs were identified encoding mature proteins of similar molecular weight (~24 kDa), but with predicted pIs ranging from acidic (pI 3.9) to basic (pI 9.3). In A. deliciosa 'Hayward' (green-fleshed kiwifruit) and A. chinensis 'Hort16A' and EM4 (gold-fleshed kiwifruit), actinidin mRNAs for acidic and basic proteins were expressed at comparable levels throughout ripening. Actinidin mRNA expression was highest in fruit at harvest, expression decreased as fruit ripened and was much lower in the core compared with outer pericarp tissue. Two-dimensional gel electrophoresis, combined with western analysis and liquid chromatography mass spectrometry (LC-MS) identified low levels of a novel basic actinidin protein in ripe A. deliciosa and A. chinensis fruit. Extremely high levels of an acidic actinidin protein were detected in A. deliciosa fruit and EM4, but this acidic protein appeared to be absent in 'Hort16A', the most important commercial cultivar of A. chinensis. Analyses on native gels indicated that both the basic and acidic actinidin isoforms in A. deliciosa were active cysteine proteases. Immunolocalisation showed that actinidin was present in small cells, but not large cells in the outer pericarp of mature A. deliciosa fruit at harvest. Within the small cells, actinidin was localised diffusely in the vacuole, associated with the plasma membrane, and in a layer in the plastids near starch granules. The presence of multiple forms of actinidin and varying protein levels in fruit will impact on the ability to breed new kiwifruit varieties with altered actinidin levels.

Cytochemistry and immunolocalisation of polysaccharides and proteoglycans in the endosperm of green Arabica coffee beans

The major noncellulosic polysaccharides and proteoglycans in the coffee bean (Coffea arabica) cell wall are (galacto)mannans and arabinogalactan proteins. Immunological and chemical probes demonstrated that the mannans and arabinogalactan proteins were located continuously across the width of the cell wall, but that the concentration of different structural epitopes within these polysaccharide types showed considerable spatial variation. For the mannans this was implied by the striated pattern demonstrated by fluctuation of the affinity between the mannan monoclonal antibody BGM C6 and (galacto)mannan. The arabinogalactan proteins labelled by the Yariv reagent and the arabinogalactan protein-specific antibody LM2 appeared to be located in all regions of the wall except the middle lamella, but showed some differences in intensity of labelling. However, the LM6 antibody, specific for (1-->5)-alpha-arabinan epitopes, was located only as a compact region adjacent to the cell lumen in the body of the endosperm; though, it did label throughout the wall of epidermal cells. This implied that either some of the more highly arabinosylated arabinogalactan proteins contained contiguous 5-arabinosyl residues or that a rhamnogalacturonan which contained 5-arabinosyl residues as side chains existed in the cell wall. In either case the polymers were very restricted in their distribution. A second category of pectin, a homogalacturonan detected by JIM7, was located only in the middle lamella region. The architecture of the wall, as revealed by resin etching, appeared to reflect the chemical heterogeneity, with three distinct physical zones identifiable in a cross section across a single wall.

Sieve element specific labeling with an anti-idiotypic monoclonal antibody mimic of the phytotoxin dothistromin

A monoclonal antibody, 12C9, an anti-idiotypic mimic of dothistromin, a toxin produced by Dothistroma pini, was found to label the cell wall of sieve elements in a number of different plant tissues and species. The antibody labeled apple leaf tissue, tobacco leaf mid vein, leaf and meristem, and Coprosma robusta leaf mid vein. Labeling was restricted to cell walls of sieve elements and did not label the companion cells or the lumen of the cells. The antibody labeled over a wide range of dilutions. This antibody could be used to differentiate sieve elements from other types of phloem. It could also be used to co-localize sieve elements and microorganisms such as phytoplasmas stained with DAPI.

Expression of biotin-binding proteins, avidin and streptavidin, in plant tissues using plant vacuolar targeting sequences

Tobacco plants have been developed which constitutively express high levels of the biotin-binding proteins, avidin and streptavidin. These plants were phenotypically normal and produced fertile pollen and seeds. The transgene was expressed and its product located in the vacuoles of most cell types in the plants. Targeting was achieved by use of N-terminal vacuolar targeting sequences derived from potato proteinase inhibitors which are known to target constitutively to vacuoles in potato tubers and, under wound-induction, in tomato leaves. Avidin was located in protein body-like structures within the vacuole and transgene protein levels remained relatively constant throughout the lifetime of the leaf. We describe two chimeric constructs with similar levels of expression. One comprised a potato proteinase inhibitor I signal peptide cDNA sequence attached to an avidin cDNA and the second a potato proteinase inhibitor II signal peptide genomic sequence (including an intron) attached to a core streptavidin synthetic sequence. We were unable to regenerate plants when transformation used constructs lacking the targeting sequences. The highest levels observed (up to 1.5% of total leaf protein) confirm the vacuole as the organelle of choice for stable storage of plant-toxic transgene products. The efficient targeting of these proteins did not result in any measured changes in plant biotin metabolism.

Dark green islands in plant virus infection are the result of posttranscriptional gene silencing

Dark green islands (DGIs) are a common symptom of plants systemically infected with a mosaic virus. DGIs are clusters of green leaf cells that are free of virus but surrounded by yellow, virus-infected tissue. We report here on two lines of evidence showing that DGIs are caused by posttranscriptional gene silencing (PTGS). First, transcripts of a transgene derived from the coat protein of Tamarillo mosaic potyvirus (TaMV) were reduced in DGIs relative to adjacent yellow tissues when the plants were infected with TaMV. Second, nontransgenic plants coinfected with TaMV and a heterologous virus vector carrying TaMV sequences showed reduced titers of the vector in DGIs compared with surrounding tissues. DGIs also were compared with recovered tissue at the top of transgenic plants because recovery has been shown previously to involve PTGS. Cytological analysis of the cells at the junction between recovered and infected tissue was undertaken. The interface between recovered and infected cells had very similar features to that surrounding DGIs. We conclude that DGIs and recovery are related phenomena, differing in their ability to amplify or transport the silencing signal.

Association of "Candidatus Phytoplasma australiense" with Sudden Decline of Cabbage Tree in New Zealand

Sudden decline of the New Zealand cabbage tree (Cordyline australis) results in the rapid death of affected plants within months of first external symptoms becoming apparent. Symptoms, which have been observed in saplings and mature trees, include vascular discoloration and leaf yellowing followed by leaf desiccation and eventual plant collapse. Previous work failed to link the disease with any causal agent. A phytoplasma has now been detected in all symptomatic saplings and some symptomatic trees tested, using one-step and nested polymerase chain reaction (PCR) to amplify portions of the 16S rRNA gene. This phytoplasma was not detected in nonsymptomatic plants. Phytoplasma DNA was found in shoot and rhizome apices, leaves and wood tissue of saplings, and in the rhizome apex and trunk tissues of adult trees. Sequencing of the PCR products from selected samples indicated that the phytoplasma is "Candidatus Phytoplasma australiense." Phytoplasma cells were detected by transmission electron microscopy in phloem sieve tubes of the rhizomes of affected saplings. One sapling with early symptoms recovered after injection with tetracycline antibiotic, but two saplings with advanced symptoms did not recover. It is concluded that "Candidatus Phytoplasma australiense" is present in symptomatic plants and is the cause of sudden decline.

Phormium Yellow Leaf Phytoplasma Is Associated with Strawberry Lethal Yellows Disease in New Zealand

A yellows disease of strawberry plants was identified in propagation beds in New Zealand. Affected plants were flatter to the ground, showed purpling of older leaves, reduced leaf size, yellowing of younger leaves, and sometimes plant death. A phytoplasma was observed in the phloem of affected plants. The 16S rRNA gene of the phytoplasma was amplified by polymerase chain reaction from symptomatic plants and from one asymptomatic plant, but not from 36 other asymptomatic plants. Nucleotide sequence analysis of the 16S rRNA gene showed that the phytoplasma is closely related or identical to the phytoplasma associated with the yellow leaf disease of New Zealand flax (Phormium tenax).

Morphology of Erysiphe pisi on leaves of Pisum sativum

Morphological details of conidium germination, hyphae, appressoria, conidiophores, and organisation within colonies of Erysiphe pisi on leaves of Pisum sativum were studied in frozen, hydrated specimens with a scanning electron microscope. The pathogen has several adaptations to enable efficient colonisation of host leaf surfaces, including production of several hyphae from each germinated conidium, unidirectional growth of individual hyphae, prolific hyphal branching at obtuse angles to lines of hyphal growth, and rapid and prolific development of appressoria and conidiophores. Hyphal cells on leaf surfaces are specialised to produce either appressoria or conidiophores and hyphal branches.

Cystic fibrosis and bronchial hyperreactivity. Concomitant defects or cause and effect?

Bronchial reactivity and lung-function tests were measured in 19 young adults with cystic fibrosis. There was moderately severe airways obstruction without hyperinflation, and mild hypoxaemia with normocapnia. Bronchial reactivity (fall in FEV1 after the administration of methacholine aerosol) was increased in about two-thirds of patients, and was markedly enhanced in nearly half of them. It was considered that the airways obstruction characteristic of cystic fibrosis can have a reversible element, and that this may provide a rationale for the use of bronchodilators in some patients. Although bronchial hyperreactivity in cystic fibrosis could represent concomitant underlying defects, a more attractive suggestion is that the chronic inflammation of cystic fibrosis has, in turn, led to acquired bronchial hyperreactivity.

Goodpasture's syndrome: normal renal diagnostic findings

A patient with Goodpasture's syndrome is described in whom pulmonary manifestations were dramatic, but in whom renal abnormalities were minor and only found on fluorescent and electron microscopy. His urine showed no proteinuria and no increase in cells in quantitative counts, and renal function was normal. It is suggested that there may be an indication for carrying out renal biopsies in patients with idiopathic pulmonary haemosiderosis and that this may lead to an early diagnosis of Goodpasture's syndrome.