Blumeria graminis f.sp. hordei (Bgh) attack disrupted stomatal behaviour, and hence leaf water conductance (g(l)), in barley genotypes Pallas and Risø-S (susceptible), P01 (with Mla1 conditioning a hypersensitive response; HR), and P22 and Risø-R (with mlo5 conditioning papilla-based penetration resistance). Inoculation caused some stomatal closure well before the fungus attempted infection. Coinciding with epidermal cell penetration, stomatal opening in light was also impeded, although stomata of susceptible and mlo5 lines remained largely able to close in darkness. Following infection, in susceptible lines stomata closed in darkness but opening in light was persistently impeded. In Risø-R, stomata recovered nearly complete function by approximately 30 h after inoculation, i.e. after penetration resistance was accomplished. In P01, stomata became locked open and unable to close in darkness shortly after epidermal cells died due to HR. In the P22 background, mlo5 penetration resistance was often followed by consequential death of attacked cells, and here too stomata became locked open, but not until approximately 24 h after pathogen attack had ceased. The influence of epidermal cell death was localized, and only affected stomata within one or two cells distance. These stomata were unable to close not only in darkness but also after application of abscisic acid and in wilted leaves suffering drought. Thus, resistance to Bgh based on HR or associated with cell death may have previously unsuspected negative consequences for the physiological health of apparently 'disease-free' plants. The results are discussed in relation to the control of stomatal aperture in barley by epidermal cells.

A new genus of the anaerobic gut fungi (Neocallimastigales), Cyllamyces aberensis gen.nov. sp.nov., with bulbous holdfast, branched sporangiophores and limited polycentric thallus development is described. The fungus was isolated from fresh cattle faeces. Free-swimming zoospores were spherical, uninucleate, and uniflagellate. After encystment, zoospores germinated and gave rise to a single, bulbous holdfast. One or several branched sporangiophores were produced from different locations on the holdfast, each bearing several spherical sporangia. DAPI staining of thalli indicated that nuclei were present in the holdfast, sporangiophores, and sporangia. As many as 12 sporangia were observed per thallus on up to 5 sporangiophores, with zoosporogenesis beginning 16–18 h after encystment. Zoospore ultrastructure was examined by transmission electron microscopy and found to be similar to that reported for other anaerobic chytrid fungi. Organelles were evenly distributed throughout the cell, except for the posteriorly attached flagellum and associated attachment apparatus, the hydrogenosomes, which were mainly situated in the posterior parts of the cell and a posteriorly directed, beak-shaped nucleus. Limited polycentric thallus development (including branched sporangiophores), the possession of a single bulbous holdfast and the absence of rhizoids were stable features of this fungus that distinguished it from the other five genera of gut fungi. Therefore, we have used these characteristics to assign the fungus to a new genus, Cyllamyces, with the specific name C. aberensis.Key words: rumen, fungal taxonomy, Neocallimastigales, chytrid, zoospore ultrastructure.

No abstract

SUMMARY Here, we consider the barley powdery mildew fungus, Blumeria graminis (DC Speer) f.sp. hordei (Marchal), and review recent research which has added to our understanding of the biology and molecular biology which underpins the asexual life cycle of this potentially devastating pathogen. We focus on the early stages of the host-pathogen interaction and report current understanding in the areas of leaf perception, fungal signal transduction and host-imposed oxidative stress management. Through this, it is becoming increasingly clear how closely and subtly both sides of the relationship are regulated. Collectively, however, this review highlights the high degree of complexity in working with an obligate parasite. Our experiences suggest that we would make more efficient progress towards understanding the basis of susceptibility and resistance to this true obligate biotroph if its genome sequence was available.

It is widely believed that the initial degradation of proteins contained in grazed forage is mediated by rumen microorganisms, but the authors' recent work suggests that the plant cells themselves contribute to their own demise. In the present study the responses of Lolium perenne leaves to the rumen environment were investigated by using an in vitro system which simulates the main stresses of the rumen but from which rumen micro-organisms were excluded. Degradation of leaf protein and the accumulation of amino acids in tissue and bathing medium occurred over a timescale that is relevant to rumen function, and in a near 1 : 1 ratio. Significant loss of nuclear material was observed after 6 h incubation and chloroplasts became morphologically more spherical as the incubation progressed. In situ localization suggested that ribulose 1,5 bisphosphate carboxylase/oxygenase was broken down within chloroplasts which from cytology were judged to be intact. We conclude from these data that plant metabolism may play a significant role in breaking down plant proteins within relatively intact organelles in the rumen. The determinations of chlorophyll content and cell viability revealed that the plant processes occurring in the simulated rumen were similar but not identical to those of natural senescence.

The senescence of leaves is characterized by yellowing as chlorophyll pigments are degraded. Proteins of the chloroplasts also decline during this phase of development. There exists a non-yellowing mutant genotype of Festuca pratensis Huds. which does not suffer a loss of chlorophyll during senescence. The fate of chloroplast membrane proteins was studied in mutant and wild-type plants by immune blotting and immuno-electron microscopy. Intrinsic proteins of photosystem II, exemplified by the light-harvesting chlorophyll a/b-binding protein (LHCP-2) and D1, were shown to be unusually stable in the mutant during senescence, whereas the extrinsic 33-kilodalton protein of the oxygen-evolving complex was equally lable in both genotypes. An ultrastructural study revealed that while the intrinsic proteins remained in the internal membranes of the chloroplasts, they ceased to display the heterogenous lateral distribution within the lamellae which was characteristic of nonsenescent chloroplasts. These observations are discussed in the light of possible mechanisms of protein turnover in chloroplasts.

Glycoprotein (GP)IIb

SUMMARYMelon necrotic leaf spot virus (MNSV) caused a major outbreak of a leaf necrosis disease of hydroponically‐grown cucumber plants at Humberside in 1983. The virus had c. 33 nm diam. particles which reacted serologically with MNSV antiserum of Dutch or American origin. Virus particles, which contained a single polypeptide (mol. wt 45 × 103) and a presumed RNA species (mol. wt 1.5 × 106), had a sedimentation coefficient (s20.w) of 134 S and a buoyant density in caesium chloride of 1.35 g/cm3. The virus was mechanically transmissible, confined to species of Cucurbitaceae, transmitted by zoospores of Olpidium radicale and retained in the resting spores of the fungus. MNSV is thus both water‐borne and soil‐borne. O. radicale zoospores were killed in <5 min in suspensions containing 20 μg/ml of the surfactant Agral (alkyl phenol ethylene oxide). The disease did not reappear in 1984 when the cucumber crops were fed with nutrients containing 20μg/ml Agral.