Common scab is one of the most important soil-borne diseases of potato and is difficult to control. Selection of potato breeding lines for resistance to common scab is also cumbersome due to environmental factors influencing symptom development and an erratic spatial distribution of the scab pathogens (Streptomyces spp.) in the field. The bacterial phytotoxin thaxtomin A, which causes scab symptoms, can be used to screen large numbers of potato seedlings for tolerance in vitro, but few studies have investigated whether the results correspond to resistance to common scab observed in the field. In this study, 120 F1 potato progeny from a single cross were screened in vitro by exposing the seedlings to thaxtomin A added to the culture medium. Eighteen genotypes were selected based on high sensitivity or tolerance using shoot growth as the criterion, multiplied in vitro, and tested for resistance to common scab caused by S. turgidiscabies and S. scabies in a glasshouse and in three different fields. Evaluation of ca. 6500 tubers showed that the 18 potato genotypes differed in scab indices and disease severity (P < 0AE0001). The relative shoot height in vitro (thaxtomin A used at 0AE5 lg mL ) and the scab index in the field showed significant correlation (r s = )0AE463, P = 0AE0528, n = 18), also consistent with the results obtained under controlled conditions in the glasshouse. Hence, the in vitro bioassay may be used to discard scab-susceptible genotypes and elevate the overall levels of common scab resistance in the potato breeding populations.
An integrated description from nucleohistone fiber level to light microscopic level of polytene chromosomes is presented. The integration is based on the results obtained by different new applications of electron microscopic techniques and on the analysis of earlier data of chromatin structure. A model for the organization of polytene chromosomes in vivo is extrapolated from the EM‐data by approximating the effects of the physico‐chemical parameters changing during preparative procedures. The nucleosome unit fiber forms loops, which are organized into about 500Å diameter coils with 24 nucleosomes per turn of the double fiber, leading to an overall packing ratio of about 1:78. The coils are packed differentially into structural units forming the polytene bands, which are separated from each other by interbands of unit fibers with no higher order coiling evident. For the general higher order structure of chromatin a model capable of explaining former observations in other eukaryotic chromosome systems is proposed. A whole mount method suitable for nucleosome fiber examination in polytene chromosomes is introduced. A sectioning method preserving the nucleosomal structures and a quick method for whole mounting polytene chromosomes retaining their light microscopic morphology are also described.
Single-stranded DNA was complexed to the single-strand binding protein (SSB) of Escherichia coli in a mass ratio of 30:1. The protein moiety of this complex can be labelled by a number of methods of which we have chosen radio-iodination and biotinylation as examples. The SSB-M13 DNA complexes, labelled to high specific activities, were used as probes in hybridization experiments in which 1.6 X 10(-18) moles of immobilized target DNA were detected. The stability of the hybrids was not severely decreased by the binding of SSB. Analysis of hybrids by electron microscopy showed that complexing of DNA with SSB could be used to allow its subsequent identification in the hybrids.
The suitability of whole mount electron microscopy for the localization of gene activity in polytene chromosomes was tested by heat shock induction in Drosophila hydei. The heat shock response of salivary glands is found out to be detrimental to surface spreading properties of salivary gland material. The major heat shock band in division 81 is determined to be 81C2. The heat shock puff is covered by amorphous material probably representing transcription complexes that partly obscure the underlying structures. After 5 min heat shock treatment, spherical particles are observed on the puff site. Successful mapping of this region, which consists of very thin bands including a developmental puff, suggests that whole mount electron microscopy is the method of choice for routine localization of gene activity in polytene chromosome systems.
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