Pollen selection for Alternaria resistance in oilseed brassicas: responses of pollen grains and leaves to a toxin of A. brassicae

Summary The effects of destruxin B, a host-specific toxin of Alternaria brassicae that causes black spot disease in oilseed brassicas, were studied on in vitro pollen germination and pollen-tube growth of Brassica campestris var brown sarson, B. juncea, B. napus cvs Westar and Cresor, B. nigra and Sinapis alba. Pollen grains of B. nigra, B. juncea and B. campestris were the most sensitive and those of S. alba the least sensitive to the toxin. Effects of the toxin were also studied on the leaves of these species, and the degree of sensitivity of leaves of different species was comparable to that of their pollen grains. The results on the responses of pollen grains as well as leaves to the toxin are in agreement with the degree of susceptibility/resistance of these species to A. brassicae reported in the literature, indicating that the genes imparting susceptibility/restistance are expressed in the pollen, a prerequisite for pollen selection. Results are also presented which show that the toxin fed to the cut end of isolated inflorescence axis is readily taken up by the developing pollen and results in the inhibition of germination of susceptible pollen. This technique offers a simple and effective method for application of selection pressure to eliminate pollen grains susceptible to the toxin from effecting fertilization.     

Replication of tomato yellow leaf curl virus (TYLCV) DNA in agroinoculated leaf discs from selected tomato genotypes

The leaf disc agroinoculation system was applied to study tomato yellow leaf curl virus (TYLCV) replication in explants from susceptible and resistant tomato genotypes. This system was also evaluated as a potential selection tool in breeding programmes for TYLCV resistance. Leaf discs were incubated with a head-to-tail dimer of the TYLCV genome cloned into the Ti plasmid ofAgrobacterium tumefaciens. In leaf discs from susceptible cultivars (Lycopersicon esculentum) TYLCV single-stranded genomic DNA and its double-stranded DNA forms appeared within 2–5 days after inoculation. Whiteflies (Bemisia tabaci) efficiently transmitted the TYLCV disease to tomato test plants following acquisition feeding on agroinoculated tomato leaf discs. This indicates that infective viral particles have been produced and have reached the phloem cells of the explant where they can be acquired by the insects. Plants regenerated from agroinfected leaf discs of sensitive tomato cultivars exhibited disease symptoms and contained TYLCV DNA concentrations similar to those present in field-infected tomato plants, indicating that TYLCV can move out from the leaf disc into the regenerating plant. Leaf discs from accessions of the wild tomato species immune to whitefly-mediated inoculation,L. chilense LA1969 andL. hirsutum LA1777, did not support TYLCV DNA replication. Leaf discs from plants tolerant to TYLCV issued from breeding programmes behaved like leaf discs from susceptible cultivars.The Hebrew University of Jerusalem, Faculty of Agriculture, Department of Field and Vegetable Crops     

Somaclonal variation within poplar

One thousand and ninety-two poplars were regenerated in vitro from callus of 13 poplar clones representing the Leuce, Aigeiros and Tacamahaca sections. At lest 44 of the regenerants differed in some way from the original clones. Somaclonal variation occurred more frequently in poplars of the Leuce section (8%) than in those of the Aigeiros or Tacamahaca sections (1%). Variation was noticed in growth habit, leaf shape or indentation but not in the reaction to four Melampsora races. However, after one growing season in the field, a few regenerants from calli of two clones (Ogy and Rap) differed in their susceptibility vis à vis the original clones. Cultivation of callus from Leuce poplars that had survived exposure to increasing concentrations of toxins from Hypoxylon mammatum gave rise to a toxin-tolerant line from which toxin tolerant plants were regenerated. Flow cytometry to measure the DNA content of nuclei showed that regenerants tended to be tetraploid.Abbreviations NAA naphthaleneacetic acid - BAP benzylaminopurine - TDZ thidiazuron - MS Murashige & Skoog medium     

Growth patterns and endogenous indole-3-acetic acid concentrations in current-year coppice shoots and seedlings of two Betula species

Apical dominance, internode elongation, radial growth and xylem cell size in coppice and apical shoots of Betula pubescens B. Pendula were determined and related to endogenous indole-3-acetic acid (IAA) levels, measured by gas chromatography-selected ion monitoring in the apical bud and at three positions along the stem. The effects of defoliation and debudding on morphological and anatomical characters and endogenous IAA levels were also investigated. The coppice shoots displayed superior stem elongation and increased branching during the initial phase of growth, after which their growth pattern was similar to that of the seedlings; however, their radial growth was greater throughout the experiment. Both plant types produced smaller-sized xylem cells at the top of the shoot than at the bottom with coppice shoots tending to form larger tracheids and smaller vessels than the seedlings. There was no consistent difference in IAA concentration between the coppice shoots and the seedlings. Defoliation and debudding reduced the IAA level in the stem within 36 h and it was still low after 25 days. Although the extent of the IAA decrease was similar in both coppice shoots and seedlings, the treatments affected the morphological and anatomical characters differently in the two plant types. The results suggest that the observed differences between seedlings and coppice shoots were not mediated through a drastic change in IAA level.     

A genetic map of Prunus based on an interspecific cross between peach and almond

A genetic linkage map of Prunus has been constructed using an interspecific F₂ population generated from self-pollinating a single F₁ plant from a cross between a dwarf peach selection (54P455) and an almond cultivar Padre. Mendelian segregations were observed for 118 markers including 1 morphological (dw), 6 isozymes, 12 plum genomic, 14 almond genomic and 75 peach mesocarp specific cDNA markers. One hundred and seven markers were mapped to 9 different linkage groups covering about 800 cM map distance, and 11 markers remained unlinked. Three loci identified by three cDNA clones, PC8, PC5 and PC68.1, were tightly linked to the dw locus in linkage group 5. Segregation distortion was observed for approximately one-third of the markers, perhaps due to the interspecific nature and the reproductive (i.e. self-incompatibility) differences between peach and almond. This map will be used for adding other markers and genes controlling important traits, identifying the genomic locations and genetic characterizing of the economically important genes in the genus Prunus, as well as for markerassisted selection in breeding populations. Of particular interest are the genes controlling tree growth and form, and fruit ripening and mesocarp development in peach and almond.     

Infection and Colonization of Sugar Cane and Other Graminaceous Plants by Endophytic Diazotrophs

Agriculturally important grasses such as sugar cane (Saccharum sp.), rice (Oryza sativa), wheat (Triticum aestivum) sorghum (Sorghum bicolor), maize (Zea mays), Panicum maximum, Brachiaria spp., and Pennisetum purpureum contain numerous diazotrophic bacteria, such as, Acetobacter diazotrophicus, Herbaspirillum spp., Azospirillum spp. These bacteria do not usually cause disease symptoms in the plants with which they are associated and the more numerous of them, for example, Herbaspirillum spp. and A. diazotrophicus, are obligate or facultative endo-phytes that do not survive well (or at all) in native soil; these are thought to be spread from plant generation to plant generation via seeds, vegetative propagation, dead plant material, and possibly by insect sap feeders. By contrast, Azospirillum spp. are not wholly endophytic but are root-associated, soil-dwelling bacteria that are also often found within plants, probably entering host plants via seeds or via wounds/cracks at lateral root junctions. Endophytic diazotrophs have been isolated from a number of grasses in which significant biological N₂ fixation (BNF) has been demonstrated, particularly Brazilian sugar cane varieties, but also in rice, maize, and sorghum. However, although the endophytic diazotrophs are held to be the causative agents of the observed BNF, direct evidence for this is lacking. Therefore, in this review we examine probable sites of bacterial multiplication and/or BNF within endophyte-containing grasses and discuss these in terms of potential benefits (or not) to both host plants and bacteria. In particular, we examine how potentially large numbers of bacteria, especially Herbaspirillum spp., A. diazotrophicus, and Azospirillum spp., can exist extracellularly within non-specialized (for symbiotic purposes) regions such as xylem vessels and intercellular spaces. The processes of infection and colonization of various grasses (particularly sugar cane) by diazotrophic endophytes are also described, and these are compared with those of important (nondiazotrophic) endophytic sugar cane pathogens such as Clavibacter xyli subsp. xyli and Xanthomonas albilineans.     

High-resolution DNA fingerprinting of parthenogenetic root-knot nematodes using AFLP analysis

Amplified fragment length polymorphism (AFLP) analysis has been used to characterize 15 root-knot nematode populations belonging to the three parthenogenetic species Meloidogyne arenaria , M. incognita and M. javanica. Sixteen primer combinations were used to generate AFLP patterns, with a total number of amplified fragments ranging from 872 to 1087, depending on the population tested. Two kinds of polymorphic DNA fragments could be distinguished: bands amplified in a single genotype, and bands polymorphic between genotypes (i.e. amplified in not all but at least two genotypes). Based on presence/absence of amplified bands and pairwise similarity values, all the populations tested were clustered according to their specific status. Significant intraspecific variation was revealed by AFLP, with DNA fragments polymorphic among populations within each of the three species tested. M. arenaria appeared as the most variable species, while M. javanica was the least polymorphic. Within each specific cluster, no general correlation could be found between genomic similarity and geographical origin of the populations. The results reported here showed the ability of the AFLP procedure to generate markers useful for genetic analysis in root-knot nematodes.     

Genetic and physical localization of the root-knot nematode resistance locus Mi in tomato

As part of a map-based cloning strategy designed to isolate the root-knot nematode resistance gene Mi, tomato F2 populations were analyzed in order to identify recombination points close to this economically important gene. A total of 21 089 F2 progeny plants were screened using morphological markers. An additional 1887 F2 were screened using PCR-based flanking markers. Fine-structure mapping of recombinants with newly developed AFLP markers, and RFLP markers derived from physically mapped cosmid subclones, localized Mi to a genomic region of about 550 kb. The low frequency of recombinants indicated that recombination was generally suppressed in these crosses and that crossovers were restricted to particular regions. To circumvent this problem, a population of Lycopersicon peruvianum, the species from which Mi was originally introgressed, that was segregating for resistance was developed. Screening of this population with PCR, RFLP and AFLP markers identified several plants with crossovers near Mi. Recombination frequency was approximately eight-fold higher in the Mi region of the L. peruvianum cross. However, even within the wild species cross, recombination sites were not uniformly distributed in the region. By combining data from the L. esculentum and L. peruvianum recombinant analyses, it was possible to localize Mi to a region of the genome spanning less than 65 kb. Received: 15 July 1997 / Accepted: 1 October 1997     

Parasites as indicators of individual feeding specialization in Arctic charr during winter in northern Norway

The relationship between infection with the food-transmitted parasites Diphyllobothrium dendriticum, D. ditremum (Cestoda) and Cystidicola farionis (Nematoda) and prey selection was studied in individual Arctic charr from Lake Takvatn, northern Norway. There was no correlation between parasites transmitted throughout prey organisms from benthic habitats (amphipods) and pelagic habitats (copepods). A strong relationship between infection with a parasite species and the corresponding intermediate host from the stomach content of individual charr, indicated an individual feeding specialization. Independent of size, charr specialized on the intermediate hosts of all three parasite species. Some charr maintained this specialization on specific prey items throughout the winter period. These parasite species are considered to be useful indicators of past prey selection.     

Diversity of Rhizobia isolated from various Hedysarum species

Cultural and physiological properties, serology, plasmid profiles and infective traits were determined for 23 strains of rhizobia isolated from various Hedysarum species: H. coronarium (common name: sulla) (16), H. carnosum (1), H. alpinum (3), H. mackenzii (2) and H. pallens (1) from Portugal, Spain, Tunisia, Alaska and Israel. Strains isolated from H. alpinum, H. mackenzii and H. pallens have slow growth rates on yeast-extract mannitol medium and were unable to nodulate H. coronarium plants, whereas the latter were effectively nodulated by all sixteen fast growing strains from sulla. Regardless of the country of origin all H. coronarium strains fell into one serogroup and were not serologically related with strains of other Hedysarum species. The RAPD (random amplified polymorphic DNA) fingerprinting method which was carried out on five H. coronarium and three H. alpinum strains allowed distinction to be made among serologically related rhizobia. No particular plasmid profile pattern was observed in relation to the host or geographical origin of the strains.