Restricted multiplication of potato leafroll virus in resistant potato genotypes

Plants of a range of potato genotypes differing in rating for field resistance to potato leafroll virus (PLRV) were inoculated with the virus by grafting or by aphids (Myzus persicae). Plants of all genotypes tested became infected by each inoculation method and PLRV was detected by ELISA in the upper leaves of all genotypes within 26 days after grafting. Most genotypes with high resistance ratings developed only mild primary and secondary symptoms whereas those with low resistance ratings developed more pronounced symptoms. However, one genotype (G7461(4)) with a high resistance rating was very severely affected. The concentrations attained by PLRV in genotypes with high resistance ratings were only 1–10% of those in genotypes with low resistance ratings. These differences in virus concentration were found in young leaves of plants with primary or secondary infection, whether inoculated by grafting or by aphids and whether grown in the glasshouse or the field. In older leaves, differences in virus concentration between genotypes were at least as pronounced as those in younger leaves. In contrast, PLRV concentration in vascular tissue at the heel end of tubers of plants with primary infection was similar for all the genotypes tested. Although low PLRV concentration was consistently associated with high resistance rating it is not the only form of resistance to PLRV occurring in potato.     

Genome affinities and epitope profiles of whitefly-transmitted geminiviruses from the Americas

The relationships among fifteen isolates of whitefly-transmitted geminiviruses (WTGs) from North, Central and South America and six from other continents were assessed (a) in nucleic acid hybridisation tests with sulphonated DNA probes for eight of the viruses, and/or (b) in triple-antibody-sandwich ELISA with panels of monoclonal antibodies (MAbs) to particles of African cassava mosaic virus (ACMV) and Indian cassava mosaic virus (ICMV). Probes specific for DNA-A of four American viruses, abutilon mosaic (AbMV), bean golden mosaic (BGMV), squash leaf curl (SLCV) and tomato golden mosaic (TGMV), detected virtually all the American viruses but reacted weakly if at all with ICMV, ACMV or tomato yellow leaf curl virus from Thailand (TYLCV-T). Conversely, the probe for ACMV DNA-A did not detect any of the American viruses, and that for TYLCV-T DNA-A reacted weakly with SLCV and TGMV0020but did not detect the others. In contrast, probes specific for DNA-B of the four American viruses or ACMV detected only the homologous virus, except for slight reactions between the AbMV DNA-B probe and both chino del tomate virus (CdTV)-DNA and SLCV-DNA. However, a probe for DNA-B of bean calico mosaic virus (BCMoV) reacted weakly with BGMV-PR DNA, and a probe for DNA-B of CdTV from Mexico detected several American viruses. Six out of 17 MAbs specific for ACMV and six out of 10 MAbs specific for ICMV reacted with one or other of the 14 American virus isolates tested. Two and-ACMV MAbs reacted with all, and one anti-ACMV MAb and two anti-ICMV MAbs reacted with nearly all the American viruses, one anti-ACMV MAb reacted with about half the American viruses and six other MAbs reacted with only one or two of them. Of the American viruses, CdTV and AbMV were the least closely related to the others. The epitope profiles of BCMoV, BGMV, cotton leaf crumple virus, serrano golden mosaic virus and SLCV were virtually indistinguishable. TGMV, potato yellow mosaic virus (PYMV) and an euphorbia virus had profiles intermediate between those of the BGMV cluster and AbMV-CdTV. In general, the epitope profiles and the results of hybridisation tests with DNA-A probes show that the similarities among the American viruses are greater than those between the American viruses and the viruses from other continents; the hybridisation tests with DNA-B probes show that substantial differences exist between individual American viruses. In America, geminivirus evolution seems to have proceeded convergently from different progenitor viruses, or divergently from one ancestral form, with DNA-B diverging to a greater extent than DNA-A and its particle-protein gene.     

Another quarter century of great progress in understanding the biological properties of plant viruses

The last quarter century saw a massive application of new molecular and cell biological, and molecular genetical, techniques in research on plant viruses. Hundreds of complete virus genomic nucleotide sequences were analysed, their constituent genes and control elements identified, and similarities and differences revealed in genome organisation, so justifying the modern taxonomic classification of the viruses. Numerous virus replication systems were described and some viral replicases isolated. Especially good progress was made in understanding cell‐to‐cell and long‐distance virus transport within plants; in cloning dominant and recessive plant genes controlling virus resistances, and identifying the cognate viral avirulence factors; in unraveling mechanisms of virus transmission by invertebrate and plasmodiophorid vectors; and in showing through these advances how viruses utilise and subvert endogenous eukaryotic processes. The discovery of gene silencing, and of viral silencing‐suppressor proteins, transformed ideas on how virus replication is controlled, and explained the phenomena of recovery from disease, cross‐protection between virus strains and synergy between unrelated viruses. Transgenic, virus‐resistant plants were created, tested successfully in field conditions and a few commercialised. Factors underlying the appearance of new disease epidemics were identified. Genetic recombination was reported and found to make an important contribution to generating virus variation, and some major virus evolutionary pathways were recognised by comparative genome analysis. The scene is set for further major advances in the coming decades.     

Survival of Strains of Soft Rot Coliform Bacteria on Microthreads Exposed in the Laboratory and in the Open Air

Survival of 5 strains of the plant pathogen, Erwinia carotovora var. atroseptica and 3 strains of the plant pathogen E. carotovora var. carotovora , grown in a liquid tryptone medium and held as 'captive'aerosols on gossamer microthreads, was determined under different atmospheric conditions in a controlled environment room and in the open air. Although these bacteria lost viability more quickly than a robust reference strain of Escherichia coli , sufficient numbers survived for 15 min or more to indicate that airborne spread of viable propagules could take place, especially under cool humid atmospheric conditions. Cells of one strain of each organism extracted from rotted potato tuber tissue were shown to behave rather like those cultured in the tryptone medium.