Arapanet2 conference report
PLXZW at VAX.CCC.NOTTINGHAM.AC.UK
PLXZW at VAX.CCC.NOTTINGHAM.AC.UK
Wed May 26 03:05:00 EST 1993
Dear Netters, Here is a general overview of the Arapanet 2
symposium which was held at Wye College, University of London
25th -27th March 1993.
Zoe A. Wilson
Summary ARAPANET 2 Meeting
The ARAPANET 2 meeting brought together approximately 60
researchers from the UK, Europe and the States, who are involved
with various aspects of plant pathology and have some link with
Arabidopsis. One notable aspect of the meeting was how many
pathogens are now able to infect this previously thought
invincible weed !
The meeting was smoothly organized by Jim Beynon, and I'm sure
that all the other participants would like to join me in thanking
him for his efforts. Special thanks to the Agricultural Food
Research Council-UK for funding support to help defray costs. I
shall now, assisted by John Lucas's much appreciated editing
comments, try to briefly summarize, in the order of presentation,
the 26 talks from the different labs that were represented. I
would like to add the "escape clause" that this summary was
compiled from rapidly taken notes during the meeting, so I am
sorry if there are any inaccuracies.
The first three presentations involved various nematode pests;
Annette Bchenhoff (Kiel) presented research into the
metabolism and nutrient uptake involved in the multinucleate
syncytial cells which are produced after infection with the sugar
beet nematode Heterodera schachtii. She is looking at this problem
by microinjection of fluorochromes and removal of cytoplasm
from the syncytium tissue, to detect specific proteins associated
with infection and pathogen metabolism.
Maria Nobre (Harpenden) is interested in the migration
patterns of the root-knot nematodes (Meloidogyne spp.) These
undergo a three phase infection process, which involves migration
to the root tip, turning and then establishment in the vascular
tissue. They have been using 5 antibodies to the plant
extracellular matrix to permit visualization of the nematode and
the disrupted tissues during the infection stages. In the near
future she aims to look at alterations in the infection process in
root morphology mutants.
Andreas Niebel (Gent) is studying the different forms of gene
expression which occur in plant tissues after the induction of
feeder cell formation during infection with root knot nematodes.
Various approaches have been used, including GUS fusions to
known cell cycle genes and the search for mutants which are
affected in the attraction process. cdc-2 was induced in giant cells
at the start of nematode feeding and was increased after 2 d, after
which time expression was reduced; in the cyst, expression was
seen close to the nematode head. Mitosis may be involved,
although the exact role of cdc-2 in DNA duplication has not been
fully established. Mutants which expressed different levels of
resistance were identified, however no natural ecotypic variation
in infection response was seen.
The theme then progressed to viroids and viruses. Fernando
Ponz (Madrid) described the interaction of Arabidopsis with
Youcai Mosaic Virus and with viroidal signals. Viroids have so far
only been identified in plants and do not encode for any specific
proteins. They have a limited host range and no viroid has yet
been described in Crucifers; varying infection ability has been
seen in Arabidopsis . Infection range is being analysed by looking
at expression using promotorless constructs and GUS fusions with
the 35S CaMV promotor.
Joachim Schiemann (Braunschweig) was screening different
Arabidopsis ecotypes for resistance to tospovirus. Out of 50
ecotypes one was identified that did not exhibit infection
symptoms, although virus accumulation still occurred. Plans are
in progress to assess the effect of environment on tolerance.
Keith Davis (Columbus) described work on the Gemini virus
Beet Curly Top Virus. This has a broad host range in dicots. It is
transmitted by leaf hoppers, but can be mechanically inoculated
although the infection rate is poor. They have been using
Agrobacterium to infect Arabidopsis with the virus genome.
Plants exhibit curling of leaves and flowers. Variation has been
observed between different ecotypical strains of Arabidopsis,
including some in which viral replication and/or movement is
Renate Schmidt (Norwich) described work in progress to
create a physical map of the Arabidopsis genome; to date they
have mapped more than 450 RFLP markers on to the top arms of
chromosomes 4 and 5, using three YAC libraries, those of Erwin
Grill (EG) , Eric Ward (EW) and Joe Ecker(yUP). Linking of YAC
contigs by chromosome walking has been shown to be extremely
inefficient. An alternative route has been adopted to try to
identify more markers and use these to link up the YAC contigs.
An example of chromosome 4 was given where 40 end-probe
markers were required to link 800kb, whilst the same number of
RFLP probes linked approximately 10Mb. Chimaeric clones of both
repetitive sequences and chloroplast sequences were identified at
approx. 20%. Cosmid contigs (Hauge and Goodman), the Dean RI
lines and cDNA mapping and sequencing are being used to provide
integrated positions for markers and aid in the linking of these
Shauna Somerville (Kln) is interested in the role that
phytoalexins (PA) have in controlling pathogen growth in the
defence response. Organic extracts from Arabidopsis were
separated on TLC plates and compounds analysed for possible
inhibition of pathogen growth. One PA, camalexin was identified,
which is related to PAs from Brassica. Shauna described the use of
tryptophan deficient mutants to determine whether tryptophan is
an intermediate for PA production. Using the trp-1 mutant which
has an early block in this pathway PA levels decreased by 50%.
This suggests that there is an alternative pathway and possibly
duplication of this enzyme. No differences in PA levels were
observed using mutants that were blocked after indole glycerol
phosphate. Feeder experiments indicate that anthranilic acid is a
precursor of camalexin. The branch point seems to be between
anthranilic acid and indole 3 glycerol phosphate.
Ulla Bonas (Gif-s-Yvette) has been working on the avrBs3
family in the pathogen Xanthomonas campestris pv. vesicatoria
(Xcv) of pepper. They have identified an operon of 6 hrp genes.
Most of these require starvation conditions for expression,
although one is constitutively expressed. Four ORF have been
identified. The possible roles these have are in chemotaxis,
transport of plant molecules, vir factors or in the export of vir
A 122kD protein has been identified and characterized. This
carries a 102bp repeat 17.5 times. Five basic repeats are present
and these seem to be involved in the specificity of the protein.
This repeat region appears very stable in vitro; there may be
some involvement of this domain in protein binding, thus
preventing recombination. A related allele has been identified in
tomato with 97% protein homology and the same repeat region.
Homology has also been detected between hrp sequences and
certain genes involved in bacterial pathogenicity to animal hosts.
However, there is no evidence for conserved promotor motifs in
Richard Mithen (JII, Norwich) described the infection process
of Plasmodiophora brassicae , the causative agent of club root, on
Arabidopsis. During the early stages of infection there is no
visible change, but after about 3 weeks leaf size and growth are
affected. Small ameoboid structures are visible in the root cortex
and cell division occurs near the endodermis in the stele. The
pathogen is present within the cell. The host nucleus becomes
surrounded by the Plasmodiophora. There is intimate association
between the pathogen and host, and horizontal DNA transfer may
occur between the pathogen and the host. Compatible reactions
are characterized by the induction of indole glucosinolates (which
serve as auxins) and the blocking of PA biosynthesis. Arabidopsis
and Brassica have extensive homology in their glucosinolates; the
mapping of the genes involved in glucosinolate production is
Julie Scholes (Sheffield) described work to determine the role
of invertases in disease resistance to biotrophic foliar and root-
infecting pathogens. The regulation of metabolic changes in
photosynthesis was determined using freeze-dried leaf discs
taken after infection and video-imaging of chlorophyll
fluorescence. Photosynthesis was inhibited, although stomatal
movements were unaffected, hence the decrease in
photosynthesis appeared associated with biochemical mechanisms.
A specific down-regulation of invertases was observed. Four
different isoforms of invertases were seen, with a decrease in
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