Arapanet2 conference report

[PLXZW at VAX.CCC.NOTTINGHAM.AC.UK]

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 
restricted.

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 
regions.

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 
factors.
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 
hrp  operons.

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 
planned.

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 
acti