LIBRO-CONGRESO-CITRUS

assay was performed to find out if the increased viroid titer was related to suppression of post-transcriptional
silencing. Transgenic Mexican limes expressing each one of these proteins were inoculated with CDVd.
Northern hybridisation analysis showed that, in plants expressing p23, viroid accumulation was enhanced
at similar levels as in plants co-infected with CTV. These results indicate that the expression of p23 via CTV
infection or via transgenic plants alters the silencing mechanism implicated in host defence, supporting the
hypothesis that viroids are targets of the RNA silencing machinery of their hosts.
S14O06
The p23 protein encoded by Citrus tristeza virus: fine dissection of its determinants for nucleolar
localization and for suppression of RNA silencing and pathogenesis
Ruiz Ruiz S. 1 , Soler N. 2 , Sánchez-Navarro J. 1 , Fagoaga C. 2 , López C. 3 , Navarro L. 2 , Moreno P. 2 , Peña L. 2 , and Flores R. 1
1Instituto de Biología Molecular y Celular de Plantas (IBMCP), Valencia, Spain ; 2Instituto Valenciano de Investigaciones Agrarias (IVIA),
Centro de Protección Vegetal y Biotecnología, Moncada, Valencia, Spain; and 3Instituto de Conservación y Mejora de la Agrodiversidad
Valenciana (COMAV), Valencia, Spain. suruirui@upvnet.upv.es
One of the three RNA silencing suppressors (RSS) encoded by Citrus tristeza virus (CTV) is p23, a RNA-binding
protein of 209 amino acids that also is a pathogenic determinant when expressed ectopically in citrus. Confocal
microscopy of Nicotiana benthamiana in which the fusion p23-GFP was transiently expressed revealed its
accumulation in nucleolus, Cajal bodies and plasmodesmata, thus being the first closterovirus protein with
a nucleolar localization signal (NoLS). Assay of seven truncated versions of p23 showed that regions 50-86
and 100-157 (excluding fragment 106-114), both with basic motifs and the first with a Zn-finger domain,
contain a NoLS bipartite. Further examination of ten point-mutants delimited this signal to three Cys of the
Zn-finger and some basic amino acids within and preceding it, and to fragment 143-155 with six basic amino
acids. Essentially all deletions and substitutions annulled RSS activity of p23 (in transgenic N. benthamiana
expressing constitutively GFP), implicating most p23 regions in this activity. Expression in N. benthamiana of
p23 as a subgenomic RNA of Potato virus X induced necrosis. This phenotype only was preserved in deletion
mutant 158-209 and in one point-mutant, showing that the Zn-finger and flanking basic motifs are part of the
pathogenic determinant. Ectopic expression of p23 and some deletion mutants in transgenic Mexican lime
delimited a similar determinant, suggesting that p23 affects related pathways in citrus and N. benthamiana.
S14O07
Viruliferous Brevipalpus phoenicis gradient from Citrus Leprosis affected trees
Bassanezi R.B. 1 , Montesino L.H. 1 , and Novelli V.M. 2
1Fundo de Defesa da Citricultura (Fundecitrus), Departamento Científico, Brazil; and 2Centro APTA Citros Sylvio Moreira - Instituto
Agronômico de Campinas (Centro APTA Citros Sylvio Moreira - IAC), Brazil. rbbassanezi@fundecitrus.com.br
Citrus leprosis, caused by Citrus leprosis virus (CiLV-C) transmitted by Brevipalpus phoenicis mite, is important
in Brazil and its control is mainly based on mitecide sprays. A weak relation between spatial distribution of
leprosis symptomatic trees and mite infested trees was observed in previous studies. Diseased trees were
much more aggregated than mite infested trees. This could be explained by a limited movement of mites
and the high dependence of viruliferous mites on symptomatic citrus trees. The aim of this work was to
assess the viruliferous mite gradient from citrus leprosis affected trees. Surveys were conducted in sweet
orange citrus blocks in which all trees were inspected for the presence of symptoms and the mite vector.
Symptomatic trees and mite infested trees were spatially located and 5-mite samples from each infested
tree were analyzed by RT-PCR for CiLV-C. From 163 samples collected on healthy trees, only 37.4% were
CiLV-C positive, and even from 337 samples from symptomatic trees, only 37.7% were positive, confirming
that most of the mite population was non-viruliferous. Out of 178 CiLV-C positive samples, 71.3% were from
symptomatic trees, 95% were found within 7 m around a symptomatic tree, and no positive sample was
found more than 23.5 m away from a diseased tree. The frequency of viruliferous mite samples (y, in %) as a
function of minimum distance from a leprosis symptomatic tree (x, in m) was fitted to a negative exponential
model y=71.exp(-0.43 x).
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