Sheet n°247 - August 2006
©IRD/Jean-Christophe Castella
ice yellow mottle virus
(RYMV) causes heavy
yield losses in rice harvests in
Africa. The best hope for significantly reducing production
losses comes from resistant
varieties. A gene responsible
for strong resistance of rice to
the virus was identified recently by Laurence Albar, IRD
geneticist. This gene, Rymv1,
is linked to a biological function vital for the plant as it
allows cellular messenger
RNAs (1) to be translated into
proteins. The research team
showed some varieties to
have small mutations in a particular locus of this gene that
confer resistance. Previous
hypotheses proposed that in
an infection-susceptible variety, the virus interacted with the
factor coded by the Rymv1
gene and used it to its own
advantage. In the long term,
this research will facilitate the
transfer of this gene by crossing between resistant and
susceptible varieties. This
result opens up great prospects for better understanding of the resistance mechanism and for use of this gene
in variety selection by IRD’s
partners such as the WARDA
(Africa Rice Centre) and national research institutes on the
African continent and in
Madagascar.
R
Discovery of the first resistance gene
to rice yellow mottle virus
Variety of rice susceptible to th Rice yellow mottle virus (RYMV).
Rice yellow mottle virus (RYMV) was
first identified in 1966 in Kenya. It has
since been reported in most African countries where rice is grown. The disease is
characterized by the appearance of mottling and then tissue death on the leaves.
The fertility and development of seeds are
affected, which causes considerable yield
losses at harvest.
Transmission of RYMV occurs by way of
insect vectors or by straightforward
contact between plants. Prevention measures, like direct sowing or the burying of
straw, have been implemented in order to
limit the spread of the disease, but the real
potential for reducing the impact of RYMV is
to be found in the use of resistant varieties.
In certain rare traditional varieties of
the Asian species of rice, Oryza sativa,
and of the African variety, O. glaberrima,
RYMV infection does not generally produce leaf symptoms, or have any impact on
the harvest production. However, these
varieties do not have the agronomic characteristics sought after for intensive irrigated cultivation or growing on low-lying
land, where the disease provokes most
damage. IRD geneticists have for several
years been applying their research to the
genetic bases of this resistance in order to
facilitate its transfer to varieties that are
agronomically useful yet susceptible to the
virus with a view to optimizing their use.
>>
Institut de recherche pour le développement - 213, rue La Fayette - F-75480 Paris cedex 10 - France - www.ird.fr
Sheet n°247- ??? 2006
For futher information
CONTACTS :
ALAIN GHESQUIÈRE OR
LAURENCE ALBAR,
IRD, "Génome et développement
des plantes", UMR5096,
+33 (0)4 67 41 61 53,
alain.ghesquiere@mpl.ird.fr or
laurence.albar@mpl.ird.fr
PRESS OFFICE:
01 48 03 75 19 ;
presse@paris.ird.fr
INDIGO BASE, IRD PICTURE LIBRARY
01 48 03 78 99 ;
indigo@paris.ird.fr
REFERENCES:
ALBAR L., BANGRATZ-REYSER
M., HEBRARD E., NDJIONDJOP
M.N., JONES M., GHESQUIERE
A.: Mutations in the
eIF(iso)4G translation initiation factor confer high resistance of rice to Rice yellow
mottle virus. (2006) The
Plant Journal 47, 417 – 426.
FOR FURTHER INFORMATION
RESISTANCE BREAKING :
HÉBRARD E., PINEL-GALZI A.,
BERSOULT A., SIRÉ C. AND
FARGETTE D.: Emergence of a
resistance-breaking isolate of
Rice yellow mottle virus
during serial inoculations is
due to single substitution in
the genome-linked viral protein VPg. (2006) J. General.
Virology 87 : 1369 -1373.
TRANSGENIC RESISTANCE:
KOUASSI N. K., CHEN L., SIRÉ
C., BANGRATZ-REYSER M.,
BEACHY R. N., FAUQUET C. M.,
AND BRUGIDOU C. :
Expression of rice yellow
mottle virus coat protein
enhances virus infection in
transgenic plants. (2006)
Archives of Virology, in
press.
KEY WORDS
RICE YELLOW MOTTLE VIRUS,
RICE, RESISTANCE.
Standard genetic studies first found
evidence that resistance was controlled
by a single recessive gene. Subsequent
genetic mapping identified a fragment of
chromosome 4 containing the resistance
gene. Data from rice genome sequencing
have been extremely useful for research
on this fragment to find out if one gene
rather than another could confer resistance to RYMV. Data from the literature indicates that gene eIF(iso)4G, involved in
cellular RNA translation and named
Rymv1, appeared to be the best candidate. Validation of the function of this resistance gene was performed by genetic
transformation. For this, a line of resistant
rice was modified by transgenically introducing the allele (2) for susceptibility of this
gene. The descendents of transformed
plants that manifested restored susceptibility always showed the presence of the
transgene.
Viruses are built with a small genome
coding for a limited number of proteins (5
in the RYMVs). They therefore need their
host’s proteins in order to accomplish
each stage of their infection cycle. One of
the proteins that the RYMV requires
appears to be the eIF(iso)4G translation
initiation factor coded by the Rymv1 gene
which is probably involved in viral protein
translation, but also perhaps in other processes such as the virus’s movement
within the cell.
The research team discovered mutations
of the gene they analysed in three different resistant varieties. These are distinct
but are situated in the same domain of the
gene in a patch on the surface favourable
for interaction with the virus. In these
varieties, the mutation does not appear to
alter the protein’s role in its primary biological functions, but can prevent its interaction with the virus which is then blocked in
one of the stages of its infectious cycle.
In parallel, a team of IRD virologists
showed that it was possible to carry out
laboratory selection of RYMV strains that
break the gene’s resistance and that the
process involved was determined by
mutations in one of the viral proteins. The
two approaches are now being combined
in order to determine the molecular
mechanisms of resistance or susceptibility
on the basis of direct interactions between
the rice protein and that of the virus.
Understanding of these mechanisms will
give clues as to the best ways of making
long-term use of this resistance gene.
Another strategy developed by the IRD
for combating this virus involves introducing part of the viral genes into the plant
genome by transgenesis, as has been
done in other plant/virus interactions, with
the aim of inducing resistance to RYMV.
The results show that the transgenic
plants have only partial resistance, and for
only a short time, and can even end up
with a heightened susceptibility. In the particular case of the rice/RYMV interaction,
the strategy of introducing a viral gene by
transgenic techniques does not bring any
advantages compared with the use of
natural resistance.
This research can find applications in
ways of improving rice production in countries hit by RYMV. Already, the IRD has
transferred the gene Rymv1, by crossing,
into some agronomically important varieties. The corresponding lineages have
been given to various national (Ivory
Coast, Senegal, Madagascar) and international research institutions such as the
African Rice Centre (Warda, Benin) for them
to use in variety selection programmes.
(1) Messenger RNA: a ribonucleic
acid transcribed from the DNA of a
gene and which serves as a model for
translation of a protein.
(2) Allele: each of the different possible forms of the same gene.
Aude Sonneville, IRD
Translation : Nicholas Flay
Marie Guillaume - Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@paris.ird.fr