www.checkbiotech.org ; www.raupp.info ; www.czu.cz

Rice 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 to be translated into proteins, October 2006
by Aude Sonneville translated by Nicholas Flay.

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.

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.

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.

[www.ird.fr] - www.checkbiotech.org

------------------------------------------
Posted to Phorum via PhorumMail