Barley yellow dwarf disease as a target of breeding for resistance (short review)

The aim of this work was to give a brief review of some points of wide knowledge of barley yellow dwarf (BYD) disease and its breeding for resistance program. Yd2 gene has been shortly characterised. Current situation in Poland has been underlined.


INTRODUCTION
Barley yellow dwarf disease is a serious treat to the cultivation of small grains.It was firstly recognized as being caused by a virus in 1951 by Oswald and Houston.Until recently Barley yellow dwarf virus (BYDV) was divided into two subgrups.Comparison of nucleotide sequences has led to definition of a new species named Cereal yellow dwarf virus (CYDV-RPV) from former serotype BYDV-RPV (D'Arcy et al., 2000).New virus was placed in genus Polerovirus.Serotypes PAV and MAV were defined as species of BYDV in genus Luteovirus.However, it is now known that genus Luteovirus shares many features with family Tombusviridae and there are some doubts about classification of that virus (M i l l e r et al., 2002).Rest of the serotypes remain unassigned in the family Luteoviridae.
BYDV has isometric icosahedral (T=3) virions 25nm in diameter, single molecule of 5.6-5.8 kb positive sense ssRNA with six open reading frames (ORFs).Proteins are: RNA dependent RNA polymerase (RdRp) (ORF 1 and 2), protein for infection phloem tissue (ORF4), coat protein (ORF3) combined with readthrough domain (ORF5) responsible for aphid transmission.Viruses are not transmitted mechanically, but by many species of cereal aphids.Serotypes were firstly defined basing on dominant vector species eg.PAV usually transmitted by Rhopalosiphum padi, MAV usually transmitted by Macrosiphum avenae.(B r u n t et al., 1996).
Main directions of research are virus replication and vector transmission.

Economic importance
BYD is spread worldwide and is an economically most important viral disease of cereals.It affects barley, oat, wheat, maize, rice and many weeds.It causes substantial losses throughout the world e.g. in wheat (17%), barley (15%), and oats (25%) (L is t e r and R a n i e r i , 1995).Between 1988 and1991 in European Russia the yields decreased by 90% because of viral epidemic which was supposed to be BYD (M oz h a e v a and K a s t a l y e v a , 2002).In England some special steps are taken every year to prevent losses (K n i g h t et al., 1996) .In Poland monitoring was performed by J e ¿ e w s k a and co-workers.In 1995 serotype PAV or mixed infections with MAV were common.Wheat and oat were affected most seriously.In 1999 MAV and PAV were detected in equal proportions, mixed infections prevailed.Prophylactical sprays were not recommended because of small number of infections observed in autumn (J e ¿ e w s k a , 2001a).In 2001 an increase of infections was noted, especially in south-east Poland (J e ¿ e w s k a , 2001b).In 2002 high degree of infections was noted in western Poland, on winter wheat and barley (J e ¿ e w s k a , 2002).

Yd2 gene
Shortly after first identification of BYD, in1959, Rasmusson and Schaller discovered in Ethiopia the first resistance gene named Yd2.It become soon, the most widely used gene in barley breeding for virus resistance.Unfortunately, the effect of this gene is satisfying only when plants are infected with PAV and MAV isolates.For RPV (now CYDV) such resistance was not detected (H e r r e r a and P l u m b , 1991).
Yd2 gene is located on chromosome 3, on the long arm, 0.5 cM from the centromere and cosegregates with RFLP loci Xwg889 and XYlp (C o l l i n s et al., 1996).The Ylp gene was characterized by Ford and co-workers (1998).It was shown that it encodes for subunit of barley vacuolar proton-translocating ATPase.Basing on cDNA comparison, a single-base-pair polymorphism was found to be correlated with difference between BYDV susceptible and unsusceptible plants.An assay for resistance detection was proposed in the same work (F o r d et al., 1998).In series of further investigations, the Ylp marker was found to be very useful, because it correlates perfectly with field resistance tests (O v e s n a et al., 2000).Second marker, used in this tests, named YLM was found less related to Yd2 gene, and it was less valuable for winter barley cultivars.For spring barley breeding program it was used with success (O v e s n a et al., 1999).This phenomenon confirmed results of mapping experiments, where Ylp marker was placed closer to Yd2 than YLM marker (F o r d at al., 1998; P a l t r i d g e et al., 1998).Further approaches toward cloning and characterisation of Yd2 gene are taken.One of the reasons for that studies is the interesting resistance mechanism of that gene.It appears to reduce the rate of replication of the virus in the phloem and on this basis it could be accounted to a new group of resistance genes (K i n g et al., 2002).
Yd2 is a semi-dominat gene.Its effect can be changed by genetic background (M a k k o u k and G h u l a m , 1992).Unfortunately it is associated with excessive plant height, limited yield and poor seed quality (C o m e a u and J e d l i n s k i , 1990).
It is big disadvantage for breeding programs, but many resistant lines and cultivars were generated.A lot of them are effects of CIMMYT (The International Maize and Wheat Improvement Center) and ICARDA (International Center for Agricultural Research in the Dry Areas) programs.Some steps were taken to transfer Yd2 gene to wheat, but no success has been achieved up to now (P l u m b , 2002).

Other natural resistance genes
Many other genetic sources of resistance are considered.Some of them are mentioned below.
In barley other genes than Yd2 usually give only moderate resistance.For example, in field tests in Czech Republic resistance in winter cultivars Pery and Sigra was found on the level of 5 in 9 steps scale (0 = no disease symptoms).When Yd2 cultivars were mostly found on the level of 3 (O v e s n a et al., 2000).In Thailand some minor resistance gene in barley were mapped on chromosomes 1 (7H), 4(4H), and 5 (1H) without significant interactions (To o j i n d a et al., 2000).
In wheat genotype no resistance, but only some tolerance, was observed.Wheatgrass (Thinopyrum intermedium, sometimes other Latin names are used) was lately found to be a good source of resistance for wheat.Broad research programs has been developed including mapping projects.Main genes of resistance have been named Bdv1 and Bdv2 .Unfortunately, cultivar Frontana with the gene Bdv1 did not show even moderate resistance in field test in Czech Republic.Some Brazilian cultivars with Rht (reduced height) gene were promising (Va c k e et al., 1996 a).
Although there was no broad genetic research program for BYDV resistance in oat, many cultivars characterised as resistant were generated and are available now.Mapping experiments were developed on University of Wisconsin (Z h u et al., 2003).Many combinations of resistance genes are available (B a r b o s a et al., 2000).Avena sterilis and its offspring lines are good source of resistance and are used from 70 s (F r e y, 1994).
The resistance to aphids can be also valuable for BYD control.

Genetic modifications
Genetic modifications are a powerful tool in breeding for resistance.The pathogen derived resistance is mainly used in case of viruses, also in case of BYDV.
Here are some examples of this type of research: oat transformed with the RNA-polymerase (K o e v et al., 1998), oat and barley transformed with coat protein sequences (M c G r a t h et al., 1997), wheat transformed with replicase, coat-protein, movement protein and a non-coding sequences (D u p r e et al., 2002).
Barley was transformed with transgene designed to produce hairpin (hp) RNA containing BYDV-PAV sequences and extreme resistance was confirmed (Wa n g et al., 2000).This method could be used for other cereals.
European and Polish law forbids any practical use of these achievements.

Conclusions
Although, barley yellow dwarf disease is not a serious economic problem in Poland, the increase of infections observed in 2001 can be significant.In England, the spread of disease was stimulated by tendency to earlier sowing.The degree of losses was highly correlated with time of inoculation, especially with winter cereals .One of the reasons, that caused earlier sowing, was the expansion of production of oilseed rape (K n i g h t et al., 1996).The conclusion is, that problem of breeding for resistance to Barley yellow dwarf virus has not been solved yet.More concern should be taken when cereal cultivars will be estimated and registered to prevent possibility of increase of importance of this disease.
Polish cultivars have not been tested for resistance and there has been no breeding program for tolerance to BYDV (J e ¿ e w s k a , 2001b).Results from Czech Republic can be taken into consideration.At the Research Institute for Crop Protection most of the registered barley cultivars from Central Europe were found susceptible or high susceptible (O v e s n a et al., 2000).Resistance has not been found in tested varieties of spring wheat from Czech group (Va c k e et al., 1996 a).64 Czech oat varieties have been tested and only 2 new lines have shown high resistance on the average of control cultivar Ogle (Va c k e et al., 1996 b).

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