Beat Keller, University of Zurich

1. How has Lr34/Yr18 conferred effective rust
resistance in wheat for so long?
BGRI Technical Workshop 2012, Beijing, 2.9.2012
Beat Keller
Institute of Plant Biology, University of Zurich, Switzerland

2. The Holy Grail of resistance breeding:
Durable resistance
Resistance is defined as „durable“ when it remains effective in cultivars
that are widely grown for long periods and in environments favorable to
the disease (Johnson 1983).
There are single, major resistance genes which are durable, but these are
exceptions
Durable resistance is mostly polygenic and caused by genes acting
quantitatively

3. Lr34: a durable leaf rust resistance gene
R S
Thatcher + Lr34 Thatcher

4. Lr34 is a durable disease resistance gene
• Has been effective against leaft and stripe rust for more than
100 years in the field, certainly 40 years at large scale
• Confers partial resistance
• Prolongs the latency period and reduces the production of
spores (slow-rusting gene)
• Is associated with the morphological trait Leaf Tip Necrosis
(Ltn1)
• Is not race specific

5. Lr34 confers resistance against multiple pathogens
Lr34
Pm38 Yr18
All these effects are caused by a
single gene
Bdv1

6. Durable leaf rust resistance in the Swiss cultivar ‘Forno’
The Lr34 gene is also an important determinant of durable resistance in
winter wheat, i.e. it is active in a broad germplasm and highly differing
climatic conditions
cv.‘Arina‘: Swiss winter wheat, susceptible cv.‘Forno‘: Swiss winter wheat, durably resistant

7. Documented history of Lr34 begins at the end of the 19th century in Italy
and is restricted to hexaploid wheat
China
Italy
Marco Polo?
Adapted from Kolmer et al. 2008. Crop Science. Pedigree of Lr34/Yr18 cultivar entries in different wheat
breeding programs.

8. Deployment of Lr34: the example of Canadian germplasm
Lr34 is present in many cultivars released since the 1970ies, but
not in older cultivars
McCallum et al. Euphytica, 2012

9. Proportion of the total Canada Western Red Spring seeded area which was
seeded to Lr34 carrying cultivars. Data from Canadian Wheat Board varietal
surveys (McCallum and DePauw 2008). No prairie wide variety surveys were
conducted 1993–1997

10. Map-based cloning of the Lr34 gene
Lr34
Wheat chrom. 7D 7DS 7DL
XSWSNP2
XSWDEL1
XSWDEL2
XSWDEL3
Xgwm1220 XSWSNP3 Lr34 XcsLVE17 XSWM10
Genetic map
chromosome 7DS
0.75 0.12 0.03 0.03
SWDEL1 SWDEL2 SWSNP2 SWDEL3
Hexose Cytochrome Cytochrome Glycosyl
carrier P450 P450 transferase (Ψ)
Physical map
‘Chinese Spring’
(363 kb) ABC Cysteine
transporter Lectin receptor proteinase (Ψ)
120,000 363,640
kinase
Eight open reading frames within a 363 kb target interval

11. Eight Lr34 mutants: for example splice site mutations
Hexose Cytochrome Cytochrome Glycosyl
carrier P450 P450 transferase (Ψ)
ABC Cysteine
transporter Lectin receptor proteinase (Ψ)
120,000
kinase
363,640
ATG TAA
4E 3E m19 2B 2G 4C 2F m21
1 kb
Infection experiments revealed that the mutants are more susceptible to leaf
rust, stripe rust, powdery mildew and stem rust and lost leaf tip necrosis

12. The molecular basis of the resistance effect of the Lr34 gene: Lr34
encodes a putative ABC transporter (ABCG or PDR) protein
What about Lr34-type of gene in susceptible lines (orthologous
region in susceptible lines)?
àSequence analysis in reference wheat lines with and without Lr34
Findings:
Lr34- lines also contain the ABC transporter coding gene (allele
Lr34sus)
There are only three sequence differences between Lr34res and
Lr34sus lines. Because of the dominant nature of Lr34res, these are
gain-of-function mutations

13. Sequence differences between susceptible and
resistant Lr34 alleles
ATG TAA
Lr34res: 5’-CCGACTT-3’ Lr34res: 5’-TCC ATC ATG-3’ Lr34res: 5’-TCG CAG CAT CGA-3’ 1 kb
Lr34sus: 5’-CCGTCTT-3’ Lr34sus: 5’-TCC ATC TTC ATG-3’ Lr34sus: 5’-TCG CAG TAT CGA-3’
Deletion of a phenylalanine Conversion of tyrosine to histidine
residue in ‘Chinese Spring’ in ‘Chinese Spring’
Resistant and susceptible allele differ by only two amino acid polymorphisms

14. Development and application of molecular markers for the
Lr34 gene:
• Perfect markers derived from the gene sequence
• Markers have been adopted in the last two years in most
wheat breeding programs worldwide where leaf and/or
stripe rust is of relevance

15. Functional studies of the Lr34 gene in transgenic wheat: cold-
treated seedlings
Seedlings of transformed
Bobwhite
Cold treatment at 4oC
Risk, Selter et al. 2012
Plant Biotech. J.

16. Functional studies of the Lr34 gene in transgenic
wheat: flag leaf of adult plant
Risk, Selter et al. 2012
Plant Biotech. J.

17. Microscopic analysis of leaf rust infection using WGA-alexa
staining
Risk, Selter et al. 2012
Plant Biotech. J.

18. Leaf tip necrosis is identical in Lr34 transgenic wheat
Risk, Selter et al. 2012
Plant Biotech. J.

19. Genetic background of transgenic wheat plays an important role:
seedling resistance in BW26SUI but not BW26AUS at 20oC
à
Additive interaction(s) with other gene(s)
Risk, Selter et al. 2012
Plant Biotech. J.

20. Conclusions from transgenic wheat lines with Lr34
• The transgene is fully functional against leaf rust and results in LTN
• This complementation demonstrates that Lr34 is indeed pleiotropic
• The genetic background can result in improved resistance: one
should broadly cross the most active transgene into a broad set of
breeding material (…normal breeding procedure…)
• Combination with other durable resistance genes in a cassette or by
crossing seems a good strategy

21. Why durable, how does the gene confer resistance?
Some evolutionary findings and conclusions

22. PDRs in Arabidopsis and rice
There are 15 PDRs in Arabidopsis and 23 PDRs in rice
Closest homolog of Lr34 in rice: PDR23 (86% identity at AS level)
second hit: OsPDR2 (55% identity)
in Arabidopsis : PDR5 / PDR9 (56% identity at AS level)
There is one clear Lr34 homolog in rice, but none in barley, Brachypodium and
Arabidopsis

23. Observations from the molecular nature of LR34:
• It is not an NBS-LRR encoding protein (such as e.g. Lr1, Lr10,
Lr21, the other cloned Lr resistance genes)
• This suggests a completely different molecular mechanism of
resistance
• For example by reducing the quality of the leaves as «food»
for biotrophic pathogens
• Transporting an antimicrobial substance (phytoalexins)
• Priming of resistance response?

24. Lr34 encodes an ATP-binding cassette (ABC) transporter
Transporters sharing a common basic structure: Nucleotide binding fold (NBF)
Trans-membrane domain (TMB)
Lr34 belongs to the subfamily of the pleiotropic drug resistance (PDR) transporters
PDRs are only found in plants and fungi.
???
TMD TMD TMD TMD plasma membrane
NBF NBF NBF NBF
ATP
substrate
ADP + P
Lr34 protein: 1402 amino acids

25. Structure of the ABC exporter Sav1866 from Staphylococcus
aureus with bound nucleotide (Dawson and Locher 2006). Source:
http://en.wikipedia.org/wiki/ATP-binding_cassette_transporter.
Most exporters in prokaryotes, such as the multidrug exporter
Sav1866, are made up of a homodimer consisting of two half-size
transporters.

26. What is Lr34 doing?
• Lr34 confers resistance against multiple obligate biotrophic pathogens.
• Lr34 is associated with reduced intercellular hyphal growth but not with a
hypersensitive response or papilla formation.
• The level of Lr34-mediated resistance for leaf rust infection correlated with the
development of leaf tip necrosis.
These observations suggest that Lr34 could be due to a general
physiological effect.
Microarray studies revealed that a similar set of genes was up-regulated in
uninfected flag leaves of Lr34 containing near-isogenic lines and senescing flag
leaves.
A. Senescence-related resistance?

27. Leaf tip necrosis in Lr34 containing lines

28. HvS40: a barley gene that is up-regulated during
leaf senescence

29. Nonfluorescent chlorophyll catabolites (NCC): hallmarks
of leaf senescence
Lr34 regulates senescence-like processes in the flag leaf of resistant plants.

30. Transcriptomics on Lr34 lines
Two transcriptomic studies on uninfected and infected wheat leaves with or
without Lr34 were made (Hulbert et al. 2007, Phytopathology; Bolton et al.
2008, MPMI)
Main conclusions:
1. After infection, there is a high demand of cellular energy (increased carbon
flux)
2. Expression of defense genes was often higher in resistant plants: is Lr34
involved in priming of defense responses?
B. Resistance priming BEFORE infection?

31. PEN3/PDR 8 contributes to non-host resistance to inappopriate pathogens
(Stein et al, Plant Cell 2006)
Pen3 mutants of Arabidopsis were more susceptible to infection with barley powdery
mildew, pea powdery mildew and potato late blight.
Lipka et al.,
Current
Opinon in
Plant Biology,
2008
C. Transport of an antimicrobial metabolite?

32. From the FIELD
To the FIELD!

33. Acknowledgments
University of Zürich CSIRO Canberra, Australia
Liselotte Selter Joanna Risk
Simon Krattinger Evans Lagudah
Thomas Wicker
Chauhan Harsh
IPK Gatersleben
Jochen Kumlehn
Götz Hensel
This work was supported by the European Research Council, GRCD and the
Swiss National Science Foundation