Lr67 and Lr34 rust resistance genes have much in common--they confer broad spectrum resistance to multiple pathogens in wheat

Abstract

Background: Adult plant rust resistance genes Lr67 and Lr34 confer race non-specific resistance to multiple fungal pathogens of wheat. Induced, susceptible mutants were characterised for both genes.

Results: Three categories of Lr34 mutants were identified that were either partial susceptible, fully susceptible or hyper-susceptible to stripe rust and leaf rust. The likely impact of the mutational change on the predicted Lr34 protein correlated with differences in response to rust infection. Four independent Lr67 mutants were recovered that were susceptible to stripe rust, leaf rust and stem rust pathogens, including one possible hyper-susceptible Lr67 mutant.

Conclusions: Detailed study of Lr34 mutants revealed that subtle changes in resistance response to multiple pathogens were correlated with mutational changes in the predicted protein. Recovery of independent Lr67 mutants indicates that as for Lr34, a single gene at the Lr67 locus is likely to confer resistance to multiple pathogens. The infection phenotypes of Lr67 mutants closely resembled that of Lr34 mutants.

Figure 1

Comparison of chitin levels in flag leaves of Lr34 and Lr67 mutants together with respective wild type RL6058 and RL6077 after infection with stripe rust. Wheat lines are indicated on the X axis while relative chitin content is equivalent to the fluorescence values indicated on the Y axis in fluorescence units. Error bars show the standard error of mean of 4 technical replicates. For mutant 2F, a resistant sib line (2F res) was analysed from the same M2 family.

Figure 2

Leaf rust infection on wheat first leaves at 25 days, 28 days and 31 days post inoculation at low temperature for RL6058 (Lr34) and three derived mutants 2G, 3E and 2F. These three mutants show partial susceptibility, full susceptibility and hyper-susceptibility to leaf rust, respectively. Infection type of susceptible Thatcher was similar to mutant 3E and leaf rust infection of a resistant sib of mutant 2F was similar to RL6058 (not shown).

Figure 3

Lr34 gene structure with open boxes indicating exons and the position of mutations was marked. RT-PCR of RNA extracted from flag leaf of mutant 2F amplified two transcripts. Sequencing of products confirmed the wild type Lr34 transcript of 1,571 bp (a) and a mis-spliced product with exon 23 (255 bp) removed (b)[15].

Figure 4

Diagram of predicted Lr34 protein showing nucleotide binding domains NBD1 and NBD2 and two transmembrane domains. The Lr34 protein differs from the protein encoded by the Thatcher allele by the absence of a phenylalanine residue (F) and a substitution of histidine (H) for tyrosine (T) in first transmembrane domain. Based on different responses to leaf rust and stripe rust infections, mutants were placed into three categories of susceptibility; partial susceptibility caused by single amino acid changes in the first or second NBD (mutants 2G, 4C and 4G), full susceptibility, similar to Thatcher, in which mutants were predicted to encode severely truncated and probably non-functional proteins (mutants 2B, 3E, 3F and 4E) and hyper-susceptibility (more susceptible than Thatcher) represented by mutant 2F which encoded a protein with a deletion of 85 amino acids in the second transmembrane domain.

Figure 5

Stripe rust infection on flag leaves of field grown of Thatcher (Tc, susceptible control), RL6077 (Lr67) and derived mutants M1723, M2255 and M2257. M2255 was more susceptible to stripe rust than Thatcher.

Figure 6

Stem rust infection on the peduncle of field grown PI 250413 (donor Lr67), Thatcher (Tc), RL6077 (Lr67) and derived mutants M1723, M2255 and M2257. M2255 was more susceptible to stem rust than Thatcher.