Stem rust resistance in wheat is suppressed by a subunit of the mediator complex

Colin W Hiebert,Tim Hewitt,Jan Vrána,Jianping Zhang,Miroslava Karafiátová,Jaroslav Doležel,Matthew N Rouse,Phon Green,Evans Lagudah,Sridhar Bhavani,Inmaculada Hernández-Pinzón ,Narayana M Upadhyaya ,Li Huang,R A Mcintosh ,Yue Jin,Vincent Pujol,Brande B H Wulff,Peng Zhang,T Fetch ,Matthew J Moscou,Burkhard Steuernagel,Wolfgang Spielmeyer

doi:10.1038/s41467-020-14937-2

Abstract

Stem rust is an important disease of wheat that can be controlled using resistance genes. The gene SuSr-D1 identified in cultivar ‘Canthatch’ suppresses stem rust resistance. SuSr-D1 mutants are resistant to several races of stem rust that are virulent on wild-type plants. Here we identify SuSr-D1 by sequencing flow-sorted chromosomes, mutagenesis, and map-based cloning. The gene encodes Med15, a subunit of the Mediator Complex, a conserved protein complex in eukaryotes that regulates expression of protein-coding genes. Nonsense mutations in Med15b.D result in expression of stem rust resistance. Time-course RNAseq analysis show a significant reduction or complete loss of differential gene expression at 24 h post inoculation in med15b.D mutants, suggesting that transcriptional reprogramming at this time point is not required for immunity to stem rust. Suppression is a common phenomenon and this study provides novel insight into suppression of rust resistance in wheat.

Highlights

Stem rust is an important disease of wheat that can be controlled using resistance genes
Seedlings of TC, CTH, and NS1 and NS2 showed clear phenotypic differences when inoculated with Pgt race QTHJC (C25), with the resistant response of the mutant lines showing no sporulation whereas the wildtype CTH showed a susceptible response with large sporulating pustules at the seedling stage (Fig. 1a)
Inoculating with Pgt races from the Ug99 group allowed some rust pustules to develop on mutants, whereas Pgt race TRTTF-elicited a susceptible response on CTH, NS1, and NS2 indicating that the resistance expressed in SuSr-D1 mutant lines was race-specific (Fig. 1b; Supplementary Table 1)

Summary

Introduction

Stem rust is an important disease of wheat that can be controlled using resistance genes. Suppression of stem rust resistance was first reported in 1980 when it was discovered that the loss of the D genome chromosomes activated resistance to several Pgt races that were virulent to the hexaploid wheat cultivar “Canthatch” (CTH)[23]. Reintroduction of the D-genome by crossing Tetra CTH with Ae. tauschii to create a synthetic hexaploid (A, B, and D genomes) resulted in a loss of resistance that was the same as CTH23 This example is analogous to instances of Sr genes from tetraploid wheat being suppressed when transferred to hexaploid wheat[11,12,13]. We establish, through genetic mapping, sequenced flowsorted chromosomes, and mutational analysis that SuSr-D1 encodes Med15b.D, a subunit of the conserved Mediator complex that regulates transcription of protein-coding genes in eukaryotic organisms. As suppression of disease resistance is frequent, the isolation of SuSr-D1 and understanding its mechanistic role will contribute to developing approaches to understand how the subgenomes of wheat interact, and how this contributes to a b

Methods

Results

Conclusion