An interspecific barberry hybrid enables genetic dissection of non-host resistance to the wheat stem rust pathogen

Authors: BARTAULA, RADHIKA - University Of New Hampshire; MELO, ARTHUR - National Center For Agriculture And Forestry Technologies (CENTA); CONNOLLY, BRYAN - National Center For Agriculture And Forestry Technologies (CENTA); Jin, Yue; HALE, IAGO - University Of New Hampshire

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2018
Publication Date: 2/26/2018
Citation: Bartaula, R., Melo, A., Connolly, B., Jin, Y., Hale, I. 2018. An interspecific barberry hybrid enables genetic dissection of non-host resistance to the wheat stem rust pathogen. Journal of Experimental Botany. 69:2483-2493. https://doi.org/10.1093/jxb/ery066.
DOI: https://doi.org/10.1093/jxb/ery066

Interpretive Summary: Stem rust is one of the most devastating diseases of wheat and other small grains globally; and the emergence of new stem rust races virulent on deployed resistance genes brings urgency to the discovery of more durable sources of genetic resistance. Despite its intrinsic durability, non-host resistance (NHR) to stem rust remains largely unexplored as a protection strategy in wheat, perhaps in part due to the challenge of developing a genetically tractable system in which NHR segregates. In this study, we approach the question of NHR through the pathogen's alternate, or sexual, host, barberry (Berberis spp.). Specifically, we characterized a natural population of Berberis ×ottawensis (B×o), an inter-specific hybrid of stem rust-susceptible European barberry (B. vulgaris) and stem rust-resistant Japanese barberry (B. thunbergii), to determine if the nothospecies can be used to dissect the genetic mechanism(s) of the apparent NHR exhibited by Japanese barberry. Parents and hybrid individuals were phenotyped for disease response to stem rust via artificial inoculation and genotyped using SNPs loci called through a de novo genotyping-by-sequencing pipeline. Inoculation experiments demonstrated that the stem rust resistance of Japanese barberry segregates into resistant, susceptible, and intermediate categories in an F1 population of B×o individuals; and the resistance is not maternally inherited. The results indicate that genetic dissection of the resistance is feasible using this unique pathosystem. Motivated by these results, effort is being made to develop genomic resources and a bi-parental B×o mapping population, with the hope that the mechanism(s) of resistance in the alternate host might inspire novel strategies for durable stem rust resistance in wheat. The information will be useful for scientists developing new approaches in effective control of wheat stem rust.

Technical Abstract: Stem rust, caused by the macrocyclic fungal pathogen Puccinia graminis (Pg), is one of the most devastating diseases of wheat and other small grains globally; and the emergence of new stem rust races virulent on deployed resistance genes brings urgency to the discovery of more durable sources of genetic resistance. Despite its intrinsic durability, non-host resistance (NHR) to Pg remains largely unexplored as a protection strategy in wheat, perhaps in part due to the challenge of developing a genetically tractable system in which Pg-NHR segregates. In this study, we approach the question of Pg-NHR through the pathogen's alternate, or sexual, host, barberry (Berberis spp.). Specifically, we characterized a natural population of Berberis ×ottawensis C.K. Scheid (B×o), an inter-specific hybrid of Pg-susceptible European barberry (B. vulgaris L.) and Pg-resistant Japanese barberry (B. thunbergii DC.; Bt), to determine if the nothospecies can be used to dissect the genetic mechanism(s) of the apparent Pg-NHR exhibited by Bt. B×o individuals were phenotyped for disease response to Pg via artificial inoculation and genotyped using a de novo genotyping-by-sequencing (GBS) pipeline. Repeated inoculations of 105 B×o accessions revealed 54 (~52%) susceptible, 37 (~35%) resistant, and 14 (~13%) intermediate genotypes. GBS analysis on a random subset of 63 of these B×o accessions showed that 54 (~87%) were first generation (F1) hybrids, among which a similar proportion of susceptible (52%), resistant (31%), and intermediate (17%) reactions were observed. Results from additional inoculations of B×o full-sibs, developed via controlled crosses, ruled out the possibility of maternal inheritance of the segregating Pg resistance. This study demonstrates that the Pg-NHR exhibited by Bt segregates in an F1 population of B×o individuals; thus its genetic dissection is feasible using this unique pathosystem. Motivated by these results, investment is being made in the development of genomic resources and a bi-parental B×o mapping population, with the hope that insight into the mechanism(s) of resistance in the alternate host might inspire novel strategies of durable stem rust resistance in wheat.