INTRODUCTION: The genetic diversity of common wheat (Triticum aestivum) has decreased sharply due to the domestication and modern breeding operations, making it more vulnerable to the threats from pests and pathogens. Leaf rust, caused by the fungal pathogen Puccinia triticina (Pt), is a devastating disease in wheat. Over 80 leaf rust resistance (Lr) genes have been identified, with nearly half originating from wheat wild relatives. However, the rapid evolution of Pt has rendered many Lr genes ineffective against prevalent Pt races. Consequently, identifying novel sources of resistance in wild relatives of common wheat remains an urgent priority for sustainable wheat breeding.

RATIONALE: As one of the most widely used relatives in the genetic improvement of wheat, decaploid Thinopyrum ponticum shows excellent resistance to multiple diseases including leaf rust. By distant hybridization and chromosome engineering, we created a wheat-Th. ponticum line WTS135. We evaluated its disease resistance with Pt race THTT, developed Th. ponticum specific markers by specific-locus amplified fragment sequencing technology and assessed its agronomic traits by phenotypic investigation. Genomic in situ hybridization (GISH)-fluorescence in situ hybridization analysis (FISH) and liquid chip analysis have been used to identify its chromosome composition.

RESULTS: WTS135 is immune to the Pt race THTT. Pedigree analysis showed that this resistance originated from the exogenous chromosome of Th. ponticum. GISH-FISH analysis revealed that the wheat chromosomes 7D were replaced by the Th. ponticum-derived chromosomes. Liquid chip analysis showed that the alien chromosomes belonged to the homoeologous group 7, and the density and abundance of the signals in the peri-centromeric region were significantly lower, which was consistent with the GISH results. Therefore, it is indicated that WTS135 is a 7St (7D) disomic substitution line. After detected by the molecular markers related to known Lr genes on wheat 7D chromosome, it is speculated that WTS135 probably carries a novel resistance gene that is different from genes Lr19 and Lr29. Ten primers specific to Th. ponticum were developed to rapidly trace the exogenous chromatin in WTS135. Phenotypic investigation showed that the yield of WTS135 was not significantly different from that of the recurrent parent Jimai 22, suggesting that this line can be useful for improving disease resistance in wheat.

CONCLUSION: Introducing resistance genes from wild relatives into wheat through distant hybridization can broaden the genetic base of wheat and provide new sources for breeding disease-resistant varieties. We developed a common wheat-Th. ponticum 7St (7D) substitution line, which possibly has a novel alien resistance gene and could be used in the breeding for enhancing wheat disease resistance.

Chromosome composition and leaf rust resistance evaluation of WTS135. (A) GISH analysis using Thinopyrum ponticum gDNA as a probe and Chinese Spring gDNA as a block; (B) Mc-FISH analysis using combined oligo probes; (C) The liquid chip analysis of WTS135; (D) Evaluation for leaf rust resistance in WTS135 and its parents (1: WTS135; 2: Xiaoyan 81; 3: Jimai 22, 4: Zhongnong 28, 5: Th. ponticum). The white arrows indicate exogenous chromosomes, purple frames indicate chromosome additions or deletions.