* Representing and correspondence author: yangping@caas.cn

Plant viruses account for almost 50% of the pathogens responsible for emerging and re-emerging plant diseases worldwide. Plant RNA viruses usually encode few proteins, and these success of viral infection depends on the deployment of host cell machineries, including host-encoded virus-compatible proteins called susceptibility factors (S genes).

Modification of S genes causes loss of susceptibility or recessive resistance. The yellow mosaic disease of wheat, caused by wheat yellow mosaic virus (WYMV) that transmitted by the soil-borne plasmodiophorid Polymyxa graminis, is a lasting and disaster threat in several countries of East Asia. Breeding for resistance varieties is the only optimal strategy for virus control.

By taking advantage of the former studies in barley that reported the susceptibility factor genes HvPDIL5-1 and HveIF4E against BaYMV/BaMMV, both in genus of Bymovirus where WYMV is present, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) technology was deployed to edit the orthologous genes TaPDIL5-1 and TaeIF4E in hexaploid wheat.

Simultaneous knockout of three homoeoalleles at either TaPDIL5-1 or TaeIF4E resulted in complete WYMV resistance, while the single- and double-mutants remained susceptible (yellow discoloration and delayed growth).These approved that the recessive resistance in polyploid species was blinded by functional redundancy of the homoeoalleles.

In addition, knockout of either of both genes is likely able to impair viral hijacking of the host-encoded partners in the photosynthesis pathway. No penalty in TaPDIL5-1 edited lines was observed, whereas an increase on plant height and delayed heading date were observed in TaeIF4E knockout lines. Several species in the genera Triticum and Aegilops were found showing the variations on WYMV accumulation, suggesting the occurrence of WYMV resistance in wheat progenitors and relatives.

Collectively, these results demonstrate a strategy to recover recessive resistance genes against viruses in hexaploid wheat by identifying the susceptibility genes in its diploid progenitors (e.g. T. urartu and A. tauschii) or relatives (e.g. A. speltoides, T. monococcum and H. vulgare), followed by manipulation of their homoeologues in transformable elite varieties via genome editing.