Resistance to the wheat curl mite and mite-transmitted viruses: challenges and future directions

TLDR: Genome sequencing will be critical to fully characterize the genetic diversity in WCM and WSMV populations to better understand the occurrence of WCM-transmitted viruses and to evaluate the potential stability of resistance genes.

Abstract: Wheat curl mite (WCM) is the only known arthropod vector of four wheat viruses, the most important of which is Wheat streak mosaic virus (WSMV). Host resistance to WCM and WSMV is limited to a small number of loci, most of which are introgressed from wild relatives and are often associated with linkage drag and temperature sensitivity. Reports of virulent WCM populations and potential resistance-breaking WSMV isolates highlight the need for more diverse sources of resistance. Genome sequencing will be critical to fully characterize the genetic diversity in WCM and WSMV populations to better understand the incidence of WCM-transmitted viruses and to evaluate the potential stability of resistance genes. Characterizing host resistance genes will help build a mechanistic understanding of wheat-WCM-WSMV interactions and inform strategies to identify and engineer more durable resistance sources.

Figures

Fig. 6. Response of wheat varieties and CSU advanced breeding lines to natural infection of wheat streak mosaic virus (WSMV) in an 867 irrigated variety trial. Bars indicate mean of three biological replicates ± SE log copies of WSMV per variety. Circles indicate average 868 WSMV visual rating on a 0-9 scale where 1= no damage and 9=severe damage. Different letters indicate significant differences 869 between varieties at P<0.05. 870

Fig. 2. Map of occurrence of wheat curl mite-transmitted viruses in eastern Colorado during 815 2019 as determined by RT-qPCR analysis Symptomatic leaf tissues were collected from various 816 locations or obtained from extension agents and growers. The number of samples tested in each 817 county ranged from one to six. Map was generated using mapchart.net. 818

Table 1. Primers used for PCR analysis in this study. 733

Fig. 3. Phylogenetic tree of wheat streak mosaic virus (WSMV) isolates using WSMV-NIb 847 region. Scale bar indicates percent genetic distances. Phylogenetic analysis by maximum 848 likelihood method was based on a sequence alignment using ClustalW in MEGAX. Bootstrap 849 values less than 65% out of 1000 replicates are not shown. 850

Table 4. Presence of virus (Wsm2) and mite resistance (WCM 6D) genes in varieties tested at the 742 Colorado State University Irrigated Variety Performance Trial. Blanks indicate that presence of 743 resistance marker was not tested in these lines. 744

Fig. 4. Amino acid variability in WSMV polyprotein amino acid sequences of Colorado isolates compared to seven Kansas isolates 854 (Fellers et.al. 2019) and the reference strains, Sidney81 (AF057533) and Type (AF285169). Black box highlights common change 855 between Sidney 81 and the other isolates reported in Fellers et.al. 2019. Alignments and visualization was performed using ClustalW. 856