Since no Huanglongbing (HLB)-resistant citrus cultivar is available in the world, selection of elite natural mutants of commercial citrus for HLB-resistance/tolerance becomes a much more appealing breeding approach, especially in HLB-epidemic regions. In this study, we have selected and evaluated more than 30 citrus mutants from commercial citrus varieties in the past eight years in Florida. After greenhouse and field trials with high HLB disease pressure, we have identified several citrus lines with improved HLB-resistance/tolerance, which can be released or used for large scale of field trials. Our analyses of these lines have revealed that citrus resistobiome plays a role in the HLB resistance/tolerance, which involves a plant virus that can enhance plant resistance and illustrated the pursuit of breeding for biocontrol and a healthy microbiome. Meanwhile, we revealed that transposons have driven the selection and diversification of sweet orange (SWO). We identified six transposon families with up to 8900-fold activity increases in modern sweet orange cultivars tracing back to a common ancestor ~500 years ago. Notably, these six families of transposons contribute significantly to the formation of major cultivar groups, with frequent independent activations or accelerations observed in the breeding history of SWO. We will discuss the molecular mechanisms underlying the improved HLB-resistance, especially how the resistobiome plays a role in the improved HLB resistance/tolerance, and how to implement this new approach by utilizing and expanding the breeding of citrus resistobiome for the control of citrus HLB.
