High natural variation of BvCR4 homologs in sea beet contrasts with narrow genetic base of sugar beet: Implications for Cercospora leaf spot resistance

Cercospora leaf spot (CLS) is a destructive disease of sugar beet (Beta vulgaris ssp. vulgaris L.) for which resistance conferred by the gene BvCR4 has been introgressed from sea beet (B. vulgaris ssp. maritima). BvCR4-mediated resistance has been broadly deployed to manage CLS globally in sugar beet; however, continuous implementation exerts strong selection pressure, increasing the risk of disease outbreaks due to resistance-breaking C. beticola populations. This study aimed to identify BvCR4 variants in public B. vulgaris spp. germplasm to assess their genetic and structural diversity. Polymerase chain reaction screening of 589 public B. vulgaris spp. germplasm detected BvCR4 homologs in 32 accessions with diverse responses to CLS including historically relevant sugar beet lines represented in breeding programs and Mediterranean sea beet populations of diverse genetic backgrounds. Analysis of the genomic sequences revealed higher diversity in sea beet compared to sugar beet. Additionally, population genetic analysis indicated that BvCR4 homologs in sugar beet likely originated from sea beet populations in Italy and Greece. We identified historical cultivated materials carrying full proteins encoded by BvCR4 alleles and delineated geographic regions where sea beet populations harbor BvCR4 homologs that may be useful in breeding programs. Structural variation was largely circumscribed within leucine-rich repeat domains, although CLS resistance phenotyping was not associated with specific polymorphisms, suggesting BvCR4-mediated resistance may rely on interactions with other co-receptors. This study provides resources for sugar beet improvement by enabling the identification of BvCR4 homologs in diverse germplasm and disclosing materials with potentially novel resistance.