How scion/rootstock biometric, anatomical, and DNA methylation dynamics affect citrus-Candidatus Liberibacter asiaticus interaction

Abstract

Huanglongbing (HLB), associated in the Americas mainly with the α-proteobacteria ‘Candidatus Liberibacter asiaticus’ (CLas), is the most destructive disease of citrus worldwide. This study investigated the impact of reciprocal grafting combinations on citrus plants infected with HLB compared to mock-inoculated ones in greenhouse conditions, focusing on biometric, anatomical, and epigenetic responses to 'Candidatus Liberibacter asiaticus' (CLas) colonization. Valencia sweet orange (SwO) and Pomeroy trifoliate orange (TrO) were reciprocally grafted as scion/rootstock, and double scion combinations with both genotypes on the same rootstock were used too. Regardless of the rootstock, results showed that SwO scions exhibited higher susceptibility to HLB, with 70–75% of plants showing infected leaves and roots compared to 28–61% in TrO scions. SwO scions also displayed reduced shoot growth (73% lower) and root weight (27% lower) when infected. Callose deposition was higher in SwO but was influenced by the rootstock genotype. Considering methylated and hemi-methylated bands altogether, CLas-inoculated plants had higher methylation frequencies over time than the mock-inoculated ones, and a higher percentage (52%) of methylated profiles was observed in SwO leaves than TrO leaves. Importantly, using TrO as scion resulted in less disease damage than SwO, emphasizing scion resistance as crucial for HLB management. Interestingly, grafting both scion varieties on the same plant (rootstock) did not offer significant advantages over single scion use. These findings underscore the importance of scion selection for HLB resistance in future genetic improvement strategies.