Can an HLB-resistant interstock block the long-distance movement of 'Candidatus Liberibacter asiaticus' within citrus trees?

Josiane Cecília Darolt^1,2Laudecir Raiol-Junior^1,2Everton Carvalho^1,2Mônica Neli Alves²Nelson Arno Wulff²Jémima Bagéa³Olivier Gros⁴Barbara HUFNAGEL³Raphael Morillon³Leandro Peña⁵Eduardo Augusto Girardi^1,2*

• ¹Brazilian Agricultural Research Corporation (EMBRAPA), Brasília, Brazil
• ²Fundo de Defesa da Citricultura, Araraquara, Brazil
• ³Centre de coopération International en Recherche Agronomique pour le Développement (CIRAD), Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), Institut Agro, Montpellier, France
• ⁴Universite des Antilles, Pointe-a-Pitre, Guadeloupe
• ⁵Ingenio CSIC-UPV, Valencia, Spain

Among citrus diseases, Huanglongbing (HLB) is recognized as the most destructive and economically damaging worldwide. It is mainly associated with 'Candidatus Liberibacter asiaticus' (CLas) being transmitted by Diaphorina citri. There are no curative treatments or commercial citrus varieties resistant to CLas. Aurantioideae species have been widely screened and, recently, Oceanian genotypes graft-and sexually compatible with Citrus were identified as HLB-resistant; however, there is no information regarding their use as interstocks of commercial varieties yet. Under greenhouse conditions, six HLB-resistant genotypes were evaluated as interstocks between 'Valencia' sweet orange scion and 'Rangpur' lime rootstock, both susceptible to HLB, with 'Valencia' interstock as control. Rootstocks were nucellar seedlings, and the scion was a commercial accession preimmunized with a mild CTV strain. Plants were assessed for CLas infection and titer in leaves, stem bark, and roots up to 12-24 months after graft-inoculation in the scion and compared to non-inoculated controls. Furthermore, molecular, anatomical, and biometric variables were investigated. As expected, the scion variety was colonized by CLas regardless of the genotype evaluated as interstock. Although bacteria were detected in the roots of most CLas-inoculated plants, CLas movement from the scion to the roots was blocked in 42 % and 86 % of composite plants when using a F1 hybrid of C. australis × C. inodora or an admixture hybrid of C. glauca, C. australis, and C. australasica as interstocks. Overall, CLas titers were similar in infected plant tissues among the evaluated genotypes, but titers were lower in interstock bark tissues compared to scion and rootstock ones. The dry weight of the root system was decreased by 50% in CLas-infected trees compared to control trees for most genotypes, and CLas + CTV infection was associated with changes in the sieve phloem and gene expression. These findings suggest that interstocks derived from some hybrids of Australian citrus types have the potential to restrict the movement of CLas from the scion into the roots of infected citrus trees. Long-term evaluation of composite plants in field conditions is necessary to assess tree performance and, ultimately, the impact of CLas blockage by interstocking on HLB disease damage.