AUTHOR=Alves Mônica N. , Raiol-Junior Laudecir L. , Girardi Eduardo A. , Miranda Maéva , Wulff Nelson A. , Carvalho Everton V. , Lopes Sílvio A. , Ferro Jesus A. , Ollitrault Patrick , Peña Leandro 

TITLE=Insight into resistance to ‘Candidatus Liberibacter asiaticus,’ associated with Huanglongbing, in Oceanian citrus genotypes

JOURNAL=Frontiers in Plant Science

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.1009350

DOI=10.3389/fpls.2022.1009350

ISSN=1664-462X

ABSTRACT=<p>Huanglongbing (HLB), the most destructive citrus disease, is associated with unculturable, phloem-limited <italic>Candidatus</italic> Liberibacter species, mainly <italic>Ca.</italic> L. asiaticus (Las). Las is transmitted naturally by the insect <italic>Diaphorina citri</italic>. In a previous study, we determined that the Oceanian citrus relatives <italic>Eremocitrus glauca</italic>, <italic>Microcitrus warburgiana</italic>, <italic>Microcitrus papuana</italic>, and <italic>Microcitrus australis</italic> and three hybrids among them and <italic>Citrus</italic> were full-resistant to Las. After 2 years of evaluations, leaves of those seven genotypes remained Las-free even with their susceptible rootstock being infected. However, Las was detected in their stem bark above the scion-rootstock graft union. Aiming to gain an understanding of the full-resistance phenotype, new experiments were carried out with the challenge-inoculated Oceanian citrus genotypes through which we evaluated: (1) Las acquisition by <italic>D. citri</italic> fed onto them; (2) Las infection in sweet orange plants grafted with bark or budwood from them; (3) Las infection in sweet orange plants top-grafted onto them; (4) Las infection in new shoots from rooted plants of them; and (5) Las infection in new shoots of them after drastic back-pruning. Overall, results showed that insects that fed on plants from the Oceanian citrus genotypes, their canopies, new flushes, and leaves from rooted cuttings evaluated remained quantitative real-time polymerase chain reaction (qPCR)-negative. Moreover, their budwood pieces were unable to infect sweet orange through grafting. Furthermore, sweet orange control leaves resulted infected when insects fed onto them and graft-receptor susceptible plants. Genomic and morphological analysis of the Oceanian genotypes corroborated that <italic>E. glauca</italic> and <italic>M. warburgiana</italic> are pure species while our <italic>M. australis</italic> accession is an <italic>M. australis</italic> × <italic>M. inodora</italic> hybrid and <italic>M. papuana is</italic> probably a <italic>M. papuana</italic> × <italic>M. warburgiana</italic> hybrid. <italic>E. glauca</italic> × <italic>C. sinensis</italic> hybrid was found coming from a cross between <italic>E. glauca</italic> and mandarin or tangor. <italic>Eremocitrus</italic> × <italic>Microcitrus</italic> hybrid is a complex admixture of <italic>M. australasica</italic>, <italic>M. australis</italic>, and <italic>E. glauca</italic> while the last hybrid is an <italic>M. australasica</italic> × <italic>M. australis</italic> admixture. Confirmation of consistent full resistance in these genotypes with proper validation of their genomic parentages is essential to map properly genomic regions for breeding programs aimed to generate new <italic>Citrus</italic>-like cultivars yielding immunity to HLB.</p>