Resistance to ' Candidatus Liberibacter asiaticus,' the Huanglongbing Associated Bacterium, in Sexually and/or Graft-Compatible Citrus Relatives

Abstract

Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated 'Candidatus Liberibacter asiaticus' (Las) becomes strategic to warrant crop sustainability, but no resistant Citrus genotypes exist. Some Citrus relatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible with Citrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible with Citrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately these Citrus relatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However, Eremocitrus glauca and Papua/New Guinea Microcitrus species as well as their hybrids and those with Citrus arose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistant Citrus rootstocks or interstocks.

Keywords: Aurantioideae; Citrus breeding; Eremocitrus; Greening; HLB; Microcitrus; Rutaceae.

FIGURE 1

Representative photographs of each of the selected Citrinae genotypes/accessions [cross-compatible (A–T); and cross-incompatible, putative graft-compatible (U–Y) with Citrus about 1 year after grafting onto ‘Rangpur’ lime rootstock]. (A) Citrus × sinensis ‘Pera’; (B) C. × sinensis ‘Tobias’; (C) C. halimii; (D) Poncirus trifoliata ‘Pomeroy’; (E) P. trifoliata ‘Benecke’; (F) P. trifoliata ‘Barnes’; (G) P. trifoliata ‘Rubidoux’; (H) Microcitrus australasica; (I) M. australasica ‘Sanguinea’; (J) M. australasica ‘True Sanguinea’; (K) ‘Faustrimedin’ hybrid; (L) M. inodora; (M) M. warburgiana; (N) M. papuana; (O) M. australis; (P) M. virgata hybrid; (Q) Microcitrus sp. × E. glauca hybrid; (R) Eremocitrus glauca; (S) E. glauca × C. × sinensis hybrid; (T) E. glauca × Microcitrus sp. hybrid; (U) Atalantia citroides; (V) A. ceylanica; (W) Limonia acidissima; (X) Citropsis gilletiana; (Y) Naringi crenulata.

FIGURE 2

(A) An Eremocitrus glauca × Citrus × sinensis scion propagated on ‘Rangpur’ lime as an example of good rootstock/scion compatibility between two genotypes. (B) Limonia acidissima scion propagated on ‘Rangpur’ lime showing overgrowth incompatibility at the graft union. (C) Profuse gum exudation in the graft union between ‘Valencia’ sweet orange budwood and Citrus halimii scion.

FIGURE 3

HLB-like symptoms in roots from ‘Rangpur’ lime rootstocks graft-inoculated either with Las-infected or healthy control budwood in composite plants with infected vs. healthy control scions from three susceptible, the partially resistant Microcitrus australasica and the full-resistant E. glauca × C. × sinensis genotypes at 12 months after challenge-inoculation. (A) Citrus × sinensis ‘Pera’, Las-infected; (B) C. × sinensis ‘Pera’, control; (C) C. × sinensis ‘Tobias’, Las-infected; (D) C. × sinensis ‘Tobias’, control; (E) Atalantia citroides, Las-infected; (F) Atalantia citroides, control; (G) Microcitrus australasica, Las-infected; (H) Microcitrus australasica, control; (I) E. glauca × C. × sinensis hybrid, Las-inoculated; (J) E. glauca × C. × sinensis hybrid, control.

FIGURE 4

Distribution on the phylogenetic tree of the Aurantioideae sub-family (NJ tree based on eight chloroplast genome fragments) based on accession responses to Las challenge inoculation according to this study (red text: susceptible, blue text: partially resistant, green text: full resistant) and Ramadugu et al. (2016) (red rectangle: susceptible –categories 6 to 8; blue rectangle: tolerant –categories 3, 4; green rectangle: resistant –categories 1, 2).