Identification of citrus immune regulators involved in defence against Huanglongbing using a new functional screening system

Abstract

Summary Huanglongbing (HLB) is the most devastating citrus disease in the world. Almost all commercial citrus varieties are susceptible to the causal bacterium, Candidatus Liberibacter asiaticus (CLas), which is transmitted by the Asian citrus psyllid (ACP). Currently, there are no effective management strategies to control HLB. HLB‐tolerant traits have been reported in some citrus relatives and citrus hybrids, which offer a direct pathway for discovering natural defence regulators to combat HLB. Through comparative analysis of small RNA profiles and target gene expression between an HLB‐tolerant citrus hybrid (Poncirus trifoliata × Citrus reticulata) and a susceptible citrus variety, we identified a panel of candidate defence regulators for HLB‐tolerance. These regulators display similar expression patterns in another HLB‐tolerant citrus relative, with a distinct genetic and geographic background, the Sydney hybrid (Microcitrus virgata). Because the functional validation of candidate regulators in tree crops is always challenging, we developed a novel rapid functional screening method, using a C. Liberibacter solanacearum (CLso)/potato psyllid/Nicotiana benthamiana interaction system to mimic the natural transmission and infection circuit of the HLB complex. When combined with efficient virus‐induced gene silencing in N. benthamiana, this innovative and cost‐effective screening method allows for rapid identification and functional characterization of regulators involved in plant immune responses against HLB, such as the positive regulator BRCA1‐Associated Protein, and the negative regulator Vascular Associated Death Protein.

Insights into the mechanism of Huanglongbing tolerance in the Australian finger lime (Citrus australasica)

“…There are seven species of Australian limes that are all native to Australia and the New Guinea islands (Forster and Smith, 2010). Australian lime species such as Citrus australasica, and the hybrid of Citrus australis and Citrus virgata (Sydney hybrid) have been reported to be HLB resistant (Ramadugu et al, 2016;Alves et al, 2020;Huang et al, 2021a). Thus, these species can provide pathogen resistancerelated genes that can be used to confer HLB tolerance into conventional citrus cultivars to produce HLB-tolerant citrus hybrid scions and rootstocks.…”

An Overview of the Mechanisms Against “Candidatus Liberibacter asiaticus”: Virulence Targets, Citrus Defenses, and Microbiome

“…Sugar Belle contained high level of secondary metabolites such as volatile organic compounds (VOC), phenolics, and flavonoids (Killiny et al, 2017;Hijaz et al, 2020;Deng et al, 2021;Huang et al, 2021b).…”

“…In addition, amino acids including phenylalanine, tyrosine, and tryptophan were accumulated on HLB-tolerant citrus relative P. trifoliata ( Killiny and Hijaz, 2016 ), which are involved in synthesis of many secondary metabolites. Furthermore, several studies demonstrated that HLB-tolerant citrus including US-942 ( P. trifoliata×Citrus reticulata ), Curry leaf [ Murraya koenigii (L.) Spreng], and LB8-9 Sugar Belle contained high level of secondary metabolites such as volatile organic compounds (VOC), phenolics, and flavonoids ( Killiny et al, 2017 ; Hijaz et al, 2020 ; Deng et al, 2021 ; Huang et al, 2021b ).…”

Molecular signatures between citrus and Candidatus Liberibacter asiaticus

“…trifoliata × C . reticulata ) and a susceptible citrus variety identified a panel of candidate defense regulators for plant immune responses against HLB, such as the positive regulator BRCA1-associated protein and the negative regulator vascular-associated death protein [ 172 ]. C Las infection and D .…”