The Candidatus Liberibacter–Host Interface: Insights into Pathogenesis Mechanisms and Disease Control

Abstract

"Candidatus Liberibacter" species are associated with economically devastating diseases of citrus, potato, and many other crops. The importance of these diseases as well as the proliferation of new diseases on a wider host range is likely to increase as the insects vectoring the "Ca. Liberibacter" species expand their territories worldwide. Here, we review the progress on understanding pathogenesis mechanisms of "Ca. Liberibacter" species and the control approaches for diseases they cause. We discuss the Liberibacter virulence traits, including secretion systems, putative effectors, and lipopolysaccharides (LPSs), as well as other important traits likely to contribute to disease development, e.g., flagella, prophages, and salicylic acid hydroxylase. The pathogenesis mechanisms of Liberibacters are discussed. Liberibacters secrete Sec-dependent effectors (SDEs) or other virulence factors into the phloem elements or companion cells to interfere with host targets (e.g., proteins or genes), which cause cell death, necrosis, or other phenotypes of phloem elements or companion cells, leading to localized cell responses and systemic malfunction of phloem. Receptors on the remaining organelles in the phloem, such as plastid, vacuole, mitochondrion, or endoplasmic reticulum, interact with secreted SDEs and/or other virulence factors secreted or located on the Liberibacter outer membrane to trigger cell responses. Some of the host genes or proteins targeted by SDEs or other virulence factors of Liberibacters serve as susceptibility genes that facilitate compatibility (e.g., promoting pathogen growth or suppressing immune responses) or disease development. In addition, Liberibacters trigger plant immunity response via pathogen-associated molecular patterns (PAMPs, such as lipopolysaccharides), which leads to premature cell death, callose deposition, or phloem protein accumulation, causing a localized response and/or systemic effect on phloem transportation. Physical presence of Liberibacters and their metabolic activities may disturb the function of phloem, via disrupting osmotic gradients, or the integrity of phloem conductivity. We also review disease management strategies, including promising new technologies. Citrus production in the presence of Huanglongbing is possible if the most promising management approaches are integrated. HLB management is discussed in the context of local, area-wide, and regional Huanglongbing/Asian Citrus Psyllid epidemiological zones. For zebra chip disease control, aggressive psyllid management enables potato production, although insecticide resistance is becoming an issue. Meanwhile, new technologies such as clustered regularly interspaced short palindromic repeat (CRISPR)-derived genome editing provide an unprecedented opportunity to provide long-term solutions.

CRISPR‐LbCas12a‐mediated modification of citrus

“…Citrus is an important fruit crop around the world, and it faces many abiotic and biotic stresses such as citrus canker and Huanglongbing disease (Ference et al ., ; Wang and Trivedi, ; Wang et al ., ,b). To promote citrus breeding, CRISPR‐SpCas9 has been employed to edit the citrus genome in several reports (Jia and Wang, ; Jia et al ., , ; Peng et al ., ; Zhang et al ., ).…”

The flagella of ‘Candidatus Liberibacter asiaticus’ and its movement in planta

“…For example, HLB has spread throughout Florida, to other major citrus‐producing states (e.g. California and Texas) in the USA, and to neighbouring countries including Mexico and countries in the Caribbean and Central America (Wang et al , ). It is paramount to understand the biology and virulence mechanism of Las to design a suitable and efficient HLB control strategy.…”

Functional and comparative genomic analysis of integrated prophage-like sequences inCandidatus Liberibacter asiaticus

“…The CLas genome size is ∼1.2 Mb with a G+C content of 36.5%; both small genome size and low G+C content are consistent with patterns of obligate intracellular bacteria [5]. The CLas genome includes genes involved in cell motility and active transport [5,6], despite the presence of these genes, only passive movement has been observed in the phloem sap [6]. Furthermore, prophage sequences have also been identified in multiple CLas strains [7,8,9,10,11,12,13,14,15].…”

“…However, strains reported to lack prophages still induce HLB symptoms [16,8]. Hence, it has been hypothesized that prophages might contribute to bacterial virulence but are not required for CLas pathogenicity [6].…”