Revisiting the Complex Pathosystem of Huanglongbing: Deciphering the Role of Citrus Metabolites in Symptom Development

Abstract

Huanglongbing (HLB), formerly known as citrus greening disease, is one of the most devastating bacterial diseases in citrus worldwide. HLB is caused by 'Candidatus Liberibacter asiaticus' bacterium and transmitted by Diaphorina citri. Both 'Ca. L. asiaticus' and its vector manipulate the host metabolism to fulfill their nutritional needs and/or to neutralize the host defense responses. Herein, we discuss the history of HLB and the complexity of its pathosystem as well as the geographical distribution of its pathogens and vectors. Recently, our recognition of physiological events associated with 'Ca. L. asiaticus' infection and/or D. citri-infestation has greatly improved. However, the roles of citrus metabolites in the development of HLB symptoms are still unclear. We believe that symptom development of HLB disease is a complicated process and relies on a multilayered metabolic network which is mainly regulated by phytohormones. Citrus metabolites play vital roles in the development of HLB symptoms through the modulation of carbohydrate metabolism, phytohormone homeostasis, antioxidant pathways, or via the interaction with other metabolic pathways, particularly involving amino acids, leaf pigments, and polyamines. Understanding how 'Ca. L. asiaticus' and its vector, D. citri, affect the metabolic pathways of their host is critical for developing novel, sustainable strategies for HLB management.

Keywords: Asian citrus psyllid; Diaphorina citri; blotchy mottle; citrus; flushing cycles; fruit drop; huanglongbing; metabolites; phytohormones; polyamines; root damage; ‘Candidatus Liberibacter’.

Figure 1

The characteristic symptoms of huanglongbing (HLB) disease. (A) Blotchy mottle symptom on HLB-positive citrus leaves with asymmetric patterns of discolorations around the leaf midvein; (B) Yellow vein symptom on HLB-positive citrus leaves; (C) Leathery leaves with enlarged corky veins from HLB-affected trees; (D) Lopsided, asymmetric, and small-sized fruits from HLB-affected trees; (E) HLB-induced preharvest fruit drop; (F,G) The root system of HLB-free (healthy) and HLB-positive trees, respectively. Photos used in panels (A–D) were provided by Dr. Pedro C. Gonzalez-Blanco whereas the photo used in panel (E) was provided by Dr. Tripti Vashisth, Department of Horticultural Sciences, Citrus Research and Education Center, University of Florida. Photos used in panels (F) and (G) were provided by Dr. Evan Johnson, Department of Plant Pathology, Citrus Research and Education Center, University of Florida.

Figure 2

The chemical structures of different phytohormones discussed throughout this review. (A) auxins, (B) salicylates, (C) cytokinins, (D) jasmonates, (E) abscisic acid, and (F) ethylene. Molecular weights (g·mol⁻¹) are mentioned between parentheses beside the chemical formula of each compound.

Figure 3

Hypothetical model of the potential roles of citrus metabolites in the development of huanglongbing (HLB) symptoms. Briefly, HLB symptoms are induced by both the pathogen and its vector due to alteration in many physiological aspects such as phytohormones, polyamines, carbohydrate status, and pigment content. The most characteristic symptom of HLB, blotchy mottle, might be due to the degradation of both chlorophylls and carotenoids. Additionally, ABA could induce leaf yellowing, which might help the development of blotchy mottle symptom. Furthermore, polyamines might be involved in HLB symptom development via the production of H₂O₂, which may eventually become toxic to the leaf tissue and cause the characteristic blotchy mottle symptom that appears after ‘Ca. L. asiaticus’ infection. Another characteristic symptom of HLB is the leathery leaves, which could be due to the extensive accumulation of starch grains in the photosynthetic cells, phloem elements, vascular parenchyma, and all other parenchyma cells of the HLB-symptomatic leaves. In addition, we suggest that auxin accumulation in HLB-infected leaves might be involved in the development of leathery leaf symptom via the formation of thicker cell wall in photosynthetic cells. Moreover, it is well-known that ‘Ca. L. asiaticus’ infection reduces the production of new flushes on infected trees. We believe that the reduced new flushes symptom could be due to the phytohormonal imbalance, particularly the auxin/cytokinin ratio. The solid lines with arrows signify positive reaction, the dashed lines with bar-ends indicate negative correlation, and round-dotted lines represent hypothetical mechanisms or uncharacterized elements, with arrows (positive) or bar-ends (negative). The red triangles signify increased levels, whereas green upside-down triangles signify decreased levels of these compounds upon the infection with ‘Ca. L. asiaticus’ and/or infestation with D. citri.

Figure 4

Preharvest fruit drop (%) from 2000 to 2019. (A) The fruit drop profile of five citrus groups including navel oranges, early-mid-season non-Valencia oranges, Valencia oranges, white, and red seedless grapefruits. The graph is based on the available information of citrus production reports in Florida from the USDA’s National Agricultural Statistics Service (NASS, 2019) available at https://www.nass.usda.gov/StatisticsbyState/Florida/Publications/Citrus/Citrus_Forecast/history.php. The dramatic increase in fruit drop during the 2004–2005 season was mainly due to the 2004 Atlantic hurricane season (Hurricane Charley, Hurricane Frances, Hurricane Ivan, and Hurricane Jeanne), whereas the high fruit drop during 2017–2018 season was due to the high winds of Hurricane Irma in September 2017. (B–F) Simple linear regression between crop season and preharvest fruit drop (%) of five citrus groups including navel oranges, early-mid-season non-Valencia oranges, Valencia oranges, white seedless grapefruits, and red seedless grapefruits, respectively. Dots present data on citrus production in Florida from the USDA’s NASS. The fitted regression line is presented as a solid line. The 95% confidence curve for the estimated linear regression is light-blue-shaded and edged by dotted lines. Regression equation, R², R²_adj, and p-values were also obtained and are presented in the lower right corner of the graph.

Figure 5

Hypothetical model of the potential roles of citrus phytohormones and carbohydrate availability in the HLB-induced preharvest fruit drop. (A) Citrus leaf and fruit abscission zones (AZs). There are five well-defined AZs in citrus. The leaf AZs are located between the shoot and the petiole (BA–AZ; also known as Branch–AZ or Basal–AZ) and between the petiole and the blade (LA–AZ; also known as Laminar–AZ), whereas the fruit AZs are located between the shoot and the peduncle (AZ–A), between the calyx and the fruit itself (AZ–C), and between the fruit and the style (AZ–STY). (B,C) Two hypothetical models that discuss the potential role of carbohydrate availability and phytohormone imbalance, respectively, in HLB-induced preharvest fruit drop. Briefly, the first hypothesis is based on that ‘Ca. L. asiaticus’ infection induces collapse and proliferation of phloem cells and plugging of sieve pores in citrus leaves causing phloem blockage, which could block the carbohydrate flow in the phloem, causing limited carbohydrate availability to the citrus fruits. The second hypothesis was built on the phytohormonal imbalance at the AZs, which might play a key role in the regulation of cell separation processes and, eventually, fruit drop. Several phytohormone groups might be implicated in abscission processes, including auxin and indole derivatives, ethylene and its precursors, abscisic acid (ABA), gibberellins (GAs), cytokinins (CKs), brassinolide (BRs), and jasmonates and their methyl ester (JAs).

Figure 6

Population dynamics of ‘Candidatus Liberibacter asiaticus’ and its insect vector, Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), in relation to the annual flushing cycles and root growth patterns of citrus trees in Florida. The data presented in this figure were adapted from previously published data by [138,139,140,141,142,143,144,145,146,147,148,149,150,151,152].