Metabolic Mechanisms of Host Species Against Citrus Huanglongbing (Greening Disease)

Abstract

Huanglongbing (HLB), previously known as citrus greening disease, is a devastating disease caused by gram negative, phloem-limited, fastidious bacteria Candidatus Liberibacter spp. HLB exists in nearly all commercially cultivated citrus, causing losses to growers by affecting tree vigor, production, fruit development, and quality. Recently, some promising HLB-tolerant germplasm has been identified, and showed particularly high concentration of metabolites, such as flavone, flavanone, aldehyde, and monoterpene. This review focuses on the citrus metabolic response against the HLB and we have summarized a comprehensive metabolic pathway that is activated in response to HLB. The antibacterial role of these metabolites, which were high in the HLB tolerant varieties are discussed. Generally, most of the amino acids, flavonoids, terpenes, and volatile compounds were significantly higher, even hundreds times of increase, in HLB-tolerant varieties. We also summarized the secondary metabolites, which were differentially altered in leaves, fruits, phloem sap (stem), and roots of infected citrus plants. Different metabolic studies have suggested that particular metabolites may play vital role in restricting the movement and multiplication of pathogens. Moreover, these metabolic signatures can be developed into tolerance markers against HLB. Genome-editing technologies should be used to confirm the functions of candidate genes responsible for increased production of compounds related to HLB tolerance. Engineering the metabolic pathways to create an ideal combination of gene alleles to propel metabolite flow for the antimicrobial activities, is an alternative tool to breed tolerant cultivars against HLB.

Keywords:

• Amino acids
• HLB
• phytochemicals
• secondary metabolites
• volatile compounds

Acknowledgements

The author likes to thank Yuantao Xu for his suggestion on Figure 1, Xiaomei Tang and Yue Huang for Figure 3.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This project was supported by the National Key Research and Development Program of China [2018YFD1000100], the innovation-driven development project of Guangxi [AA18118027], and the fundamental research funds for the central universities.