The pathogens in question are a select group of bacteria that have made the plant vascular system their exclusive home. Xylella fastidiosa colonizes the xylem, the water-transport network of the plant, while Spiroplasmas, Phytoplasmas and Liberibacters confine themselves to the phloem, the sugar-rich tissue that distributes photosynthetic products. These bacteria cannot move between plants on their own. They depend on piercing-sucking insects, mostly hemipterans such as leafhoppers, planthoppers, spittlebugs and psyllids, whose needle-like stylets penetrate directly into the vascular tissues. In doing so, the insects ingest sap along with any microbes it contains and later deposit the bacteria into new hosts. Intriguingly, within the insect suborder Sternorrhyncha, only psyllids are known to transmit bacterial pathogens, while aphids and whiteflies, their close relatives, are primarily virus carriers.

The economic stakes are enormous. Citrus huanglongbing, or HLB, caused by the psyllid-transmitted Liberibacter species, has cut citrus yields in Florida by 74 percent since its detection there in 2005. In Italy, Xylella fastidiosa triggers olive quick decline syndrome, a disease that has killed millions of olive trees in the southern region of Puglia. Genetic analysis traced that outbreak to a Central American introduction of the pathogen around 2008, and phylogenetic work suggests the bacterium actually arrived in Europe as early as 1993. Xylella infects more than 700 plant species, including grape, in which it causes Pierce’s disease, and almond and citrus, where it produces leaf scorch and variegated chlorosis. The review also highlights the recent recognition of Xylella taiwanensis as a distinct species rather than a subspecies of X. fastidiosa.

Each pathogen has evolved a distinctive lifestyle suited to its vascular niche. Xylella, a Gram-negative bacterium related to Xanthomonas albilineans, uses Type IV pili to twitch its way through the nutrient-poor xylem, secreting cell wall-degrading enzymes such as polygalacturonase and endoglucanases to dissolve the pit membranes that separate adjacent vessels. It then forms sticky biofilms, anchored by extracellular polysaccharides and adhesins, that physically obstruct water flow. The resulting drought stress, stomatal closure and reduced photosynthesis eventually kill the plant. Spiroplasmas, by contrast, are helical, wall-less bacteria of the class Mollicutes that swim by propagating kinks along their bodies. Spiroplasma citri, discovered in 1970 and the first vector-borne bacterium ever cultured, causes citrus stubborn disease, while S. kunkelii produces maize stunt and S. phoeniceum infects periwinkle.

Phytoplasmas are stranger still. First observed in 1967 as mycoplasma-like organisms in infected phloem, they remain unculturable in the laboratory, forcing researchers to rely on DNA extracted from infected plants and on 16S rRNA gene sequencing for classification. Lacking both cell walls and flagella, they cannot produce the classic molecular patterns that plant immune receptors normally detect, and they appear to elicit immune responses through internal patterns instead. Their most dramatic effects are developmental: infected plants produce witches’ brooms, dense clusters of stunted twigs, or phyllody, in which flowers transform into leafy structures. These deformities prevent normal reproduction and turn the plant into a breeding ground for both the pathogen and its vectors. Phytoplasmas compensate for their lack of motility genes by using their immunodominant membrane protein to bind plant actin, apparently hitching rides through the host’s own cytoskeletal transport system.

The Liberibacters tell an evolutionary story of their own. Six species share a common ancestor, and genome analysis suggests the non-pathogenic Liberibacter crescens gave rise to the pathogenic Candidatus Liberibacter lineages. The pathogenic species are fastidious and unculturable, likely because their highly reduced genomes lack genes for thiamine and essential amino acids that L. crescens retains. Candidatus Liberibacter asiaticus is the primary HLB agent, transmitted by the Asian citrus psyllid Diaphorina citri, while Ca. L. solanacearum exists as at least six haplotypes: A and B, spread by the potato psyllid Bactericera cockerelli, cause zebra chip disease of potato, named for the dark streaks that appear in tubers after frying, and haplotypes C, D and E, carried by the carrot psyllid B. trigonica, attack umbelliferous crops.

Central to the review is the molecular arms race between these bacteria and plant immunity. Plants deploy pattern recognition receptors that detect bacterial flagellin and other pathogen-associated molecular patterns, triggering a first line of defense. Bacteria strike back with effector proteins. Phytoplasma effectors are among the best characterized: SAP05 hijacks the host 26S proteasome to degrade SPL and GATA transcription factors, promoting lateral shoots and sterile flowers; SAP11 and related proteins degrade TCP transcription factors to stimulate bud growth; SAP54 converts flowers into leaf-like structures; and SRP1 from rice orange leaf phytoplasma impairs chlorophyll precursor biosynthesis, producing yellow leaves that attract leafhoppers. Liberibacter effectors are equally sophisticated. CLas secretes SDE15 to target the programmed cell death regulator CsACD2, SDE1 to suppress papain-like cysteine proteases, and SDE3 and SDE4405 to manipulate autophagy, while the effector CLas4425 undermines salicylic acid signaling. For CLso, the effector Lso-HPE1 disrupts the tomato ubiquitin-proteasome system, and CKC_05701 suppresses programmed cell death and reactive oxygen production.

The relationship between pathogen and vector is just as intricate. Xylella is unique among these bacteria in forming biofilms in the foregut of its sharpshooter and spittlebug vectors, a semi-persistent, non-circulative association. It degrades chitin and uses adhesins such as XadA1 and XadA2, which bind strongly to chitin and cellulose, to anchor itself in the insect’s feeding apparatus. The phloem-limited bacteria, by contrast, establish persistent, circulative relationships: they cross the insect gut epithelium, survive in the hemolymph and eventually reach the salivary glands for reinoculation. Spiroplasma citri uses adhesion proteins including P89 and ScARPs for receptor-mediated endocytosis, and its phosphoglycerate kinase interacts with insect actin to promote colonization. Phytoplasmas enter insect cells through clathrin-mediated endocytosis, and their antigenic membrane proteins form complexes with insect microfilaments that determine vector specificity.

Liberibacters manipulate their psyllid hosts in remarkable ways. CLas localizes along the actin cytoskeleton of D. citri gut cells, and roughly 95 percent of its genes are active during gut colonization. In adult psyllids, CLas triggers apoptosis in the gut, which may reduce their transmission capacity, whereas nymphs show no such response and transmit more efficiently. CLso takes a preemptive approach: it induces expression of the anti-apoptotic gene IAPP5.2 early in infection, suppressing gut cell death to facilitate its own acquisition. The two zebra chip haplotypes provoke distinct immune responses in the potato psyllid, with CLsoB repressing apoptosis and achieving higher titers and more efficient transmission. Even insect symbionts matter: Wolbachia correlates positively with CLas titer in D. citri, and psyllids lacking Wolbachia acquire and transmit CLso less efficiently, suggesting microbiome-based control strategies.

The three-way interplay culminates in pathogen-driven manipulation of plant chemistry to recruit vectors. Phytoplasma effectors dampen jasmonic acid signaling and increase leafhopper fecundity, while SAP54 makes infected plants more attractive to leafhopper females. CLas induces citrus to release specific volatiles that deceptively attract D. citri and alters metabolite emissions to shift vector preference. Current management leans on insecticides and antibiotics, including oxytetracycline and streptomycin, approved by the U.S. EPA for Florida citrus, with trunk injection proving more efficient than foliar sprays. Promising alternatives include antimicrobial peptides such as nisin, which disrupts the Xylella lipid bilayer, and MaSAMP, a heat-stable peptide from HLB-resistant Microcitrus that both kills CLas and primes plant immunity. The authors point to CRISPR/Cas9 gene editing, plant immunomodulators such as salicylic acid analogues and GABA supplementation, and even light-engineering technologies as future directions. But with many of these pathogens still unculturable and genetically intractable, the review’s central message stands: cracking the bacterium-insect-plant triangle is the essential next step toward protecting global food security.

Subject of Research: Interactions among insect vectors, bacterial plant pathogens and host plants

Article Title: Bacterial vector-borne plant diseases: global issues caused by three-way interactions

Article References: Huang, Y., Hao, J., & Tang, X. (2025). Bacterial vector-borne plant diseases: global issues caused by three-way interactions. Crop Health, 3(1), Article 10. https://doi.org/10.1007/s44297-025-00049-0

Image Credits: AI Generated

DOI: 10.1007/s44297-025-00049-0

Keywords: Xylella fastidiosa, huanglongbing, Liberibacter, phytoplasma, Spiroplasma, insect vectors, psyllids, leafhoppers, plant immunity, effector proteins, phloem, xylem