Study identifies target gene for controlling the Huanglongbing psyllid

Researchers at South China Agricultural University have silenced a key gene in the insect. diaphorina citri, known as the citrus psyllid, and recorded lethal and sterilizing effects. The gene in question, Syntaxin-1A (Syx1A), regulates vital processes in the insect. By blocking its expression through RNA interference (RNAi), scientists caused mortality of over 70% in adults, weight loss, ovarian atrophy, and reproductive failure.

The psyllid is a vector of Candidatus Liberibacter asiaticus, the bacterium that causes Huanglongbing (HLB), a disease with no known cure that devastates citrus groves. The RNAi technique tested paves the way for more specific and sustainable pest control, suggesting a future alternative to traditional insecticides.

The Syx1A gene belongs to the Qa-SNARE family, whose functions include vesicle fusion and neurotransmitter release. In D. citri, its expression peaks in the salivary glands, but also appears in other tissues and in all phases of the life cycle.

Cloning and characterization

Scientists cloned and characterized the gene from the insect's transcriptome. The sequence encodes a protein with 309 amino acids and approximately 35,7 kDa. Bioinformatics analyses revealed domains typical of the SNARE family, such as SynN and the transmembrane region. Similarity with other insect species exceeds 78%.

Expression analyses using RT-qPCR showed that Syx1A is intensely expressed in the egg stage, fourth- and fifth-instar nymphs, and young adults. In tissues, salivary glands concentrate the highest levels of messenger RNA, followed by the intestine and testes in adults. This predominance indicates that the gene actively participates in salivary secretion and the insect's interaction with host plants.

Physiological functions

To investigate the gene's physiological functions, the scientists produced double-stranded RNA (dsRNA) and administered it via microinjection into fifth-instar nymphs and newly emerged adults. After 48 hours, they observed a 39% decrease in Syx1A expression in nymphs and a 58% decrease in adults. The mortality rate among nymphs reached 58% on the fifth day. In adults, 73% died by the seventh day. In both groups, body weight decreased significantly.

Gene suppression also compromised reproduction. Females treated with dsRNA showed a significant decline in egg laying from day three onward. Between days 3 and 11, fecundity declined progressively, and the cumulative egg count per female was drastically lower than in the control group.

Morphological analyses of the ovaries revealed evident degeneration. Ovarioles of control females contained mature oocytes with intense yellow pigmentation. In treated females, the ovaries were smaller, with failures in yolk deposition and oocytes in early stages of development.

Reproductive changes correlate with the regulation of genes involved in vitellogenesis. The team measured the expression of Vg1, VgA, and VgR—genes encoding vitellogenin and its receptor. After silencing Syx1A, all three showed a marked decrease in transcription, indicating that the gene directly interferes with yolk production and uptake by oocytes.

The observed mechanism suggests that Syx1A acts as an upstream regulator in the reproductive cascade by indirectly controlling vitellogenic genes. In other insects, such as Migratory locust e Drosophila melanogaster, the same gene has similar functions in neurological and reproductive processes.

RNA interference

RNA interference, a mechanism that specifically degrades messenger RNAs, has already demonstrated efficacy against pests of various orders, such as Diptera and Coleoptera. The agricultural use of this technology is advancing on two fronts: in transgenic plants with dsRNA genes and in topical formulations such as sprays. In this study, application was made by microinjection—a technique suitable for laboratory testing.

Known barriers to RNAi efficacy include dsRNA's instability in the insect gut and its low systemic absorption. To overcome this, researchers are studying the use of nanocarriers such as cationic liposomes, chitosan, and polymeric nanoparticles, capable of protecting and facilitating dsRNA penetration.

In the case of the citrus psyllid, multiple RNAi strategies have already been successfully tested. Genes such as cathepsin D, hexokinase, V-ATPase-E, and chitin synthase have proven vulnerable to the technology. The Syx1A gene, however, stands out for causing lethal effects combined with reproductive failure, a rare and effective combination for population control programs.

More information at doi.org/10.3390/insects16090901