Stopping citrus greening with peptide therapy

Citrus greening disease (also known as Huanglongbing) has had a huge impact on citrus fruit production worldwide, with Florida particularly hard hit. The disease is caused by insect-vector-spread bacteria, including Candidatus Liberibacter asiaticus (CLas). There is some genetic variability in susceptibility, which led Zhao et al. to investigate the role of an E3-ubiquitin ligase, PUB21, in disease susceptibility. Higher levels of PUB21, through both natural variation and induced expression, increased susceptibility. They also identified a naturally occurring dominant-negative PUB21 allele that encodes a variant without ubiquitin-ligase activity, and showed that it positively correlates with disease resistance. When they screened for potential targets of PUB21 ubiquitination, they identified the transcription factor MYB2, which regulates signaling by the defense hormone jasmonate. They also identified a bacterial effector that promotes the interaction between PUB21 and MYB2, leading to increased susceptibility. Finally, they identified an antiproteolysis peptide (APP), APP3-14, that disrupts the interaction between PUB21 and MYB2 in the presence of the bacterial effector. Remarkably, application of APP3-14 to infected trees increases MYB2 expression and jasmonate defenses, thereby decreasing bacterial growth and disease progression. This exciting work opens many avenues for control of citrus greening disease. (Summary by Mary Williams @PlantTeaching.bsky.social) Science 10.1126/science.adq7203