Figure: An orange tree that is infected with huanglongbing. Leaf symptoms begin on one or a few branches and slowly spread to the rest of the tree. Georgios Vidalakis, UC Riverside

Researchers at the University of California, Riverside have taken an important step in understanding the molecular mechanism of huanglongbing (HLB), the destructive disease that is a serious threat to the citrus industry worldwide. HLB, also known as citrus greening disease, is associated with the bacteria Candidatus Liberibacter asiaticus (CLas), which is transmitted by the Asian citrus psyllid (ACP). Once infected, trees typically die within three to five years.

A team led by Wenbo Ma, a professor of plant pathology in UCR, reported a significant breakthrough in understanding the disease mechanism of HLB. They discovered that the bacterium secretes a protein called Sec-delivered effector 1 (SDE1) that helps infect plants. SDE1 works by attacking papain-like cysteine proteases (PLCPs) that help the citrus trees resist infection.

Diseased trees that were studied by Ma's team showed increased protein levels of some PLCPs, presumably attempting to combat the bacterial infection. The bacterium fights back, by inhibiting the enzymatic activity of PLCPs through SDE1. The team used the model plant Arabidopsis thaliana and the bacterial pathogen Pseudomonas syringae that was genetically engineered to produce SDE1. They showed that SDE1 promotes bacterial infection.

"This study represents an important step towards better understanding of the HLB disease mechanism, which will help us develop novel approaches to control this unstoppable disease," Ma said. Their study is among the first to describe the molecular tactics employed by CLas to colonize citrus plants.

For more details, read the news release at UCR Today.