Researchers Find How HLB Bypasses Citrus Immune System

citrus greening

Ma

Researchers at the University of California, Riverside (UCR) claim they have made an important step in understanding the molecular mechanism of huanglongbing (HLB). They believe they have found the mechanism by which the disease evades detection and destruction by the natural immune system of citrus trees.

HLB, or citrus greening disease, has devastated groves in Asia, South America and the southern United States. Since 2012, the disease has been spreading in California’s residential areas, prompting serious concerns about the state’s commercial citrus groves.

HLB is associated with a species of bacteria called Candidatus Liberibacter asiaticus (CLas), which is transmitted by the tiny Asian citrus psyllid. An important step to developing HLB-resistant citrus varieties is to better understand how the bacterium infects trees and causes disease.

“Citrus trees, like all plants, have complex immune systems to prevent pathogenic infection, so the question is ‘how does the CLas pathogen evade that immunity so it can cause disease?’” said Wendo Ma, a professor of plant pathology in UCR’s College of Natural and Agricultural Sciences.

In a paper published recently in the science journal Nature Communications, a team led by Ma reported a significant breakthrough in understanding the disease mechanism of HLB. The team discovered that the bacterium secretes a protein, called Sec-delivered effector 1 (SDE1), that helps infect plants. SDE1 works by attacking specific proteases, called papain-like cysteine proteases (PLCPs), that could otherwise help the citrus trees resist infection.

“In the diseased trees we studied, the protein levels of some PLCPs were increased, presumably attempting to combat the bacterial infection,” Ma said. “However, the bacterium fights back, by inhibiting the enzymatic activity of PLCPs through SDE1.”

“This study represents an important step toward better understanding the HLB disease mechanism, which will help us develop novel approaches to control this unstoppable disease,” Ma said.

The team is now investigating the molecular details of how SDE1 suppresses citrus PLCPs with the aim to use the CRISPR gene editing system to modify the proteases to become resistant to the inhibitory effects of SDE1.

The title of the paper is “An effector from the Huanglongbing-associated pathogen targets citrus proteases.” In addition to Ma, contributing authors were from UCR, UC Davis, University of Florida and University of Oxford. Funding was from the United States Department of Agriculture National Institute of Food and Agriculture.

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