The devastating disease huanglongbing, or citrus greening, looms darkly over the United States, threatening to wipe out the nation's citrus industry, whose fresh fruit alone was valued at more than $3.4 billion in 2012.
Recently, however, a research team led by a University of California, Davis, plant scientist used DNA sequencing technologies to paint a broad picture of how citrus greening impacts trees before they even show signs of infection, offering hope for developing diagnostic tests and treatments for the currently incurable disease.
The new findings indicate that the bacterial disease interferes with starch and sugar metabolism in young and mature leaves and fruit, while also wreaking havoc with hormonal networks that are key to the trees' ability to fend off infections. Study results were reported September 25 in the PLOS ONE journal.
In this new study, the researchers studied four categories of healthy and diseased citrus trees, with the goal of better understanding how HLB affects trees physiologically during the very early stages of infection. The researchers used gene sequencing technology to study the "transcriptome," which is the collection of RNA found in the tree leaves and fruit.
Their analysis confirmed that in infected trees, HLB disease caused starch to accumulate in the leaves, blocking nutrient transport through the phloem and decreasing photosynthesis. They also found that normal metabolism of sucrose, a sugar also key to photosynthesis, was disrupted.
Furthermore, the researchers discovered that HLB interfered with the regulation of hormones such as salicylic acid, jasmonic acid and ethylene, which are "the backbone" of the plant innate immune response. And they found that infected trees also had changes in the metabolism of important amino acids that serve as a reservoir for organic nitrogen in many plants. The nitrogen is required to stimulate the plant immune response.