Amino acids in orange juice may hold clues to the mostly secret, highly successful attack strategy of a powerful plant pathogen (Candidatus Liberibacter asiaticus), which causes the devastating disease HLB in citrus.
USDA/ARS chemist Andrew P. Breksa III and University of California-Davis professor Carolyn M. Slupsky have compared the amino acid composition of juice from commercially grown oranges. They used nuclear magnetic resonance spectroscopy to study juice from oranges grown on either HLB-positive trees or HLB-negative trees. Their investigation is apparently the first to do so, the scientists say.
The research has yielded distinctive profiles of the kinds and amounts of 11 different amino acids in three types of oranges: 1) fruit from healthy trees 2) symptom-free fruit from HLB-positive trees 3) fruit with HLB symptoms from HLB-positive trees.
With further research, the profiles may prove to be “a reliable, rapid, and early indicator of the presence of the HLB pathogen in an orchard,” Breksa says. For growers, an early indicator of HLB would be valuable because the disease can go undetected for years in groves.
The amino acid profiles may have another use, as well. They may reveal clues to mechanisms underlying the microbe’s mode of attack.“No one understands precisely how the pathogen overcomes the defense system a citrus tree can ordinarily mobilize when it’s under siege,” Breksa says.
Trees need amino acids for growth, development, and defense. But what if the HLB pathogen were causing havoc with the trees’ ability to create, use, and recycle these amino acids? This information could be used as a starting point for a tightly focused counterattack strategy, Breksa points out.
Building up and tearing down amino acids is part of the everyday life of a citrus tree. For instance, a tree can convert the amino acid phenylalanine into cinnamic acid, a precursor to compounds thought to be important to the tree’s defense system. But juice from oranges of HLB-positive trees “had significantly higher concentrations of phenylalanine,” Breksa notes. “This means that the HLB pathogen may have interfered with the orderly conversion of phenylalanine to cinnamic acid.”
Juice from oranges grown on HLB-positive trees also contained significantly less of the amino acid proline. Says Breksa, “When a tree ‘knows’ something is wrong, it synthesizes proline. In the case of HLB-infected trees, however, the pathogen might be outsmarting the tree by undermining proline synthesis.”