One telltale sign of citrus greening infection in citrus trees is leaf chlorosis, which is characterized by yellow blotches showing up on green citrus leaves. While chlorosis can have other causes, researchers believed it was caused by the bacterial infection of citrus greening. UF/IFAS plant pathologist Nabil Killiny, with UF/IFAS Citrus Research and Education Center (CREC), conducted research on the relationship between citrus greening and leaf chlorosis.
Leaf Chlorosis and Plant Signaling Molecules
In the study, Killiny’s research team compared sap from healthy Valencia sweet orange trees with sap from similar Valencia citrus trees infected with citrus greening. The team identified 39 different compounds, including higher amounts of many acids from the sap of the orange trees infected with citrus greening.
One compound in particular was found in elevated levels in both the xylem tissue and the phloem tissue of the orange trees; tissue that are respectively responsible for circulating water and nutrients through a tree, much like veins in the human body. Killiny hypothesized this compound, an organic acid, could be working as a plant signaling molecule in citrus trees infected by the bacteria that cause citrus greening.
A plant signaling molecule is a method that a plant’s cells use to communicate with other cells in the plant. They play a role in controlling plant and fruit development, and they act as a stress signal in response to wounding, infection, and more. Essentially, the organic acid travels through a tree’s tissues to alert other cells, cells not yet infected by the citrus greening bacteria, that an infection is present in the tree and thus causing the leaf chlorosis.
An additional study confirmed the hypothesis, with the organic acid being applied to healthy Valencia trees and leaf chlorosis resulting. Killiny believes the increased production of organic acid in citrus trees infected with citrus greening points to a negative effect on the trees’ biochemical systems that are in control of regulating leaf pigment.
The research can be used to develop management strategies for citrus greening-infected trees and to improve tree health, such as through reversing citrus greening’s symptoms and improving citrus tree growth and production.