Published 2019-04-19
Abstract
Currently, Huanglongbing is the major disease in Florida citrus. The disease is caused by the phloem-restricted, uncultivable bacterium, Candidatus Liberibacter asiaticus and transmitted by the Asian citrus psyllid (ACP), Diaphorina citri. Many of the phloem sap-feeders occasionally feed on the xylem sap. It has been shown that ACP also spends time on xylem activities and a proportion of psyllids feeds from xylem indicating that xylem sap contains the essential nutrients needed for psyllid. We studied the chemical composition of citrus xylem sap and carried out a comparison with citrus phloem sap. Xylem sap was collected by centrifugation. The collected sap was derivatized with trimethylsilyl (TMS) and analyzed with gas chromatography-mass spectrometry (GC-MS). Five amino acids (proline, glycine, threonine, GABA, and serine), six sugars (sucrose, glucose, fructose, and three inositol isomers), and six organic acids (succinic acid, fumaric acid, malic acid, citric acid, quinic acid, and threonic acid), were detected in the xylem sap. Mannose and galactose were found in trace amounts. The profile of the xylem sap was similar to that of the phloem sap. However, the xylem sap was less concentrated than the phloem sap. Because the xylem sap contains most of the components found in the phloem sap, it could support the survival and the growth of ACP. This information expands our knowledge about the nutrition requirements for citrus phloem-sap feeders, such as ACP, and can help define a suitable minimal artificial diet. The diet will greatly help in laboratory studies such as testing the efficiency of RNAi and antimicrobial peptides.