The Occurrence of Non-protein Amino Acids in the Phloem Sap of Different Citrus Varieties

摘要

Huanglongbing (HLB), or citrus greening disease, is caused by a phloem limited gram-negative bacterium, Candidatus Liberibacter asiaticus (CLas) which is transmitted by the Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) (ACP), aphloem sap feeder. CLas and ACP obtain most of their nutrients from the citrus phloem sap which contains many sugars, organic acids, protein amino acids (PAAs), non-protein amino acids (NPAAs), and many other metabolites. Recently more studies have beenconducted to investigate the occurrence, metabolism, and the roles of NPAAin plants. These studies showed NPAAs may play important roles in the plant's defense against insects, pathogens, and protect plants against stress. In the current study, we studied the NPAAs profile in the phloem sap of many citrus varieties with different degrees of CLas tolerance to determine if any of these NPAAs could be implicated in citrus tolerance to CLas. The phloem sap was collected by centrifligation, derivatized usingtrimethylsilyl (TMS) reagent after being dried under nitrogen stream, and analyzed by gas chromatography-mass spectrometry (GC-MS). Six amino acids were detected in the phloem sap of citrus varieties, y-aminobutyric acid (GABA) was the most abundant ofthe NPAAs and was detected in all of the selected varieties. The concentration of GABA varied widely among varieties and was higher in CLas-tolerant varieties. Anthranilic acid was found in low amounts (< 2 mM) and it was also higher in CLas-tolerant varieties. Synephrine level in citrus varieties varied widely and tended to be higher in CLas-susceptible varieties. N-methyl-L-proline and acetyl-L-proline were the highest in Citrus latipes (Swingle). The levels of putrescine, N-carboxy-glycine, and fi-aminopropanol did not vary a lot among varieties. In our future research, we will investigate the role of these NPAAs in citrus defense against CLas and ACP.