Effects of Citrus tristeza closterovirus infection on phloem sap and released volatile organic compounds in Citrus macrophylla

Abstract

Severe Citrus tristeza virus (CTV) infection can induce change in patterns of growth and cellular differentiation in citrus spp. On the other hand, mild CTV strains do not produce any visible symptom in some citrus species. However, it is not clear whether this asymptomatic infection affects the primary and secondary metabolites of citrus plants. In this study, we investigated the effects of CTV-36 (mild stain) on the phloem sap and released volatiles of Citrus macrophylla. The phloem sap was collected by centrifugation and dried under a nitrogen stream. The dried phloem sap was then derivatized with methoxamine hydrochloride in pyridine (MOX) followed by N-methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA), and was analyzed by gas chromatography-mass spectrometry (GC-MS). Volatiles released from intact young leaves were collected using a solid phase micro extraction (SPME) fiber and were analyzed using GC-MS. Although no visible symptoms were observed on the infected plants, many changes in the phloem sap and released volatile composition were observed. The levels of many amino acids (l-valine, l-threonine, l-lysine, l-asparagine, and γ-aminobutyric acid) and organic acids (oxalic, malic, and citric acid) were higher in CTV-infected plants. Sugar acids were increased, while glucose and sugar alcohols were decreased as a result of CTV infection. Similarly, CTV infection increased some volatiles and decreased others. Interestingly, most of the aldehyde volatiles were affected in CTV-infected plants; decanal and menthadienal were increased, whereas citronellal and isogeranial were decreased. Understanding the interactions between CTV and citrus is of particular interest for the development of this virus as a vector to express antimicrobial peptides or for virus-induced gene silencing.