Abstract
Plants produce considerable amounts of volatile organic compounds (VOCs) with several biological functions, including protection against biotic agents such as viruses and their vectors. In citrus species, these metabolites can be related with their different susceptibility/tolerance toward the Tristeza virus (CTV), one of the main biotic constraints for the citrus industry. The objective of this study was to compare the VOCs pattern from the leaves of a CTV-susceptible citrus variety such as Citrus aurantium and from three CTV-tolerant varieties: Citrus volkameriana, Carrizo citrange, and Forner-Alcaide no. 5. The VOCs emitted were analyzed via the headspace SPME method, while plant metabolites sequestered in the leaves were analyzed by heptane extraction followed by GC-MS. The results indicated that the majority of the VOCs emitted and sequestered in the leaves of the varieties tolerant and susceptible to CTV are constituted mainly by volatile terpenes (VTs) that exhibit strong qualitative/quantitative differences among the profiles of the four citrus species. In detail, the VOC emission indicated different patterns between C. aurantium and C. volkameriana and from both of them in comparison with Forner-Alcaide no. 5 and Carrizo citrange that exhibited more similarities, with the last two characterized by a higher presence of sesquiterpenes. The data obtained from the analysis of the VOCs sequestered in leaf tissues of the CTV-tolerant varieties indicated a higher presence of monoterpenes such as limonene, α-pinene, and p-cymene, known to be the main components of several plant extracts showing deterrent properties toward viruses and insect vectors. As VOC evaluation is a fast and noninvasive measure of phenotypic dynamics, allowing the association of plant phenotypes in accordance to plant disease resistance and/or stress tolerance, the possible implications of such differences in terms of tolerance grade to CTV and/or its related vectors are discussed.
Highlights
Plants produce and emit a large variety of secondary metabolites with several functions, including communication and protection against biotic agents [1,2]
The results indicated that the majority of the volatile organic compounds (VOCs) emitted and sequestered in the leaves of the varieties tolerant and susceptible to citrus tristeza virus (CTV) are constituted mainly by volatile terpenes (VTs) that exhibit strong qualitative/quantitative differences among the profiles of the four citrus species
Monoterpenes were more abundant in the VOC profile of C. aurantium and C. volkameriana, with 92.47% and 92.31% of the total peak area, respectively; the remainder was mainly composed by sesquiterpenes
Summary
Plants produce and emit a large variety of secondary metabolites with several functions, including communication and protection against biotic agents [1,2]. These compounds are known to have a primary role in the ecological interactions between plants and their phytophagous insects [3,4]. VOC profile modulations implicate changes in the plant defense response to herbivore feeding [6,7]. The importance of the role of such secondary plant substances to determine the patterns of host plant utilization was first speculated by Fraenkel [11] in the famous article “The raison d’etre of secondary plant substances.” On the other hand, the qualitative and quantitative modulation of VOC emission is involved in the plant defense mechanisms, allowing it to affect the food searching of insects [12], deterring the herbivore (direct defense) [13,14], through antixenosis mechanisms [15,16], or calling its natural enemies (indirect defense) [17,18,19]
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