Introduction
Plants
interact with the environment by releasing various volatiles that
possess physiological and ecological functions (O’Connor, 2015). The
theory of herbivore-induced plant volatiles (HIPVs), proposed by Ehrlich
and Raven (1964), states that plants release various volatiles to
communicate and defend against herbivore attack. Thereafter, studies
have been increasingly focused on relationships between plant compounds
and insects. A recent report showed that the cottons emitting increased
levels of volatile compounds have fewer eggs of Helicoverpa
armigera (Liu et al. 2018), suggesting that volatiles play
important roles in protecting plants from pests and enhancing plant
survival. Indeed, numerous plant-derived volatile compounds,
particularly terpenes and terpenoids, are documented to protect plants.
For example, farnesene protects plants from aphids (Myzus
persicae ) because it acts as an alarm pheromone and attracts the
predatory ladybugs (Harmonia axyridis ) (Francis et al.2004; Zhu et al. 2005). Another common compound in flowers and
fruits is linalool, which has studied in Arabidopsis thaliana ,
indicating it attracts thrips; however, oxides of linalool, catalysed by
CYP71D, can repel these insects (Boachon et al . 2015). Meanwhile,
HIPVs, including mint-derived volatiles such as limonene, 1,8-cineole,
and carvone, can also attract species that predate on herbivores, such
as predatory mites that feed on two-spotted spider mites
(Togashi et al. 2019).
Therefore, volatile compounds may be part of a defence system used
directly or indirectly by plants to protect themselves via tritrophic
interactions (Meena et al . 2017). Furthermore, as they deter
herbivores, these plant volatiles are likely expressed in higher
concentrations during early stages of development than in mature stages
and tissues (Meena et al. 2017); thereafter, expression of these
compounds is maintained at a constant level or reduced( Niederbacher et al . 2015). This shows that plants
dynamically regulate expression of metabolites in order to adapt to the
environment and reproduce.
Most HIPVs are terpenes/terpenoids and are biosynthesized via two
pathways. The first is the mevalonate (MVA) pathway, which occurs in the
cytosol/endoplasmic reticulum, and leads to the formation of farnesyl
diphosphate (FPP) as a precursor to sesquiterpenoids. The other is
2-C-methyl-D-erythritol-4-phosphate (MEP) pathway, occurring in
plastids; this pathway generates geranyl diphosphate (GPP) as a
precursor to monoterpenoids, catalysed by the associated terpene
synthases (TPSs) (McGarvey and Croteau, 1995). These TPSs are a
mid-sized gene family in plants, consisting of TPS-a, -b, -c, -d, -e/f,
and -g (Chen et al . 2011).
Recently, neryl diphosphate (NPP), which is an isomer of GPP, and
(Z,Z)-FPP, were shown to be precursors to several terpenes in the tomato
plant
(Schilmilleret al. 2009; Akhtar et al . 2013). This indicates that
terpene/terpenoid biosynthesis needs to be further investigated in
various plant species. Moreover, the structure and function of terpenes
that are post-processed by co-expressed genes have been extensively
studied in the peppermint (Mentha x piperita ) (Rodneyet al . 2005), Artemisia annua L. (Teoh et al .
2006), and A. thaliana (Boachon et al , 2019). However, few
studies have examined these compounds in the lavender.
Lavender (Lavandula angustifolia ) is an important aromatic plant
generating as many as 70 volatile metabolites, including limonene,α- pinene, linalool. Lavender has also been proposed as a model to
study the regulation of terpene biosynthesis (Guitton et al.2010a). Although lavender metabolism has been investigated for several
years, enzymes, such as 3-carene
synthase, fenchol synthase, α -pinene synthase, andβ -phellandrene synthase are mainly identified usingin-vitro assays (Demissie ZAet al . 2011; Benabdelkaderet al . 2014;
Adal
AM et al . 2017). In addition, terpenes synthesized by TPSs are
potential to be transported into endoplasmic reticulum where these
compounds are converted by cytochrome P450 (CYP) into their respective
derivatives (Karunanithi and Zerbe 2019). This enables plants to
increase the expression of metabolites that can perform multiple
functions. Although the CYP enzyme family, which accounts for 1% of the
genome in most plants (Nelson and Werck-Reichhart 2011), has not been
functionally identified in the lavender, 30 unique CYPs genes were
predicted using sequence tags (ESTs) (Lane et al. 2010).
Physiological functions of volatiles and their genes within
tritrophic interactions in lavenders remain elusive. Previous studies
have described volatile organic compound (VOC) content during
inflorescence ontogeny, and revealed that the terpenes involved fell
into three groups identifiable via three developmental phases of
flowering. 3-carene, limonene, myrcene, bornyl acetate, borneol,
camphor, 1,8-cineol, and trans -ocimene belonged to the first
group and may play a protective role in repelling damaging insects
(Guittonet al . 2010b). Previously, we found that aphids and ladybugs were
dominant insects in lavender fields during early spring in Beijing. This
suggests probable tritrophic
interaction among predators, preys and volatiles (Li et al.2019) . Y-tube olfactometer experiments showedβ -trans-ocimene and
(+)-R-limonene get rid of 74.71%
and 78.41% aphids (Li et al. 2019).
In this study, we investigated the tactics used by the lavender plant to
defend against herbivore attack during immature developmental stages and
in young plant tissues.