1 INTRODUCTION
An organism’s capacity to survive in an ecosystem depends on its ability
to respond quickly and efficiently to external stimuli and to develop
effective and sustainable defences (Zebelo & Maffei, 2015). For this
reason, plants have developed numerous mechanisms to react specifically
to each biotrophic attack, and cell-to-cell communication between
distant tissues is essential to coordinate activities in response to the
environment. Thus, plants need to produce a signalling mechanism to
integrate perception, transmission, and response to biotic and abiotic
actions that occur in the ecosystem (Baluska et al., 2015; Brenner et
al., 2006; Pelagio-Flores et al., 2011; Maffei et al., 2007). Electrical
signals have been shown to be associated with responses to herbivory
(Pachú, 2020), leading to the activation of multiple organism defences
(Maffei et al., 2004). However, studies that better characterize the
electrical signalling mechanisms in plants attacked by herbivores, such
as aphids, are still scarce.
The aphid Aphis gossypii Glover (Hemiptera: Aphididae) is a
cotton-damaging pest (Malaquias et
al., 2017a) of cotton and one of the most important nontarget species of
Bt cotton, which is a genetically modified cultivar expressing proteins
derived from Bacillus thuringiensis (Berliner) (Bt), which gives
them high efficiency against some lepidopteran species (Malaquias et
al., 2017b; Malaquias et al., 2020). However, researchers have raised
concerns about their potential impact on nontarget organisms such as
aphids (Hagenbucher et al., 2013)
Studies addressing the impact of Bt cotton on the population dynamics of
aphids (Udikeri et al., 2012) or
on growth or developmental
characteristics (Zhao et al.,
2016) have shown that aphid feeding on Bt cotton plants can cause
morphological, physiological, biochemical and molecular changes in
cotton plants and possibly different biological response patterns
between Bt and non-Bt cotton cultivars.
Different environmental stimuli cause specific responses in living cells
that are capable of transmitting electrical signals (Lautner et al.,
2005). Among the signals involved in electrophysiological responses,
variation potentials are characterized by rapid depolarization and
subsequent slow repolarization. The amplitude and shape of the variation
potentials (VPs) vary with the stimulus intensity. In addition, the
magnitude and speed of responses decrease as the signal moves away from
the stimulus site, and its induction depends on the type of damage
sustained (Stahlberg et al., 2006).
Although the methods used to produce transgenic crops are being
continuously improved, it is currently not possible to control the exact
stability, integration and expression of the gene inserted in plant
genomes. Plant physiological traits may be altered; thus, it is crucial
to understand how plants produce different signs of stress and convert
them into appropriate specific responses. Therefore, it is essential to
characterize the type of electrical signalling of Bt and non-Bt cotton
plants as a function of the stress caused by A. gossypii and
provide insights to understand how plants convert these different
signals into appropriate physiological reactions. In our study, we
characterized the production of electrical signals on Bt [cultivar
WideStrike®] cotton plants and their non-Bt isoline [cultivar FM
993] infested with A. gossypii in alternating light–dark
cycles. The aphid A. gossypii was used as a model insect for the
study because insect feeding occurs at the phloem level, and the
biological interactions between the herbivore and its host plant can be
considered unique. Additionally, we characterized A. gossypiidispersal behaviour to relate this behaviour to plant signalling
responses.