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.