Introduction
Aphids are important plant pests that often have a broad host range. In addition, they significantly reduce the yields of susceptible plants via nutrient depletion and feeding damage to host tissues and the spread of viruses (Smith and Chuang 2014). Aphids are phloem feeders, with a probing stylet that can damage as the insect searches for the phloem. In addition, the stylet secretes saliva that contains elicitors and proteins that modulate plant defensive responses (Kaloshian and Walling 2016). Nevertheless, aphid feeding triggers the host multi-level immune system that operates to mitigate the adverse effects of pathogens and insects (Zust and Agrawal 2016). The host defensive networks that underpin plant responses to aphids have been characterised in a number of plant species (Nguyen et al. 2016; Kiani and Szczepaniec 2018, Sanchez-Arcos et al. 2019; Koch et al., 2020). Aphid feeding results in extensive changes to the leaf transcriptome and metabolome signatures (Foyer et al., 2015), revealing a complex interplay between the different hormones regulating plant basal immunity. Changes in the levels of phytohormones such as jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA) and indole acetic acid (IAA) have been documented (Kerchev et al., 2013; Foyer et al., 2015). However, little attention has been paid to the role of strigolactones (SL) in plant-aphid interactions. SLs fulfil many important roles in the control of plant growth and architecture, seed dormancy and senescence, as well as abiotic stress tolerance (Al-Babili & Bouwmeester, 2015; Waterset al ., 2017; Machin & Bennett 2020). These carotenoid-derived phytohormones are also critical regulators of plant–microbe interactions in the rhizosphere, such as the symbiosis with arbuscular mycorrhizal fungi (Aliche et al., 2020). They also serve functions in plant responses to biotic stresses to bacterial and fungal pathogens (Torres-Vera et al. 2014; Stes et al., 2015; Marzec, 2016).
Climate change is an important global challenge that will have important impacts on insects as well as their host plants. The failure to curb fossil fuel CO2 emissions, together with the inadequacy of planned mitigation measures required to achieve the United Nations Framework Convention on Climate Change Paris Agreement targets for limiting global warming, has led to strategies for carbon dioxide removal from the atmosphere (Beerling et al. 2020). The global average CO2 level is about 420 μmol mol-1 and it is predicted to reach 730-1000 μmol mol-1 towards the end of this century (IPCC, 2014). Plant pathogens are responsible for about 25% crop losses globally in agricultural ecosystems. Crop losses due to insect herbivores are also significant particularly in African countries (Botha et al., 2019). These losses are predicted to be even higher in some crops in the future as a result of climate change and this will have serious implications for food security (Martinelli et al. , 2015). While a better understanding of the molecular events that determine how elevated CO2 (eCO2) influences plant-insect interactions is essential for the improvement of plant resilience traits, it is difficult to predict how this change will affect phloem feeding insects such as aphids (Sun and Ge, 2011).
The effects of high CO2 on aphid fecundity reported in literature are highly variable (Guo et al., 2014; Ryalls, et al., 2017). High atmospheric CO2 had little impact on aphid performance in oilseed rape (Himanen et al., 2008), while the performance of the pea aphid (Acyrthosiphon pisum ) was increased under eCO2 on Medicago sativa (Ryalls, et al., 2017). Growth under eCO2 increased the resistance of twoM. truncatula genotypes to pea aphids by increasing SA-dependent defences and decreasing JA and ethylene-dependent signalling pathways, as well as increasing the density of non‐glandular and glandular trichomes (Guo et al., 2014). In contrast, the performance of the pea aphid was decreased in a free air enrichment (FACE) studies of performance on Vicia faba (Mondor et al., 2005). Since photosynthesis is stimulated by high CO2 it could be predicted that assimilate transport through the phloem would be increased, benefitting phloem feeding aphids. However, aphid fecundity might be decreased by growth under elevated atmospheric CO2 because these activate SA-mediated defence pathways (Mhamdi and Noctor, 2016). Moreover, plants may perceive high CO2 as a stress, not least because growth under high CO2 alters the balance of cellular redox signalling (Foyer and Noctor, 2020).
The silencing of SL biosynthesis genes compromised plant defences against the root knot nematode (RKN) Meloidogyne incognita (Xu et al., 2019). JA and ABA, which are positive and negative regulators of RKN resistance, were both suppressed by SL in nematode-infected roots (Xu et al., 2019). However, there have been no reports of whether SLs mediate plant defences against insects or how SL signalling contributes to plant responses to high atmospheric CO2 levels. However, SLs were recently found to play a role in the stomatal closure induced by pathogens and high CO2 concentrations (Kalliola et al., 2020). These authors reported that the SL biosynthesis (max3 and max4 ) and the SL perception (max2 anddwarf14 ) Arabidopsis mutants were more sensitive to infection by the biotrophic pathogen Pseudomonas syringae DC3000. Moreover, stomatal conductance was higher in the SL mutants and the stomatal responses to high CO2 were impaired in the SL biosynthesis and perception mutants (Kalliola et al., 2020).
In this study, we firstly compared the phenotypes of wild type peas and different pea ramosus mutants (rms ; Johnson et al ., 2006) to growth under high atmospheric CO2 levels. Shoot architecture was compared in wild type peas and mutants that are deficient in either SL synthesis (rms1-2 and rms5-3 ) or SL signalling (rms3-1 and rms4-1 ). We next compared aphid fecundity on the different lines grown under high CO2conditions to test the hypothesis that SL-dependent pathways are important in plant defences against aphids. The results of these studies show that aphid numbers are greater on SL mutants than the wild type under both growth conditions and this is related to decreased levels of gibberellic acid.
Taken together, these data demonstrate that while SL had little effect on plant acclimation to high [CO2], in terms of dry biomass accumulation, they apparently affect shoot water status. The role of SL in the resistance of peas to aphid infestation is perhaps not surprising given that these phytohormone have important functions in plant interactions with bacteria, fungi and nematodes. The identification of SL as a key player in plant defences against aphids paves the way for new strategies for the development of more aphid resistant crops.