Introduction
Insects are commonly associated with symbiotic microbes which could
bring some advantages to themselves. The pea aphid Acyrthosiphon
pisum , a model insect for endosymbiont studies, usually hosts one
obligate symbiont and several facultative symbionts (Douglas & Prosser,
1992; La Pena, Vandomme, & Frago, 2014). Since phloem sap is a
suboptimal diet for aphids due to imbalanced amino acids: carbohydrates
proportions, the obligate symbiont, Buchnera aphidicola that
resides in aphid bacteriocytes offers
essential amino acids
to aphids (Douglas, 1998). By contrast, facultative symbionts are
usually not required for survival and reproduction of aphids but are
able to improve their fitness particularly when they are confronted by
various biotic and abiotic challenges. Studies indicate that some
facultative symbionts increase heat tolerance (Doremus & Olive, 2017),
defense against the natural enemies and fungi (Łukasik, Van Asch, Guo,
Ferrari, & Godfray, 2013; Oliver, Russell, Moran, & Hunter, 2003), and
host plant adaptation of aphids (Leonardo & Muiru, 2003). Previous
research shows that an elicitor protein GroEL from B. aphidicolatriggers plant resistance against aphids (Chaudhary, Atamian, Shen,
Briggs, & Kaloshian, 2014). Hence, endosymbionts may modify the
components of salivary proteins that modulate aphid feeding on plants.
Very little is known, however, how facultative endosymbionts affect
aphid feeding by altering aphid salivary proteins.
Aphids use their stylets to obtain nutrients from phloem sieve elements
of the host plants (Tjallingii & Esch, 1993). While probing, aphids
secrete saliva to inhibit plant defense response caused by wounding due
to aphid feeding as well as clogging of the fluid in the phloem tissue
usually induced by Ca2+ influx (Aidemark, Andersson,
Rasmusson, & Widell, 2009). Plants typically initiate rapid cytosolic
Ca2+ sparks upon aphid probing (Vincent et al., 2017),
resulting in the activation of local defenses including reactive oxygen
species (ROS) and phytohormone signaling (Tian et al., 2019).
ROS-induced cell death prevents the aphid stylet from reaching the
phloem (Laitinen et al., 2017) and triggers the salicylic acid (SA)
signaling pathway to confer the local plant resistance to aphids
(Hogenhout & Bos, 2011; Jaouannet et al., 2014). In addition,
Ca2+ is also a long-distance signal that can be
transmitted from injured cells to uninfected tissues, activating the
wound-related systemic jasmonic acid (JA) signaling pathway (Farmer,
Gasperini, & Acosta, 2014). Both synergistic and antagonistic JA- and
SA-regulated defense have been reported in plant responses to aphids
(Moran and Thompson,2001; Moran et al., 2002; De Vos et al., 2005; Mewis
et al., 2006; Pegadaraju, 2005). Conversely,
Ca2+-binding proteins secreted by aphids into plant
tissues have been shown to facilitate aphids feeding. For example,
Armet, a salivary protein from the pea aphid sequesters
Ca2+ to counteract the
Ca2+-triggered occlusion in Vicia fabae , which
prolongs aphid phloem feeding time (Wang et al., 2015). Effectors
containing Ca2+-binding domains presumably could
efficiently quench the cytosolic Ca2+ elevation of
host plants, which consequently improves aphid feeding.
The facultative symbiont Serratia symbiotica increases heat
tolerance of A. pisum , and enhances resistance to parasitoids and
predators (Costopoulos, Kovacs, Kamins, & Gerardo, 2014; Hopper et al.,
2018; Montllor, Maxmen, & Purcell, 2002), but little in known of
whether and how S. symbiotica benefits aphid feeding. Previous
studies showed that pea aphid clones collected from 11 legume plants
differed in their facultative symbionts and infectious rate (Frantz,
Calcagno, Mieuzet, Plantegenest, & Simon, 2009; Henry et al.,
2013; Simon et al., 2003), suggesting that host plant species impact the
facultative symbionts associated with aphids. During July-August of
2015-2019, we intended to determine the prevalence of endosymbionts of
pea aphids in Medicago sativa fields in Yinchuan city, Ningxia
province, China, and identified 5 endosymbionts within 154 sampled pea
aphids (Fig. S1). S. symbiotica had a relatively high infectious
rate (85%) in all sampled aphids, we hypothesized that infection ofS. symbiotica may benefit the aphid growth on Medicagoplants by facilitating aphid feeding. To test the hypothesis, we
monitored aphid feeding behavior, performed aphid salivary gland
transcriptomic analysis in the presence and absence of S.
symbiotica . Histidine-rich calcium-binding protein-like (GeneID:
103308203) (A. pisum HRC , ApHRC ) that encodes a
salivary protein was up-regulated by S. symbiotica .
Further investigation indicated that ApHRC suppressed
Ca2+ elevation, which otherwise would have induced the
plant defense response. Our study has demonstrated that S.
symbiotica infection can modify aphid salivary composition by
modulating salivary gland gene expression, which suppressed plant
defense and benefited aphid feeding.