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Abstract:
The pea aphid (Acyrthosiphon pisum) is a complex of at least 15 genetically different
host races that are native to specific legume plants, but can all develop on the universal
host plant Vicia faba. Despite much research, it is still unclear why pea aphid host races
(biotypes) are able to colonize their native hosts while other host races are not. All aphids
penetrate the plant and salivate into plant cells when they test plant suitability. Thus
plants might react differently to the various pea aphid host races. To find out whether
legume species vary in their defense responses to different pea aphid host races, we
measured the amounts of salicylic acid (SA), the jasmonic acid-isoleucine conjugate
(JA-Ile), other jasmonate precursors and derivatives, and abscisic acid (ABA) in four
different species (Medicago sativa, Trifolium pratense, Pisum sativum, V. faba) after
infestation by native and non-native pea aphid clones of various host races. Additionally,
we assessed the performance of the clones on the four plant species. On M. sativa and
T. pratense, non-native clones that were barely able to survive or reproduce, triggered
a strong SA and JA-Ile response, whereas infestation with native clones led to lower
levels of both phytohormones. On P. sativum, non-native clones, which survived or
reproduced to a certain extent, induced fluctuating SA and JA-Ile levels, whereas the
native clone triggered only a weak SA and JA-Ile response. On the universal host V. faba
all aphid clones triggered only low SA levels initially, but induced clone-specific patterns
of SA and JA-Ile later on. The levels of the active JA-Ile conjugate and of the other JApathway
metabolites measured showed in many cases similar patterns, suggesting that
the reduction in JA signaling was due to an effect upstream of OPDA. ABA levels were
downregulated in all aphid clone-plant combinations and were therefore probably not
decisive factors for aphid-plant compatibility. Our results suggest that A. pisum clones
manipulate plant-defense signaling to their own advantage, and perform better on their
native hosts due to their ability to modulate the SA- and JA-defense signaling pathways.
