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Abstract
In nature, plants are often exposed to multiple stress factors at the same time. Yet, little is known about how plants modulate their physiology to counteract simultaneous abiotic and biotic stresses, such as soil salinity and insect herbivory. In this study, insect performance bioassays, phytohormone measurements, quantification of transcripts, and protein determination were employed to study the phenotypic variations of two alfalfa (Medicago sativa) cultivars in response to insect Spodoptera litura feeding under normal and salt stress condition. When being cultivated in normal soil, the salt-tolerant alfalfa cultivar Zhongmu-1 exhibited lower insect resistance than did the salt-sensitive cultivar Xinjiang Daye. Under salinity stress, the defense responses of Xinjiang Daye were repressed, whereas Zhongmu-1 did not show changes in resistance levels. It is likely that salinity influenced the resistance of Xinjiang Daye through suppressing the accumulation of jasmonic acid-isoleucine (JA-Ile), which is the bioactive hormone inducing herbivore defense responses, leading to attenuated trypsin proteinase inhibitor (TPI) activity. Furthermore, exogenous ABA supplementation suppressed the insect herbivory-induced JA/JA-Ile accumulation and levels of JAR1 (jasmonate resistant 1) and TPI, and further decreased the resistance of Xinjiang Daye, whereas Zhongmu-1 showed very little response to the increased ABA level. We propose a mechanism, in which high levels of abscisic acid induced by salt treatment may affect the expression levels of JAR1 and consequently decrease JA-Ile accumulation and thus partly suppress the defense of Xinjiang Daye against insects under salt stress. This study provides new insight into the mechanism by which alfalfa responds to concurrent abiotic and biotic stresses.
Highlights
Plants are constantly exposed to various abiotic and biotic stresses in nature [1], including insect herbivory and salt stress
To evaluate the resistance of different alfalfa varieties to the generalist lepidopteran insect Spodoptera litura, a no-choice bioassay was performed on seven alfalfa cultivars
To determine whether Zhongmu-1 is preferred over Xinjiang Daye by S. litura, a two-way choice test was performed, in which the larvae could freely feed on both cultivars
Summary
Plants are constantly exposed to various abiotic and biotic stresses in nature [1], including insect herbivory and salt stress. Insect attack results in changes in plant morphologies and/or chemical compositions, which are mediated by molecular processes such as phytohormone signaling and transcriptomic rearrangements [3, 5]. Among these responses, jasmonic acid (JA) signaling plays a critical role in plant defense against insects. Abscisic acid (ABA) signaling is one of the most important regulators in plant tolerance to abiotic stress, such as drought and salinity [9, 10]. Plants impaired in ABA biosynthesis showed increased susceptibility to salt stress at the vegetative stage [11]
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