Salt-tolerant and -sensitive alfalfa (<i>Medicago sativa</i>) cultivars have large variations in defense responses to the lepidopteran insect <i>Spodoptera litura</i> under normal and salt stress condition

<div><p>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 (<i>Medicago sativa</i>) cultivars in response to insect <i>Spodoptera litura</i> 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 <i>JAR1</i> (<i>jasmonate resistant 1</i>) 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 <i>JAR1</i> 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.</p></div

<i>S</i>. <i>litura</i> growth and ABA contents under normal and salt-stress condition.

<p>Zhongmu-1 and Xinjiang Daye were irrigated with 250 mM NaCl or water and thereafter cultivated for a week. (A) Masses of <i>S</i>. <i>litura</i> feeding on these plants (n = 150). (B) ABA levels in Zhongmu-1 and Xinjiang Daye, 1 h after W+OS treatment (untreated plants served as controls; n = 5). Different lowercase letters represent significant differences among the combinations of abiotic stresses and cultivars. Different uppercase letters indicate significant differences between biotic stresses within the same cultivar and abiotic treatment (Tukey HSD test; P < 0.05).</p

JA and JA-Ile contents in two alfalfa cultivars in response to <i>S</i>. <i>litura</i> feeding.

<p>Zhongmu-1 and Xinjiang Daye were treated with W+OS, and samples were harvested at 0, 0.5, 1, 1.5 and 3 h, and the JA (A) and JA-Ile (B) levels were determined (n = 5). Asterisks indicate significances between two cultivars with the same treatment and time point (Unpaired <i>t</i>-test; *, P < 0.05; ***, P < 0.001).</p

Insect performance on seven different alfalfa cultivars and a two-way choice assay on Zhongmu-1 and Xinjiang Daye.

<p>(A) Larval masses of <i>S</i>. <i>litura</i> feeding on seven different cultivars of alfalfa (n = 150). (B) A photograph and (C) a bar-graph showing the two-way choice test assessing the leaf tissue consumption of <i>S</i>. <i>litura</i> larvae on Zhongmu-1 and Xinjiang Daye. Different lowercase letters indicate significant differences among different cultivars (Tukey HSD test; P < 0.05); asterisks indicate significantly different levels between two cultivars (paired <i>t</i>-test; ***, P < 0.001).</p