Abstract
The activation of the abscisic acid (ABA) signaling pathway reduces water loss from plants challenged by drought stress. The effect of drought-induced ABA signaling on the defense and nutrition allocation of plants is largely unknown. We postulated that these changes can affect herbivorous insects. We studied the effects of drought on different feeding stages of pea aphids in the wild-type A17 of Medicago truncatula and ABA signaling pathway mutant sta-1. We examined the impact of drought on plant water status, induced plant defense signaling via the abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) pathways, and on the host nutritional quality in terms of leaf free amino acid content. During the penetration phase of aphid feeding, drought decreased epidermis/mesophyll resistance but increased mesophyll/phloem resistance of A17 but not sta-1 plants. Quantification of transcripts associated with ABA, JA and SA signaling indicated that the drought-induced up-regulation of ABA signaling decreased the SA-dependent defense but increased the JA-dependent defense in A17 plants. During the phloem-feeding phase, drought had little effect on the amino acid concentrations and the associated aphid phloem-feeding parameters in both plant genotypes. In the xylem absorption stage, drought decreased xylem absorption time of aphids in both genotypes because of decreased water potential. Nevertheless, the activation of the ABA signaling pathway increased water-use efficiency of A17 plants by decreasing the stomatal aperture and transpiration rate. In contrast, the water potential of sta-1 plants (unable to close stomata) was too low to support xylem absorption activity of aphids; the aphids on sta-1 plants had the highest hemolymph osmolarity and lowest abundance under drought conditions. Taken together this study illustrates the significance of cross-talk between biotic-abiotic signaling pathways in plant-aphid interaction, and reveals the mechanisms leading to alter aphid fecundity in water stresses plants.
Highlights
Drought is a serious agricultural problem because 40% of the world’s agricultural land lies in arid or semi-arid regions (Ehsanpour and Razavizadeh, 2005)
We examined the impact of drought on plant water status, induced plant defense signaling via the abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) pathways, and on the host nutritional quality in terms of leaf free amino acid content
Relative to a well-watered condition, the abundance of pea aphids associated with A17 and sta-1 was decreased by 35.1% and 78.2%, respectively, under drought stress beginning 15 d post-infestation
Summary
Drought is a serious agricultural problem because 40% of the world’s agricultural land lies in arid or semi-arid regions (Ehsanpour and Razavizadeh, 2005). Drought harms plants and reduces crop yields by causing cellular water deficits, membrane injury, and reduced enzyme activity (Su et al, 2013). By modifying the quality of host plants, drought can alter the performance of herbivorous insects. Plants challenged by drought have decreased water potential and water content in their leaf tissues (Morgan, 1984). Drought-induced carbon starvation may result in the re-allocation of primary metabolites into secondary metabolites, which can change host plant nutrition, palatability, and resistance with respect to herbivores (McDowell, 2011). Especially in legumes, inhibits biological N fixation and in turn decreases N accumulation in plant tissue (Serraj et al, 1999). Plant responses to water stress, including changes in the plant’s nutritive value, resistance, and water status, should be examined when considering the effect of water stress on plant–insect interactions
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