Abstract
cis-Jasmone (CJ) is a natural plant product that activates defence against herbivores in model and crop plants. In this study, we investigated whether CJ could prime defence in maize, Zea mays, against the leafhopper, Cicadulina storeyi, responsible for the transmission of maize streak virus (MSV). Priming occurs when a pre-treatment, in this case CJ, increases the potency and speed of a defence response upon subsequent attack on the plant. Here, we tested insect responses to plant volatile organic compounds (VOCs) using a Y-tube olfactometer bioassay. Our initial experiments showed that, in this system, there was no significant response of the herbivore to CJ itself and no difference in response to VOCs collected from unexposed plants compared to CJ exposed plants, both without insects. VOCs were then collected from C. storeyi-infested maize seedlings with and without CJ pre-treatment. The bioassay revealed a significant preference by this pest for VOCs from infested seedlings without the CJ pre-treatment. A timed series of VOC collections and bioassays showed that the effect was strongest in the first 22 h of insect infestation, i.e. before the insects had themselves induced a change in VOC emission. Chemical analysis showed that treatment of maize seedlings with CJ, followed by exposure to C. storeyi, led to a significant increase in emission of the defensive sesquiterpenes (E)-(1R,9S)-caryophyllene, (E)-α-bergamotene, (E)-β-farnesene and (E)-4,8-dimethyl-1,3,7-nonatriene, known to act as herbivore repellents. The chemical analysis explains the behavioural effects observed in the olfactometer, as the CJ treatment caused plants to emit a blend of VOCs comprising more of the repellent components in the first 22 h of insect infestation than control plants. The speed and potency of VOC emission was increased by the CJ pre-treatment. This is the first indication that CJ can prime plants for enhanced production of defensive VOCs antagonist towards herbivores.
Highlights
Maize, Zea mays L. (Poaceae), is the third major cereal crop in the world after wheat and rice, and supplies 50% of the calorific intake in sub-Saharan Africa
In view of our earlier studies showing a role for CJ in inducing indirect and direct defence in Triticum aestivum, another poaceous plant [16,17,18], and the previously reported priming of a specific subset of jasmonic acid (JA)-inducible pathways in maize by defence volatile organic compounds (VOCs) [13], we investigated the ability of CJ to induce defence pathways in maize and to prime maize for enhanced defence against the leafhopper C. storeyi
There was no significant difference in the amount of time spent by adult C. storeyi in the region of the olfactometer containing volatile organic compounds (VOCs) collected from CJtreated maize seedlings, compared with the solvent control (P.0.05) or VOCs collected from untreated seedlings (P.0.05) (Table 1)
Summary
Zea mays L. (Poaceae), is the third major cereal crop in the world after wheat and rice, and supplies 50% of the calorific intake in sub-Saharan Africa. A farmer’s entire crop can be wiped out by maize streak virus (MSV) (Geminiviridae: Mastrevirus), an endemic pathogen of native African grasses and the most important plant virus disease of maize in sub-Saharan Africa [1]. It is acquired and transmitted by nine leafhopper species (Homoptera: Cicadellidae) in the genus Cicadulina, in a persistent manner. The negative impact of leafhopper populations upon maize production makes the search for alternative control methods extremely urgent. The potential for exploiting semiochemicals (naturally-occurring behaviour or development-modifying chemicals) in maize/leafhopper interactions, with a view to their deployment in leafhopper control, was investigated in our earlier work [5]
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