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Plants are often confronted by different groups of herbivores, which threaten their growth and reproduction. However, they are capable of mounting defenses against would-be attackers which may be heightened upon attack. Resistance to insects often varies among plant species, with different genotypes exhibiting unique patterns of chemical and physical defenses. Within this framework, plant access to nutrients may be critical for maximal functioning of resistance mechanisms and are likely to differ among plant genotypes. In this study, we aimed to test the hypothesis that access to nutrition would alter the expression of plant resistance to insects and alter insect performance in a manner consistent with fertilization regime. We used two maize (Zea mays) genotypes possessing different levels of resistance and the fall armyworm (Spodoptera frugiperda) as model systems. Plants were subjected to three fertilization regimes prior to assessing insect-mediated responses. Upon reaching V4 stage, maize plants were separated into two groups, one of which was infested with fall armyworm larvae to induce plant defenses. Plant tissue was collected and used in insect bioassays and to measure the expression of defense-related genes and proteins. Insect performance differed between the two plant genotypes substantially. For each genotype, fertilization altered larval performance, where lower fertilization rates hindered larval growth. Induction of plant defenses by prior herbivory substantially reduced naïve fall armyworm growth in both genotypes. The effects between fertilization and induced defenses were complex, with low fertilization reducing induced defenses in the resistant maize. Gene and protein expression patterns differed between the genotypes, with herbivory often increasing expression, but differing between fertilization levels. The soluble protein concentrations did not change across fertilization levels but was higher in the susceptible maize genotype. These results demonstrate the malleability of plant defenses and the cascading effects of plant nutrition on insect herbivory.

Abstract  The oral secretions or regurgitant of caterpillars contain potent elicitors of plant induced responses. These elicitors are recognized by host plants to differentiate between simple mechanical injury and the presence of herbivores. In some cases, this level of recognition is highly specific. Despite the in-depth chemical characterization of these elicitors, little is known about the amounts delivered in regurgitant during feeding. In this study, we use a fluorescent dye to label regurgitant in order to visualize caterpillar regurgitation during feeding. The procedure is highly sensitive and allows us to visualize nanoliter amounts of regurgitant. We examined the propensity of larval Helicoverpa zea, Heliothis virescens, Spodoptera exigua, Spodoptera frugiperda, and Manduca sexta to regurgitate on various host plants. These species were selected because they have been among the most intensely studied in terms of elicitors. Our results indicate that most larvae did not regurgitate following a brief feeding bout (∼10 min) during which they ate ca. 0.40 cm2 of leaf. When larvae did regurgitate, it was typically less than 10 nl. This is several orders of magnitude less than is typically used in most studies on oral secretions. The frequency of regurgitation appears to vary depending upon the host plant. Larval H. zea are less likely to regurgitate when feeding on tomato leaves compared to corn mid-whorl tissue. Our results have importance in understanding the role of oral secretions in plant recognition of herbivory. Because caterpillars did not routinely regurgitate during feeding, it is likely that they avoid the elicitation of some plant defensive responses during most feeding bouts. [ABSTRACT FROM AUTHOR]