This study aims to explore the relationship between the resistance of maize (Zea mays) to Spodoptera frugiperda and the content of secondary metabolites (ferulic acid, caffeic acid, chlorogenic acid and DIMBOA) in leaves, as well as the gene expression of key enzymes ZmPALs and ZmBXs in their synthesis pathways, so as to provide theoretical guidance for the breeding of insect-resistant maize varieties.

The resistance difference of six maize varieties cultivated in Shaanxi Province to S. frugiperda was evaluated by using artificial insect inoculation at maize whorl stage and leaf-feeding methods, with plant damage ratings, and development and reproduction parameters of S. frugiperda after feeding as indicators. Contents of three phenolic acids and DIMBOA were determined by colorimetry and high-performance liquid chromatography (HPLC), respectively. Real-time quantitative PCR (RT-qPCR) was used to determine mRNA expression levels of ZmPAL6, ZmPAL7, ZmBX10, and ZmBX11 in maize leaves of different varieties at 0, 6, and 18 h after infestation. And then, the relationships between secondary metabolite contents and resistance indicators measured were studied by correlation analysis.

Among the maize varieties tested, Zhengdan958, Qinlong14 and Shanke16 showed less damage, with mean ratings lower than 4.20 at 7-14 d after S. frugiperda infestation, while Zhengda12, Jiyuan1 and Dedan123 suffered the heaviest damage, with ratings from 4.31 to 7.22. Moreover, S. frugiperda larvae and pupae had the fastest development and significantly greater weight; adults had significantly longer lifespan and oviposition duration, as well as higher fecundity on Zhengda12 and Jiyuan1 compared to other varieties. By contrast, the slowest larvae development, significantly lower larval and pupal weight, longer adult preoviposition period lifespan, shorter oviposition duration and lower fecundity occurred on Zhengdan958, followed by Qinlong14 and Shanke16. Correlation analysis results showed that contents of chlorogenic acid, ferulic acid and caffeic acid in maize leaves showed significantly negative correlation with plant damage rating and larval weight of S. frugiperda, and significantly positive correlation with larval developmental period. In addition, chlorogenic acid content exhibited significantly negative correlation with pupal weight. DIMBOA content had significantly negative correlation with plant damage rating. The expressions of ZmPAL6, ZmPAL7, ZmBX10 and ZmBX11 were significantly induced by larval feeding. Whether non-infested or 6 and 18 h after infested plants, the expression levels of these four genes in Zhengdan958 and Qinlong14 were significantly higher than those in Jiyuan1 and Zhengda12.

Maize varieties Zhengdan958, Qinlong14 and Shanke16 planted in Shaanxi Province exhibit certain resistance to damage and growth and development of S. frugiperda, and their resistance is closely related to the contents of three phenolic acids and DIMBOA as well as the expression levels of ZmPALs and ZmBXs in leaves.