Inducible direct defense responses in maize to pink stem borer (Sesamia inferens Walker): challenges and opportunities for sustainable solutions

Abstract

Maize (Zea mays L.) is the third most important cereal food crop after wheat and rice, grown in an area of 9.2 million ha with the production of 30.2 million MT (DES, 2021). Pink stem borer (PSB), Sesamia inferens Walker is one of the major insect pests in the winter maize resulting in 25.7-78.9 percent yield losses (Siddiqui and Marwaha 1993). The larvae feed inside the leaf sheath in groups and subsequently bore into the central shoot resulting in the formation of dead hearts. Understanding the mechanisms of Host Plant Resistance (HPR) can provide an opportunity to manage PSB. HPR is an economically viable, environmentally friendly, and sustainable strategy and is one of the most effective components of integrated pest management (IPM) modules. Understanding the plant-insect interactions is of utmost importance for developing effective pest management approaches. Therefore, the present study was undertaken to study whether the resistance was inducted in response to plant-insect and/or non-insect interactions. Materials and methods The experiment was conducted in two different sets one each to record the response of genotypes upon PSB infestation and defense responses of the genotypes against different treatments in field and pot, respectively. To investigate the maize defense responses, five treatments were designed including insect and non-insect interactions namely (i) control (untreated plants) (ii) PSB larvae infestation, (iii) mechanical wounding, (iv) mechanical wounding plus PSB regurgitation, (v) exposure to methyl jasmonate. The treatments were imposed at two stages i.e.V3 and V6 stages in resistant, moderately resistant and susceptible genotypes. Maize genotypes were categorized The quantification of two major defense related cell wall bound phenolic compounds namely p-Coumaric acid (p-CA) and ferulic acid (FA) was carried out through ultra-fast liquid chromatography (UFLC). The effect of induced treatments on p-CA and FA content at each sampling time were subjected to two-way ANOVA by using a general linear model (PROC GLM) using SAS version 9.3. Results and Discussion The p-CA content induced in leaf tissues of maize genotypes were intrinsically higher when challenged by PSB attack at V3 and V6 stages in short- and long-term responses. Higher p-CA content was observed in stalk tissues upon wounding and regurgitation in short- and long-term responses at V3 and V6 stages. Significant accumulation of FA content was also observed in leaf tissues in response to PSB feeding at V3 stage in long-term response while at V6 stage it was observed both in short- and long-term responses. In stalk tissues, methyl jasmonate induced higher FA content in short-term response at V3 stage. However, at V6 stage PSB feeding induced FA accumulation in the short-term while, wounding and regurgitation treatment-induced defense responses in the long-term. The study indicates that phenolic mediated defense responses in maize are induced by PSB attack followed by wounding and regurgitation and the response observed was more in resistant genotypes compared to susceptible ones. Breeding for the restoration of these defense chemicals or cross species transfer of defense metabolite production may harness the chemical defense capabilities of maize resulting in effective pest management.