Influence of Plant Growth-Promoting Rhizobacteria (PGPR) Inoculation on Phenolic Content and Key Biosynthesis-Related Processes in Ocimum basilicum Under Spodoptera frugiperda Herbivory

Abstract

Plants are naturally subjected to various types of biotic stresses, includingpathogenic microorganisms and herbivory by insects, which trigger different signalingpathways and related defense mechanisms. Inoculation with microorganisms, such asplant growth-promoting rhizobacteria (PGPR), can be seen as a form of stress because ittriggers a systemic resistance response in plants similar to that caused by insect herbivory.However, these interactions have typically been studied independently, which has limitedthe understanding of their combined effects. This study examines the effects of Bacillusamyloliquefaciens GB03 inoculation and Spodoptera frugiperda herbivory on the total phenoliccontents of Ocimum basilicum. We also analyze the levels of endogenous phytohormonesand the activity of phenylalanine ammonia-lyase (PAL), a crucial enzyme involved in thebiosynthesis of phenolic defense-related metabolites. The results indicate that the totalphenolic content significantly increased only in plants that were both inoculated by GB03and damaged by larvae. Additionally, PAL activity showed an increase in plants that weredamaged by larvae and in those subjected to the combined treatment of larval damage andinoculation with GB03. Regarding phytohormones, in plants damaged by insects, the levelsof salicylic acid (SA) increased, regardless of whether they were inoculated or not, whilethe levels of jasmonic acid–isoleucine (JA-ile) rose in all treatments compared to the control.This study highlights the intricate relationships among beneficial microbes, herbivores,and plant defense mechanisms, emphasizing their potential impact on improving plantresilience and the production of secondary metabolites. Furthermore, understanding theindependent effects of PGPR inoculation, beyond its interaction with herbivory, couldprovide valuable insights into its role as a sustainable alternative for enhancing plantdefense responses and promoting crop productivity.