The rhizosphere harbors abundant plant growth-promoting rhizobacteria (PGPR) that are vital for plant health. In this study, we screened growth-promoting bacteria from tomato rhizosphere soil, verified their functions, and constructed the optimal combination of growth-promoting bacteria for promoting tomato growth. Furthermore, the effects of these bacteria on various physiological and biochemical parameters of tomato plants were evaluated. A total of 36 strains of rhizobacteria were isolated from tomato rhizosphere soil and their abilities to produce indole-3-acetic acid (IAA), solubilize phosphate and iron carriers were assessed. The bacterial strains with the highest capacities for IAA production (R62, R317), phosphate solubilization (R41, R219), and siderophore production (R25, R325) were selected to form three bacterial combinations: R62 + R219 + R317 + R325 (T1), R62 + R325 (T5), and R317 + R325 (T8). Fifteen days after inoculation, all three combinations showed a stimulatory effect on seedling growth compared to the un-inoculated control. Inoculation with T1, T5 and T8 increased the seedling vigor index by 173.7%, 204.1%, and 168.7%, respectively. Compared to the un-inoculated control, the T1 combination increased the activities of polyphenol oxidase, peroxidase, and the net photosynthetic rate by 132.7%, 18.7%, 58.5%, and upregulated the relative expression levels of the photosynthetic assimilation-related genes RbcL, RbcS, FBPase and FDA by 22.2-, 6.6-, 1.95-, and 2.0-fold, respectively. Our findings provide a potential for constructing rhizobacterial combinations of different functional groups for improving crop growth.

The release of herbivore-induced plant volatiles (HIPVs) has been recognized to be an important strategy for plant adaptation to herbivore attack. However, whether these induced volatiles are beneficial to insect herbivores, particularly insect larvae, is largely unknown. We used the two important highly polyphagous lepidopteran pests Spodoptera frugiperda and S. litura to evaluate the benefit on xenobiotic detoxification of larval exposure to HIPVs released by the host plant maize (Zea mays). Larval exposure of the invasive alien species S. frugiperda to maize HIPVs significantly enhanced their tolerance to all three of the well-known defensive compounds 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), chlorogenic acid, and tannic acid in maize and the two commonly used insecticides methomyl and chlorpyrifos. HIPV exposure also improved the larval tolerance of S. litura third instars to chlorogenic and tannic acids. Furthermore, larval exposure to either maize HIPVs or DIMBOA induced the activities of cytochrome P450 enzymes (P450s), glutathione-s-transferase (GST), and carboxylesterase (CarE) in the midguts and fat bodies of the two insects, while the induction was significantly higher by the two components together. In addition, the expression of four genes encoding uridine diphosphate (UDP)-glycosyltransferases (UGT33F28, UGT40L8) and P450s (CYP4d8, CYP4V2) showed similar induction patterns in S. frugiperda. Cis-3-hexen-1-ol, an important component in maize HIPVs, also showed the same functions as maize HIPVs, and its exposure increased larval xenobiotic tolerance and induced the detoxification enzymes and gene expression. Our findings demonstrate that HIPVs released by the pest-infested host plants are conductive to the xenobiotic tolerance of lepidopteran insect larvae. Hijacking the host plant HIPVs is an important strategy of the invasive alien polyphagous lepidopteran pest to counter-defend against the host plant’s chemical defense.

Herbivore-induced plant volatiles (HIPVs) are well recognized for their ability to recruit natural enemies of insect herbivores; however, their function in priming plant anti-herbivore defenses remains poorly elucidated. Here, we demonstrate that volatile exposure to a commercial β-ocimene isomer mixture (hereinafter referred to as β-ocimene) enhances anti-herbivore defense responses in tomato (Solanum lycopersicum) against the polyphagous lepidopteran pest Spodoptera litura (Fabricius). Larval infestation by S. litura significantly upregulated the expression of ten terpene synthase (TPS) genes that govern the biosynthesis of four dominant monoterpene HIPVs: β-ocimene, (-)-α-phellandrene, myrcene, and limonene. Preliminary bioassays revealed that pre-exposure to these four monoterpenes significantly enhances tomato resistance to S. litura larvae, with β-ocimene exhibiting the most pronounced effect. A 24 h pre-exposure to β-ocimene substantially augmented tomato antiherbivore resistance, and this protective response persisted for up to 10 days. Biochemical and molecular analyses revealed that pre-exposure to β-ocimene elevated the endogenous accumulation of jasmonic acid (JA) and jasmonoyl-isoleucine, along with increased transcript levels of the JA biosynthetic genes LOXD and AOS. Consistently, the JA biosynthesis-deficient mutant spr8 failed to exhibit strengthened antiherbivore resistance upon β-ocimene pre-exposure. Collectively, our results demonstrate that β-ocimene pre-exposure enhances tomato antiherbivore resistance via activating JA biosynthesis, highlighting the potential ecological function of HIPVs in priming plant defense against insect pests.