Worldwide, cadmium (Cd) contamination severely threatens rice production and public health. Jasmonic acid (JA) is recognized to be involved in rice Cd stress responses, but the underlying mechanism remains unclear. In this study, we show that JA positively regulates Cd tolerance in rice by repressing Cd uptake and root-to-shoot translocation. Cd exposure rapidly elevated the endogenous JA in rice roots, which was associated with increased expression of JA synthesis and JA-responsive genes. Moreover, silencing the expression of either allene oxide synthase (OsAOS; active in JA biosynthesis) or CORONATINE INSENSITIVE1 (OsCOI1; active in JA perception) resulted in aggravated Cd toxicity and increased Cd accumulation in both the roots and shoots, as well as increased translocation from the root to the shoots. A short-term uptake experiment revealed that silencing of OsAOS and OsCOI1 enhanced root Cd uptake ability. Furthermore, the elevated transcript levels of genes for Cd uptake (OsNramp5, OsNramp1, and OsIRT1) and root-to-shoot translocation (OsHMA2) were observed in OsAOS and OsCOI1 RNAi plants in comparison with wild-type plants. Taken together, our findings suggest that JA enhances rice cadmium tolerance by suppressing Cd uptake and translocation.

Polyamines (PAs) are ubiquitous low-molecular-weight aliphatic polycations in all living organisms, which are crucial for plant response to abiotic and biotic stresses. The role of PAs in plant disease resistance has been well documented. However, their involvement in plant–pest interactions remains unclear. Here, the role of PAs in rice against striped stem borer (SSB, Chilo suppressalis Walker), a destructive pest in rice production worldwide, was investigated. SSB larval infestation led to a substantial accumulation of free putrescine (Put) in rice seedlings, which was in parallel with an elevated expression of host PA biosynthesis genes Arginine Decarboxylase1 (ADC1) and ADC2. Moreover, SSB larval oral secretion application with wounding further raised the transcripts of ADC1 and ADC2 in rice compared with wounding treatment alone. The larval growth on both rice plants and artificial diet was promoted by the exogenous application of PA and inhibited by a PA biosynthesis inhibitor. On the other hand, the rice defense responses, including polyphenol oxidase (PPO) and peroxidase (POD) activities, as well as protease inhibitor level, were enhanced by a Put supplement and reduced by an ADC inhibitor. Our results indicate that SSB herbivory triggers polyamine accumulation in host rice plants, which is beneficial to SSB in rice–SSB interaction.

Silicon (Si) protects plants against insect herbivores; however, the underlying mechanisms remain unclear. Polyamines (PAs) play a crucial role in plant–insect interactions. Here, the involvement of Si in putrescine (Put) metabolism and its role in rice resistance against striped stem borer (SSB, Chilo suppressalis Walker) were investigated. The results showed that SSB larval infestation led to a substantial accumulation of free Put in rice seedlings. Si application increased rice resistance against SSB and repressed the SSB attack-induced accumulation of Put, in parallel with a decreased expression of Put biosynthesis genes encoding arginine decarboxylase (ADC1 and ADC2). Moreover, Si application had no significant effect on the wounding-induced expression of ADC1 and ADC2, but attenuated the further elevation in the transcription of ADC1 and ADC2 induced by SSB larvae oral secretion. Simultaneously, Si addition reduced the Put and spermidine contents in SSB-attacked plants. Furthermore, the exogenous application of Put attenuated Si-enhanced resistance against SSB larvae, whereas exogenous D-arginine, an inhibitor of ADC, showed similar effects to Si on rice resistance against SSB. Our findings indicate that Si improves rice resistance to SSB, at least partly by reducing herbivory-stimulated putrescine accumulation.

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.