Salicylhydroxamic acid (SHAM) negatively mediates tea herbivore-induced direct and indirect defense against the tea geometrid Ectropis obliqua

Tissue-specific regulation of volatile emissions moves predators from flowers to attacked leaves

Plant-predator mutualisms have been widely described in nature.^1,2 How plants fine-tune their mutualistic interactions with the predators they recruit remains poorly understood. In the wild potato (Solanum kurtzianum), predatory mites, Neoseiulus californicus, are recruited to flowers of undamaged plants but rapidly move downward when the herbivorous mites, Tetranychus urticae, damage leaves. This "up-down" movement within the plant corresponds to the shift of N. californicus from palynivory to carnivory, as they change from feeding on pollen to herbivores when moving between different plant organs. This up-down movement of N. californicus is mediated by the organ-specific emissions of volatile organic compounds (VOCs) in flowers and herbivory-elicited leaves. Experiments with exogenous applications, biosynthetic inhibitors, and transient RNAi revealed that salicylic acid and jasmonic acid signaling in flowers and leaves mediates both the changes in VOC emissions and the up-down movement of N. californicus. This alternating communication between flowers and leaves mediated by organ-specific VOC emissions was also found in a cultivated variety of potato, suggesting the agronomic potential of using flowers as reservoirs of natural enemies in the control of potato pests.

Pseudomonas fluorescens BsEB-1: an endophytic bacterium isolated from the root of Bletilla striata that can promote its growth

An endophytic Pseudomonas fluorescens (BsEB-1) was obtained from the roots of Bletilla striata. We investigated its growth-promoting properties and observed the impact of its inoculation on both the growth and polysaccharide content of Bletilla striata tubers. It was found that BsEB-1 possessed three growth-promoting activities: phosphate-solubilizing, produced indoleacetic acid (IAA) and siderophores, but had no nitrogen-fixing activity. BsEB-1 could rapidly attach to the root hairs of Bletilla striata tissue culture seedlings and endophytically colonize the region of maturation in the roots. It also significantly promoted the rooting and transplant survival rate of the seedlings, as well as the growth and expansion of the tubers, but did not increase their polysaccharide content. Pseudomonas fluorescens BsEB-1 exhibits potential for applications in the artificial planting of Bletilla striata.

A novel inhibitor of the JA signaling pathway represses herbivore resistance in tea plants

The jasmonic acid (JA) signaling pathway plays a vital role in mediating plant resistance to herbivores. Tea plant (Camellia sinensis) is one of the most important woody cash crops in the world. Due to the lack of genetic transformation systems for tea plants, how the JA signaling pathway works in tea plants has not yet been determined. Now, with the development of cross-disciplines, chemical biology provides new means for analysing the JA signaling pathway. In the present study, the small molecule isoquinoline compound ZINC71820901 (lyn3) was obtained from the ZINC molecular library through virtual screening based on the structure of the crystal COI1-JAZ1 co-receptor and was found to act as an inhibitor of the JA signaling pathway both in Arabidopsis and tea plants. Our results revealed that lyn3 repressed tea plant resistance to Ectropis grisescens mainly by decreasing the accumulation of (-)-epicatechin (EC) and (-)-epigallocatechin (EGC) via repression of the JA signaling pathway, which functioned in the different modulation manner to the already known inhibitor SHAM. As a novel inhibitor of JA signaling pathway, lyn3 provides a specific option for further research on the JA pathway.

The Jasmonic Acid Signaling Pathway is Associated with Terpinen-4-ol-Induced Disease Resistance against Botrytis cinerea in Strawberry Fruit

Terpinen-4-ol, the main component of tea tree oil, markedly increases the disease resistance of postharvest strawberry fruit. To understand the mechanism underlying the enhancement of disease resistance, a high-throughput RNA-seq was used to analyze gene transcription in terpinen-4-ol-treated and untreated fruit. The results show that terpinen-4-ol induces the expression of genes in the jasmonic acid (JA) biosynthesis pathway, secondary metabolic pathways such as phenylpropanoid biosynthesis, and pathways involved in plant-pathogen interactions. Terpinen-4-ol treatment reduced disease incidence and lesion diameter in strawberry fruit inoculated with Botrytis cinerea. Terpinen-4-ol treatment enhanced the expression of genes involved in JA synthesis (FaLOX, FaAOC, and FaOPR3) and signaling (FaCOI1), as well as genes related to disease defense (FaPAL, FaCHI, and FaGLU). In contrast, treatment with the JA biosynthesis inhibitor salicylhydroxamic acid (SHAM) accelerated disease development and inhibited the induction of gene expressions by terpinen-4-ol. We conclude that the JA pathway participates in the induction of disease resistance by terpinen-4-ol in strawberry fruit. More generally, the results illuminate the mechanisms by which disease resistance is enhanced by essential oils.

Indole primes defence signalling and increases herbivore resistance in tea plants

Upon herbivore attack, plants emit herbivore-induced plant volatiles (HIPVs). HIPVs can prime defences and resistance of intact plants. However, how HIPVs are decoded and translated into functional defence responses is not well understood, especially in long-lived woody plants. Here, we investigated the impact of the aromatic HIPV indole on defence-related early signalling, phytohormone accumulation, secondary metabolite biosynthesis and herbivore resistance in tea plants. We find that tea plants infested with tea geometrid caterpillars release indole at concentrations >450 ng*hr^-1 . Exposure to corresponding doses of synthetic indole primes the expression of early defence genes involved in calcium (Ca²⁺ ) signalling, MPK signalling and jasmonate biosynthesis. Indole exposure also primes the production of jasmonates and defence-related secondary metabolites. These changes are associated with higher herbivore resistance of indole-exposed tea plants. Chemical inhibition of Ca²⁺ and jasmonate signalling provides evidence that both are required for indole-mediated defence priming and herbivore resistance. Our systematic assessment of the impact of indole on defence signalling and deployment shows that indole acts by boosting Ca²⁺ signalling, resulting in enhanced jasmonate-dependent defence and resistance in a woody plant. Our work extends the molecular basis of HIPV-induced defence priming from annual plants to an economically important tree species.

The Jasmonic Acid Pathway Positively Regulates the Polyphenol Oxidase-Based Defense against Tea Geometrid Caterpillars in the Tea Plant (Camellia sinensis)

Polyphenol oxidases (PPOs) as inducible defense proteins, contribute to tea (Camellia sinensis) resistance against tea geometrid larvae (Ectropis grisescens), and this resistance has been associated with the jasmonic acid (JA) signaling by testing geometrid performance in our previous work. However, the regulation of PPO-based defense by JA and other hormone signaling underlying these defense responses is poorly understood. Here, we investigated the role of phytohormones in regulating the PPO response to tea geometrids. We profiled levels of defense hormones, PPO activity and CsPPO genes in leaves infested with tea geometrids. Then, hormone levels were manipulated by exogenous application of methyl jasmonate (MeJA), gibberellin acid (GA₃), abscisic acid (ABA), JA biosynthesis inhibitors (sodium diethyldithiocarbamate trihydrate, DIECA and salicylhydroxamic acid, SHAM) and GA inhibitor (uniconazole, UNI). Upon geometrid attack, JA levels significantly increased, whereas GA levels notably decreased and ABA level was slightly decreased. And the PPO activity significantly increased in line with the transcript levels of CsPPO2 and CsPPO4 but not CsPPO1. There were an obvious antagonistic cross-talk between JA and GA signals and an association among JA signals, PPO response and herbivore resistance in tea plants. Pretreatment with MeJA increased PPO activity by activating the transcripts of CsPPO2 and CsPPO4, whereas application of JA inhibitor DIECA suppressed PPO activity. GA₃ strongly enhanced PPO activity, but ABA did not alter PPO activity. These findings strongly suggest that JA is a central player in PPO-mediated tea resistance against tea geometrids in a manner that prioritizes defense over growth.

Effects of Chemical Insecticide Imidacloprid on the Release of C6 Green Leaf Volatiles in Tea Plants (Camellia sinensis)

Chemical insecticides are widely used for pest control worldwide. However, the impact of insecticides on indirect plant defense is seldom reported. Here, using tea plants and the pesticide imidacloprid, effects of chemical insecticides on C₆-green leaf volatiles (GLVs) anabolism and release were investigated first time. Compared with the non-treated control plants, the treatment of imidacloprid resulted in the lower release amount of key GLVs: (Z)-3-hexenal, n-hexenal, (Z)-3-hexene-1-ol and (Z)-3-Hexenyl acetate. The qPCR analysis revealed a slight higher transcript level of the CsLOX3 gene but a significantly lower transcript level of CsHPL gene. Our results suggest that imidacloprid treatment can have a negative effect on the emission of GLVs due to suppressing the critical GLVs synthesis-related gene, consequently affecting plant indirect defense.

Tea green leafhopper, Empoasca vitis, chooses suitable host plants by detecting the emission level of (3Z)-hexenyl acetate

Green leaf volatiles (GLVs) have been reported to play an important role in the host-locating behavior of several folivores that feed on angiosperms. However, next to nothing is known about how the green leafhopper, Empoasca vitis, chooses suitable host plants and whether it detects differing emission levels of GLV components among genetically different tea varieties. Here we found that the constitutive transcript level of the tea hydroperoxide lyase (HPL) gene CsiHPL1, and the amounts of (Z)-3-hexenyl acetate and of total GLV components are significantly higher in tea varieties that are susceptible to E. vitis (Enbiao (EB) and Banzhuyuan (BZY)) than in varieties that are resistant to E. vitis (Changxingzisun (CX) and Juyan (JY)). Moreover, the results of a Y-tube olfactometer bioassay and an oviposition preference assay suggest that (Z)-3-hexenyl acetate and (Z)-3-hexenol offer host and oviposition cues for E. vitis female adults. Taken together, the two GLV components, (Z)-3-hexenol and especially (Z)-3-hexenyl acetate, provide a plausible mechanism by which tea green leafhoppers distinguish among resistant and susceptible varieties. Future research should be carried out to obtain the threshold of the above indices and then assess their reasonableness. The development of practical detection indices would greatly improve our ability to screen and develop tea varieties that are resistant to E. vitis.