A major bottleneck in identifying therapies to control citrus greening and other devastating plant diseases caused by fastidious pathogens is our inability to culture the pathogens in defined media or axenic cultures. As such, conventional approaches for antimicrobial evaluation (genetic or chemical) rely on time-consuming, low-throughput and inherently variable whole-plant assays. Here, we report that plant hairy roots support the growth of fastidious pathogens likeCandidatusLiberibacter spp., the presumptive causal agents of citrus greening, potato zebra chip and tomato vein greening diseases. Importantly, we leverage the microbial hairy roots for rapid, reproducible efficacy screening of multiple therapies. We identify six antimicrobial peptides, two plant immune regulators and eight chemicals which inhibitCandidatusLiberibacter spp. in plant tissues. The antimicrobials, either singly or in combination, can be used as near- and long-term therapies to control citrus greening, potato zebra chip and tomato vein greening diseases.

Above‐ground plant tissues produce characteristic blends of volatile compounds in response to insect herbivory. These herbivore‐induced plant volatiles (HIPVs) function in plant defence and mediate foraging decisions by herbivores and their natural enemies. The ecological roles of HIPVs as foraging cues for different trophic levels highlight an important conflict for herbivores that need to locate suitable host plants while avoiding competition and predation. Plant roots also emit HIPVs following herbivory, but our understanding of root‐produced volatiles and their ecological functions in soil environments remains limited. Moreover, recent studies have documented the effects of temporal dynamics of plant volatile production on ecological interactions, but little is known about how root HIPVs change throughout herbivory or the resulting ecological implications from such changes. In this study, we examined the roles of HIPVs from roots of cucumber plants Cucumis sativus as foraging cues for a specialist herbivore, striped cucumber beetle Acalymma vittatum and its natural enemies, entomopathogenic nematodes (EPNs). We predicted HIPVs from A. vittatum‐damaged roots would attract EPNs, while repelling conspecific larvae that avoid competition, induced plant defences and increased risk of predation by EPNs. To capture the temporal dynamics of root HIPVs, we determined how HIPV‐mediated interactions change over time with sustained herbivory. Initially (after 24 hr), A. vittatum herbivory on C. sativus, or mechanical wounding, induced greater production of root volatiles. These root HIPVs recruited EPNs and repelled foraging A. vittatum larvae, although larval performance was not affected by prior damage. Sustained (7 days) herbivory by larvae reduced HIPVs to levels indistinguishable from undamaged control roots while mechanically damaged roots continued to produce higher levels of volatiles. Attenuation of HIPVs impaired indirect defence responses of C. sativus by reducing recruitment of EPNs and deterrence of A. vittatum larvae. These results suggest that root HIPVs function as honest signals that indicate the presence of herbivores, induction of indirect plant defences and increased risk of predation by natural enemies. However, some herbivores may overcome this line of plant defence by attenuating production of HIPVs and thus altering the outcomes of subsequent interactions among plants, herbivores and natural enemies. A free Plain Language Summary can be found within the Supporting Information of this article.

Two oxylipins 12-OPDA (12-Oxo-10(Z),15(Z)-phytodienoic acid) and an α-ketol,9,10-KODA (10-oxo-9-hydroxy-12(Z), 15(Z)-octadecadienoic acid) were recently identified as important long-distance-induced systemic resistance (ISR) signals in Trichoderma virens-treated maize. On the other hand, jasmonic acid (JA), long believed to be a major signal of ISR, was not involved, as the JA-deficient mutant, opr7 opr8, retained the capacity for T. virens-triggered ISR. In order to further understand the biochemical basis for ISR priming in maize leaves, diverse oxylipins and phytohormones in the leaves of wild-type maize or ISR-deficient lox10-3 mutants treated with T. virens were quantified. This analysis revealed that 12-OPDA and two novel α-ketols, 9,12-KODA (12-Oxo-9-hydroxy-10(E)-octadecenoic acid) and 9,12-KOMA (12-Oxo-9-hydroxy-10(E),15(Z)-octadecadienoic acid), accumulated at high levels in ISR-positive plants. In support of the notion that 12-OPDA serves as a priming agent for ISR in addition to being a xylem-mobile signal, leaf pretreatment with this JA precursor resulted in increased resistance to Colletotrichum graminicola. Furthermore, the injection of 9,12-KODA or 9,12-KOMA in wild-type plants enhanced resistance against C. graminicola infection, suggesting that they play roles in ISR priming.

In plants, embryo size is determined via interactions between metabolic and developmental signals. Maize (Zea mays) big embryo 6 (bige6) enhances embryo size while sharply reducing plant growth. Here, we show that BigE6 encodes a plastidial prephenate aminotransferase (PPA-AT), a key enzyme in the arogenate pathway for L-phenylalanine (Phe) and L-tyrosine (Tyr) biosynthesis. The maize BigE6 paralog, BigE6Like, encodes a cytosol-localized PPA-AT, revealing Phe and Tyr biosynthesis via cytosolic arogenate as a potential alternative to the known cytosolic phenylpyruvate pathway. Moreover, the single PPA-AT gene of Arabidopsis (Arabidopsis thaliana) encodes plastidial and cytosolic enzymes by alternative splicing. Transgenic rescue of a ppa-at mutant in Arabidopsis demonstrates that the plastidial PPA-AT is indispensable for seed formation due, in part, to its essential role in the female gametophyte. Leaves of bige6 maize maintained overall homeostasis for aromatic amino acids and downstream metabolites, revealing a resilience of mechanisms that scale growth to a limiting supply of Phe and Tyr. In bige6 seeds, broad perturbation of amino acid homeostasis is associated with transcriptomic upregulation of growth processes in the embryo and endosperm, implicating amino acid signaling in the regulation of embryo size. Our findings reveal the complexity and developmental dependence of growth responses to limiting amino acid biosynthesis.

Plant organ growth results from the combined activity of cell division and cell expansion. The co-ordination of these two processes depends on the interplay between multiple hormones that determine the final organ size. Using the semidominant Hairy Sheath Frayed1 (Hsf1) maize mutant that hypersignals the perception of cytokinin (CK), we show that CK can reduce leaf size and growth rate by decreasing cell division. Linked to CK hypersignaling, the Hsf1 mutant has an increased jasmonic acid (JA) content, a hormone that can inhibit cell division. The treatment of wild-type seedlings with exogenous JA reduces maize leaf size and growth rate, while JA-deficient maize mutants have increased leaf size and growth rate. Expression analysis revealed the increased transcript accumulation of several JA pathway genes in the Hsf1 leaf growth zone. A transient treatment of growing wild-type maize shoots with exogenous CK also induced the expression of JA biosynthetic genes, although this effect was blocked by the co-treatment with cycloheximide. Together, our results suggest that CK can promote JA accumulation, possibly through the increased expression of specific JA pathway genes.

Methyl salicylate (MeSA) is a component of the herbivore-induced volatile blend of tomato (Solanum lycopersicum), and an intermediate in long distance salicylic acid (SA) signaling. To investigate the role of MeSA in interactions between tomato and the lepidopteran pests Helicoverpa zea and Manduca sexta, we used transgenic tomato lines that were antisense (AS) or overexpressors (OE) of the tomato SA carboxyl methyl transferase SlSAMT1, which under- or over-produce MeSA, respectively. Transgenic AS plants produced less MeSA and were more susceptible to both insects compared to wild-type plants, while OE plants had elevated MeSA production but displayed increased resistance only to M. sexta. Pairwise choice and toxicity assays revealed that MeSA itself impacted the larvae as a deterrent rather than a toxin, and metabolic profiling indicated that increased susceptibility in the AS lines and resistance in the OE lines may be related to altered jasmonic acid and ABA mediated herbivore-induced defense signaling, and modified production of inducible defense compounds such as flavonoids and phenolamides. To expand beyond Lepidoptera, the settling preference of the piercing-sucking insect Bemisia tabaci was also examined and it displayed a strong preference for lines with less SlSAMT1 expression. These data indicate that herbivore-inducible SlSAMT1 expression and its associated MeSA production play an important and complex role in the defense of tomato against insect pests.