A large number of plants accumulate N-acylated polyamines (phenolamides [PAs]) in response to biotic and/or abiotic stress conditions. In the native tobacco (Nicotiana attenuata), the accumulation of two major PAs, caffeoylputrescine and dicaffeoylspermidine (DCS), after herbivore attack is known to be controlled by a key transcription factor, MYB8. Using a broadly targeted metabolomics approach, we show that a much larger spectrum of PAs composed of hydroxycinnamic acids and two polyamines, putrescine and spermidine, is regulated by this transcription factor. We cloned several novel MYB8-regulated genes, annotated as putative acyltransferases, and analyzed their function. One of the novel acyltransferases (AT1) is shown to encode a hydroxycinnamoyl-coenzyme A:putrescine acyltransferase responsible for caffeoylputrescine biosynthesis in tobacco. Another gene (acyltransferase DH29), specific for spermidine conjugation, mediates the initial acylation step in DCS formation. Although this enzyme was not able to perform the second acylation toward DCS biosynthesis, another acyltransferase gene, CV86, proposed to act on monoacylated spermidines, was isolated and partially characterized. The activation of MYB8 in response to herbivore attack and associated signals required the activity of LIPOXYGENASE3, a gene involved in jasmonic acid (JA) biosynthesis in N. attenuata. These new results allow us to reconstruct a complete branch in JA signaling that defends N. attenuata plants against herbivores: JA via MYB8's transcriptional control of AT1 and DH29 genes controls the entire branch of PA biosynthesis, which allows N. attenuata to mount a chemically diverse (and likely efficient) defense shield against herbivores.

SummaryThe MYB75 transcription factor is a regulator of anthocyanin biosynthesis. Overexpressing MYB75 results in re-channelling of quercetin/kaempferol metabolites including reduced accumulation of kaempferol-3,7-dirhamnoside, a novel defensive metabolite against a specialist caterpillar.

Intense selection by pesticides and antibiotics has resulted in a global epidemic of evolved resistance. In agriculture and medicine, using mixtures of compounds from different classes is widely accepted as optimal resistance management. However, this strategy may promote the evolution of more generalist resistance mechanisms. Here we test this hypothesis at a national scale in an economically important agricultural weed: blackgrass (Alopecurus myosuroides), for which herbicide resistance is a major economic issue. Our results reveal that greater use of herbicide mixtures is associated with lower levels of specialist resistance mechanisms, but higher levels of a generalist mechanism implicated in enhanced metabolism of herbicides with diverse modes of action. Our results indicate a potential evolutionary trade-off in resistance management, whereby attempts to reduce selection for specialist resistance traits may promote the evolution of generalist resistance. We contend that where specialist and generalist resistance mechanisms co-occur, similar trade-offs will be evident, calling into question the ubiquity of resistance management based on mixtures and combination therapies.

SUMMARYFor most plant hormones, biological activity is suppressed by reversible conjugation to sugars, amino acids and other small molecules. In contrast, the conjugation of jasmonic acid (JA) to isoleucine (Ile) is known to enhance the activity of JA. Whereas hydroxylation and carboxylation of JA-Ile permanently inactivates JA-Ilemediated signaling in plants, the alternative deactivation pathway of JA-Ile by its direct hydrolysis to JA remains unstudied. We show that Nicotiana attenuata jasmonoyl-L-isoleucine hydrolase 1 (JIH1), a close homologue of previously characterized indoleacetic acid alanine resistant 3 (IAR3) gene in Arabidopsis, hydrolyzes both JA-Ile and IAA-Ala in vitro. When the herbivory-inducible NaJIH1 gene was silenced by RNA interference, JA-Ile levels increased dramatically after simulated herbivory in irJIH1, compared with wild-type (WT) plants. When specialist (Manduca sexta) or generalist (Spodoptera littoralis) herbivores fed on irJIH1 plants they gained significantly less mass compared with those feeding on wild-type (WT) plants. The poor larval performance was strongly correlated with the higher accumulation of several JA-Ile-dependent direct defense metabolites in irJIH1 plants. In the field, irJIH1 plants attracted substantially more Geocoris predators to the experimentally attached M. sexta eggs on their leaves, compared with empty vector plants, which correlated with higher herbivory-elicited emissions of volatiles known to function as indirect defenses. We conclude that NaJIH1 encodes a new homeostatic step in JA metabolism that, together with JA and JA-Ilehydroxylation and carboxylation of JA-Ile, rapidly attenuates the JA-Ile burst, allowing plants to tailor the expression of direct and indirect defenses against herbivore attack in nature.

No abstract

Jasmonic acid (JA) and ethylene (ET) are known to play important roles in mediating plant defense against herbivores, but how they affect development in herbivore-attacked plants is unknown. We used JA-deficient (silenced in LIPOXYGENASE3 [asLOX3]) and ET-insensitive (expressing a mutated dominant negative form of ETHYLENE RESPONSE1 [mETR1]) Nicotiana attenuata plants, and their genetic cross (mETR1asLOX3), to examine growth and development of these plants under simulated herbivory conditions. At the whole plant level, both hormones suppressed leaf expansion after the plants had been wounded and the wounds had been immediately treated with Manduca sexta oral secretions (OS). In addition, ectopic cell expansion was observed around both water-and OS-treated wounds in mETR1asLOX3 leaves but not in mETR1, asLOX3, or wild-type leaves. Pretreating asLOX3 leaves with the ET receptor antagonist 1-methylcyclopropane resulted in local cell expansion that closely mimicked the mETR1asLOX3 phenotype. We found higher auxin (indole-3-acetic acid) levels in the elicited leaves of mETR1asLOX3 plants, a trait that is putatively associated with enhanced cell expansion and leaf growth in this genotype. Transcript profiling of OS-elicited mETR1asLOX3 leaves revealed a preferential accumulation of transcripts known to function in cell wall remodeling, suggesting that both JA and ET act as negative regulators of these genes. We propose that in N. attenuata, JA-ET cross talk restrains local cell expansion and growth after herbivore attack, allowing more resources to be allocated to induced defenses against herbivores.

Several grass and broadleaf weed species around the world have evolved multiple-herbicide resistance at alarmingly increasing rates. Research on the biochemical and molecular resistance mechanisms of multiple-resistant weed populations indicate a prevalence of herbicide metabolism catalyzed by enzyme systems such as cytochrome P450 monooxygenases and glutathioneS-transferases and, to a lesser extent, by glucosyl transferases. A symposium was conducted to gain an understanding of the current state of research on metabolic resistance mechanisms in weed species that pose major management problems around the world. These topics, as well as future directions of investigations that were identified in the symposium, are summarized herein. In addition, the latest information on selected topics such as the role of safeners in inducing crop tolerance to herbicides, selectivity to clomazone, glyphosate metabolism in crops and weeds, and bioactivation of natural molecules is reviewed.

Summary2‐C‐Methyl‐d‐erythritol‐2,4‐cyclodiphosphate (MEcDP) is an intermediate of the plastid‐localized 2‐C‐methyl‐d‐erythritol‐4‐phosphate (MEP) pathway which supplies isoprenoid precursors for photosynthetic pigments, redox co‐factor side chains, plant volatiles, and phytohormones. The Arabidopsis hds‐3 mutant, defective in the 1‐hydroxy‐2‐methyl‐2‐(E)‐butenyl‐4‐diphosphate synthase step of the MEP pathway, accumulates its substrate MEcDP as well as the free tetraol 2‐C‐methyl‐d‐erythritol (ME) and glucosylated ME metabolites, a metabolic diversion also occurring in wild type plants. MEcDP dephosphorylation to the free tetraol precedes glucosylation, a process which likely takes place in the cytosol. Other MEP pathway intermediates were not affected in hds‐3. Isotopic labeling, dark treatment, and inhibitor studies indicate that a second pool of MEcDP metabolically isolated from the main pathway is the source of a signal which activates salicylic acid induced defense responses before its conversion to hemiterpene glycosides. The hds‐3 mutant also showed enhanced resistance to the phloem‐feeding aphid Brevicoryne brassicae due to its constitutively activated defense response. However, this MEcDP‐mediated defense response is developmentally dependent and is repressed in emerging seedlings. MEcDP and ME exogenously applied to adult leaves mimics many of the gene induction effects seen in the hds‐3 mutant. In conclusion, we have identified a metabolic shunt from the central MEP pathway that diverts MEcDP to hemiterpene glycosides via ME, a process linked to balancing plant responses to biotic stress.