MYC transcription factors coordinate tryptophan-dependent defence responses and compromise seed yield in Arabidopsis.

Robust plant immunity negatively affects other fitness traits, including growth and seed production. Jasmonate (JA) confers broad-spectrum protection against plant consumers by stimulating the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins, which in turn relieves repression on transcription factors (TFs) coincident with reduced growth and fecundity. The molecular mechanisms underlying JA-mediated decreases in fitness remain largely unknown. To assess the contribution of MYC TFs to growth and reproductive fitness at high levels of defence, we mutated three MYC genes in a JAZ-deficient mutant (jazD) of Arabidopsis thaliana that exhibits strong defence and low seed yield. Genetic epistasis analysis showed that de-repression of MYC TFs in jazD not only conferred strong resistance to insect herbivory but also reduced shoot and root growth, fruit size and seed yield. We also provided evidence that the JAZ-MYC module coordinates the supply of tryptophan with the production of indole glucosinolates and the proliferation of endoplasmic reticulum bodies that metabolise glucosinolates through the action of ß-glucosidases. Our results establish MYCs as major regulators of growth- and reproductive-defence trade-offs and further indicate that these factors coordinate tryptophan availability with the production of amino acid-derived defence compounds.

The steroid hormone ecdysone regulates growth rate in response to oxygen availability.

In almost all animals, physiologically low oxygen (hypoxia) during development slows growth and reduces adult body size. The developmental mechanisms that determine growth under hypoxic conditions are, however, poorly understood. Here we show that the growth and body size response to moderate hypoxia (10% O2) in Drosophila melanogaster is systemically regulated via the steroid hormone ecdysone. Hypoxia increases level of circulating ecdysone and inhibition of ecdysone synthesis ameliorates the negative effect of low oxygen on growth. We also show that the effect of ecdysone on growth under hypoxia is through suppression of the insulin/IGF-signaling pathway, via increased expression of the insulin-binding protein Imp-L2. These data indicate that growth suppression in hypoxic Drosophila larvae is accomplished by a systemic endocrine mechanism that overlaps with the mechanism that slows growth at low nutrition. This suggests the existence of growth-regulatory mechanisms that respond to general environmental perturbation rather than individual environmental factors.

A Phytochrome B-Independent Pathway Restricts Growth at High Levels of Jasmonate Defense.

The plant hormone jasmonate (JA) promotes resistance to biotic stress by stimulating the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins, which relieves repression on MYC transcription factors that execute defense programs. JA-triggered depletion of JAZ proteins in Arabidopsis (Arabidopsis thaliana) is also associated with reduced growth and seed production, but the mechanisms underlying these pleiotropic growth effects remain unclear. Here, we investigated this question using an Arabidopsis JAZ-deficient mutant (jazD; jaz1-jaz7, jaz9, jaz10, and jaz 13) that exhibits high levels of defense and strong growth inhibition. Genetic suppressor screens for mutations that uncouple growth-defense tradeoffs in the jazD mutant identified nine independent causal mutations in the red-light receptor phytochrome B (phyB). Unlike the ability of the phyB mutations to completely uncouple the mild growth-defense phenotypes in a jaz mutant (jazQ) defective in JAZ1, JAZ3, JAZ4, JAZ9, and JAZ10, phyB null alleles only weakly alleviated the growth and reproductive defects in the jazD mutant. phyB-independent growth restriction of the jazD mutant was tightly correlated with upregulation of the Trp biosynthetic pathway but not with changes in central carbon metabolism. Interestingly, jazD and jazD phyB plants were insensitive to a chemical inhibitor of Trp biosynthesis, which is a phenotype previously observed in plants expressing hyperactive MYC transcription factors that cannot bind JAZ repressors. These data provide evidence that the mechanisms underlying JA-mediated growth-defense balance depend on the level of defense, and they further establish an association between growth inhibition at high levels of defense and dysregulation of Trp biosynthesis.

Stimulation of Insect Herbivory by Elevated Temperature Outweighs Protection by the Jasmonate Pathway.

Rising global temperatures are associated with increases in the geographic range, population size, and feeding voracity of insect herbivores. Although it is well established that the plant hormone jasmonate (JA) promotes durable resistance to many ectothermic herbivores, little is known about how JA-mediated defense is influenced by rising temperatures. Here, we used the Arabidopsis-Trichoplusia ni (cabbage looper) interaction to investigate the relative contribution of JA and elevated temperature to host resistance. Video monitoring of T. ni larval behavior showed that elevated temperature greatly enhanced defoliation by increasing the bite rate and total time spent feeding, whereas loss of resistance in a JA-deficient mutant did not strongly affect these behaviors. The acceleration of insect feeding at elevated temperature was not attributed to decreases in wound-induced JA biosynthesis, expression of JA-responsive genes, or the accumulation of defensive glucosinolates prior to insect challenge. Quantitative proteomic analysis of insect frass, however, provided evidence for a temperature-dependent increase in the production of T. ni digestive enzymes. Our results demonstrate that temperature-driven stimulation of T. ni feeding outweighs the protective effects of JA-mediated resistance in Arabidopsis, thus highlighting a potential threat to plant resilience in a warming world.

Insect herbivory antagonizes leaf cooling responses to elevated temperature in tomato.

As global climate change brings elevated average temperatures and more frequent and extreme weather events, pressure from biotic stresses will become increasingly compounded by harsh abiotic stress conditions. The plant hormone jasmonate (JA) promotes resilience to many environmental stresses, including attack by arthropod herbivores whose feeding activity is often stimulated by rising temperatures. How wound-induced JA signaling affects plant adaptive responses to elevated temperature (ET), however, remains largely unknown. In this study, we used the commercially important crop plant Solanum lycopersicum (cultivated tomato) to investigate the interaction between simulated heat waves and wound-inducible JA responses. We provide evidence that the heat shock protein HSP90 enhances wound responses at ET by increasing the accumulation of the JA receptor, COI1. Wound-induced JA responses directly interfered with short-term adaptation to ET by blocking leaf hyponasty and evaporative cooling. Specifically, leaf damage inflicted by insect herbivory or mechanical wounding at ET resulted in COI1-dependent stomatal closure, leading to increased leaf temperature, lower photosynthetic carbon assimilation rate, and growth inhibition. Pharmacological inhibition of HSP90 reversed these effects to recapitulate the phenotype of a JA-insensitive mutant lacking the COI1 receptor. As climate change is predicted to compound biotic stress with larger and more voracious arthropod pest populations, our results suggest that antagonistic responses resulting from a combination of insect herbivory and moderate heat stress may exacerbate crop losses.

JAZ repressors of metabolic defense promote growth and reproductive fitness in Arabidopsis.

Plant immune responses mediated by the hormone jasmonoyl-l-isoleucine (JA-Ile) are metabolically costly and often linked to reduced growth. Although it is known that JA-Ile activates defense responses by triggering the degradation of JASMONATE ZIM DOMAIN (JAZ) transcriptional repressor proteins, expansion of the JAZ gene family in vascular plants has hampered efforts to understand how this hormone impacts growth and other physiological tasks over the course of ontogeny. Here, we combined mutations within the 13-member Arabidopsis JAZ gene family to investigate the effects of chronic JAZ deficiency on growth, defense, and reproductive output. A higher-order mutant (jaz decuple, jazD) defective in 10 JAZ genes (JAZ1-7, -9, -10, and -13) exhibited robust resistance to insect herbivores and fungal pathogens, which was accompanied by slow vegetative growth and poor reproductive performance. Metabolic phenotypes of jazD discerned from global transcript and protein profiling were indicative of elevated carbon partitioning to amino acid-, protein-, and endoplasmic reticulum body-based defenses controlled by the JA-Ile and ethylene branches of immunity. Resource allocation to a strong defense sink in jazD leaves was associated with increased respiration and hallmarks of carbon starvation but no overt changes in photosynthetic rate. Depletion of the remaining JAZ repressors in jazD further exaggerated growth stunting, nearly abolished seed production and, under extreme conditions, caused spreading necrotic lesions and tissue death. Our results demonstrate that JAZ proteins promote growth and reproductive success at least in part by preventing catastrophic metabolic effects of an unrestrained immune response.

Regulation of growth-defense balance by the JASMONATE ZIM-DOMAIN (JAZ)-MYC transcriptional module.

The plant hormone jasmonate (JA) promotes the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins to relieve repression on diverse transcription factors (TFs) that execute JA responses. However, little is known about how combinatorial complexity among JAZ-TF interactions maintains control over myriad aspects of growth, development, reproduction, and immunity. We used loss-of-function mutations to define epistatic interactions within the core JA signaling pathway and to investigate the contribution of MYC TFs to JA responses in Arabidopsis thaliana. Constitutive JA signaling in a jaz quintuple mutant (jazQ) was largely eliminated by mutations that block JA synthesis or perception. Comparison of jazQ and a jazQ myc2 myc3 myc4 octuple mutant validated known functions of MYC2/3/4 in root growth, chlorophyll degradation, and susceptibility to the pathogen Pseudomonas syringae. We found that MYC TFs also control both the enhanced resistance of jazQ leaves to insect herbivory and restricted leaf growth of jazQ. Epistatic transcriptional profiles mirrored these phenotypes and further showed that triterpenoid biosynthetic and glucosinolate catabolic genes are up-regulated in jazQ independently of MYC TFs. Our study highlights the utility of genetic epistasis to unravel the complexities of JAZ-TF interactions and demonstrates that MYC TFs exert master control over a JAZ-repressible transcriptional hierarchy that governs growth-defense balance.