Abstract
The jasmonate (JA) phytohormone signaling system is an important mediator of plant defense against herbivores. Plants deficient in JA signaling are more susceptible to herbivory as a result of deficiencies in defensive trait expression. Recent studies have implicated the circadian clock in regulating JA-mediated defenses, but the molecular mechanisms linking the clock to JA signaling are unclear. Here, we report that wild tobacco (Nicotiana attenuata) plants rendered deficient in the clock component ZEITLUPE (ZTL) by RNA interference have attenuated resistance to the generalist herbivore Spodoptera littoralis This effect can be attributed in part to reduced concentrations of nicotine, an abundant JA-regulated toxin produced in N. attenuata roots and transported to shoots. RNA interference targeting ZTL dramatically affects the root circadian clock and reduces the expression of nicotine biosynthetic genes. Protein-protein interaction experiments demonstrate that ZTL regulates JA signaling by directly interacting with JASMONATE ZIM domain (JAZ) proteins in a CORONATINE-INSENSITIVE1- and jasmonoyl-isoleucine conjugate-independent manner, thereby regulating a JAZ-MYC2 module that is required for nicotine biosynthesis. Our study reveals new functions for ZTL and proposes a mechanism by which a clock component directly influences JA signaling to regulate plant defense against herbivory.
Highlights
The jasmonate (JA) phytohormone signaling system is an important mediator of plant defense against herbivores
ZTL Regulates JA-Mediated Herbivore Defense leaves; second, ir-ztl plants have a longer period in free-running conditions; third, hypocotyl length is increased in the seedlings of ir-ztl plants compared with empty vector (EV) plants; and NaZTL interacts directly with NaTOC1 (Somers et al, 2004; Yon et al, 2012, 2016)
Recent studies have revealed the importance of the plant circadian clock in regulating constitutive defenses that contribute to herbivore resistance (Goodspeed et al, 2012, 2013a)
Summary
The jasmonate (JA) phytohormone signaling system is an important mediator of plant defense against herbivores. Some defense-related metabolites regulated by JA signaling, including glucosinolates and terpenes, oscillate and are abundant at midday in several plant species, at least under diurnal conditions (Loughrin et al, 1994; Martin et al, 2003; Joo et al, 2018). These fluctuations may coincide with the behavior of some herbivores or the herbivores’ natural enemies (Goodspeed et al, 2012; Joo et al, 2018). A recent study found that the circadian evening complex comprising LUX, EARLY FLOWERING3 (ELF3), and ELF4 suppresses JA-mediated senescence in Arabidopsis by binding directly to the promoter of MYC2 (Zhang et al, 2018), indicating that MYC2 may be a direct target of clock regulation
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