Abstract
Oxygenated membrane fatty acid derivatives termed oxylipins play important roles in plant defense against biotic and abiotic cues. Plants challenged by insect pests, for example, synthesize a blend of different defense compounds that include volatile aldehydes and jasmonic acid (JA), among others. Because all oxylipins are derived from the same pathway, we investigated how their synthesis might be regulated, focusing on two closely related atypical cytochrome P450 enzymes designated CYP74A and CYP74B, respectively, allene oxide synthase (AOS) and hydroperoxide lyase (HPL). These enzymes compete for the same substrate but give rise to different products: the final product of the AOS branch of the oxylipin pathway is JA, while those of the HPL branch comprise volatile aldehydes and alcohols. AOS and HPL are plastid envelope enzymes in Arabidopsis thaliana but accumulate at different locations. Biochemical experiments identified AOS as a constituent of complexes also containing lipoxygenase 2 (LOX2) and allene oxide cyclase (AOC), which catalyze consecutive steps in JA precursor biosynthesis, while excluding the concurrent HPL reaction. Based on published X-ray data, the structure of this complex was modelled and amino acids involved in catalysis and subunit interactions predicted. Genetic studies identified the microRNA 319-regulated clade of TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) transcription factor genes and CORONATINE INSENSITIVE 1 (COI1) as controlling JA production through the LOX2-AOS-AOC2 complex. Together, our results define a molecular branch point in oxylipin biosynthesis that allows fine-tuning of the plant's defense machinery in response to biotic and abiotic stimuli.
Highlights
Jasmonic acid (JA) and its derivatives are cyclopentanone compounds of widespread occurrence and ubiquitous function in plants (Böttcher and Pollmann, 2009; Reinbothe et al, 2009; Wasternack and Hause, 2013;Yan et al, 2013)
We show that lipoxygenase 2 (LOX2), allene oxide synthase (AOS), and AOC2, the enzymes that catalyze consecutive steps in jasmonic acid (JA) precursor biosynthesis (Fig. 1), are co-localized in the inner envelope of Arabidopsis chloroplasts (Fig. 2 and Supplementary Fig. S7; see Springer et al, 2016), and form complexes operating in cis-(+)-12-oxo-phytodienoic acid (OPDA) synthesis from α-linolenic acid (α-LeA) (Figs 3 and 5; Supplementary Fig. S8)
In contrast to those studies, in which AOS and AOC2 were randomly bound to the matrix, excluding tight substrate channeling, in our experiments the enzymes were associated in an orderly fashion, thereby allowing the channeling of α-LeA
Summary
Jasmonic acid (JA) and its derivatives are cyclopentanone compounds of widespread occurrence and ubiquitous function in plants (Böttcher and Pollmann, 2009; Reinbothe et al, 2009; Wasternack and Hause, 2013;Yan et al, 2013). (+)-7-iso-JA-Ile (JA-Ile) is the physiologically active compound of JA signaling (Fonseca et al, 2009) It triggers changes in gene expression including the activation of defense genes and inhibition of photosynthetic genes (Reinbothe et al, 1993a, b; Rustgi et al, 2014). Key regulatory elements in JA signaling involve the F-box protein COI1 (CORONATINE INSENSITIVE 1) acting as the JA-Ile receptor (Xie et al, 1998;Yan et al, 2009), the E3 ubiquitin-ligase Skp-Cullin-Fbox complex SCFCOI1, and the JASMONATE ZIM-domain (JAZ) transcriptional repressors, which normally suppress the expression of JA response genes (Chini et al, 2007;Thines et al, 2007; Chung et al, 2008). Binding of JA-Ile to COI1 elicits the degradation of JAZ transcriptional repressors through the 26S proteasome and permits the expression of JA response genes, driven by a number of MYC transcription factors (Chini et al, 2009; Fernández-Calvo et al, 2011; Hoffmann et al, 2011; Schweizer et al, 2013)
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