Summary The F‐box protein CORONANTINE INSENSITIVE1 (COI1) serves as the receptor for the plant hormone jasmonoyl‐isoleucine (JA‐Ile). COI1, its co‐receptors of the JASMONATE ZIM‐domain (JAZ) protein family, and JA‐Ile form a functional unit that regulates growth or defense mechanisms in response to various stress cues. Strikingly, COI1, but not JA‐Ile, is required for susceptibility of Arabidopsis thaliana towards the soil‐borne vascular pathogen Verticillium longisporum. In order to obtain marker genes for further analysis of this JA‐Ile‐independent COI1 function, transcriptome analysis of roots of coi1 and allene oxide synthase (aos) plants (impaired in JA biosynthesis) was performed. Intriguingly, nearly all of the genes that are differentially expressed in coi1 versus aos and wild type are constitutively more highly expressed in coi1. To support our notion that COI1 acts independently of its known downstream signaling components, coi1 plants were complemented with a COI1 variant (COI1AA) that is compromised in its interaction with JAZs. As expected, these plants showed only weak induction of the expression of the JA‐Ile marker gene VEGETATIVE STORAGE PROTEIN2 after wounding and remained sterile. On the other hand, genes affected by COI1 but not by JA‐Ile were still strongly repressed by COI1AA. We suggest that COI1 has a potential moonlighting function that serves to repress gene expression in a JA‐Ile‐ and JAZ‐independent manner.

CORONATINE INSENSITIVE 1 (COI1) perceives the plant hormone jasmonoyl-isoleucine (JA-Ile) together with proteins of the JASMONATE ZIM-domain (JAZ) family. JA-Ile induces signalling cascades in defence and developmental processes. It has been shown that in Arabidopsis thaliana, COI1 without its ligand conveys susceptibility to the soil-born vascular pathogen Verticillium longisporum. Grafting experiments have shown that presence of COI1 in roots mediates susceptibility to the pathogen. Root transcriptome analysis has revealed that a number of salicylic acid defence-associated genes are constitutively expressed in coi1. The observation that COI1 acts as a JA-Ile-independent repressor of root gene expression led us to postulate that this novel COI1 function operates independently of the canonical JA signalling machinery.In this thesis, we show that coi1 plants complemented with a COI1 protein, that was severely impaired in its interaction with JAZ proteins (COI1 AA ), were compromised in wound-induced induction of the JA-signalling marker gene VEGETATIVE STORAGE PROTEIN 2 (VSP2).Moreover, COI1 AA could not restore fertility in sterile coi1 plants. In contrast, COI1 AA was able to repress gene expression in roots. Hence, in roots, COI1 has a second function other than its role in JA-Ile perception, in which it acts as a suppressor of defence gene expression independently of JA-Ile and most likely independently of JAZ proteins. We furthermore show that after infection with V. longisporum, approximately half of the COI1-repressed genes in roots are induced to similar levels as in coi1. We hence postulate that COI1-mediated repression is inactivated upon infection with V. longisporum leading to induction of these genes. Gene induction requires the transcription factor SYSTEMIC ACQUIRED RESISTANCE 1 (SARD1) which is itself repressed by COI1. Equally, constitutive expression of genes in coi1 was abolished by mutations in SARD1 and its close homologue CALMODULIN BINDING PROTEIN 60-LIKE G. In contrast, overexpression of SARD1 in wild-type roots did not lead to activation of gene expression, likely because the repressive effect of COI1 on gene expression could not be overcome. The repressor function of COI1 was only observed in roots and not in shoots. As roots need to balance perception of microbe-associated molecular patterns with maintaining an intact rhizosphere, we speculate that COI1 acts as a regulator of the onset of defence responses in roots.