Coronatine

The present experiment evaluated how coronatine (COR) elicitation affects chemical and biological properties of cumin (Cuminum cyminum L.) seed essential oil (CSEO). Following isolation of the EO, its chemical composition was analyzed by gas chromatography-mass spectrometry; also, its bioactivities in terms of antimicrobial/antifungal, cytotoxic (measured by MTT assay) and antioxidant effects (evaluated by DPPH, β-carotene bleaching (BCB) and TBARS methods) were evaluated. COR-elicitation significantly increased CSEO yield and the level of its chemical components, especially cumin aldehyde which is the main component of CSEO. Results showed that COR-elicitation significantly reduced the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of CSEO against 4 Gram-positive and 3 Gram-negative bacteria and 2 fungi. Moreover, elicitation markedly enhanced the antioxidant and in vitro cytotoxic activity of CSEO. Therefore, COR may be regarded as a useful biotic elicitor for improving EO chemical and biological properties.

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.

Recent studies establish a crucial role of the circadian clock in regulating plant defense against pathogens. Whether pathogens modulate host circadian clock as a potential strategy to suppress host innate immunity is not well understood. Coronatine is a toxin produced by the bacterial pathogen Pseudomonas syringae that is known to counteract Arabidopsis defense through mimicking defense signaling molecules, jasmonates (JAs). We report here that COR preferentially suppresses expression of clock-related genes in high throughput gene expression studies, compared with the plant-derived JA molecule methyl jasmonate (MJ). COR treatment dampens the amplitude and lengthens the period of all four reporters tested while MJ and another JA agonist JA-isoleucine (JA-Ile) only affect some reporters. COR, MJ, and JA-Ile act through the canonical JA receptor COI1 in clock regulation. These data support a stronger role of the pathogen-derived molecule COR than plant-derived JA molecules in regulating Arabidopsis clock. Further study shall reveal mechanisms underlying COR regulation of host circadian clock.