Jasmonate Signaling

The transition to flowering is governed by different pathways integrating endogenous and exogenous signals. Here, we evaluated the role of the phytohormone cytokinin (CK) in regulating Arabidopsis thaliana flowering time. By analyzing key mutants in CK metabolism, transport and signalling, we found that the hormone promotes flowering under both long-day (LD) and short-day (SD) conditions, with a stronger impact on flowering under SDs. Genetic analyses indicated that both trans- and cis-zeatin regulate the floral transition, while isopentenyladenine plays a minor role. Blocking CK export from roots and reciprocal grafting experiments revealed that root-derived CK is an important flowering signal. Perception and transmission of the CK flowering signal depended on distinct CK receptors, phosphotransmitter proteins and several B-type response regulators. Further, CK functioned through floral integrators such as OVEREXPRESSION OF CONSTANS1 (SOC1) and components of the age pathway. The CK status of plants affected the levels of the age pathway microRNAs miR156 and miR172. Cytokinin-promoted flowering required the miR156-target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE15 (SPL15) and miR172, and the late-flowering phenotype of LD-grown CK-deficient plants depended on miR172-targeted APETALA2 (AP2)-like genes encoding floral repressors. Collectively, this study shows that CK regulates flowering time through the two-component signaling system and components of the age pathway, providing a genetic framework for future investigations.

Small-molecule phytohormones exert control over plant growth, development, and stress responses by coordinating the patterns of gene expression within and between cells. Increasing evidence indicates that currently recognized plant hormones are part of a larger group of regulatory metabolites that have acquired signaling properties during the evolution of land plants. This rich assortment of chemical signals reflects the tremendous diversity of plant secondary metabolism, which offers evolutionary solutions to the daunting challenges of sessility and other unique aspects of plant biology. A major gap in our current understanding of plant regulatory metabolites is the lack of insight into the direct targets of these compounds. Here, we illustrate the blurred distinction between classical phytohormones and other bioactive metabolites by highlighting the major scientific advances that transformed the view of jasmonate from an interesting floral scent to a potent transcriptional regulator. Lessons from jasmonate research generally apply to other phytohormones and thus may help provide a broad understanding of regulatory metabolite–protein interactions. In providing a framework that links small-molecule diversity to transcriptional plasticity, we hope to stimulate future research to explore the evolution, functions, and mechanisms of perception of a broad range of plant regulatory metabolites.

Jasmonates are oxygenated lipids (oxylipins) that control defense gene expression in response to cell damage in plants. How mobile are these potent mediators within tissues? Exploiting a series of 13-lipoxygenase (13-lox) mutants in Arabidopsis (Arabidopsis thaliana) that displays impaired jasmonic acid (JA) synthesis in specific cell types and using JA-inducible reporters, we mapped the extent of the transport of endogenous jasmonates across the plant vegetative growth phase. In seedlings, we found that jasmonate (or JA precursors) could translocate axially from wounded shoots to unwounded roots in a LOX2-dependent manner. Grafting experiments with the wild type and JA-deficient mutants confirmed shoot-to-root oxylipin transport. Next, we used rosettes to investigate radial cell-to-cell transport of jasmonates. After finding that the LOX6 protein localized to xylem contact cells was not wound inducible, we used the lox234 triple mutant to genetically isolate LOX6 as the only JA precursor-producing LOX in the plant. When a leaf of this mutant was wounded, the JA reporter gene was expressed in distal leaves. Leaf sectioning showed that JA reporter expression extended from contact cells throughout the vascular bundle and into extravascular cells, revealing a radial movement of jasmonates. Our results add a crucial element to a growing picture of how the distal wound response is regulated in rosettes, showing that both axial (shoot-to-root) and radial (cell-to-cell) transport of oxylipins plays a major role in the wound response. The strategies developed herein provide unique tools with which to identify intercellular jasmonate transport routes.