From little things big things grow: karrikins and new directions in plant development

Stereospecific reduction of 2'S-configured strigolactones by cowpea OPR3 enzymes

Strigolactones (SLs), plant-derived apocarotenoids, serve dual roles as phytohormones and rhizosphere signaling molecules. While exogenous administration of SLs to plants aids in studying their functions, the metabolic destiny of these administered SLs remains poorly elucidated. Our previous research demonstrated that among synthetic SL GR24 stereoisomers administered to cowpea (Vigna unguiculata), 2'-epi-GR24 undergoes selective reduction at the C-3',4' double bond in its D-ring. In this investigation, we isolated proteins from cowpea roots based on SL reducing activity and identified 12-oxophytodienoate reductase 3 homologs (VuOPR3s) as contributors to this reduction. Enzymatic assays conducted with recombinant proteins revealed that VuOPR3s exhibited a preference for reducing activity toward 2'S-configured SLs, including 2'-epi-GR24. This specificity for 2'S-configured SLs was congruent with that observed for orobanchol produced by cowpea and its stereoisomers. These findings suggest that exogenously administered SLs undergo enzymatic stereoselective reduction, underscoring the importance of considering stereospecificity when interpreting data obtained from SL usage.

SMAX1 interacts with DELLA protein to inhibit seed germination under weak light conditions via gibberellin biosynthesis in Arabidopsis

Karrikins (KARs) were first identified as a class of small-molecule chemicals derived from smoke that promote seed germination. However, the implied mechanism is still not well understood. Here, we find that KAR signaling mutants have a lower germination percentage than that of wild type under weak light conditions, and KARs promote seed germination through transcriptional activation of gibberellin (GA) biosynthesis via SMAX1. SMAX1 interacts with the DELLA proteins REPRESSOR of ga1-3-LIKE 1 (RGL1) and RGL3. The interaction enhances the transcriptional activity of SMAX1 and inhibits GIBBERELLIN 3-oxidase 2 (GA3ox2) gene expression. The KAR signaling mutant seed germination defect under weak light is partially rescued by exogenous application of GA₃ or by GA3ox2 overexpression, and the rgl1 rgl3 smax1 triple mutant exhibits higher germination rates under weak light than the smax1 mutant. Thus, we show a crosstalk between KAR and GA signaling pathways via a SMAX1-DELLA module in regulating seed germination in Arabidopsis.

Perception of karrikins by plants: a continuing enigma

Karrikins are small butenolide molecules with the capacity to promote germination and enhance seedling establishment. Generated abiotically from partial combustion of vegetation, karrikins are comparatively rare in the environment, but studying their mode of action has been most informative in revealing a new regulatory pathway for plant development that uses the karrikin perception machinery. Recent studies suggest that the karrikin receptor protein KAI2 and downstream transcriptional co-repressors in the SMXL family influence seed germination, seedling photomorphogenesis, root morphology, and responses to abiotic stress such as drought. Based on taxonomic distribution, this pathway is ubiquitous and likely to be evolutionarily ancient, originating prior to land plants. However, we still do not have a good grasp on how karrikins actually activate the receptor protein, and we have yet to discover the assumed endogenous ligand for KAI2 that karrikins are thought to mimic. This review covers recent progress in this field, as well as current gaps in our knowledge.

An allelic series at the KARRIKIN INSENSITIVE 2 locus of Arabidopsis thaliana decouples ligand hydrolysis and receptor degradation from downstream signalling

Karrikins are butenolide compounds present in post-fire environments that can stimulate seed germination in many species, including Arabidopsis thaliana. Plants also produce endogenous butenolide compounds that serve as hormones, namely strigolactones (SLs). The receptor for karrikins (KARRIKIN INSENSITIVE 2; KAI2) and the receptor for SLs (DWARF14; D14) are homologous proteins that share many similarities. The mode of action of D14 as a dual enzyme receptor protein is well established, but the nature of KAI2-dependent signalling and its function as a receptor are not fully understood. To expand our knowledge of how KAI2 operates, we screened ethyl methanesulphonate (EMS)-mutagenized populations of A. thaliana for mutants with kai2-like phenotypes and isolated 13 new kai2 alleles. Among these alleles, kai2-10 encoded a D184N protein variant that was stable in planta. Differential scanning fluorimetry assays indicated that the KAI2 D184N protein could interact normally with bioactive ligands. We developed a KAI2-active version of the fluorescent strigolactone analogue Yoshimulactone Green to show that KAI2 D184N exhibits normal rates of ligand hydrolysis. KAI2 D184N degraded in response to treatment with exogenous ligands, suggesting that receptor degradation is a consequence of ligand binding and hydrolysis, but is insufficient for signalling activity. Remarkably, KAI2 D184N degradation was hypersensitive to karrikins, but showed a normal response to strigolactone analogues, implying that these butenolides may interact differently with KAI2. These results demonstrate that the enzymatic and signalling functions of KAI2 can be decoupled, and provide important insights into the mechanistic events that underpin butenolide signalling in plants.

Comprehensive Analysis of DWARF14-LIKE2 (DLK2) Reveals Its Functional Divergence from Strigolactone-Related Paralogs

Strigolactones (SLs) and related butenolides, originally identified as active seed germination stimulants of parasitic weeds, play important roles in many aspects of plant development. Two members of the D14 α/β hydrolase protein family, DWARF14 (D14) and KARRIKIN INSENSITIVE2 (KAI2) are essential for SL/butenolide signaling. The third member of the family in Arabidopsis, DWARF 14-LIKE2 (DLK2) is structurally very similar to D14 and KAI2, but its function is unknown. We demonstrated that DLK2 does not bind nor hydrolyze natural (+)5-deoxystrigol [(+)5DS], and weakly hydrolyzes non-natural strigolactone (-)5DS. A detailed genetic analysis revealed that DLK2 does not affect SL responses and can regulate seedling photomorphogenesis. DLK2 is upregulated in the dark dependent upon KAI2 and PHYTOCHROME INTERACTING FACTORS (PIFs), indicating that DLK2 might function in light signaling pathways. In addition, unlike its paralog proteins, DLK2 is not subject to rac-GR24-induced degradation, suggesting that DLK2 acts independently of MORE AXILLARY GROWTH2 (MAX2); however, regulation of DLK2 transcription is mostly accomplished through MAX2. In conclusion, these data suggest that DLK2 represents a divergent member of the DWARF14 family.

Strigolactones in the Rhizosphere: Friend or Foe?

Strigolactones are well-known endogenous plant hormones that play a major role in planta by influencing different physiological processes. Moreover, ex planta, strigolactones are important signaling molecules in root exudates and function as host detection cues to launch mutualistic interactions with arbuscular mycorrhizal fungi in the rhizosphere. However, parasitic plants belonging to the Orobanchaceae family hijacked this communication system to stimulate their seed germination when in close proximity to the roots of a suitable host. As a result, the secretion of strigolactones by the plant can have both favorable and detrimental outcomes. Here, we discuss these dual positive and negative effects of strigolactones and we provide a detailed overview on the role of these molecules in the complex dialogs between plants and different organisms in the rhizosphere.