Hormonal cues regulate many aspects of plant growth and development, as well as maintain the ability to systemically respond to environmental changes. Elucidating the molecular mechanisms governing these signaling pathways is crucial to the understanding of how plants function. Structural and functional biology have been essential keys to decode plant genetic findings and reveal the precise molecular action at the protein level. Plant hormones (aka phytohormones) comprise a set of structurally unrelated small organic compounds. Interestingly, most phytohormone signaling pathways are tightly regulated by highly coordinated intracellular protein degradation machine known as the ubiquitin-proteasome system (UPS). The specificity of UPS is conferred by the action of a family of E3 ubiquitin ligase enzymes that target specific proteins for destruction in a timely manner. Read our lab’s review (Tal et al, 2020. Plant Physiology), and a short review (Shabek et al., 2014. Nature Structural Molecular biology) for more information.

The Shabek lab is particularly interested in a novel class of plant hormones, Strigolactones (SL) as well as the Karrikin signaling pathway. SLs regulate distinct aspects of plant development, promote symbiotic associations with arbuscular mycorrhizal fungi, and trigger germination of parasitic weeds. The central questions concerning the exact molecular mechanisms of SL perception and signal transduction remain to be answered and our long-term research goal is to determine how SLs control these processes at the molecular level. SLs’ perception and signal propagation are coordinated by three evolutionarily conserved proteins: the ubiquitin ligase MAX2, the SL receptor and hydrolase D14, and the novel AAA+ ATPase transcriptional repressors SMXL6/7/8 (SMXLs). Loss of function mutations in these genes result in SL-insensitive plants with numerous developmental defects including enhanced seed dormancy, reduced seedling photomorphogenesis, increased axillary branching, reduced plant height, delayed leaf senescence, and altered leaf and root development.


The current model suggests that the MAX2-D14 complex mediates SL response by ubiquitinating and degrading SMXLs, thus alleviating repression of SL-signaling genes. In one of our studies (published in Nature), we isolated the MAX2-D14-SL complex and determined multiple functional conformational states. These findings now open up new avenues of research and applications.




In our recent work (published in Nature Plants) we elucidate the functional dynamics of ASK1–D3/MAX2 in SL signalling by leveraging conformational switch mutants both in vitro and in plants. We determine the crystal structure of a dislodged D3/MAX2 mutant and demonstrate that disruptions in CTH plasticity via either CRISPR–Cas9 genome editing or expression of point mutation mutants result in impairment of SL signalling. We show that the conformational switch in ASK1–D3/MAX2 CTH directly regulates ubiquitin-mediated protein degradation. Interstingly, we uncovered an organic acid metabolite that can directly trigger the conformational switch.



Karrikins (KARs) are a family of butenolide small molecules produced from the combustion of vegetation and are bio-active components of smoke. These molecules are capable of inducing germination of numerous plant species, even those not associated with fire or fire-prone environments. There are striking similarities between KAR and strigolactone (SL) signaling pathways as both dependent on similar key proteins: a KAR receptor α/β hydrolase KARRIKIN INSENSITIVE2 (KAI2), MAX2 ubiquitin ligase, and the target of ubiquitination and degradation, the transcriptional corepressor SMAX1/SMXL2.  KAR signaling is important in the regulation of a number of plant developmental processes including seedling development, leaf shape, cuticle formation, and root development . Furthermore, they play critical roles in arbuscular mycorrhiza (AM) symbiosis and abiotic stress response. Our lab investigates the karrikin signaling using multiple strategies in including structural biology, biochemistry and plant biology.


In another study, in collaboration with multiple groups, we identified and characterized divergent KAI2 receptors in legume (pisum/pea), revealed their distinct function and ligand sensitivities, and illuminated the KAI2s enzymatic mechanism. Our research shed light on the evolution of plant hydrolase receptors and their functional adaptation in KAR/KL/SL sensing, in particular in a key crop.