SMAX1 Integrates Karrikin and Light Signals into GA-Mediated Hypocotyl Growth during Seedling Establishment

HTL/KAI2 signaling substitutes for light to control plant germination

Plants monitor multiple environmental cues, such as light and temperature, to ensure they germinate at the right time and place. Some specialist plants, like ephemeral fire-following weeds and root parasitic plants, germinate primarily in response to small molecules found in specific environments. Although these species come from distinct clades, they use the same HYPOSENSITIVE TO LIGHT/KARRIKIN INSENSITIVE 2 (HTL/KAI2) signaling pathway, to perceive different small molecules suggesting convergent evolution on this pathway. Here, we show that HTL/KAI2 signaling in Arabidopsis thaliana bypasses the light requirement for germination. The HTL/KAI2 downstream component, SUPPRESSOR OF MAX2 1 (SMAX1) accumulates in the dark and is necessary for PHYTOCHROME INTERACTING FACTOR 1/PHYTOCHROME INTERACTING FACTOR 3-LIKE 5 (PIF1/PIL5) to regulate hormone response pathways conducive to germination. The interaction of HTL/KAI2 and light signaling may help to explain how specialist plants like ephemeral and parasitic weeds evolved their germination behaviour in response to specific environments.

Non-transcriptional regulatory activity of SMAX1 and SMXL2 mediates karrikin-regulated seedling response to red light in Arabidopsis

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs mediate karrikin and strigolactone signaling by directly binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetectable or weak transcriptional repression activities but still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of the smax1 smxl2 mutant. SMAX1 and SMXL2 directly interact with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 to enhance their protein stability by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were then identified by treatment with GR24ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1, which is independent of the EAR motif, had a global effect on gene expression. Taken together, these results indicate that non-transcriptional regulatory activities of SMAX1 and SMXL2 mediate karrikin-regulated seedling response to red light.

Environmentally adaptive reshaping of plant photomorphogenesis by karrikin and strigolactone signaling

Coordinated morphogenic adaptation of growing plants is critical for their survival and propagation under fluctuating environments. Plant morphogenic responses to light and warm temperatures, termed photomorphogenesis and thermomorphogenesis, respectively, have been extensively studied in recent decades. During photomorphogenesis, plants actively reshape their growth and developmental patterns to cope with changes in light regimes. Accordingly, photomorphogenesis is closely associated with diverse growth hormonal cues. Notably, accumulating evidence indicates that light-directed morphogenesis is profoundly affected by two recently identified phytochemicals, karrikins (KARs) and strigolactones (SLs). KARs and SLs are structurally related butenolides acting as signaling molecules during a variety of developmental steps, including seed germination. Their receptors and signaling mediators have been identified, and associated working mechanisms have been explored using gene-deficient mutants in various plant species. Of particular interest is that the KAR and SL signaling pathways play important roles in environmental responses, among which their linkages with photomorphogenesis are most comprehensively studied during seedling establishment. In this review, we focus on how the phytochemical and light signals converge on the optimization of morphogenic fitness. We also discuss molecular mechanisms underlying the signaling crosstalks with an aim of developing potential ways to improve crop productivity under climate changes.

Karrikin signalling: impacts on plant development and abiotic stress tolerance

Plants rely upon a diverse range of metabolites to control growth and development, and to overcome stress that results from suboptimal conditions. Karrikins (KARs) are a class of butenolide compounds found in smoke that stimulate seed germination and regulate various developmental processes in plants. KARs are perceived via a plant α/β-hydrolase called KARRIKIN INSENSITIVE2 (KAI2), which also functions as a receptor for a postulated phytohormone, provisionally termed KAI2 ligand (KL). Considered natural analogues of KL, KARs have been extensively studied for their effects on plant growth and their crosstalk with plant hormones. The perception and response pathway for KAR-KL signalling is closely related to that of strigolactones, another class of butenolides with numerous functions in regulating plant growth. KAR-KL signalling influences seed germination, seedling photomorphogenesis, root system architecture, abiotic stress responses, and arbuscular mycorrhizal symbiosis. Here, we summarize current knowledge of KAR-KL signalling, focusing on its role in plant development, its effects on stress tolerance, and its interaction with other signalling mechanisms.

KAI2 Can Do: Karrikin Receptor Function in Plant Development and Response to Abiotic and Biotic Factors

The α/β hydrolase KARRIKIN INSENSITIVE 2 (KAI2) functions as a receptor for a yet undiscovered phytohormone, provisionally termed KAI2 ligand (KL). In addition, it perceives karrikin, a butenolide compound found in the smoke of burnt plant material. KAI2-mediated signaling is involved in regulating seed germination and in shaping seedling and adult plant morphology, both above and below ground. It also governs responses to various abiotic stimuli and stresses and shapes biotic interactions. KAI2-mediated signaling is being linked to an elaborate cross-talk with other phytohormone pathways such as auxin, gibberellin, abscisic acid, ethylene and salicylic acid signaling, in addition to light and nutrient starvation signaling. Further connections will likely be revealed in the future. This article summarizes recent advances in unraveling the function of KAI2-mediated signaling and its interaction with other signaling pathways.

Complexity of SMAX1 signaling during seedling establishment

Karrikins (KARs) are small butenolide compounds identified in the smoke of burning vegetation. Along with the stimulating effects on seed germination, KARs also regulate seedling vigor and adaptive behaviors, such as seedling morphogenesis, root hair development, and stress acclimation. The pivotal KAR signaling repressor, SUPPRESSOR OF MAX2 1 (SMAX1), plays central roles in these developmental and morphogenic processes through an extensive signaling network that governs seedling responses to endogenous and environmental cues. Here, we summarize the versatile roles of SMAX1 reported in recent years and discuss how SMAX1 integrates multiple growth hormone signals into optimizing seedling establishment. We also discuss the evolutionary relevance of the SMAX1-mediated signaling pathways during the colonization of aqueous plants to terrestrial environments.

Karrikin perception and signalling

Karrikins (KARs) are a class of butenolide compounds found in smoke that were first identified as seed germination stimulants for fire-following species. Early studies of KARs classified the germination and postgermination responses of many plant species and investigated crosstalk with plant hormones that regulate germination. The discovery that Arabidopsis thaliana responds to KARs laid the foundation for identifying mutants with altered KAR responses. Genetic analysis of KAR signalling revealed an unexpected link to strigolactones (SLs), a class of carotenoid-derived plant hormones. Substantial progress has since been made towards understanding how KARs are perceived and regulate plant growth, in no small part due to advances in understanding SL perception. KAR and SL signalling systems are evolutionarily related and retain a high degree of similarity. There is strong evidence that KARs are natural analogues of an endogenous signal(s), KAI2 ligand (KL), which remains unknown. KAR/KL signalling regulates many developmental processes in plants including germination, seedling photomorphogenesis, and root and root hair growth. KAR/KL signalling also affects abiotic stress responses and arbuscular mycorrhizal symbiosis. Here, we summarise the current knowledge of KAR/KL signalling and discuss current controversies and unanswered questions in this field.