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Authors: Yuqi Liu, Shangyu Chen, Sikander Pal, Jingquan Yu, Yanhong Zhou, Lam-Son Phan Tran and Xiaojian Xia.
New Crops (2024)
Abstract: "Plants have evolved varied structures for environmental adaptation. Shoot branching, as a part of plant architecture, influences the allocation of sugars produced by photosynthesis and thus greatly impacts crop yields. The activity of axillary meristem, and apical dominance governs the shoot branching patterns. In this review, we summarize the key factors involved in the formation of lateral branches, and the mechanisms of how these factors are interconnected. In particular, we focus on recent advances in understanding how sugar and environmental signals affect the hormonal signaling network to regulate apical dominance. Ultimately, we propose that epigenetic modifications are critical mechanisms underlying the plasticity of shoot branching, and that precise targeted gene editing is promising for shaping the ideal plant architecture."
Authors: Qi Chen, Xiaojun Pu, Xiaomin Li, Rongrong Li, Qian Yang, Xinjia Wang, Miao Guan and Zed Rengel.
Journal of Experimental Botany (2022)
Abstract: "Phytomelatonin is a new plant hormone, and its primary functions in plant growth and development remain relatively poorly appraised. Phytomelatonin is a master regulator of the reactive oxygen species (ROS) signaling and acts as a darkness signal in circadian stomatal closure. Plants exhibit at least three interrelated patterns of interactions between phytomelatonin and ROS production. Exogenous melatonin could induce flavonoid biosynthesis, which might be required for maintenance of antioxidant capacity under stress, after harvest and in leaf senescence conditions. However, several genetic studies provided direct evidence that phytomelatonin plays a negative role in the biosynthesis of flavonoids under normal growth conditions. Phytomelatonin delays flowering time in both dicot and monocot plants, probably via its receptor PMTR1 and interactions with the gibberellin (GA), strigolactone (SL) and ROS signaling pathways. Furthermore, phytomelatonin signaling also functions in hypocotyl and shoot growth in skotomorphogenesis and UV-B exposure; the G protein α-subunit (arabidopsis GPA1 and rice RGA1) and Constitutive Photomorphogenic1 (COP1) are important signal components during this process. Taken together, phytomelatonin acts as a darkness signal with important regulatory roles in circadian stomatal closure, flavonoid biosynthesis, flowering, and hypocotyl and shoot growth."
Authors: Young-Joon Park, Jae Young Kim and Chung-Mo Park.
The Plant Cell (2022)
One-sentence summary: SMAX1 functions as a 'thermosensitizer' that modulates the thermal sensitivity of plant morphogenesis.
Abstract: "Plant thermosensors help optimize plant development and architecture for ambient temperatures, and morphogenic adaptation to warm temperatures has been extensively studied in recent years. Phytochrome B (phyB)-mediated thermosensing and the gene regulatory networks governing thermomorphogenic responses are well understood at the molecular level. However, it is unknown how plants manage their responsiveness to fluctuating temperatures in inducing thermomorphogenic behaviors. Here, we demonstrate that SUPPRESSOR OF MAX2 1 (SMAX1), known as a karrikin signaling repressor, enhances the thermosensitivity of hypocotyl morphogenesis in Arabidopsis thaliana. Hypocotyl thermomorphogenesis was largely disrupted in SMAX1-deficient mutants. SMAX1 interacts with phyB to alleviate its suppressive effects on the transcription factor activity of PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), promoting hypocotyl thermomorphogenesis. Interestingly, the SMAX1 protein is slowly destabilized at warm temperatures, preventing hypocotyl overgrowth. Our findings indicate that the thermodynamic control of SMAX1 abundance serves as a molecular gatekeeper for phyB function in thermosensitizing PIF4-mediated hypocotyl morphogenesis."
Authors: Xinpeng Zhou, Zhanming Tan, Yaguang Zhou, Shirong Guo, Ting Sang, Yu Wang and Sheng Shu.
BMC Plant Biology (2022)
Abstract: "Strigolactone is a newly discovered type of plant hormone that has multiple roles in modulating plant responses to abiotic stress. Herein, we aimed to investigate the effects of exogenous GR24 (a synthetic analogue of strigolactone) on plant growth, photosynthetic characteristics, carbohydrate levels, endogenous strigolactone content and antioxidant metabolism in cucumber seedlings under low light stress. The results showed that the application of 10 μM GR24 can increase the photosynthetic efficiency and plant biomass of low light-stressed cucumber seedlings. GR24 increased the accumulation of carbohydrates and the synthesis of sucrose-related enzyme activities, enhanced antioxidant enzyme activities and antioxidant substance contents, and reduced the levels of H2O2 and MDA in cucumber seedlings under low light stress. These results indicate that exogenous GR24 might alleviate low light stress-induced growth inhibition by regulating the assimilation of carbon and antioxidants and endogenous strigolactone contents, thereby enhancing the tolerance of cucumber seedlings to low light stress."
Authors: Yuliya Krasylenko, George Komis, Sofiia Hlynska, Tereza Vavrdová, Miroslav Ovečka, Tomáš Pospíšil and Jozef Šamaj.
Frontiers in Plant Science (2021)
Abstract: "Strigolactones are plant hormones regulating cytoskeleton-mediated developmental events in roots, such as lateral root formation and elongation of root hairs and hypocotyls. The latter process was addressed herein by the exogenous application of a synthetic strigolactone, GR24, and an inhibitor of strigolactone biosynthesis, TIS108, on hypocotyls of wild-type Arabidopsis and a strigolactone signaling mutant max2-1 (more axillary growth 2-1). Owing to the interdependence between light and strigolactone signaling, the present work was extended to seedlings grown under a standard light/dark regime, or under continuous darkness. Given the essential role of the cortical microtubules in cell elongation, their organization and dynamics were characterized under the conditions of altered strigolactone signaling using fluorescence microscopy methods with different spatiotemporal capacities, such as confocal laser scanning microscopy (CLSM) and structured illumination microscopy (SIM). It was found that GR24-dependent inhibition of hypocotyl elongation correlated with changes in cortical microtubule organization and dynamics, observed in living wild-type and max2-1 seedlings stably expressing genetically encoded fluorescent molecular markers for microtubules. Quantitative assessment of microscopic datasets revealed that chemical and/or genetic manipulation of strigolactone signaling affected microtubule remodeling, especially under light conditions. The application of GR24 in dark conditions partially alleviated cytoskeletal rearrangement, suggesting a new mechanistic connection between cytoskeletal behavior and the light-dependence of strigolactone signaling."
Authors: Sylwia Struk, Carolien De Cuyper, Anse Jacobs, Lukas Braem, Alan Walton, Annick De Keyser, Stephen Depuydt, Lam Dai Vu, Ive De Smet, François-Didier Boyer, Dominique Eeckhout, Geert Persiau, Kris Gevaert, Geert De Jaeger and Sofie Goormachtig.
Molecular & Cellular Proteomics (2021)
Abstract: "The F-box protein MORE AXILLARY GROWTH 2 (MAX2) is a central component in the signaling cascade of strigolactones (SLs) as well as of the smoke derived karrikins (KARs) and the so far unknown endogenous KAI2 ligand (KL). The two groups of molecules are involved in overlapping and unique developmental processes, and signal-specific outcomes are attributed to perception by the paralogous α/β-hydrolases DWARF14 (D14) for SL and KARRIKIN INSENSITIVE 2/ HYPOSENSITIVE TO LIGHT (KAI2/HTL) for KAR/KL. Additionally, depending on which receptor is activated, specific members of the SUPPRESSOR OF MAX2 1 (SMAX1) – LIKE (SMXL) family control KAR/KL and SL responses. As proteins that function in the same signal transduction pathway often occur in large protein complexes, we aimed at discovering new players of the MAX2, D14 and KAI2 protein network by tandem affinity purification using Arabidopsis cell cultures. When using MAX2 as a bait, various proteins were co-purified among which general components of the Skp1-Cullin-F-box complex and members of the CONSTITUTIVE PHOTOMORPHOGENIC 9 signalosome. Here, we report the identification of a novel interactor of MAX2, a type 5 serine/threonine protein phosphatase, designated PHYTOCHROME-ASSOCIATED PROTEIN PHOSPHATASE 5 (PAPP5). Quantitative affinity purification pointed at PAPP5 as being more present in KAI2 rather than D14 protein complexes. In agreement, mutant analysis suggests that PAPP5 modulates KAR/KL-dependent seed germination in suboptimal conditions and seedling development. Additionally, a phosphopeptide enrichment experiment revealed that PAPP5 might dephosphorylate MAX2 in vivo independently of the synthetic strigolactone analog, rac-GR24. Together, by analyzing the protein complexes to which MAX2, D14 and KAI2 belong, we revealed a new MAX2 interactor, PAPP5, that might act through dephosphorylation of MAX2 to control mainly KAR/KL- related phenotypes and, hence, provide another link with the light pathway."
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Authors: Kartikye Varshney and Caroline Gutjahr.
Plant and Cell Physiology (2023)
Abstract: "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 signalling 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-signalling is being linked to an elaborate cross-talk with other phytohormone pathways such as auxin, gibberellin, abscisic acid, ethylene, and salicylic acid signalling, in addition to light and nutrient starvation signalling. Further connections will likely be revealed in the future. This article summarizes recent advances in unravelling the function of KAI2-mediated signalling and its interaction with other signalling pathways."
Author: Michela Osnato.
The Plant Cell (2022)
Excerpts: "Plants cannot migrate but have developed several cooling capacities to cope with warmer climates throughout their life cycle. A set of adaptive strategies—collectively defined as thermomorphogenesis—includes stem elongation to move the delicate shoot apex away from the warm soil, and upward bending of leaves to increase air circulation near green organs."
"In this issue, Young-Joon Park and colleagues (Park et al., 2022) carried out thermomorphogenic as-says to identify mutants with impaired hypocotyl elongation at warm temperature (28ºC) versus normal temperature (23ºC). Mutations in SUPPRESSOR OF MAX2 1 (SMAX1), a gene involved in seed germination and photomorphogenesis downstream of the KARRIKIN INSENSITIVE2 (KAI2) receptor (Stanga et al., 2013), reduced thermal sensitivity of hypocotyl growth. Indeed, smax1 mutants displayed shorter hypocotyls as compared to wildtype seedlings, not only at higher temperatures but also in different light conditions. Transcriptomic analyses revealed that smax1 mutants grown at warmer temperatures showed differential expression of auxin-related genes, and specifically downregulation of the auxin biosynthetic gene YUCCA8 (YUC8), a downstream target of PIF4. These findings suggested a functional link between SMAX1 and the PhyB-PIF4 module in thermal induction of hypocotyl growth, further corroborated by genetic analyses of higher order mutants."
Authors: Maxime Hamon-Josse, Jose Antonio Villaecija Aguilar, Karin Ljung, Ottoline Leyser, Caroline Gutjahr and Tom Bennett.
New Phytologist (2022)
Abstract: "Photomorphogenic remodelling of seedling growth is a key developmental transition in the plant life cycle. The α/β-hydrolase signalling protein KARRIKIN-INSENSITIVE2 (KAI2), a close homologue of the strigolactone receptor DWARF14 (D14), is involved in this process, but it is unclear how the effects of KAI2 on development are mediated. Here, using a combination of physiological, pharmacological, genetic and imaging approaches in Arabidopsis thaliana (Heynh.) we show that kai2 phenotypes arise because of a failure to downregulate auxin transport from the seedling shoot apex towards the root system, rather than a failure to respond to light per se. We demonstrate that KAI2 controls the light-induced remodelling of the PIN-mediated auxin transport system in seedlings, promoting a reduction in PIN7 abundance in older tissues, and an increase of PIN1/PIN2 abundance in the root meristem. We show that removing PIN3, PIN4 and PIN7 from kai2 mutants, or pharmacological inhibition of auxin transport and synthesis, is sufficient to suppress most kai2 seedling phenotypes. We conclude that KAI2 regulates seedling morphogenesis by its effects on the auxin transport system. We propose that KAI2 is not required for the light-mediated changes in PIN gene expression but is required for the appropriate changes in PIN protein abundance within cells."
Authors: Shibei Ge, Liqun He, Lijuan Jin, Xiaojian Xia, Lan Li, Golam Jalal Ahammed, Zhenyu Qi, Jingquan Yu and Yanhong Zhou.
New Phytologist (2022)
Abstract: "Light quality affects mutualisms between plant roots and arbuscular mycorrhizal fungi (AMF), which modify nutrient acquisition in plants. However, the mechanisms by which light systemically modulates root colonization by AMF and phosphate uptake in roots remain unclear. We used a range of approaches, including grafting techniques, protein immunoblot analysis, electrophoretic mobility shift assay, chromatin immunoprecipitation, and dual-luciferase assays to unveil the molecular basis of light signal transmission from shoot to root that mediates arbuscule development and phosphate uptake in tomato. The results show that shoot phytochrome B (phyB) triggers shoot-derived mobile ELONGATED HYPOCOTYL5 (HY5) protein accumulation in roots, and HY5 further positively regulates transcription of strigolactone (SL) synthetic genes, thus forming a shoot phyB-dependent systemic signaling pathway that regulates the synthesis and accumulation of SLs in roots. Further experiments with ccd7 mutants and supplementary red light confirm that SLs are indispensable in the red light-regulated mycorrhizal symbiosis in roots. Our results reveal a phyB-HY5-SLs systemic signaling cascade that facilitates mycorrhizal symbiosis and phosphate utilization in plants. The findings provide new prospects on the potential application of AMF and light manipulation to effectively improve nutrient utilization and minimize the use of chemical fertilizers and associated pollution.
Authors: Maxime Hamon-Josse, Jose Villaecija-Aguilar, Karin Ljung, Ottoline Leyser, Caroline Gutjahr and Tom Bennett.
bioRxiv (2021)
Abstract: "The photomorphogenic remodelling of seedling growth upon exposure to light is a key developmental transition in the plant life cycle. The α/β-hydrolase signalling protein KARRIKIN-INSENSITIVE2 (KAI2), a close homologue of the strigolactone receptor DWARF14 (D14), is involved in this process, and kai2 mutants have strongly altered seedling growth as a result. KAI2 and D14 both act through the MAX2 (MORE AXILLARY BRANCHING2) F-box protein to target proteins of the SMAX1-LIKE (SUPPRESSOR OF MAX2 1) (SMXL) family for degradation, but the signalling events downstream of this step are unclear in both pathways. Here, we show that kai2 phenotypes arise because of a failure to downregulate auxin transport from the seedling shoot apex towards the root system, rather than a failure to respond to light per se. We demonstrate that KAI2 controls the light-induced remodelling of the PIN-mediated auxin transport system in seedlings, promoting the reduction of PIN3, PIN4, and PIN7 abundance in older tissues, and the increase of PIN1, PIN2, PIN3, and PIN7 abundance in the root meristem, consistent with transition from elongation-mediated growth in the dark to meristematically-mediated growth in the light. We show that removing PIN3, PIN4 and PIN7 from kai2 mutants, or pharmacological inhibition of auxin transport and synthesis, is sufficient to suppress most kai2 seedling phenotypes. KAI2 is not required for the light-mediated changes in PIN gene expression but is required for the changes in PIN protein abundance at the plasma membrane; we thus propose that KAI2 acts to promote vesicle trafficking, consistent with previous suggestions about D14-mediated signalling in the shoot."
Authors: Yuliya A. Krasylenko, George Komis, Sonya Hlynska, Tereza Vavrdová, Miroslav Ovečka, Tomáš Pospíšil and Jozef Šamaj.
bioRxiv (2020)
Abstract: "Strigolactones are phytohormones involved in shoot branching and hypocotyl elongation. The latter phenomenon was addressed herein by the exogenous application of a synthetic strigolactone GR24 and an inhibitor of strigolactone biosynthesis TIS108 on hypocotyls of wild type Arabidopsis and a strigolactone signalling mutant max2-1 (more axillary growth 2-1). Owing to the interdependence between light and strigolactone signalling, the present work was extended to seedling cultivation under a standard light/dark regime, or under continuous darkness. Given the essential role of the cortical microtubules in cell elongation, their organization and dynamics were characterized under the conditions of altered strigolactone signalling using fluorescence microscopy methods with different spatiotemporal capacities such as confocal laser scanning microscopy and structured illumination microscopy. It was found that the strigolactone-dependent inhibition of hypocotyl elongation correlated with changes in cortical microtubule organization and dynamics, visualized in living wild type and max2-1 seedlings stably expressing genetically-encoded fluorescent molecular markers for microtubules. Quantitative analysis of microscopic datasets revealed that chemical and/or genetic manipulation of strigolactone signalling affected microtubule remodelling, especially under light conditions. The application of GR24 and TIS108 in dark conditions partially alleviated cytoskeletal rearrangement, suggesting a new mechanistic connection between the cytoskeletal behaviour and the light-dependence of strigolactone signalling."
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