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Thigmomorphogenesis involves plants altering their growth and development in response to mechanical stimuli, such as touch, wind, or rain. This process begins with the perception of mechanical forces by cellular , followed by their transduction into signal transduction pathways cascades, and culminating in changes in gene expression and hormone activity. The response integrates diverse molecular components, including mechanosensitive , receptor-like kinases, the cytoskeleton, , and transcription factors, which collectively drive both immediate physiological and long-term morphological .
Early observations noted that greenhouse-grown plants were often taller and more slender than stockier plants grown outdoors, where they were exposed to natural mechanical stresses. The term "thigmomorphogenesis" is derived from Ancient Greek θιγγάνω (thingánō, "to touch"), μορφή ( morphê, "shape"), and γένεσις (génesis, "creation").
While Microfilament are less directly involved in initial mechanical sensing, they are crucial for maintaining cellular integrity and facilitating localized growth responses. During tendril coiling, for example, actin filaments help stabilize cytoskeletal architecture and direct asymmetric growth along the dorsal and ventral sides of the tendril. Disrupting actin does not inhibit the coiling response but affects Cell turgor, suggesting a supportive rather than primary role in mechanosensation. Together, the microtubule and actin networks provide a structural framework for the mechanotransduction machinery, ensuring efficient integration of external mechanical forces into the plant's development and physiology.
In plants, three major families of MCAss have been identified: MscS-like channels (MSLs), Mid1-complementing activity proteins (MCAs), and two-pore potassium (TPK) channels. The MSL family, which shares homology with bacterial MscS Ion channel, includes members such as MSL8, MSL9, and MSL10. These proteins are localized to various cellular membranes and respond to mechanical stress by gating ionic flux. Arabidopsis MSL8, for example, is expressed in pollen and regulates Cell turgor during hydration and germination. Mutations in MSL8 result in high rates of pollen bursting during hydration, highlighting its role as a turgor regulator. Similarly, MSL9 and MSL10 are expressed in root cells and play critical roles in mediating responses to osmotic shock and mechanical stress. These channels regulate ion fluxes across the plasma membrane, contributing to the plant's ability to adjust to changing environmental conditions. While MSL9 and MSL10 share similar gating properties, they appear to have distinct physiological roles, with MSL10 also implicated in reactive oxygen species (ROS) generation and stress signaling.
MCAs are another key family of MCAss, known for their role in enhancing Ca²⁺ influx upon mechanical perturbation. Arabidopsis MCA1 and MCA2 are localized to the plasma membrane and are essential for root penetration into hard or compacted soils. By gating Ca²⁺ influx, these channels facilitate downstream processes such as cytoskeletal rearrangements and hormone signaling that enable root growth under challenging conditions.
TPK channels, a less-studied but significant family, modulate potassium flux in response to mechanical forces. This activity influences guard cell function, root cell turgor, and other mechanical responses critical to maintaining cellular homeostasis under stress.
These calcium-binding proteins activate effectors like calcium-dependent protein kinasess (CDPKs) and calcineurin B-like proteinss (CBLs), which fine-tune cellular responses such as ion transport, ROS generation, and gene expression. The rapid, transient nature of calcium spikes helps plants differentiate between fleeting disturbances and sustained forces, preventing overstimulation of downstream pathways. By integrating with other signaling pathways, including ROS and hormonal signaling, calcium signals orchestrate a coordinated response to mechanical stress.
Ethylene and auxin complement JA by influencing cell elongation and asymmetric growth, both critical for thigmomorphogenesis. Ethylene production increases in mechanically stimulated plants and is linked to radial expansion and stem thickening, traits that enhance structural stability against mechanical forces like wind. Auxin, on the other hand, modulates differential growth responses, such as tendril coiling. In tendrils, JA and auxin establish opposing gradients, with JA promoting growth inhibition on one side and auxin stimulating elongation on the other, driving the coiling response.
In parallel, MYC transcription factors play a central role in JA-dependent transcriptional networks. These factors mediate the expression of genes involved in hormone biosynthesis, defense responses, and mechanical stress adaptation. Mutants deficient in MYC2, MYC3, and MYC4 exhibit impaired thigmomorphogenic responses, highlighting the importance of these transcription factors in integrating hormonal and mechanical signaling. MYC-regulated genes include those encoding jasmonate-responsive proteins, such as JAZ repressors, which modulate JA signaling pathways.
Enhanced lignification, a hallmark of thigmomorphogenesis, strengthens plant cell walls, contributing to mechanical resistance against environmental stressors. In species like Phaseolus vulgaris and Arabidopsis thaliana, mechanical stimulation induces enzymes involved in lignin biosynthesis, leading to thicker and more rigid stems. These structural changes also improve plant defenses against herbivory and pathogen invasion, as lignified tissues are more resistant to mechanical penetration by pests and microbes.
Delayed flowering is another adaptation linked to thigmomorphogenesis, allowing plants to allocate resources toward fortifying their structures before investing in reproduction. Touch-induced delays in flowering are regulated by hormonal pathways involving JA and Gibberellin (GA). For instance, Arabidopsis plants subjected to mechanical stimulation exhibit lower bioactive GA levels and higher JA levels, both of which contribute to delayed floral transition and enhanced resilience.
Mechanosensation responses also enhance root anchorage, crucial for stabilizing plants against mechanical forces such as wind. Mechanical stimulation increases root dry weight and branching, likely mediated by changes in auxin and ethylene signaling pathways. In environments with compacted soils, MCAss facilitate calcium-dependent responses that promote root penetration and growth. Together, these changes optimize root architecture for effective water and nutrient uptake while resisting uprooting by mechanical stresses.
These physiological adaptations underscore the evolutionary significance of thigmomorphogenesis as a survival strategy. By integrating mechanical signals with growth and defense pathways, plants achieve a balance between structural reinforcement, stress tolerance, and resource allocation. This dynamic response enhances individual fitness and contributes to the ecological success of plants in diverse and fluctuating environments.
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