Growth and molecular responses to long-distance stimuli in poplars: bending vs flame wounding

Deciphering the role of mechanosensitive channels in plant root biology: perception, signaling, and adaptive responses

MAIN CONCLUSION

Mechanosensitive channels are integral membrane proteins that rapidly translate extrinsic or intrinsic mechanical tensions into biological responses. They can serve as potential candidates for developing smart-resilient crops with efficient root systems. Mechanosensitive (MS) calcium channels are molecular switches for mechanoperception and signal transduction in all living organisms. Although tremendous progress has been made in understanding mechanoperception and signal transduction in bacteria and animals, this remains largely unknown in plants. However, identification and validation of MS channels such as Mid1-complementing activity channels (MCAs), mechanosensitive-like channels (MSLs), and Piezo channels (PIEZO) has been the most significant discovery in plant mechanobiology, providing novel insights into plant mechanoperception. This review summarizes recent advances in root mechanobiology, focusing on MS channels and their related signaling players, such as calcium ions (Ca2+), reactive oxygen species (ROS), and phytohormones. Despite significant advances in understanding the role of Ca2+ signaling in root biology, little is known about the involvement of MS channel-driven Ca2+ and ROS signaling. Additionally, the hotspots connecting the upstream and downstream signaling of MS channels remain unclear. In light of this, we discuss the present knowledge of MS channels in root biology and their role in root developmental and adaptive traits. We also provide a model highlighting upstream (cell wall sensors) and downstream signaling players, viz., Ca2+, ROS, and hormones, connected with MS channels. Furthermore, we highlighted the importance of emerging signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), and neurotransmitters (NTs), and their association with root mechanoperception. Finally, we conclude with future directions and knowledge gaps that warrant further research to decipher the complexity of root mechanosensing.

The Course of Mechanical Stress: Types, Perception, and Plant Response

Mechanical stimuli, together with the corresponding plant perception mechanisms and the finely tuned thigmomorphogenetic response, has been of scientific and practical interest since the mid-17th century. As an emerging field, there are many challenges in the research of mechanical stress. Indeed, studies on different plant species (annual/perennial) and plant organs (stem/root) using different approaches (field, wet lab, and in silico/computational) have delivered insufficient findings that frequently impede the practical application of the acquired knowledge. Accordingly, the current work distils existing mechanical stress knowledge by bringing in side-by-side the research conducted on both stem and roots. First, the various types of mechanical stress encountered by plants are defined. Second, plant perception mechanisms are outlined. Finally, the different strategies employed by the plant stem and roots to counteract the perceived mechanical stresses are summarized, depicting the corresponding morphological, phytohormonal, and molecular characteristics. The comprehensive literature on both perennial (woody) and annual plants was reviewed, considering the potential benefits and drawbacks of the two plant types, which allowed us to highlight current gaps in knowledge as areas of interest for future research.

Stem bending generates electrical response in poplar

Under natural conditions, plants experience external mechanical stresses such as wind and touch that impact their growth. A remarkable feature of this mechanically induced growth response is that it may occur at a distance from the stimulation site, suggesting the existence of a signal propagating through the plant. In this study, we investigated the electrical response of poplar trees to a transient controlled bending stimulation of the stem that mimics the mechanical effect of wind. Stem bending was found to cause an electrical response that we called "gradual" potential, similar in shape to an action potential. However, this signal can be distinguished from the well-known plant action potential by its propagation up to 20 cm along the stem and its strong dumping in velocity and amplitude. Two hypotheses regarding the mode of propagation of the "gradual" potential are discussed.

Between Stress and Response: Function and Localization of Mechanosensitive Ca2+ Channels in Herbaceous and Perennial Plants

Over the past three decades, how plants sense and respond to mechanical stress has become a flourishing field of research. The pivotal role of mechanosensing in organogenesis and acclimation was demonstrated in various plants, and links are emerging between gene regulatory networks and physical forces exerted on tissues. However, how plant cells convert physical signals into chemical signals remains unclear. Numerous studies have focused on the role played by mechanosensitive (MS) calcium ion channels MCA, Piezo and OSCA. To complement these data, we combined data mining and visualization approaches to compare the tissue-specific expression of these genes, taking advantage of recent single-cell RNA-sequencing data obtained in the root apex and the stem of Arabidopsis and the Populus stem. These analyses raise questions about the relationships between the localization of MS channels and the localization of stress and responses. Such tissue-specific expression studies could help to elucidate the functions of MS channels. Finally, we stress the need for a better understanding of such mechanisms in trees, which are facing mechanical challenges of much higher magnitudes and over much longer time scales than herbaceous plants, and we mention practical applications of plant responsiveness to mechanical stress in agriculture and forestry.

Potential of astigmatid mites (Acari: Astigmatina) as prey for rearing edaphic predatory mites of the families Laelapidae and Rhodacaridae (Acari: Mesostigmata)

Laelapidae and Rhodacaridae are important families of edaphic predatory mites and species of these families have been considered for use in biological control programs of soil pests. Mites of Cohort Astigmatina (Acari: Sarcoptiformes) have been largely used as factitious prey in the mass rearing of various edaphic or plant-inhabiting predatory mites. Stratiolaelaps scimitus (Womersley) (Mesostigmata: Laelapidae) (widely commercialized for the control of fungus gnats and thrips) and Protogamasellopsis zaheri Abo-Shnaf, Castilho and Moraes (Mesostigmata: Rhodacaridae) (not available commercially but promising for the control of thrips and nematodes) are known to be reared on Tyrophagus putrescentiae (Schrank) (Astigmatina: Acaridae), but the possibility to find a perhaps more efficient prey has not been evaluated. The objective of this paper was to evaluate different astigmatid species as prey for these predators. S. scimitus and P. zaheri oviposited on all evaluated astigmatids and the acarid mites T. putrescentiae and Aleuroglyphus ovatus (Tropeau) were the most suitable prey; to confirm the effect of prey on oviposition rates, pregnant females of the predators were kept under starvation conditions and oviposition was negligible or null. Survivorship was always higher than 78 % and was not influenced by prey species or starvation.

Mechanosensitive control of plant growth: bearing the load, sensing, transducing, and responding

As land plants grow and develop, they encounter complex mechanical challenges, especially from winds and turgor pressure. Mechanosensitive control over growth and morphogenesis is an adaptive trait, reducing the risks of breakage or explosion. This control has been mostly studied through experiments with artificial mechanical loads, often focusing on cellular or molecular mechanotransduction pathway. However, some important aspects of mechanosensing are often neglected. (i) What are the mechanical characteristics of different loads and how are loads distributed within different organs? (ii) What is the relevant mechanical stimulus in the cell? Is it stress, strain, or energy? (iii) How do mechanosensing cells signal to meristematic cells? Without answers to these questions we cannot make progress analyzing the mechanobiological effects of plant size, plant shape, tissue distribution and stiffness, or the magnitude of stimuli. This situation is rapidly changing however, as systems mechanobiology is being developed, using specific biomechanical and/or mechanobiological models. These models are instrumental in comparing loads and responses between experiments and make it possible to quantitatively test biological hypotheses describing the mechanotransduction networks. This review is designed for a general plant science audience and aims to help biologists master the models they need for mechanobiological studies. Analysis and modeling is broken down into four steps looking at how the structure bears the load, how the distributed load is sensed, how the mechanical signal is transduced, and then how the plant responds through growth. Throughout, two examples of adaptive responses are used to illustrate this approach: the thigmorphogenetic syndrome of plant shoots bending and the mechanosensitive control of shoot apical meristem (SAM) morphogenesis. Overall this should provide a generic understanding of systems mechanobiology at work.