Plants are exposed to various environmental stresses. The sensing of environmental cues and the transduction of stress signals into intracellular signaling are initial events in the cellular signaling network. As a second messenger, Ca2+ links environmental stimuli to different biological processes, such as growth, physiology, and sensing of and response to stress. An increase in intracellular calcium concentrations ([Ca2+]i) is a common event in most stress-induced signal transduction pathways. In recent years, significant progress has been made in research related to the early events of stress signaling in plants, particularly in the identification of primary stress sensors. This review highlights current advances that are beginning to elucidate the mechanisms by which abiotic environmental cues are sensed via Ca2+ signals. Additionally, this review discusses important questions about the integration of the sensing of multiple stress conditions and subsequent signaling responses that need to be addressed in the future.
Although
responsive actuators have been intensively investigated,
it remains challenging to enable rapid and self-oscillating actuation
under ambient circumstances without human intervention analogous to
living organisms. By hybridizing a unique type of two-dimensional
nanomaterials (i.e., MXene) with a particular hydrophilic polymer,
a smart and flexible conductive composite was produced with rapid
actuation and spontaneous oscillation near a moist surface. Due to
the presence of layered microstructures and the moisture-sensitivity
improved by surface roughness and intercalated polymeric layers, the
composites could reversibly bend up to 180° in 2 s or 210°
in 10 s on demand when the circumstantial humidity was varied, being
superior or comparable to many actuators in the literature. More importantly,
the composite was capable not only of flipping upside down repeatedly
on the moist surface but also of self-oscillating ceaselessly under
ambient gradient humidity without human intervention, e.g., an oscillation
between 30 and 100° with an oscillation frequency of 0.08 Hz.
This self-oscillation resulted from the occurrence of rapid asymmetrical
hydration and dehydration of the composite between the regions of
high and low humidity, which could further be modulated both by different
hydrophilic polymers and by photoradiation owing to the photothermal
effect of MXene nanosheets. Because of the ubiquitous presence of
humidity gradient near the moist surface, this type of smart composite
may not only offer a strategy for designing artificial materials that
are capable of spontaneous actuation under ambient circumstance without
human intervention but also promise potential applications in artificial
muscles, autonomous robotics, and energy harvesting from environments.
Nanofluidics in two-dimensional (2-D) heterogeneous layered materials with hybrid overlapping structures exhibit promising potential in filtration and separation applications. However, molecular transport across the heterogeneous interlayer galleries remains largely unexplored, in particular, there exists disputation in the function and performance of hybrid graphene oxide (GO)-based laminate membrane for the water transport. Herein, heterogeneous 2-D GO-based nanochannels were employed as a typical platform to investigate the water flow by nonequilibrium molecular dynamics (MD) simulation. It is demonstrated that both heterogeneous and homogeneous GO nanochannels exhibit similar reduced water flow behavior, even if one surface of the 2-D channel is the pristine graphene. In particular, the flow rate in the hybrid GO/pristine graphene nanochannels does not lie between those of the oxidized and the pristine regions, and the highfriction GO surface suppresses the water transport and controls the entire flow performance. This result is qualitatively consistent with the recent experimental observation. By comparing with the MD simulation, a hydrodynamic model was developed to describe the flow rate for 2-D heterogeneous nanochannels. The reduced water transport has been revealed as the distinct vertical dragging effect, arising from the synergistic effect between the interfacial affinity from GO surfaces and the interlayer molecular interaction. Our results provide novel physical pictures for the molecular transport inside heterogeneous 2-D nanochannels.
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