Molecular evolution of calcium signaling and transport in plant adaptation to abiotic stress

Adaptation to unfavorable abiotic stresses is one of the key processes in the evolution of plants. Calcium (Ca2+) signaling is characterized by the spatiotemporal pattern of Ca2+ distribution and the activities of multi-domain proteins in integrating environmental stimuli and cellular responses, which are crucial early events in abiotic stress responses in plants. However, a comprehensive summary and explanation for evolutionary and functional synergies in Ca2+ signaling remains elusive in green plants. We review mechanisms of Ca2+ membrane transporters and intracellular Ca2+ sensors with evolutionary imprinting and structural clues. These may provide molecular and bioinformatics insights for the functional analysis of some non-model species in the evolutionarily important green plant lineages. We summarize the chronological order, spatial location, and characteristics of Ca2+ functional proteins. Furthermore, we highlight the integral functions of calcium-signaling components in various nodes of the Ca2+ signaling pathway through conserved or variant evolutionary processes. These ultimately bridge the Ca2+ cascade reactions into regulatory networks, particularly in the hormonal signaling pathways. In summary, this review provides new perspectives towards a better understanding of the evolution, interaction and integration of Ca2+ signaling components in green plants, which is likely to benefit future research in agriculture, evolutionary biology, ecology and the environment

Steady Wind Load on External Surface and Its Effect on Wind-Induced Response for a 200 m High Natural-Draught Cooling Tower

Wind tunnel tests were carried out to measure the wind pressure of a 200 m high natural-draught cooling tower. An analysis of the distribution characteristics of external pressure was then conducted to determine the pressure coefficients Cp(θ, z) in a given wind profile. Finally, the effect on the response of the shell and the buckling safety of the shell, applying the simplified height-constant pressure coefficient Cp(θ) and the realistic pressure Cp(θ, z), was determined. Taking the wind load specified in the code as an example, the influence of the distribution of external pressure on the wind-induced response was further analyzed. The results indicate that the pressure distribution varies with not only the height z but also the circumferential angle θ, and the wind load of both ends of the tower is significantly greater than that of its middle. Moreover, the wind-induced static responses of the tower under the action of the realistic pressure distribution Cp(θ, z) and the simplified approach Cp(θ) are basically consistent, because the wind load distribution is more important than its magnitude for the wind-induced response of cooling tower, and the wind-induced response of the cooling tower is dominated by the local shell deformation

K+ uptake, H+-ATPase pumping activity and Ca2+ efflux mechanism are involved in drought tolerance of barley

Chemical signals play a significant role in improving plant water use efficiency under drought stress. Hydroponic and pot experiments were conducted using three barley genotypes to study genotypic differences in K+, Ca2+ and H+ fluxes and physiological and biochemical traits of drought tolerant Tibetan wild barley XZ5 and cv Tadmor and drought sensitive cv ZJU9 in response to drought. Transient and steady-state ion fluxes were measured by noninvasive ion-selective microelectrode MIFE technique. We showed that exogenous PEG (polyethylene glycol 6000) and mannitol and soil drought stress all resulted in an immediate K+ uptake from root epidermis and leaf mesophyll, with much more uptake in XZ5. Long-term drought stress are more detrimental to root K+ homeostasis, and the degree of K+ uptake differed due to severity of drought stress and was less presented in XZ5. Barley subjected to drought stress caused a large H+ efflux in root epidermis and H+ influx in leaf mesophyll, with significantly less alteration in XZ5 and Tadmor than in ZJU9. Meanwhile a dramatic Ca2+ efflux was observed in root epidermis and leaf mesophyll under drought stress. PEG and mannitol treatments induced marked increases in H+-K+- ATPase in XZ5 and Tadmor. Our results demonstrate that K+ uptake, Ca2+ efflux and leaf H+ influx/ alkalization of apoplastic pH could be a chemical signal in barley in response to drought stress, and that stimulated H+-K+-ATPase and K+ uptake, but less Ca2+ efflux and H+ alteration under drought, when concerning ionic mechanisms underlying drought tolerance, play an important role in drought tolerance in XZ5

Genotypic difference in the influence of aluminum and low pH on ion flux, rhizospheric pH and ATPase activity between Tibetan wild and cultivated barley

Low-pH and Al3+ toxicity are the major factors causing the inhibition of root elongation and plant growth in acid soils. However, it is still unclear about the adaptive mechanisms for plant roots to cope with simultaneous low-pH and Al3+ toxicity in the acid soil. In this study, changes in root tip H+, K+, and Ca2+ fluxes and rhizospheric pH of three barley genotypes, including two Tibetan wild barley (XZ16, Al-tolerant; XZ61, Al-sensitive), and Al-tolerant cv Dayton, were investigated using microelectrode ion flux estimation (MIFE) technology. The results showed that Al-induced ion flux changes were mainly occurred at the elongation zone. Under low-pH treatment, at 24 h, the rhizosphere pH of low-pH tolerant XZ16 and Dayton were 0.7 and 0.3 units higher than low-pH sensitive XZ61, respectively. The H+ influx of tolerant XZ16 and Dayton was higher than that of sensitive XZ61 during the whole process of both low-pH and Al treatments. Furthermore, the Al stress significantly increased the Ca2+ efflux, inhibited the K+ efflux and ATPase activities, especially in the Al-sensitive XZ61. These results indicated that the higher low-pH tolerance in XZ16 was connected with the higher ability of H+ uptake and rhizospheric alkalization. The different mechanisms between low-pH and Al tolerance were probably tightly modulated by Al-induced increase of Ca2+ efflux and inhibition of ATPase activities

HvAKT2 and HvHAK1 confer drought tolerance in barley through enhanced leaf mesophyll H+ homoeostasis

Plant K+ uptake typically consists low—affinity mechanisms mediated by Shaker K+ channels (AKT/KAT/KC) and high-affinity mechanisms regulated by HAK/KUP/KT transporters, which are extensively studied. However, the evolutionary and genetic roles of both K+ uptake mechanisms for drought tolerance are not fully explored in crops adapted to dryland agriculture. Here, we employed evolutionary bioinformatics, biotechnological and electrophysiological approaches to determine the role of two important K+ transporters HvAKT2 and HvHAK1 in drought tolerance in barley. HvAKT2 and HvHAK1 were cloned and functionally characterized using barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) in drought-tolerant wild barley XZ5 and agrobacterium-mediated gene transfer in the barley cultivar Golden Promise. The hallmarks of the K+ selective filters of AKT2 and HAK1 are both found in homologues from strepotophyte algae, and they are evolutionarily conserved in strepotophyte algae and land plants. HvAKT2 and HvHAK1 are both localized to the plasma membrane and have high selectivity to K+ and Rb+ over other tested cations. Overexpression of HvAKT2 and HvHAK1 enhanced K+ uptake and H+ homoeostasis leading to drought tolerance in these transgenic lines. Moreover, HvAKT2- and HvHAK1-overexpressing lines showed distinct response of K+, H+ and Ca2+ fluxes across plasma membrane and production of nitric oxide and hydrogen peroxide in leaves as compared to the wild type and silenced lines. High- and low-affinity K+ uptake mechanisms and their coordination with H+ homoeostasis play essential roles in drought adaptation of wild barley. These findings can potentially facilitate future breeding programs for resilient cereal crops in a changing global climate