@phdthesis{Li2023,
  author    = {Li, Kunkun},
  title     = {Dissecting the interconnection of Ca\(^{2+}\) and pH signaling in plants with a novel biosensor for dual imaging},
  doi       = {10.25972/OPUS-24973},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-249736},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Calcium ion (Ca2+) and protons (H+) are both regarded as second messengers, participating in plant growth and stress mechanisms. However, H+ signals in plant physiology are less well investigated compared to Ca2+ signals. If interconnections between these two second messengers exist remains to be uncovered because appropriate imaging tools to monitor Ca2+ and H+ simultaneously in the same cell as well as accurate bioinformatics analysis remain to be developed. To overcome this problem and unravel the role and possible interconnection of Ca2+ and H+ in plants, a new biosensor named CapHensor was developed and optimized to visualize intracellular Ca2+ and H+ changes simultaneously and ratiometrically in the same cell. The CapHensor consisted of an optimized green fluorescent pH sensor (PRpHluorin) and an established red fluorescent Ca2+ sensor (R-GECO1) that were combined in one construct via a P2A sequence. A P2A self-cleavage site between the two sensors allowed to express equal amounts but spatially separated sensors, which enabled artifact-free and ratiometric imaging of cellular Ca2+ and pH side-by-side. The function of the CapHensor was verified in pollen tubes, since they possess standing Ca2+ and pH gradients. We found better imaging quality and the signal-to-noise ratio to be enhanced in live-cell imaging when two R-GECO1 proteins were fused in tandem within the CapHensor construct. To guarantee exclusive subcellular localization and avoid mixed signals from different compartments, Nuclear Export Sequence (NES) and Nuclear Localization Sequence (NLS) were used to target PRpHluorin and R-GECO1 to distinct compartments. After optimization and verification its function, CapHensor was successfully expressed in different cell types to investigate the role of Ca2+ and H+ signals to control polar growth of pollen tube, stomatal movement or leaf defense signaling. Results obtained in the past indicated both Ca2+ gradients and pH gradients in pollen tubes play roles in polar growth. However, the role and temporal relationship between the growth process and changes in Ca2+ and pH have not been conclusively resolved. Using CapHensor, I found cytosolic acidification at the tip could promote and alkalization to suppress growth velocity in N. tabacum pollen tubes, indicating that cytosolic H+ concentrations ([H+]cyt) play an important role in regulation pollen tubes growth despite the accompanied changes in cytosolic Ca2+ concentrations ([Ca2+]cyt). Moreover, growth correlated much better with the tip [H+]cyt regime than with the course of the tip [Ca2+]cyt regime. However, surprisingly, tip-focused [Ca2+]cyt andII [H+]cyt oscillations both lagged behind growth oscillations approximately 33 s and 18 s, respectively, asking for a re-evaluation of the role that tip [Ca2+]cyt may play in pollen tube growth. Live-cell CapHensor imaging combined with electrophysiology uncovered that oscillatory membrane depolarization correlated better with tip [H+]cyt oscillations than with tip [Ca2+]cyt oscillations, indicative for a prominent role of [H+]cyt to also control electrogenic membrane transport. Using CapHensor, reading out cellular movement at the same time enabled to provide a precise temporal and spatial resolution of ion signaling events, pointing out a prominent role of [H+]cyt in pollen tube tip growth. For leaf cells, a special CapHensor construct design had to be developed, containing additional NES localization sequences to avoid overlapping of fluorescense signals from the nucleus and the cytosol. Once this was achieved, the role of Ca2+ and pH changes in guard cells, another typical single-cell system was investigated. Cytosolic pH changes have been described in stomatal movement, but the physiological role of pH and the interaction with changing Ca2+ signals were still unexplored. Combining CapHensor with the here developed technique to monitor stomatal movement in parallel, the role of Ca2+ and H+ in stomatal movement was studied in detail and novel aspects were identified. The phytohormone ABA and the bacterial elicitor flagellin (flg22) are typical abiotic and biotic stresses, respectively, to trigger stomatal closure. What kind of Ca2+ and H+ signals by ABA and flg22 are set-off in guard cells and what their temporal relationship and role for stomatal movement is were unknown. Similar [Ca2+]cyt increases were observed upon ABA and flg22 triggered stomatal closure, but [H+]cyt dynamics differed fundamentally. ABA triggered pronounced cytosolic alkalization preceded the [Ca2+]cyt responses significantly by 57 s while stomata started to close ca. 205 s after phytohormone application. With flg22, stomatal closure was accompanied only with a mild cytosolic alkalization but the [Ca2+]cyt response was much more pronounced compared to the ABA effects. Where the cytosolic alkalization originates from was unclear but the vacuole was speculated to contribute in the past. In this thesis, vacuolar pH changes were visualized by the dye BCECF over time, basically displaying exactly the opposite course of the concentration shift in the vacuole than observed in the cytosol. This is indicative for the vacuolar pH dynamics to be coupled strongly to the cytosolic pH changes. In stomatal closure signalling, reactive oxygen species (ROS) were proposed to play a major role, however, only very high concentration of H2O2 (> 200 µM), which resulted in the loss of membrane integrity, induced stomatal closure. Unexpectedly, physiological concentrations of ROS led to cytosolic acidificationIII which was associated with stomatal opening, but not stomatal closure. To study the role of [H+]cyt to steer stomatal movement in detail, extracellular and intracellular pH variations were evoked in N. tabacum guard cells and their behaviour was followed. The results demonstrated cytosolic acidification stimulated stomatal opening while cytosolic alkalization triggered stomatal closure accompanied by [Ca2+]cyt elevations. This demonstrated pH regulation to be an important aspect in stomatal movement and to feed-back on the Ca2+-dynamics. It was remarkable that cytosolic alkalization but not [Ca2+]cyt increase seemed to play a crucial role in stomatal closure, because more pronounced cytosolic alkalization, evoked stronger stomatal closure despite similar [Ca2+]cyt increases. Increases in [Ca2+]cyt, which are discussed as an early stomatal closure signal in the past, could not trigger stomatal closure alone in my experiments, even when extremely strong [Ca2+]cyt signals were triggered. Regarding the interaction between the two second messengers, [Ca2+]cyt and [H+]cyt were negatively correlated most of the times, which was different from pollen tubes showing positive correlation of [Ca2+]cyt and [H+]cyt regimes. [Ca2+]cyt elevations were always associated with a cytosolic alkalization and this relationship could be blocked by the presence of vanadate, a plasma membrane H+-pump blocker, indicating plasma membrane H+-ATPases to contribute to the negative correlation of [Ca2+]cyt and [H+]cyt. To compare with guard cells, cytosolic and nuclear versions of CapHensor were expressed in N. benthamiana mesophyll cells, a multicellular system I investigated. Mesophyll cell responses to the same stimuli as tested in guard cells demonstrated that ABA and H2O2 did not induce any [Ca2+]cyt and [H+]cyt changes while flg22 induced an increase in [Ca2+]cyt and [H+]cyt, which is different from the response in guard cells. I could thus unequivocally demonstrate that guard cells and mesophyll cells do respond differently with [Ca2+]cyt and [H+]cyt changes to the same stimuli, a concept that has been proposed before, but never demonstrated in such detail for plants. Spontaneous Ca2+ oscillations have been observed for a long time in guard cells, but the function or cause is still poorly understood. Two populations of oscillatory guard cells were identified according to their [Ca2+]cyt and [H+]cyt phase relationship in my study. In approximately half of the oscillatory cells, [H+]cyt oscillations preceded [Ca2+]cyt oscillations whereas [Ca2+]cyt was the leading signal in the other half of the guard cells population. Strikingly, natural [H+]cyt oscillations were dampened by ABA but not by flg22. This effect could be well explained by dampening of vacuolar H+ oscillations in the presence of ABA, but not through flg22. Vacuolar pH contributes to spontaneous [H+]cyt oscillations and ABA but not flg22 can block the interdependence of naturalIV [Ca2+]cyt and [H+]cyt signals. To study the role of [Ca2+]cyt oscillations in stomatal movement, solutions containing high and low KCl concentrations were applied aiming to trigger [Ca2+]cyt oscillations. The triggering of [Ca2+]cyt oscillations by this method was established two decades ago leading to the dogma that [Ca2+]cyt increases are the crucial signal for stomatal closure. However, I found stomatal movement by this method was mainly due to osmotic effects rather than [Ca2+]cyt increases. Fortunately, through this methodology, I found a strong correlation between cytosolic pH and the transport of potassium across the plasma membrane and vacuole existed. The plasma membrane H+-ATPases and H+-coupled K+ transporters were identified as the cause of [H+]cyt changes, both very important aspects in stomata physiology that were not visualized experimentally before. Na+ transport is also important for stomatal regulation and leaves generally since salt can be transported from the root to the shoot. Unlike well-described Ca2+- dependent mechanisms in roots, how leaves process salt stress is not at all understood. I applied salt on protoplasts from leaves, mesophyll cells and guard cells and combined live-cell imaging with Vm recordings to understand the transport and signaling for leaf cells to cope with salt stress. In both, mesophyll and guard cells, NaCl did not trigger Ca2+-signals as described for roots but rather triggered Ca2+ peaks when washing salt out. However, membrane depolarization and pronounced alkalinization were very reliably triggered by NaCl, which could presumably act as a signal for detoxification of high salt concentrations. In line with this, I found the vacuolar cation/H+ antiporter NHX1 to play a role in sodium transport, [H+]cyt homeostasis and the control of membrane potential. Overexpression of AtNHX1 enabled to diminish [H+]cyt changes and resulted in a smaller depolarization responses druing NaCl stress. My results thus demonstrated in contrast to roots, leaf cells do not use Ca2+-dependent signalling cascades to deal with salt stress. I could show Na+ and K+ induced [H+]cyt and Vm responses and Cl- transport to only have a minor impact. Summing all my results up briefly, I uncovered pH signals to play important roles to control pollen tube growth, stomatal movement and leaf detoxification upon salt. My results strongly suggested pH changes might be a more important signal than previously thought to steer diverse processes in plants. Using CapHensor in combination with electrophysiology and bioinformatics tools, I discovered distinct interconnections between [Ca2+]cyt and [H+]cyt in different cell types and distinct [Ca2+]cyt and [H+]cyt signals are initiated through diverse stimuli and environmental cues. The CapHensor will be very useful in the future to further investigate the coordinated role of Ca2+ and pH changes in controlling plant physiology.},
  subject      = {Pflanzen},
  language  = {en}
}