Salinity-Induced Cytosolic Alkaline Shifts in Arabidopsis Roots Require the SOS Pathway

Abstract

Plants have evolved elaborate mechanisms to sense, respond to and overcome the detrimental effects of high soil salinity. The role of calcium transients in salinity stress signaling is well established, but the physiological significance of concurrent salinity-induced changes in cytosolic pH remains largely undefined. Here, we analyzed the response of Arabidopsis roots expressing the genetically encoded ratiometric pH-sensor pHGFP fused to marker proteins for the recruitment of the sensor to the cytosolic side of the tonoplast (pHGFP-VTI11) and the plasma membrane (pHGFP-LTI6b). Salinity elicited a rapid alkalinization of cytosolic pH (pHcyt) in the meristematic and elongation zone of wild-type roots. The pH-shift near the plasma membrane preceded that at the tonoplast. In pH-maps transversal to the root axis, the epidermis and cortex had cells with a more alkaline pHcyt relative to cells in the stele in control conditions. Conversely, seedlings treated with 100 mM NaCl exhibited an increased pHcyt in cells of the vasculature relative to the external layers of the root, and this response occurred in both reporter lines. These pHcyt changes were substantially reduced in mutant roots lacking a functional SOS3/CBL4 protein, suggesting that the operation of the SOS pathway mediated the dynamics of pHcyt in response to salinity.

The Morphological Parameters and Cytosolic pH of Cells of Root Zones in Tobacco Plants (Nicotiana tabacum L.): Nonlinear Effects of NaCl Concentrations

“…Notably, this pHcyt response to NaCl in tobacco plants, up to 100 mM, bears a resemblance to the pHcyt changes observed in arabidopsis cells, both in terms of direction and within a range of up to 0.6 pH units [25,27]. At the same time, Rombolá-Caldentey et al [28] reported cytosolic alkalinization in the cells of the meristematic and elongation root zones of arabidopsis plants when exposed to 50 mM or 100 mM NaCl during the day. Nevertheless, it is inconclusive whether the nonlinear pHcyt response to NaCl concentration is a distinctive feature of tobacco plants or a characteristic shared by other species, as data about the dependence of pH change on NaCl concentration in wide range are absent in the literature.…”

“…Changes in pH are also observed in the case of salinity and have been recorded in the root and shoot cells of various plant species, including Arabidopsis [25][26][27][28], tobacco and potato [29], quince protoplasts [30], beans protoplasts [31], quinoa and peas protoplasts [32] and rice protoplasts [33]. Salinity-induced pHcyt changes can be of either direction.…”

“…Salinity-induced pHcyt changes can be of either direction. Some studies report cytosolic acidification [25][26][27]29,32], while others indicate cytosolic alkalinization [28,[30][31][32]. Investigations focusing on salinity-induced tissue-specific pH changes in intact roots across a wide range of NaCl concentrations have rarely been published and they have mainly been conducted on Arabidopsis [27,28].…”

“…Some studies report cytosolic acidification [25][26][27]29,32], while others indicate cytosolic alkalinization [28,[30][31][32]. Investigations focusing on salinity-induced tissue-specific pH changes in intact roots across a wide range of NaCl concentrations have rarely been published and they have mainly been conducted on Arabidopsis [27,28]. This scarcity is primarily due to the limitations of traditional methods, such as potentiometric analysis and fluorescent chemical probes, which are unsuitable for long-lasting measurements on intact plants.…”

Salt-Induced Early Changes in Photosynthesis Activity Caused by Root-to-Shoot Signaling in Potato

“…Simultaneously with the hyperpolarization in the root cells, the cytosol was alkalized and then acidified (Figure 13A). This fact may indicate a short-term increase in the activity of plasmalemma H + -ATPase [68,69], which induced hyperpolarization in the root and calcium influx through voltage-gated ion channels [65,70,71]. H + -ATPase can be activated both as part of the activation of the salt overly sensitive (SOS) signaling pathway of Na + exclusion from the cell [61] and by reducing the phosphoinositol concentration due to the activation of phosphatidylinositol-4-kinase (PI4K) [72,73]; both pathways are likely to involve Ca 2+ .…”