Making and breaking of boron bridges in the pectic domain rhamnogalacturonan-II at apoplastic pH in vivo and in vitro

Cross-linking of the cell-wall pectin domain rhamnogalacturonan-II (RG-II) via boron bridges between apiose residues is essential for normal plant growth and development, but little is known about its mechanism or reversibility. We characterized the making and breaking of boron bridges in vivo and in vitro at 'apoplastic' pH. RG-II (13-26 μm) was incubated in living Rosa cell cultures and cell-free media with and without 1.2 mm H3 BO3 and cationic chaperones (Ca2+ , Pb2+ , polyhistidine, or arabinogalactan-protein oligopeptides). The cross-linking status of RG-II was monitored electrophoretically. Dimeric RG-II was stable at pH 2.0-7.0 in vivo and in vitro. In-vitro dimerization required a 'catalytic' cation at all pHs tested (1.75-7.0); thus, merely neutralizing the negative charge of RG-II (at pH 1.75) does not enable boron bridging. Pb2+ (20-2500 μm) was highly effective at pH 1.75-4.0, but not 4.75-7.0. Cationic peptides were effective at approximately 1-30 μm; higher concentrations caused less dimerization, probably because two RG-IIs then rarely bonded to the same peptide molecule. Peptides were ineffective at pH 1.75, their pH optimum being 2.5-4.75. d-Apiose (>40 mm) blocked RG-II dimerization in vitro, but did not cleave existing boron bridges. Rosa cells did not take up d-[U-14 C]apiose; therefore, exogenous apiose would block only apoplastic RG-II dimerization in vivo. In conclusion, apoplastic pH neither broke boron bridges nor prevented their formation. Thus boron-starved cells cannot salvage boron from RG-II, and 'acid growth' is not achieved by pH-dependent monomerization of RG-II. Divalent metals and cationic peptides catalyse RG-II dimerization via co-ordinate and ionic bonding respectively (possible and impossible, respectively, at pH 1.75). Exogenous apiose may be useful to distinguish intra- and extra-protoplasmic dimerization.

Inhibition of shoot-expressed NRT1.1 improves reutilization of apoplastic iron under iron-deficient conditions

Iron deficiency is a major constraint for plant growth in calcareous soils. The interplay between NO₃ ^- and Fe nutrition affects plant performance under Fe-deficient conditions. However, how NO₃ ^- negatively regulates Fe nutrition at the molecular level in plants remains elusive. Here, we showed that the key nitrate transporter NRT1.1 in Arabidopsis plants, especially in the shoots, was markedly downregulated at post-translational levels by Fe deficiency. However, loss of NRT1.1 function alleviated Fe deficiency chlorosis, suggesting that downregulation of NRT1.1 by Fe deficiency favors plant tolerance to Fe deficiency. Further analysis showed that although disruption of NRT1.1 did not alter Fe levels in both the shoots and roots, it improved the reutilization of apoplastic Fe in shoots but not in roots. In addition, disruption of NRT1.1 prevented Fe deficiency-induced apoplastic alkalization in shoots by inhibiting apoplastic H⁺ depletion via NO₃ ^- uptake. In vitro analysis showed that reduced pH facilitates release of cell wall-bound Fe. Thus, foliar spray with an acidic buffer promoted the reutilization of Fe in the leaf apoplast to enhance plant tolerance to Fe deficiency, while the opposite was true for the foliar spray with a neutral buffer. Thus, downregulation of the shoot-part function of NRT1.1 prevents apoplastic alkalization to ensure the reutilization of apoplastic Fe under Fe-deficient conditions. Our findings may provide a basis for elucidating the link between N and Fe nutrition in plants and insight to scrutinize the relevance of shoot-expressed NRT1.1 to the plant response to stress.

PDX1.1-dependent biosynthesis of vitamin B6 protects roots from ammonium-induced oxidative stress

Despite serving as a major inorganic nitrogen source for plants, ammonium causes toxicity at elevated concentrations, inhibiting root elongation early on. While previous studies have shown that ammonium-inhibited root development relates to ammonium uptake and formation of reactive oxygen species (ROS) in roots, it remains unclear about the mechanisms underlying the repression of root growth and how plants cope with this inhibitory effect of ammonium. In this study, we demonstrate that ammonium-induced apoplastic acidification co-localizes with Fe precipitation and hydrogen peroxide (H₂O₂) accumulation along the stele of the elongation and differentiation zone in root tips, indicating Fe-dependent ROS formation. By screening ammonium sensitivity in T-DNA insertion lines of ammonium-responsive genes, we identified PDX1.1, which is upregulated by ammonium in the root stele and whose product catalyzes de novo biosynthesis of vitamin B₆. Root growth of pdx1.1 mutants is hypersensitive to ammonium, while chemical complementation or overexpression of PDX1.1 restores root elongation. This salvage strategy requires non-phosphorylated forms of vitamin B₆ that are able to quench ROS and rescue root growth from ammonium inhibition. Collectively, these results suggest that PDX1.1-mediated synthesis of non-phosphorylated B₆ vitamers acts as a primary strategy to protect roots from ammonium-dependent ROS formation.

Biochemical and biophysical pH clamp controlling Net H+ efflux across the plasma membrane of plant cells

P-type H⁺ ATPases mediate active H⁺ efflux from plant cells. They generate a proton motive force across the plasma membrane, providing the free energy to drive the transport of other solutes, partly by coupling to H⁺ influx. Wegner & Shabala (2020) recently suggested that passive H⁺ influx can exceed pump-driven efflux due to 'active buffering', that is, cytosolic H⁺ scavenging and apoplastic H⁺ generation by metabolism ('biochemical pH clamp'). Charge balance is provided by K⁺ efflux or anion influx. Here, this hypothesis is extended to net H⁺ efflux: even though H⁺ pumping is faster than backflow via symporters and antiporters, a progressive increase in the transmembrane pH gradient is avoided. Cytosolic H⁺ release is associated with bicarbonate formation from CO₂ . Bicarbonate serves as substrate for the PEPCase, catalyzing the reaction from phosphoenolpyruvate to oxaloacetate, which is subsequently reduced to malate. Organic anions such as malate and citrate are released across the plasma membrane and are (partly) protonated in the apoplast, thus limiting pump-induced acidification. Moreover, a 'biophysical pH clamp' is introduced, that is, adjustment of apoplastic/cytosolic pH involving net H⁺ fluxes across the plasma membrane, while the gradient between compartments is maintained. The clamps are not mutually exclusive but are likely to coexist.

Phenolic compound abundance in Pak choi leaves is controlled by salinity and dependent on pH of the leaf apoplast

Onset of salinity induces the pH of the leaf apoplast of Pak choi transiently to increase over a period of 2 to 3 hr. This pH event causes protein abundances in leaves to increase. Among them are enzymes that are key for the phenylpropanoid pathway. To answer the questions whether this short-term salt stress also influences contents of the underlying phenylpropanoids and for clarifying as to whether the apoplastic pH transient plays a role for such a putative effect, Pak choi plants were treated with 37.5 mM CaCl2 against a non-stressed control. A third experimental group, where the leaf apoplast of plants treated with 37.5 mM CaCl2, was clamped in the acidic range by means of infiltration of 5 mM citric acid/sodium citrate (pH 3.6), enabled validation of pH-dependent effects. Microscopy-based live cell imaging was used to quantify leaf apoplastic pH in planta. Phenolics were quantified shortly after the formation of the leaf apoplastic pH transient by means of HPLC-DAD-ESI-MS. Results showed that different phenolic compounds were modulated at 150 and 200 min after the onset of chloride salinity. A pH-independent reduction in phenolic acid abundance as well as an accumulation of phenolic acid:malate conjugates was quantified after 200 min of salt stress. However, at 150 min after the onset of salt stress, flavonoids were significantly reduced by salinity in a pH-dependent manner. These results provided a strong indication that the pH of the apoplast is a relevant component for the short-term metabolic response to chloride salinity.

Chemical inhibition of xylem cellular activity impedes the removal of drought-induced embolisms in poplar stems - new insights from micro-CT analysis

In drought-stressed plants a coordinated cascade of chemical and transcriptional adjustments occurs at the same time as embolism formation. While these processes do not affect embolism formation during stress, they may prime stems for recovery during rehydration by modifying apoplast pH and increasing sugar concentration in the xylem sap. Here we show that in vivo treatments modifying apoplastic pH (stem infiltration with a pH buffer) or reducing stem metabolic activity (infiltration with sodium vanadate and sodium cyanide; plant exposure to carbon monoxide) can reduce sugar accumulation, thus disrupting or delaying the recovery process. Application of the vanadate treatment (NaVO_3, an inhibitor of many ATPases) completely halted recovery from drought-induced embolism for up to 24 h after re-irrigation, while partial recovery was observed in vivo in control plants using X-ray microcomputed tomography. Our results suggest that stem hydraulic recovery in poplar is a biological, energy-dependent process that coincides with accumulation of sugars in the apoplast during stress. Recovery and damage are spatially coordinated, with embolism formation occurring from the inside out and refilling from the outside in. The outside-in pattern highlights the importance of xylem proximity to the sugars within the phloem to the embolism recovery process.

Evidence for multiple receptors mediating RALF-triggered Ca2+ signaling and proton pump inhibition

Acidification of the apoplastic space facilitates cell wall loosening and is therefore a key step in cell expansion. PSY1 is a growth-promoting secreted tyrosine-sulfated glycopeptide whose receptor directly phosphorylates and activates the plasma membrane H⁺ -ATPase, which results in acidification and initiates cellular expansion. Although the mechanism is not clear, the Rapid Alkalinization Factor (RALF) family of small, secreted peptides inhibits the plasma membrane H⁺ -ATPase, leading to alkalinization of the apoplastic space and reduced growth. Here we show that treating Arabidopsis thaliana roots with PSY1 induced the transcription of genes encoding the RALF peptides RALF33 and RALFL36. A rapid burst of intracellular Ca²⁺ preceded apoplastic alkalinization in roots triggered by RALFs, with peptide-specific signatures. Ca²⁺ channel blockers abolished RALF-induced alkalinization, indicating that the Ca²⁺ signal is an obligatory part of the response and that it precedes alkalinization. As expected, fer mutants deficient in the RALF receptor FERONIA did not respond to RALF33. However, we detected both Ca²⁺ and H⁺ signatures in fer mutants upon treatment with RALFL36. Our results suggest that different RALF peptides induce extracellular alkalinization by distinct mechanisms that may involve different receptors.

Priming xylem for stress recovery depends on coordinated activity of sugar metabolic pathways and changes in xylem sap pH

Some plant species are capable of significant reduction of xylem embolism during recovery from drought despite stem water potential remains negative. However, the functional biology underlying this process is elusive. We subjected poplar trees to drought stress followed by a period of recovery. Water potential, hydraulic conductivity, gas exchange, xylem sap pH, and carbohydrate content in sap and woody stems were monitored in combination with an analysis of carbohydrate metabolism, enzyme activity, and expression of genes involved in sugar metabolic and transport pathways. Drought resulted in an alteration of differential partitioning between starch and soluble sugars. Upon stress, an increase in the starch degradation rate and the overexpression of sugar symporter genes promoted the efflux of disaccharides (mostly maltose and sucrose) to the apoplast. In turn, the efflux activity of the sugar-proton cotransporters caused a drop in xylem pH. The newly acidic environment induced the activity of apoplastic invertases leading to the accumulation of monosaccharides in the apoplast, thus providing the main osmoticum necessary for recovery. During drought and recovery, a complex network of coordinated molecular and biochemical signals was activated at the interface between xylem and parenchyma cells that appeared to prime the xylem for hydraulic recovery.

How Does pH Fit in with Oscillating Polar Growth?

Polar growth in root hairs and pollen tubes is an excellent model for investigating plant cell size regulation. While linear plant growth is historically explained by the acid growth theory, which considers that auxin triggers apoplastic acidification by activating plasma membrane P-type H⁺-ATPases (AHAs) along with cell wall relaxation over long periods, the apoplastic pH (_apopH) regulatory mechanisms are unknown for polar growth. Polar growth is a fast process mediated by rapid oscillations that repeat every ∼20-40s. In this review, we explore a reactive oxygen species (ROS)-dependent mechanism that could generate oscillating _apopH gradients in a coordinated manner with growth and Ca²⁺ oscillations. We propose possible mechanisms by which _apopH oscillations are coordinated with polar growth together with ROS and Ca²⁺ waves.

Auxin steers root cell expansion via apoplastic pH regulation in Arabidopsis thaliana

Plant cells are embedded within cell walls, which provide structural integrity, but also spatially constrain cells, and must therefore be modified to allow cellular expansion. The long-standing acid growth theory postulates that auxin triggers apoplast acidification, thereby activating cell wall-loosening enzymes that enable cell expansion in shoots. Interestingly, this model remains heavily debated in roots, because of both the complex role of auxin in plant development as well as technical limitations in investigating apoplastic pH at cellular resolution. Here, we introduce 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) as a suitable fluorescent pH indicator for assessing apoplastic pH, and thus acid growth, at a cellular resolution in Arabidopsis thaliana roots. Using HPTS, we demonstrate that cell wall acidification triggers cellular expansion, which is correlated with a preceding increase of auxin signaling. Reduction in auxin levels, perception, or signaling abolishes both the extracellular acidification and cellular expansion. These findings jointly suggest that endogenous auxin controls apoplastic acidification and the onset of cellular elongation in roots. In contrast, an endogenous or exogenous increase in auxin levels induces a transient alkalinization of the extracellular matrix, reducing cellular elongation. The receptor-like kinase FERONIA is required for this physiological process, which affects cellular root expansion during the gravitropic response. These findings pinpoint a complex, presumably concentration-dependent role for auxin in apoplastic pH regulation, steering the rate of root cell expansion and gravitropic response.

Extracellular Alkalinization as a Defense Response in Potato Cells

A quantitative and robust bioassay to assess plant defense response is important for studies of disease resistance and also for the early identification of disease during pre- or non-symptomatic phases. An increase in extracellular pH is known to be an early defense response in plants. In this study, we demonstrate extracellular alkalinization as a defense response in potatoes. Using potato suspension cell cultures, we observed an alkalinization response against various pathogen- and plant-derived elicitors in a dose- and time-dependent manner. We also assessed the defense response against a variety of potato pathogens, such as protists (Phytophthora infestans and Spongospora subterranea) and fungi (Verticillium dahliae and Colletotrichum coccodes). Our results show that extracellular pH increases within 30 min in proportion to the number of pathogen spores added. Consistently with the alkalinization effect, the higher transcription level of several defense-related genes and production of reactive oxygen species was observed. Our results demonstrate that the alkalinization response is an effective marker to study early stages of defense response in potatoes.

Chloride-inducible transient apoplastic alkalinizations induce stomata closure by controlling abscisic acid distribution between leaf apoplast and guard cells in salt-stressed Vicia faba

Chloride stress causes the leaf apoplast transiently to alkalize, an event that is presumed to contribute to the ability of plants to adapt to saline conditions. However, the initiation of coordinated processes downstream of the alkalinization is unknown. We hypothesize that chloride-inducible pH dynamics are a key chemical feature modulating the compartmental distribution of abscisic acid (ABA) and, as a consequence, affecting stomata aperture. Apoplastic pH and stomata aperture dynamics in intact Vicia faba leaves were monitored by microscopy-based ratio imaging and porometric measurements of stomatal conductance. ABA concentrations in leaf apoplast and guard cells were compared with pH dynamics by gas-chromatography-mass-spectrometry (GC-MS) and liquid-chromatography-tandem-mass spectrometry (LC-MS/MS). Results demonstrate that, upon chloride addition to roots, an alkalizing factor that initiates the pH dynamic propagates from root to leaf in a way similar to xylem-distributed water. In leaves, it induces a systemic transient apoplastic alkalinization that causes apoplastic ABA concentration to increase, followed by an elevation of endogenous guard cell ABA. We conclude that the transient alkalinization, which is a remote effect of chloride stress, modulates the compartmental distribution of ABA between the leaf apoplast and the guard cells and, in this way, is instrumental in inducing stomata closure during the beginning of salinity.

In comparison with nitrate nutrition, ammonium nutrition increases growth of the frostbite1 Arabidopsis mutant

Ammonium nutrition inhibits the growth of many plant species, including Arabidopsis thaliana. The toxicity of ammonium is associated with changes in the cellular redox state. The cellular oxidant/antioxidant balance is controlled by mitochondrial electron transport chain. In this study, we analysed the redox metabolism of frostbite1 (fro1) plants, which lack mitochondrial respiratory chain complex I. Surprisingly, the growth of fro1 plants increased under ammonium nutrition. Ammonium nutrition increased the reduction level of pyridine nucleotides in the leaves of wild-type plants, but not in the leaves of fro1 mutant plants. The observed higher activities of type II NADH dehydrogenases and cytochrome c oxidase in the mitochondrial electron transport chain may improve the energy metabolism of fro1 plants grown on ammonium. Additionally, the observed changes in reactive oxygen species (ROS) metabolism in the apoplast may be important for determining the growth of fro1 under ammonium nutrition. Moreover, bioinformatic analyses showed that the gene expression changes in fro1 plants significantly overlap with the changes previously observed in plants with a modified apoplastic pH. Overall, the results suggest a pronounced connection between the mitochondrial redox system and the apoplastic pH and ROS levels, which may modify cell wall plasticity and influence growth.

Receptor kinase-mediated control of primary active proton pumping at the plasma membrane

Acidification of the cell wall space outside the plasma membrane is required for plant growth and is the result of proton extrusion by the plasma membrane-localized H+-ATPases. Here we show that the major plasma membrane proton pumps in Arabidopsis, AHA1 and AHA2, interact directly in vitro and in planta with PSY1R, a receptor kinase of the plasma membrane that serves as a receptor for the peptide growth hormone PSY1. The intracellular protein kinase domain of PSY1R phosphorylates AHA2/AHA1 at Thr-881, situated in the autoinhibitory region I of the C-terminal domain. When expressed in a yeast heterologous expression system, the introduction of a negative charge at this position caused pump activation. Application of PSY1 to plant seedlings induced rapid in planta phosphorylation at Thr-881, concomitant with an instantaneous increase in proton efflux from roots. The direct interaction between AHA2 and PSY1R observed might provide a general paradigm for regulation of plasma membrane proton transport by receptor kinases.

Real-Time Imaging of Leaf Apoplastic pH Dynamics in Response to NaCl Stress

Knowledge concerning apoplastic ion concentrations is important for the understanding of many processes in plant physiology. Ion-sensitive fluorescent probes in combination with quantitative imaging techniques offer opportunities to localize, visualize, and quantify apoplastic ion dynamics in situ. The application of this technique to the leaf apoplast is complicated because of problems associated with dye loading. We demonstrate a more sophisticated dye loading procedure that enables the mapping of spatial apoplastic ion gradients over a period of 3 h. The new technique has been used for the real-time monitoring of pH dynamics within the leaf apoplast in response to NaCl stress encountered by the roots.

Nanoinfusion: an integrating tool to study elicitor perception and signal transduction in intact leaves

•  To study elicitor effects in intact leaves of Hordeum vulgare cv. Ingrid, chitin fragments were delivered to substomatal cavities with a micropipette. Responses were monitored by a calibrated reference microelectrode and a pH-sensitive microelectrode, simply positioned below neighbouring open stomata in the air-filled space of the substomatal cavities. •  Flooding of a leaf spot of approx. 600 × 300 µm with physiological aqueous solutions caused an immediate transient polarization of the extracellular solution in the order of -25 ± 12 mV. Immediately after the pipette solution was consumed, the extracellular solution was again polarized, still before the cavities dried out. In dry cavities, the extracellular equilibrium potential was -34 ± 10 mV. On flooding, the extracellular pH rose to 5.7 ± 0.3 after approx. 12 ± 7 min and returned to a stable level of 5.2 ± 0.3 after 30 min. •  Sequential infusion on the same leaf spot, first with elicitor-free solution, then with the same solution containing 25 µmN-acetyl-chitooctaose, into the still-flooded cavities yielded an elicitor-specific pH maximum between 6 and 7 approx. 10 min after flooding. A pronounced pH maximum > 7 occurred between 40 and 240 min after flooding in wild-type plants. •  The use of elicitor nanoinfusion for the integrated development of resistance inducers in cereals, wine and poplar is discussed.