Kunkun Li

Summary: The brassinosteroid pathway promotes anisotropic cell expansion; however, the effectors in this process remain unclear. Candidates include ARABINOGALACTAN PROTEIN ( AGP ) genes, which are prominent brassinosteroid‐responsive transcriptional targets, and EXORDIUM (EXO)‐LIKE ( EXL ) genes. Here, we examined whether AGP and EXO/EXL genes mediate anisotropic cell expansion. AGPs are evolutionarily ancient glycoproteins, which are secreted and attached to the plasma membrane. Plant genomes contain dozens of potentially redundant AGP genes. Their roles have mostly been inferred from the phenotypes of beta‐glycosyltransferase mutants with impaired AGP glycosylation. Compared with AGPs, little is known about the smaller family of secreted EXO/EXL proteins. Here, we investigated brassinosteroid‐dependent AGP and EXO/EXL gene expression patterns in Arabidopsis roots and created loss‐of‐function mutants for preponderant genes. Whereas single, double and triple agp mutants appeared wild‐type, combinatorial quadruple to sextuple agp2/4/10/11/13/22/24 mutants displayed longer, frequently aberrant root hairs and shorter yet thicker roots. These traits reflect an underlying reduced cellular anisotropy associated with increased organ stiffness, which were also observed in hypocotyls. Combinatorial exo/exl1/2/3/4/5/6/7 sextuple or septuple mutants displayed a qualitatively similar but quantitatively weaker phenotype than combinatorial agp mutants. Collectively, our data suggest that classical/peptide AGPs and EXO/EXLs promote brassinosteroid‐dependent anisotropic growth to different extent.

Brassinosteroid signaling is essential for plant growth as exemplified by the dwarf phenotype of loss-of-function mutants in BRASSINOSTEROID INSENSITIVE 1 ( BRI1 ), a ubiquitously expressed Arabidopsis brassinosteroid receptor gene. Complementation of brassinosteroid-blind receptor mutants by BRI1 expression with various tissue-specific promoters implied that local brassinosteroid signaling may instruct growth non–cell autonomously. Here, we performed such rescues with a panel of receptor variants and promoters, in combination with tissue-specific transgene knockouts. Our experiments demonstrate that brassinosteroid receptor expression in several tissues is necessary but not sufficient for rescue. Moreover, complementation with tissue-specific promoters requires the genuine BRI1 gene body sequence, which confers ubiquitous expression of trace receptor amounts that are sufficient to promote brassinosteroid-dependent root growth. Our data, therefore, argue for a largely cell-autonomous action of brassinosteroid receptors.

Summary: Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca²⁺ and pH changes are largely unclear. Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live‐cell ion imaging and computational simulations. K⁺ and anion fluxes at both, PM and VM, affected H⁺ and Ca²⁺, as changes in extracellular KCl or KNO₃ concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca²⁺]_cyt and the membrane potential. At both membranes, the K⁺ transporter networks mediated an antiport of K⁺ and H⁺. By contrast, net transport of anions was accompanied by parallel H⁺ transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K⁺/H⁺ transport but cell type‐specific differences in [H⁺]_cyt and pH‐dependent [Ca²⁺]_cyt signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM. Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H⁺/K⁺ homeostasis and points to transceptor‐like Ca²⁺/H⁺‐based ion signaling in plant cells.

Polar growth and guidance of pollen tubes (PTs) is regulated by chemical and electrical signaling cues. Central regulators controlling cellular protrusion of PTs tip-focused Ca ²⁺ and H ⁺ cytosolic gradients and ion fluxes depend, but also feedback, on membrane voltage ( V _m ). However, the functional relationship between V _m , Ca ²⁺ , and H ⁺ dynamics is largely unclear. We carried out parallel live-cell Ca ²⁺ and H ⁺ -imaging together with V _m recordings followed by cross-correlation analysis to study their interconnection in tobacco PTs. Upon electrode impalement at the tip or shank, PT growth ceased but sustained oscillatory behavior of all three signals continued and remained mostly in phase, although shifted in time. The Ca ²⁺ oscillations were largely restricted to the apex, temporally leading the H ⁺ increases by ∼18 s, whereas H ⁺ tip-focused peaks spread into the shank. Voltage recordings in the shank (50–80 μm behind the tip) revealed transient bell-shaped depolarizations that correlated strongly with H ⁺ but not Ca ²⁺ oscillations. In contrast, V _m oscillations readout at the tip correlated equally with tip Ca ²⁺ - and H ⁺ -signals, having action potential-like appearances that were approximately five times faster compared with those in the shank. Our multiparameter readout approach identified distinct relationships of chemical and electric signals along PTs, where at the tip changes in both Ca ²⁺ and H ⁺ correlated with V _m dynamics, whereas at the shank only H ⁺ correlated with V _m . We propose that spatially confined chemo-electric coupling support the manifestation of propagating voltage and pH waves along nongrowing PTs, which are preceded by short-lasting but high amplitude tip Ca ²⁺ spikes.

Summary: Salt stress is a major abiotic stress, responsible for declining agricultural productivity. Roots are regarded as hubs for salt detoxification, however, leaf salt concentrations may exceed those of roots. How mature leaves manage acute sodium chloride (NaCl) stress is mostly unknown. To analyze the mechanisms for NaCl redistribution in leaves, salt was infiltrated into intact tobacco leaves. It initiated pronounced osmotically‐driven leaf movements. Leaf downward movement caused by hydro‐passive turgor loss reached a maximum within 2 h. Salt‐driven cellular water release was accompanied by a transient change in membrane depolarization but not an increase in cytosolic calcium ion (Ca²⁺) level. Nonetheless, only half an hour later, the leaves had completely regained turgor. This recovery phase was characterized by an increase in mesophyll cell plasma membrane hydrogen ion (H⁺) pumping, a salt uptake‐dependent cytosolic alkalization, and a return of the apoplast osmolality to pre‐stress levels. Although, transcript numbers of abscisic acid‐ and Salt Overly Sensitive pathway elements remained unchanged, salt adaptation depended on the vacuolar H⁺/Na⁺‐exchanger NHX1. Altogether, tobacco leaves can detoxify sodium ions (Na⁺) rapidly even under massive salt loads, based on pre‐established posttranslational settings and NHX1 cation/H⁺ antiport activity. Unlike roots, signaling and processing of salt stress in tobacco leaves does not depend on Ca²⁺ signaling.

Summary: Whereas the role of calcium ions (Ca²⁺) in plant signaling is well studied, the physiological significance of pH‐changes remains largely undefined. Here we developed CapHensor, an optimized dual‐reporter for simultaneous Ca²⁺ and pH ratio‐imaging and studied signaling events in pollen tubes (PTs), guard cells (GCs), and mesophyll cells (MCs). Monitoring spatio‐temporal relationships between membrane voltage, Ca²⁺‐ and pH‐dynamics revealed interconnections previously not described. In tobacco PTs, we demonstrated Ca²⁺‐dynamics lag behind pH‐dynamics during oscillatory growth, and pH correlates more with growth than Ca²⁺. In GCs, we demonstrated abscisic acid (ABA) to initiate stomatal closure via rapid cytosolic alkalization followed by Ca²⁺ elevation. Preventing the alkalization blocked GC ABA‐responses and even opened stomata in the presence of ABA, disclosing an important pH‐dependent GC signaling node. In MCs, a flg22‐induced membrane depolarization preceded Ca²⁺‐increases and cytosolic acidification by c. 2 min, suggesting a Ca²⁺/pH‐independent early pathogen signaling step. Imaging Ca²⁺ and pH resolved similar cytosol and nuclear signals and demonstrated flg22, but not ABA and hydrogen peroxide to initiate rapid membrane voltage‐, Ca²⁺‐ and pH‐responses. We propose close interrelation in Ca²⁺‐ and pH‐signaling that is cell type‐ and stimulus‐specific and the pH having crucial roles in regulating PT growth and stomata movement.

Loquat ( Eriobotrya japonica Lindl.) is an economically important subtropical fruit tree, originating and commercially produced mainly in China and Japan. The aseptic seed germination of 13 loquat cultivars, including 9 white-fleshed cultivars and 4 red-fleshed ones, was studied, and the relationship between the germination rate and the content of endogenous ABA in seeds was explored. The germination rate and the seedling height at 80 days after sowing of white-fleshed cultivars were generally higher than that of red-fleshed ones. The ABA content in seeds was generally lower in white-fleshed cultivars, and the ABA content negatively correlated with the germination rate at 80 days after sowing. A moderate detrimental effect of 75% ethanol disinfection on aseptic germination of loquat seeds was observed, especially, for the cultivar Dahongpao, and the germination rate and seedling height were reduced by around half and one-third, respectively. The addition of plant preservative mixture (PPM) to the germination medium at a final concentration of 25 mg·L ⁻¹ effectively controlled contamination when the 75% ethanol disinfection procedure was omitted.