ABA signalling and metabolism are not essential for dark-induced stomatal closure but affect response speed

Rapid spatial assessment of leaf-absorbed irradiance

Image-based high-throughput phenotyping promises the rapid determination of functional traits in large plant populations. However, interpretation of some traits - such as those related to photosynthesis or transpiration rates - is only meaningful if the irradiance absorbed by the measured leaves is known, which can differ greatly between different parts of the same plant and within canopies. No feasible method currently exists to rapidly measure absorbed irradiance in three-dimensional plants and canopies. We developed a method and protocols to derive absorbed irradiance at any visible part of a canopy with a thermal camera, by fitting a leaf energy balance model to transient changes in leaf temperature. Leaves were exposed to short light pulses (30 s) that were not long enough to trigger stomatal opening but strong enough to induce transient changes in leaf temperature that was proportional to the absorbed irradiance. The method was successfully validated against point measurements of absorbed irradiance in plant species with relatively simple architecture (sweet pepper, cucumber, tomato, and lettuce). Once calibrated, the model was used to produce absorbed irradiance maps from thermograms. Our method opens new avenues for the interpretation of plant responses derived from imaging techniques and can be adapted to existing high-throughput phenotyping platforms.

Night-time water relations and gas exchange in cut shoots of five boreal dwarf shrub species: impact of soil water availability

Recent findings suggest that drought may affect plants' daytime and night-time stomatal regulation differently. However, knowledge of night-time stomatal behaviour in dwarf shrubs growing in boreal ecosystems is lacking. We sampled cut shoots from dwarf shrub species to elucidate their capacity to transpire at night and the effect of drought on stomatal regulation. The shoots' water relations and gas exchange were measured under controlled conditions in a growth chamber. The studied species demonstrated considerable differences in their diurnal water use. The night-time water use percentage of daytime water use (NWU) reached up to 90% in Andromeda polifolia and Vaccinium uliginosum. In Rhododendron tomentosum, Vaccinium myrtillus and Chamaedaphne calyculata, the NWU was 62, 27 and 26%, respectively. The shoots of C. calyculata showed a significant increase (P < 0.001) in the transpiration rate (E) during the night. However, in R. tomentosum, a decrease (P < 0.05) in nightly E was observed. The shoot conductance (g) at the end of the night was lower than daytime g in all studied species, but the difference was not significant for V. uliginosum. Across the species, NWU was negatively related (P < 0.001) to the soil volumetric water content (SWC) in the plant habitat. However, daytime E and g were positively related (P < 0.05) to the habitat SWC. Only in V. myrtillus was night-time E higher (P < 0.05) in dry conditions than in wet conditions. Our results demonstrate high variability in diurnal water relations in dwarf shrubs, which can keep stomata open in the dark even when drought limits daytime g and E.

Cell wall fucosylation in Arabidopsis influences control of leaf water loss and alters stomatal development and mechanical properties

The Arabidopsis sensitive-to-freezing8 (sfr8) mutant exhibits reduced cell wall (CW) fucose levels and compromised freezing tolerance. To examine whether CW fucosylation also affects the response to desiccation, we tested the effect of leaf excision in sfr8 and the allelic mutant mur1-1. Leaf water loss was strikingly higher than in the wild type in these, but not other, fucosylation mutants. We hypothesized that reduced fucosylation in guard cell (GC) walls might limit stomatal closure through altering mechanical properties. Multifrequency atomic force microscopy (AFM) measurements revealed a reduced elastic modulus (E'), representing reduced stiffness, in sfr8 GC walls. Interestingly, however, we discovered a compensatory mechanism whereby a concomitant reduction in the storage modulus (E'') maintained a wild-type viscoelastic time response (tau) in sfr8. Stomata in intact leaf discs of sfr8 responded normally to a closure stimulus, abscisic acid, suggesting that the time response may relate more to closure properties than stiffness does. sfr8 stomatal pore complexes were larger than those of the wild type, and GCs lacked a fully developed cuticular ledge, both potential contributors to the greater leaf water loss in sfr8. We present data that indicate that fucosylation-dependent dimerization of the CW pectic domain rhamnogalacturonan-II may be essential for normal cuticular ledge development and leaf water retention.

Changes in endogenous abscisic acid and stomata of the resurrection fern, Pleopeltis polypodioides, in response to de- and rehydration

PREMISE

While angiosperms respond uniformly to abscisic acid (ABA) by stomatal closure, the response of ferns to ABA is ambiguous. We evaluated the effect of endogenous ABA, hydrogen peroxide (H₂ O₂ ), nitric oxide (NO), and Ca²⁺ , low and high light intensities, and blue light (BL) on stomatal opening of Pleopeltis polypodioides.

METHODS

Endogenous ABA was quantified using gas chromatography-mass spectrometry; microscopy results and stomatal responses to light and chemical treatments were analyzed with Image J.

RESULTS

The ABA content increases during initial dehydration, peaks at 15 h and then decreases to one fourth of the ABA content of hydrated fronds. Following rehydration, ABA content increases within 24 h to the level of hydrated tissue. The stomatal aperture opens under BL and remains open even in the presence of ABA. Closure was strongly affected by BL, NO, and Ca²⁺ , regardless of ABA, H₂ O₂ effect was weak.

CONCLUSIONS

The decrease in the ABA content during extended dehydration and insensitivity of the stomata to ABA suggests that the drought tolerance mechanism of Pleopeltis polypodioides is independent of ABA.

Light prevents pathogen-induced aqueous microenvironments via potentiation of salicylic acid signaling

Many plant pathogens induce water-soaked lesions in infected tissues. In the case of Pseudomonas syringae (Pst), water-soaking effectors stimulate abscisic acid (ABA) production and signaling, resulting in stomatal closure. This reduces transpiration, increases water accumulation, and induces an apoplastic microenvironment favorable for bacterial growth. Stomata are sensitive to environmental conditions, including light. Here, we show that a period of darkness is required for water-soaking, and that a constant light regime abrogates stomatal closure by Pst. We find that constant light induces resistance to Pst, and that this effect requires salicylic acid (SA). Constant light did not alter effector-induced accumulation of ABA, but induced greater SA production, promoting stomatal opening despite the presence of ABA. Furthermore, application of a SA analog was sufficient to prevent pathogen-induced stomatal closure and water-soaking. Our results suggest potential approaches for interfering with a common virulence strategy, as well as providing a physiological mechanism by which SA functions in defense against pathogens.

COP1 mutation causes low leaf temperature under various abiotic stresses in Arabidopsis thaliana

Stomata are microscopic pores on epidermal cells of leaves and stems that regulate water loss and gas exchange between the plant and its environment. Constitutive photomorphogenic 1 (COP1) is an E3 ubiquitin ligase that is involved in plant growth and development and multiple abiotic stress responses by regulating the stability of various target proteins. However, little is known about how COP1 controls stomatal aperture and leaf temperature under various environmental conditions. Here, we show that COP1 participates in leaf temperature and stomatal closure regulation under normal and stress conditions in Arabidopsis. Leaf temperature of cop1 mutants was relatively lower than that of wild type (WT) under drought, salt, and heat stress and after abscisic acid (ABA), CaCl₂, and H₂O₂ treatments. However, leaf temperature was generally higher in both WT and cop1 mutants after abiotic stress and chemical treatment than that of untreated WT and cop1 mutants. Stomatal aperture was wider in cop1 mutants than that in WT under all conditions tested, although the extent of stomatal closure varied between WT and cop1 mutants. Under dark conditions, leaf temperature was also lower in cop1 mutants than that in WT. Expression of the genes encoding ABA receptors, ABA biosynthesis proteins, positive regulators of stomatal closure and heat tolerance, and ABA-responsive proteins was lower in cop1 mutants that that in WT. In addition, expression of respiration-related genes was lower in cop1 mutants that that in WT. Taken together, the data provide evidence that mutations in COP1 lead to wider stomatal aperture and higher leaf temperature under normal and stress conditions, indicating that leaf temperature is highly correlated with stomatal aperture.

Elevated CO2 induces rapid dephosphorylation of plasma membrane H+ -ATPase in guard cells

Light induces stomatal opening, which is driven by plasma membrane (PM) H⁺ -ATPase in guard cells. The activation of guard-cell PM H⁺ -ATPase is mediated by phosphorylation of the penultimate C-terminal residue, threonine. The phosphorylation is induced by photosynthesis as well as blue light photoreceptor phototropin. Here, we investigated the effects of cessation of photosynthesis on the phosphorylation level of guard-cell PM H⁺ -ATPase in Arabidopsis thaliana. Immunodetection of guard-cell PM H⁺ -ATPase, time-resolved leaf gas-exchange analyses and stomatal aperture measurements were carried out. We found that light-dark transition of leaves induced dephosphorylation of the penultimate residue at 1 min post-transition. Gas-exchange analyses confirmed that the dephosphorylation is accompanied by an increase in the intercellular CO₂ concentration, caused by the cessation of photosynthetic CO₂ fixation. We discovered that CO₂ induces guard-cell PM H⁺ -ATPase dephosphorylation as well as stomatal closure. Interestingly, reverse-genetic analyses using guard-cell CO₂ signal transduction mutants suggested that the dephosphorylation is mediated by a mechanism distinct from the established CO₂ signalling pathway. Moreover, type 2C protein phosphatases D6 and D9 were required for the dephosphorylation and promoted stomatal closure upon the light-dark transition. Our results indicate that CO₂ -mediated dephosphorylation of guard-cell PM H⁺ -ATPase underlies stomatal closure.

CBF4/DREB1D represses XERICO to attenuate ABA, osmotic and drought stress responses in Arabidopsis

Water stress can severely impact plant growth, productivity and yield. Consequently, plants have evolved various strategies through which they can respond and adapt to their environment. XERICO (XER) is a stress-responsive RING E3 ubiquitin ligase that modulates abscisic acid (ABA) levels and promotes drought tolerance when overexpressed. To better understand the biological role of XER in stress responses, we characterized a xer-1 hypomorphic mutant and a CRISPR/Cas9-induced xer-2 null mutant in Arabidopsis. Both xer mutant alleles exhibited increased drought sensitivity, supporting the results from overexpression studies. Furthermore, we discovered that both xer mutants have greater stomatal indices and that XER is expressed in epidermal cells, indicating that XER functions in the epidermis to repress stomatal development. To explore XER spatiotemporal and stress-dependent regulation, we conducted a yeast one-hybrid screen and found that CBF4/DREB1D associates with the XER 5' untranslated region (5'-UTR). We generated three cbf4 null mutants with CRISPR/Cas9 and showed that CBF4 negatively regulates ABA responses, promotes stomatal development and reduces drought tolerance, in contrast to the roles shown for XER. CBF4 is induced by ABA and osmotic stress, and localizes to the nucleus where it downregulates XER expression via the DRE element in its 5'-UTR. Lastly, genetic interaction studies confirmed that xer is epistatic to cbf4 in stomatal development and in ABA, osmotic and drought stress responses. We propose that the repression of XER by CBF4 functions to attenuate ABA signaling and stress responses to maintain a balance between plant growth and survival under adverse environmental conditions.