Isolation and whole-cell patch clamping of Arabidopsis guard cell protoplasts

Exogenous application of pectin triggers stomatal closure and immunity in Arabidopsis

Pectin has been extensively studied in animal immunity, and exogenous pectin as a food additive can provide protection against inflammatory bowel disease. However, the utility of pectin to improve immunity in plants is still unstudied. Here, we found exogenous application of pectin triggered stomatal closure in Arabidopsis in a dose- and time-dependent manner. Additionally, pectin activated peroxidase and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase to produce reactive oxygen species (ROS), which subsequently increased cytoplasmic Ca2+ concentration ([Ca2+ ]cyt ) and was followed by nitric oxide (NO) production, leading to stomatal closure in an abscisic acid (ABA) and salicylic acid (SA) signalling-dependent mechanism. Furthermore, pectin enhanced the disease resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) with mitogen-activated protein kinases (MPKs) MPK3/6 activated and upregulated expression of defence-responsive genes in Arabidopsis. These results suggested that exogenous pectin-induced stomatal closure was associated with ROS and NO production regulated by ABA and SA signalling, contributing to defence against Pst DC3000 in Arabidopsis.

The Cyclophilin ROC3 Regulates ABA-Induced Stomatal Closure and the Drought Stress Response of Arabidopsis thaliana

Drought causes a major constraint on plant growth, development, and crop productivity. Drought stress enhances the synthesis and mobilization of the phytohormone abscisic acid (ABA). Enhanced cellular levels of ABA promote the production of reactive oxygen species (ROS), which in turn induce anion channel activity in guard cells that consequently leads to stomatal closure. Although Cyclophilins (CYPs) are known to participate in the biotic stress response, their involvement in guard cell ABA signaling and the drought response remains to be established. The Arabidopsis thaliana gene ROC3 encodes a CYP. Arabidopsis roc3 T-DNA mutants showed a reduced level of ABA-activated S-type anion currents, and stomatal closure than wild type (WT). Also, roc3 mutants exhibited rapid loss of water in leaf than wild type. Two complementation lines of roc3 mutants showed similar stomatal response to ABA as observed for WT. Both complementation lines also showed similar water loss as WT by leaf detached assay. Biochemical assay suggested that ROC3 positively regulates ROS accumulation by inhibiting catalase activity. In response to ABA treatment or drought stress, roc3 mutant show down regulation of a number of stress responsive genes. All findings indicate that ROC3 positively regulates ABA-induced stomatal closure and the drought response by regulating ROS homeostasis and the expression of various stress-activated genes.

Secreted Peptide PIP1 Induces Stomatal Closure by Activation of Guard Cell Anion Channels in Arabidopsis

Plant stomata which consist of a pair of guard cells, are not only finely controlled to balance water loss as transpiration and CO₂ absorption for photosynthesis, but also serve as the major sites to defend against pathogen attack, thus allowing plants to respond appropriately to abiotic and biotic stress conditions. The regulatory signaling network for stomatal movement is complex in nature, and plant peptides have been shown to be involved in signaling processes. Arabidopsis secreted peptide PIP1 was previously identified as an endogenous elicitor, which induced immune response through its receptor, RLK7. PIP1-RLK7 can activate stomatal immunity against the bacterial strain Pst DC3118. However, the molecular mechanism of PIP1 in stomatal regulation is still unclear and additional new factors need to be discovered. In this study, we further clarified that PIP1 could function as an important regulator in the induction of stomatal closure. The results showed that PIP1 could promote stomata to close in a certain range of concentrations and response time. In addition, we uncovered that PIP1-RLK7 signaling regulated stomatal response by activating S-type anion channel SLAC1. PIP1-induced stomatal closure was impaired in bak1, mpk3, and mpk6 mutants, indicating that BAK1 and MPK3/MPK6 were required for PIP1-regulated stomatal movement. Our research further deciphered that OST1 which acts as an essential ABA-signaling component, also played a role in PIP1-induced stomatal closure. In addition, ROS participated in PIP1-induced stomatal closure and PIP1 could activate Ca²⁺ permeable channels. In conclusion, we reveal the role of peptide PIP1 in triggering stomatal closure and the possible mechanism of PIP1 in the regulation of stomatal apertures. Our findings improve the understanding of the role of PIP1 in stomatal regulation and immune response.

Calcium-dependent protein kinase CPK9 negatively functions in stomatal abscisic acid signaling by regulating ion channel activity in Arabidopsis

In this manuscript, we demonstrated the negative role of CPK9 in stomatal ABA signaling, and both CPK9 and CPK33 for accurate guard cell function was explored via cpk9/cpk33 double mutants' phenotype. Abscisic acid (ABA) can inhibit stomatal opening and promote stomatal closure by regulating ion channel activity in guard cell membranes. As an important second messenger, calcium (Ca²⁺) is essentially needed in ABA regulation of stomatal movement. Calcium-dependent protein kinases (CDPKs) have been proposed to contribute to central Ca²⁺ signal transduction in plants. Here, we report the functional characterization of CPK9 in Arabidopsis stomatal ABA signaling. CPK9 had high expression in guard cells and the protein was subcellularly located in the cell membrane. A loss-of-function mutant cpk9 showed a much more sensitive phenotype to ABA regulation of stomatal movement and ion channel activity, while CPK9 overexpression lines had opposite phonotypes. These findings demonstrated the negative role of CPK9 in stomatal ABA signaling. As the closest homolog of CPK33, we also proved that stomatal movement of the cpk9/cpk33 double mutants was more sensitive to ABA than either single mutants. These results revealed the role of CPK9 in guard cells, and the need of both CPK9 and CPK33 for accurate guard cell function.

A guideline for reporting experimental protocols in life sciences

Experimental protocols are key when planning, performing and publishing research in many disciplines, especially in relation to the reporting of materials and methods. However, they vary in their content, structure and associated data elements. This article presents a guideline for describing key content for reporting experimental protocols in the domain of life sciences, together with the methodology followed in order to develop such guideline. As part of our work, we propose a checklist that contains 17 data elements that we consider fundamental to facilitate the execution of the protocol. These data elements are formally described in the SMART Protocols ontology. By providing guidance for the key content to be reported, we aim (1) to make it easier for authors to report experimental protocols with necessary and sufficient information that allow others to reproduce an experiment, (2) to promote consistency across laboratories by delivering an adaptable set of data elements, and (3) to make it easier for reviewers and editors to measure the quality of submitted manuscripts against an established criteria. Our checklist focuses on the content, what should be included. Rather than advocating a specific format for protocols in life sciences, the checklist includes a full description of the key data elements that facilitate the execution of the protocol.

Mitochondrial pyruvate carrier 1 mediates abscisic acid-regulated stomatal closure and the drought response by affecting cellular pyruvate content in Arabidopsis thaliana

BACKGROUND

Stomata are micropores surrounded by pairs of guard cells, and their opening is finely controlled to balance water vapor loss as transpiration and CO₂ absorption for photosynthesis. The regulatory signaling network for stomatal movement is complicated, and increasing numbers of new genes have been shown to be involved in this process. Our previous study indicated that a member of the plant putative mitochondrial pyruvate carrier (MPC) family, NRGA1, is a negative regulator of guard cell abscisic acid (ABA) signaling. In this study, we identified novel physiological roles of pyruvate and MPC1, another member of the MPC family, in the regulation of stomatal closure in Arabidopsis.

RESULTS

Loss-of-function mutants of MPC1 (mpc1) were hypersensitive to ABA-induced stomatal closure and ABA-activated guard cell slow-type anion currents, and showed a reduced rate of water loss upon drought treatment compared with wild-type plants. In contrast, plants overexpressing MPC1 showed a hyposensitive ABA response and increased sensitivity to drought stress. In addition, mpc1 mutants accumulated more pyruvate after drought or ABA treatment. The increased pyruvate content also induced stomatal closure and activated the slow-type anion channels of guard cells, and this process was dependent on the function of RbohD/F NADPH oxidases and reactive oxygen species concentrations in guard cells.

CONCLUSIONS

Our findings revealed the essential roles of MPC1 and pyruvate in stomatal movement and plant drought resistance.

CML20, an Arabidopsis Calmodulin-like Protein, Negatively Regulates Guard Cell ABA Signaling and Drought Stress Tolerance

Guard cells shrink in response to drought and abscisic acid (ABA), which is caused by efflux of ions that in turn reduces stomatal aperture and improves the plant's ability to retain moisture. Cytosolic free calcium is an essential secondary messenger in guard cell ABA signaling, but the details of this regulatory pathway remain sketchy. Here, the calmodulin-like protein CML20, which has four EF-hand domains and calcium-binding activity in vitro, was found to be a negative regulator of ABA-induced stomatal movement in Arabidopsis. The guard cells of cml20 loss-of-function mutant plants were hypersensitive to both ABA-activated S-type anion currents, and ABA inhibited inward K⁺ currents than those of wild type. Additional, due to smaller stomatal aperture, cml20 showed less water loss from the leaves than wild type. These phenotypes of CML20 overexpressing plants contrasted with wild type in the opposite direction. In the cml20 mutant, the transcripts of stress responsive genes, such as MYB2, RAB18, ERD10, COR47, and RD29A were up-regulated in response to drought and ABA, while down-regulated of APX2 transcription and higher reactive oxygen species (ROS) accumulation. These observations support the CML20, a functional Ca²⁺ sensor, is a negative regulator in guard cell ABA signaling.

Preparation of Epidermal Peels and Guard Cell Protoplasts for Cellular, Electrophysiological, and -Omics Assays of Guard Cell Function

Bioassays are commonly used to study stomatal phenotypes. There are multiple options in the choice of plant materials and species used for observation of stomatal and guard cell responses in vivo. Here, detailed procedures for bioassays of stomatal responses to abscisic acid (ABA) in Arabidopsis thaliana are described, including ABA promotion of stomatal closure, ABA inhibition of stomatal opening, and ABA promotion of reaction oxygen species (ROS) production in guard cells. We also include an example of a stomatal bioassay for the guard cell CO2 response using guard cell-enriched epidermal peels from Brassica napus. Highly pure preparations of guard cell protoplasts can be produced, which are also suitable for studies on guard cell signaling, as well as for studies on guard cell ion transport. Small-scale and large-scale guard cell protoplast preparations are commonly used for electrophysiological and -omics studies, respectively. We provide a procedure for small-scale guard cell protoplasting from A. thaliana. Additionally, a general protocol for large-scale preparation of guard cell protoplasts, with specifications for three different species, A. thaliana, B. napus, and Vicia faba is also provided.

Abscisic Acid as an Internal Integrator of Multiple Physiological Processes Modulates Leaf Senescence Onset in Arabidopsis thaliana

Many studies have shown that exogenous abscisic acid (ABA) promotes leaf abscission and senescence. However, owing to a lack of genetic evidence, ABA function in plant senescence has not been clearly defined. Here, two-leaf early-senescence mutants (eas) that were screened by chlorophyll fluorescence imaging and named eas1-1 and eas1-2 showed high photosynthetic capacity in the early stage of plant growth compared with the wild type. Gene mapping showed that eas1-1 and eas1-2 are two novel ABA2 allelic mutants. Under unstressed conditions, the eas1 mutations caused plant dwarf, early germination, larger stomatal apertures, and early leaf senescence compared with those of the wild type. Flow cytometry assays showed that the cell apoptosis rate in eas1 mutant leaves was higher than that of the wild type after day 30. A significant increase in the transcript levels of several senescence-associated genes, especially SAG12, was observed in eas1 mutant plants in the early stage of plant growth. More importantly, ABA-activated calcium channel activity in plasma membrane and induced the increase of cytoplasmic calcium concentration in guard cells are suppressed due to the mutation of EAS1. In contrast, the eas1 mutants lost chlorophyll and ion leakage significant faster than in the wild type under treatment with calcium channel blocker. Hence, our results indicate that endogenous ABA level is an important factor controlling the onset of leaf senescence through Ca(2+) signaling.

Heterotrimeric G-protein regulation of ROS signalling and calcium currents in Arabidopsis guard cells

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are important signalling agents in both animals and plants. In plants, G proteins modulate numerous responses, including abscisic acid (ABA) and pathogen-associated molecular pattern (PAMP) regulation of guard cell ion channels and stomatal apertures. Previous analyses of mutants deficient in the sole canonical Arabidopsis Gα subunit, GPA1, have shown that Gα-deficient guard cells are impaired in ABA inhibition of K(+) influx channels, and in pH-independent activation of anion efflux channels. ABA-induced Ca(2+) uptake through ROS-activated Ca(2+)-permeable channels in the plasma membrane is another key component of ABA signal transduction in guard cells, but the question of whether these channels are also dependent on Gα for their ABA response has not been evaluated previously. We used two independent Arabidopsis T-DNA null mutant lines, gpa1-3 and gpa1-4, to investigate this issue. We observed that gpa1 mutants are disrupted both in ABA-induced Ca(2+)-channel activation, and in production of reactive oxygen species (ROS) in response to ABA. However, in response to exogenous H(2)O(2) application, I(Ca) channels are activated normally in gpa1 guard cells. In addition, H(2)O(2) inhibition of stomatal opening and promotion of stomatal closure are not disrupted in gpa1 mutant guard cells. These data indicate that absence of GPA1 interrupts ABA signalling between ABA reception and ROS production, with a consequent impairment in Ca(2+)-channel activation.