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Plant hormones are signalling compounds that regulate crucial aspects of growth, development and environmental stress responses. Abiotic stresses, such as drought, salinity, heat, cold and flooding, have profound effects on plant growth and survival. Adaptation and tolerance to such stresses require sophisticated sensing, signalling and stress response mechanisms. In this Review, we discuss recent advances in understanding how diverse plant hormones control abiotic stress responses in plants and highlight points of hormonal crosstalk during abiotic stress signalling. Control mechanisms and stress responses mediated by plant hormones including abscisic acid, auxin, brassinosteroids, cytokinins, ethylene and gibberellins are discussed. We discuss new insights into osmotic stress sensing and signalling mechanisms, hormonal control of gene regulation and plant development during stress, hormone-regulated submergence tolerance and stomatal movements. We further explore how innovative imaging approaches are providing insights into single-cell and tissue hormone dynamics. Understanding stress tolerance mechanisms opens new opportunities for agricultural applications.Abiotic stresses, such as drought, salinity, heat, cold and flooding, have profound effects on plant growth and survival. Adaptation and tolerance to such stresses require sophisticated sensing, signalling and stress response mechanisms. Shroeder and colleagues discuss recent insights into how plant hormones control such responses. Understanding these mechanisms opens opportunities for agricultural applications.

Abiotic stresses, including drought and salinity, trigger a complex osmotic-stress and abscisic acid (ABA) signal transduction network. The core ABA signalling components are snf1-related protein kinase2s (SnRK2s), which are activated by ABA-triggered inhibition of type-2C protein-phosphatases (PP2Cs). SnRK2 kinases are also activated by a rapid, largely unknown, ABA-independent osmotic-stress signalling pathway. Here, through a combination of a redundancy-circumventing genetic screen and biochemical analyses, we have identified functionally-redundant MAPKK-kinases (M3Ks) that are necessary for activation of SnRK2 kinases. These M3Ks phosphorylate a specific SnRK2/OST1 site, which is indispensable for ABA-induced reactivation of PP2C-dephosphorylated SnRK2 kinases. ABA-triggered SnRK2 activation, transcription factor phosphorylation and SLAC1 activation require these M3Ks in vitro and in plants. M3K triple knock-out plants show reduced ABA sensitivity and strongly impaired rapid osmotic-stress-induced SnRK2 activation. These findings demonstrate that this M3K clade is required for ABA- and osmotic-stress-activation of SnRK2 kinases, enabling robust ABA and osmotic stress signal transduction. SnRK2 kinases activate abiotic stress responses in plants following ABA-dependent phosphatase inhibition or ABA-independent osmotic stress signalling. Here Takahashi et al. show that MAPKK-kinases phosphorylate and activate SnRK2s thus enabling robust ABA and osmotic stress signal transduction.

A central question is how specificity in cellular responses to the eukaryotic second messenger Ca2+ is achieved. Plant guard cells, that form stomatal pores for gas exchange, provide a powerful system for in depth investigation of Ca2+-signaling specificity in plants. In intact guard cells, abscisic acid (ABA) enhances (primes) the Ca2+-sensitivity of downstream signaling events that result in activation of S-type anion channels during stomatal closure, providing a specificity mechanism in Ca2+-signaling. However, the underlying genetic and biochemical mechanisms remain unknown. Here we show impairment of ABA signal transduction in stomata of calcium-dependent protein kinase quadruple mutant plants. Interestingly, protein phosphatase 2Cs prevent non-specific Ca2+-signaling. Moreover, we demonstrate an unexpected interdependence of the Ca2+-dependent and Ca2+-independent ABA-signaling branches and the in planta requirement of simultaneous phosphorylation at two key phosphorylation sites in SLAC1. We identify novel mechanisms ensuring specificity and robustness within stomatal Ca2+-signaling on a cellular, genetic, and biochemical level. Plant leaves have tiny openings or pores called stomata, which allow carbon dioxide, water vapor and other gases to diffuse in and out of the plant. Two cells called guard cells surround each stoma and control the opening and closing of the pore. If a plant is losing excessive amounts of water, for example during a drought, the plant produces a hormone called abscisic acid that promotes the closure of its stomata. When abscisic acid is present, the guard cells are sensitive to changes in their internal concentration of calcium ions so that calcium ions can activate a protein called SLAC1. This leads to responses in the guard cells that close the stoma. The calcium ions activate SLAC1 by stimulating enzymes called calcium-dependent protein kinases (CPKs). However, abscisic acid can also trigger other enzymes that can activate SLAC1 independently of the calcium ions. Calcium ions are also reported to be involved in the opening of stomata, when abscisic acid is not present. Therefore, it is not clear how abscisic acid works to specifically ‘prime’ guard cells to close the stomata in response to increases in calcium ions during drought. Brandt, Munemasa et al. studied stomata in a plant called Arabidopsis thaliana. The experiments show that, in the presence of abscisic acid, mutant plants that lack four different CPK enzymes are impaired in the activation of SLAC1 and the closing of stomata in response to increases in calcium ions. Further experiments found that other enzymes called the PP2Cs—which are switched off by abscisic acid—are responsible for regulating the Ca2+ sensitivity of guard cells. Switching off PP2Cs enables closing of the stomata in response to calcium ions. It has been suggested previously that the CPKs and the calcium-independent enzymes are involved in two separate pathways that promote the closure of stomata. However, Brandt, Munemasa et al. found that the calcium-independent enzymes are required for calcium ions to activate SLAC1 in guard cells, revealing that these two pathways are linked. Brandt, Munemasa et al.'s findings reveal how abscisic acid is able to specifically prime guard cells to close stomata in response to calcium ions. The next challenge is to understand how the CPKs and calcium-independent enzymes work together during the closure of stomata.

The present study aimed to investigate the potential protective role of oxalic acid (OA) against Cadmium (Cd) toxicity. Chickpea (Cicer arietinum L.) seeds were exposed to 200 µM Cd stress for 6 days or to co-treatment with 200 µM Cd + 100 µM OA for 3 days following to 3-day Cd stress. The application of OA ameliorated the growth of both roots and shoots of Cd-treated seedlings. This effect was mediated by the restriction of Cd accumulation in plant tissues. Besides, malondialdehyde (MDA) and carbonyl group contents decreased in OA-treated seedlings, suggesting that OA reversed the Cd-induced oxidative stress and its detrimental effect on cell membrane integrity. These results were further confirmed by the reduction by 2- and 1.7-fold of hydrogen peroxide (H2O2) accrual in roots and shoots, respectively, when compared to Cd-challenged seedlings. Moreover, OA corrected the Cd-imposed imbalance of the glutathione redox state, mainly via the restoration of the glutathione pool. This achievement seems to be the result of the modulation of glutathione peroxidase (GPX) and glutathione reductase (GR) activities. Likewise, OA counteracted the adverse effect of Cd on nicotinamides redox state reflected by the restoration of the balance between oxidized [NAD(P)] and reduced [NAD(P)H] forms. Taken together, our results suggest that the exogenous supply of OA to germinating seeds can be a promising alternative to improve plant tolerance to heavy metal stress.

Stomata regulate gas exchange between plants and atmosphere by integrating opening and closing signals. Stomata open in response to low CO 2 concentrations to maximize photosynthesis in the light; however, the mechanisms that coordinate photosynthesis and stomatal conductance have yet to be identified. Here we identify and characterize CBC1/2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO 2 1/2), two kinases that link BL, a major component of photosynthetically active radiation (PAR), and the signals from low concentrations of CO 2 in guard cells. CBC1/CBC2 redundantly stimulate stomatal opening by inhibition of S-type anion channels in response to both BL and low concentrations of CO 2 . CBC1/CBC2 function in the signaling pathways of phototropins and HT1 (HIGH LEAF TEMPERATURE 1). CBC1/CBC2 interact with and are phosphorylated by HT1. We propose that CBCs regulate stomatal aperture by integrating signals from BL and CO 2 and act as the convergence site for signals from BL and low CO 2. Stomata open in response to low CO 2 conditions in the light to maximise photosynthesis. Here, Hiyama et al. identify two kinases that promote stomatal opening by inhibiting S-type anion channels downstream of phototropin and HT1 thereby acting as a convergence point for blue light and CO 2 signaling.

Increased drug metabolism and elimination are prominent mechanisms mediating multidrug resistance (MDR) to not only chemotherapy drugs but also anti-cancer natural products, such as benzyl isothiocyanate (BITC). To evaluate the possibility of combined utilization of a certain compound to overcome this resistance, we focused on glutathione S-transferase (GST)-dependent metabolism of BITC. The pharmacological treatment of a pi-class GST-selective inhibitor, 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX), significantly increased BITC-induced toxicity in human colorectal cancer HCT-116 cells. However, NBDHEX unexpectedly increased the level of the BITC–glutathione (GSH) conjugate as well as BITC-modified proteins, suggesting that NBDHEX might increase BITC-modified protein accumulation by inhibiting BITC–GSH excretion instead of inhibiting GST. Furthermore, NBDHEX significantly potentiated BITC-induced apoptosis with the enhanced activation of apoptosis-related pathways, such as c-Jun N-terminal kinase and caspase-3 pathways. These results suggested that combination treatment with NBDHEX may be an effective way to overcome MDR with drug efflux and thus induce the biological activity of BITC at lower doses.

• Low concentrations of CO₂ cause stomatal opening, whereas [CO₂] elevation leads to stomatal closure. Classical studies have suggested a role for Ca2+ and protein phosphorylation in CO₂-induced stomatal closing. Calcium-dependent protein kinases (CPKs) and calcineurin-B-like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca2+ into specific phosphorylation events. However, Ca2+-binding proteins that function in the stomatal CO₂ response remain unknown. • Time-resolved stomatal conductance measurements using intact plants, and guard cell patch-clamp experiments were performed. • We isolated cpk quintuple mutants and analyzed stomatal movements in response to CO₂, light and abscisic acid (ABA). Interestingly, we found that cpk3/5/6/11/23 quintuple mutant plants, but not other analyzed cpk quadruple/quintuple mutants, were defective in high CO₂-induced stomatal closure and, unexpectedly, also in low CO₂-induced stomatal opening. Furthermore, K⁺-uptake-channel activities were reduced in cpk3/5/6/11/23 quintuple mutants, in correlation with the stomatal opening phenotype. However, light-mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO₂-regulated stomatal movement kinetics were not clearly affected in plasma membrane-targeted cbl1/4/5/8/9 quintuple mutant plants. • Our findings describe combinatorial cpk mutants that function in CO₂ control of stomatal movements and support the results of classical studies showing a role for Ca2+ in this response.

It is still unclear whether or how quercetin influences the toxic events induced by acetaldehyde in hepatocytes, though quercetin has been reported to mitigate alcohol-induced mouse liver injury. In this study, we evaluated the modulating effect of quercetin on the cytotoxicity induced by acetaldehyde in mouse hepatoma Hepa1c1c7 cells, the frequently used cellular hepatocyte model. The pretreatment with quercetin significantly inhibited the cytotoxicity induced by acetaldehyde. The treatment with quercetin itself had an ability to enhance the total ALDH activity, as well as the ALDH1A1 and ALDH3A1 gene expressions. The acetaldehyde treatment significantly enhanced the intracellular reactive oxygen species (ROS) level, whereas the quercetin pretreatment dose-dependently inhibited it. Accordingly, the treatment with quercetin itself significantly up-regulated the representative intracellular antioxidant-related gene expressions, including heme oxygenase-1 (HO-1), glutamate-cysteine ligase, catalytic subunit (GCLC), and cystine/glutamate exchanger (xCT), that coincided with the enhancement of the total intracellular glutathione (GSH) level. Tin protoporphyrin IX (SNPP), a typical HO-1 inhibitor, restored the quercetin-induced reduction in the intracellular ROS level, whereas buthionine sulphoximine, a representative GSH biosynthesis inhibitor, did not. SNPP also cancelled the quercetin-induced cytoprotection against acetaldehyde. These results suggest that the low-molecular-weight antioxidants produced by the HO-1 enzymatic reaction are mainly attributable to quercetin-induced cytoprotection.

In this study, we examined the involvement of endogenous abscisic acid (ABA) in methyl jasmonate (MeJA)-induced stomatal closure using an inhibitor of ABA biosynthesis, fluridon (FLU), and an ABA-deficient Arabidopsis (Arabidopsis thaliana) mutant, aba2-2. We found that pretreatment with FLU inhibited MeJA-induced stomatal closure but not ABA-induced stomatal closure in wild-type plants. The aba2-2 mutation impaired MeJA-induced stomatal closure but not ABA-induced stomatal closure. We also investigated the effects of FLU and the aba2-2 mutation on cytosolic free calcium concentration ([Ca²⁺] cyt ) in guard cells using a Ca²⁺-reporter fluorescent protein, Yellow Caméléon 3.6. In wild-type guard cells, FLU inhibited MeJA-induced [Ca²⁺] cyt elevation but not ÁÂÁ-induced [Ca²⁺] cyt elevation. The aba2-2 mutation did not affect ABA-elicited [Ca²⁺] t elevation but suppressed MeJA-induced [Ca²⁺] cyt elevation. We also tested the effects of the aba2-2 mutation and FLU on the expression of MeJA-inducible VEGETATIVE STORAGE PROTEIN1 (VSP1). In the aba2-2 mutant, MeJA did not induce VSP1 expression. In wild-type leaves, FLU inhibited MeJA-induced VSP1 expression. Pretreatment with ABA at 0.1 μm, which is not enough concentration to evoke ABA responses in the wild type, rescued the observed phenotypes of the aba2-2 mutant. Finally, we found that in wild-type leaves, MeJA stimulates the expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE3, which encodes a crucial enzyme in ABA biosynthesis. These results suggest that endogenous ABA could be involved in MeJA signal transduction and lead to stoma tal closure in Arabidopsis guard cells.

Reactive oxygen species (ROS) mediate abscisic acid (ABA) signaling in guard cells. To dissect guard cell ABA-ROS signaling genetically, a cell type-specific functional genomics approach was used to identify 2 MAPK genes, MPK9 and MPK12, which are preferentially and highly expressed in guard cells. To provide genetic evidence for their function, Arabidopsis single and double TILLING mutants that carry deleterious point mutations in these genes were isolated. RNAi-based gene-silencing plant lines, in which both genes are silenced simultaneously, were generated also. Mutants carrying a mutation in only 1 of these genes did not show any altered phenotype, indicating functional redundancy in these genes. ABA-induced stomatal closure was strongly impaired in 2 independent RNAi lines in which both MPK9 and MPK12 transcripts were significantly silenced. Consistent with this result, mpk9-1/12-1 double mutants showed an enhanced transpirational water loss and ABA- and H₂O₂-insensitive stomatal response. Furthermore, ABA and calcium failed to activate anion channels in guard cells of mpk9-1/12-1, indicating that these 2 MPKs act upstream of anion channels in guard cell ABA signaling. An MPK12-YFP fusion construct rescued the ABA-insensitive stomatal response phenotype of mpk9-1/12-1, demonstrating that the phenotype was caused by the mutations. The MPK12 protein is localized in the cytosol and the nucleus, and ABA and H₂O₂ treatments enhance the protein kinase activity of MPK12. Together, these results provide genetic evidence that MPK9 and MPK12 function downstream of ROS to regulate guard cell ABA signaling positively.