Plants in nature are constantly exposed to organisms that touch them and wound them. A highly conserved response to these stimuli is a rapid collapse of membrane potential (i.e. a decrease of electrical field strength across membranes). This can be coupled to the production and/or action of jasmonate or ethylene. Here, the various types of electrical signals in plants are discussed in the context of hormone responses. Genetic approaches are revealing genes involved in woundinduced electrical signalling. These include clade 3 GLUTAMATE RECEPTOR-LIKE (GLR) genes, Arabidopsis H +-ATPases (AHAs), RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), and genes that determine cell wall properties. We briefly review touch-and wound-induced increases in cytosolic Ca 2+ concentrations and their temporal relationship to electrical activities. We then look at the questions that need addressing to link mechanostimulation and woundinduced electrical activity to hormone responses. Utilizing recently published results, we also present a hypothesis for wound-response leaf-to-leaf electrical signalling. This model is based on rapid electro-osmotic coupling between the phloem and xylem. The model suggests that the depolarization of membranes within the vascular matrix triggered by physical stimuli and/or chemical elicitors is linked to changes in phloem turgor and that this plays vital roles in leaf-to-leaf electrical signal propagation.

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Stomata are the major gates in plant leaf that allow water and gas exchange, which is essential for plant transpiration and photosynthesis. Stomatal movement is mainly controlled by the ion channels and transporters in guard cells. In Arabidopsis, the inward Shaker K channels, such as KAT1 and KAT2, are responsible for stomatal opening. However, the characterization of inward K channels in maize guard cells is limited. In the present study, we identified two KAT1-like Shaker K channels, KZM2 and KZM3, which were highly expressed in maize guard cells. Subcellular analysis indicated that KZM2 and KZM3 can localize at the plasma membrane. Electrophysiological characterization in HEK293 cells revealed that both KZM2 and KZM3 were inward K (K ) channels, but showing distinct channel kinetics. When expressed in Xenopus oocytes, only KZM3, but not KZM2, can mediate inward K currents. However, KZM2 can interact with KZM3 forming heteromeric K channel. In oocytes, KZM2 inhibited KZM3 channel conductance and negatively shifted the voltage dependence of KZM3. The activation of KZM2-KZM3 heteromeric channel became slower than the KZM3 channel. Patch-clamping results showed that the inward K currents of maize guard cells were significantly increased in the KZM2 RNAi lines. In addition, the RNAi lines exhibited faster stomatal opening after light exposure. In conclusion, the presented results demonstrate that KZM2 functions as a negative regulator to modulate the K channels in maize guard cells. KZM2 and KZM3 may form heteromeric K channel and control stomatal opening in maize.

Stomatal closure is an important process to prevent water loss in plants response to drought stress, which is finely modulated by ion channels together with their regulators in guard cells, especially the S-type anion channel AtSLAC1 in Arabidopsis. However, the functional characterization and regulation analyses of anion channels in gramineous crops, such as in maize guard cells are still limited. In this study, we identified an S-type anion channel ZmSLAC1 that was preferentially expressed in maize guard cells and involved in stomatal closure under drought stress. We found that two Ca 2+ -dependent protein kinases ZmCPK35 and ZmCPK37 were expressed in maize guard cells and localized on the plasma membrane. Lesion of ZmCPK37 resulted in drought-sensitive phenotypes. Mutation of ZmSLAC1 and ZmCPK37 impaired ABAactivated S-type anion currents in maize guard cells, while the S-type anion currents were increased in the guard cells of ZmCPK35-and ZmCPK37-overexpression lines. Electrophysiological characterization in maize guard cells and Xenopus oocytes indicated that ZmCPK35 and ZmCPK37 could activate ZmSLAC1-mediated Cland NO 3 currents. The maize inbred and hybrid lines overexpressing ZmCPK35 and ZmCPK37 exhibited enhanced tolerance and increased yield under drought conditions. In conclusion, our results demonstrate that ZmSLAC1 plays crucial roles in stomatal closure in maize, whose activity is regulated by ZmCPK35 and ZmCPK37. Elevation of ZmCPK35 and ZmCPK37 expression levels is a feasible way to improve maize drought tolerance as well as reduce yield loss under drought stress.

Potassium and nitrogen are essential mineral elements for plant growth and development. The protein kinase LKS1/CIPK23 is involved in both K+ and NH4+ uptake in Arabidopsis root. The transcripts of LKS1 can be induced by low K+ (0.1 mM) and high NH4+ (30 mM); however, the molecular mechanism is still unknown. In this study, we isolated the transcription factor STOP1 that positively regulates LKS1 transcription in Arabidopsis responses to both low-K+ and high-NH4+ stresses. STOP1 proteins can directly bind to the LKS1 promoter, promoting its transcription. The stop1 mutants displayed a leaf chlorosis phenotype similar to lks1 mutant when grown on low-K+ and high-NH4+ medium. On the other hand, STOP1 overexpressing plants exhibited a similar tolerant phenotype to LKS1 overexpressing plants. The transcript level of STOP1 was only upregulated by low K+ rather than high NH4+; however, the accumulation of STOP1 protein in the nucleus was required for the upregulation of LKS1 transcripts in both low-K+ and high-NH4+ responses. Our data demonstrate that STOP1 positively regulates LKS1 transcription under low-K+ and high-NH4+ conditions; therefore, LKS1 promotes K+ uptake and inhibits NH4+ uptake. The STOP1/LKS1 pathway plays crucial roles in K+ and NH4+ homeostasis, which coordinates potassium and nitrogen balance in plants in response to external fluctuating nutrient levels.