Convergence and Divergence of Signaling Events in Guard Cells during Stomatal Closure by Plant Hormones or Microbial Elicitors

Abstract

Dynamic regulation of stomatal aperture is essential for plants to optimize water use and CO2 uptake. Stomatal opening or closure is accompanied by the modulation of guard cell turgor. Among the events leading to stomatal closure by plant hormones or microbial elicitors, three signaling components stand out as the major converging points. These are reactive oxygen species (ROS), cytosolic free Ca(2+), and ion channels. Once formed, the ROS and free Ca(2+) of guard cells regulate both downstream and upstream events. A major influence of ROS is to increase the levels of NO and cytosolic free Ca(2+) in guard cells. Although the rise in NO is an important event during stomatal closure, the available evidences do not support the description of NO as the point of convergence. The rise in ROS and NO would cause an increase of free Ca(2+) and modulate ion channels, through a network of events, in such a way that the guard cells lose K(+)/Cl(-)/anions. The efflux of these ions decreases the turgor of guard cells and leads to stomatal closure. Thus, ROS, NO, and cytosolic free Ca(2+) act as points of divergence. The other guard cell components, which are modulated during stomatal closure are G-proteins, cytosolic pH, phospholipids, and sphingolipids. However, the current information on the role of these components is not convincing so as to assign them as the points of convergence or divergence. The interrelationships and interactions of ROS, NO, cytosolic pH, and free Ca(2+) are quite complex and need further detailed examination. Our review is an attempt to critically assess the current status of information on guard cells, while emphasizing the convergence and divergence of signaling components during stomatal closure. The existing gaps in our knowledge are identified to stimulate further research.

Keywords: ABA; ROS; cytosolic free Ca2+; cytosolic pH; guard cells; ion channels; nitric oxide; secondary messengers.

Figure 1

Key points of convergence and divergence during stomatal closure in response to plant hormones and elicitors. Stomatal closure is the result of ion efflux out of guard cells, loss of their turgor, and forms the ultimate step during signal transduction. We suggest that ROS, cytosolic free Ca²⁺, and ion channels form points of convergence during stomatal closure by a variety of abiotic/abiotic factors. Similarly, ROS, NO, and cytosolic Ca²⁺ are identified as points of divergence. The activation of NADPH oxidase and ROS production are among the earliest events. Similarly, the modulation of ion channels, influx of free Ca²⁺ along with efflux of K⁺ and anions, are the final steps, leading to the loss of ions/turgor of guard cells. The binding of ABA to RCAR/PYR or Flg22 to FLS2 or SA to S-receptor are well established, while receptors of cryptogein, chitosan, and PAs are yet to be characterized. ROS: When ABA binds to the receptor (RCAR/PYR/PYL), PP2C becomes non-functional, leading to phosphorylation, and activation of OST1 protein kinase. The elevated kinase activity along with Ca²⁺, activates NADPH oxidase, and subsequently elevates ROS production. Besides NADPH oxidase, CuAO/PAO are also involved in the increase of ROS in guard cells. The levels of ROS can be elevated by also peroxidase, for e.g., upon salicylic acid binding to its receptor. Further, G-protein alpha subunit induces the ROS production through the activation of NADPH oxidase. Modulation of ROS levels by NO, cytosolic Ca²⁺, cytosolic pH can occur by direct or indirect mechanisms but these reactions need to be established. Cytosolic free Ca²⁺: the rise in the levels of ROS and NO, can increase the levels of cytosolic free Ca²⁺, by either release of Ca²⁺from internal stores or influx of external Ca²⁺ through plasma membrane ${\text{Ca}}_{\text{in}}^{2+}$channels. Ca²⁺ also activates SLAH3 and SLAC1 ion channels, while inhibiting ${\text{K}}_{\text{in}}^{+}$ ion channels. Ion channels: the modulation of cation/anion channels results in the net efflux of K⁺/Cl⁻/ malate and influx of Ca²⁺, making guard cells to lose turgor and causing stomatal closure. NO: NR, nitrate reductase; NOA, nitric oxide associated 1 are the sources of NO. Although there are suggestions that ROS, cytosolic Ca²⁺ or cytosolic pH can elevate NO levels, the mechanism is not known. The rise in NO leads to divergent actions, namely the rise in cytosolic Ca²⁺, activation of PLD, and subsequently NADPH oxidase. Further, NO activates ${\text{K}}_{\text{out}}^{+}$ ion channels, inhibits K⁺ channels, and activates ${\text{Ca}}_{\text{in}}^{2+}$ ion channels. Other components: The role of cytosolic pH is not completely understood. The available evidence suggests that the cytosolic pH may act parallely with the events involving ROS/NO/cytosolic free Ca²⁺. Similarly, G-proteins, phospholipids, phospholipases, phosphatidyl inositol kinases, sphingolipids, and MAP kinases also act in such a way to cause the loss of turgor in guard cells and stomatal closure. Solid arrows represent the events which are documented, while broken arrows represent the possible effects/suggestions.